The Physics of Sex Widget Game

In the tradition of physicists through the ages, I've attempted to make a very simple model of a complex system.

This image is screen shot from a game that simulates some of the feedback issues involved in sex. It's the Physics of Sex equivalent of a spherical cow.

To play the game, you have to stimulate the floating ball with your mouse. If you do it properly, the meter on the left will show your progress. Ultimately the meter will top out and the ball will turn red and throb. I'm not going to tell you exactly how to stimulate the ball. Just like learning about the birds and bees in real life, you'll have to discover some of the details on your own. Also just like real life, it's not that hard to figure out.

If you want to give it a try, click the image to download the Widget file. If you've never installed a Widget before, you'll first have to install the free Yahoo Widget 4 engine. It should work automatically on a Mac.

The game is a bit simple and not too challenging at the lower levels(although I haven't beaten it at Level 12 yet), but it illustrates three things about sex

1. Positive feedback (provided by the level meter) helps you achieve the ultimate goal.

2. Negative feedback is necessary to help you follow the ball and apply the appropriate stimulation at the apprpriate place. That is, when the mouse cursor is too far from the ball you adjust by bringing it back.

3. The simplified orgasms simulated by playing the game, which I claim are similar to the type that men have most of the time and women have at least some of the time, are essentially the result of integrate-and-fire circuits.

The first two points are probably familiar to most folks, but integrate-and-fire circuits are a bit more obscure. Basically, this type of circuit measures some input and when it reaches a trigger point it fires.

Avalanches look a lot like integrate-and-fire circuits - snow builds and builds on a mountain until it's unstable, then the slightest disturbance can send it careening down the slope.

Neurons are often described as leaky integrate-and-fire circuits. That means that the correct input can push the neuron toward firing, but if the stimulation stops the neuron will gradually lose memory of the stimulus and return to its resting state.

Picture it in terms of a leaky balloon. If you blow it up far enough, it will eventually pop. But if you take a break before it blows, the air will slowly escape and the balloon will deflate.

I made my widget leaky too. If you stimulate the ball, the meter will climb, but stop for a while and the meter will slowly drop back to zero.

Another aspect of integrate-and-fire circuits is the fact they often experience a refractory period after firing. During that time, they don't respond to any stimulation at all. If you blow up a leaky balloon until it pops, the refractory period corresponds to the time it takes you to find another balloon. Men are intimately familiar with the refractory period that follows sex, and older men know that it seems to take longer to find their balloons with every passing year.

I added a refractory period to my model as well. If you manage to get the ball to throb, you have to wait a few moments before it's ready for you to start again.

I adapted the game from an even simpler game called Focus developed by Aaron McBride of MIT.

Check out the thousands of other widget in the Yahoo gallery. Lot's of them are as useless as the Physics of Sex widget, but plenty of them are handy and/or cool, and most are made by amateur programmers with an idea and a little free time.

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A Climactic Hiccup Cure (and your chance to help test it)

"Sex is good for lots of things - now it seems we can add hiccup cure to the list."

If you follow the annual presentation of the tongue-in-cheek Ig Nobel Prize, then you already know that modern medicine has come up with at least one promising hiccup cure. As is often the case for the Nobel Prizes that the Ig Nobel parodies, the recognition of Francis Fesmire's work came much later than it should have.

Back in 1988 Fesmire published a revolutionary paper entitled Termination of intractable hiccups with digital rectal massage in the journal Annals of Emergency Medicine (annals . . . that's funny).

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I've heard jokes about how he might have discovered the effect, but it's not really such a stretch. The key to Fesmire's discovery may be stimulation of the vagus nerve. Other researchers have noted the connection between the vagus nerve and hiccups. Unlike most of the nerves that make their way from your brain to other parts of your body through the spinal column, the vagus nerve is a major nerve bundle that starts at your brain stem and winds its way through your abdomen. In fact the FDA has approved an implantable vagus nerve stimulator for controlling hiccups with electrical bursts.

Personally, I think Fesmire's discovery is a much cleverer way to stimulate the nerve. It makes sense because, among other things, the vagus nerve connects to the sphincter muscles of the gastrointestinal system (including the anus) as well as many muscles and organs involved in hiccuping.

For those of you uncomfortable with the massage, there's an alternative. In 2000, Roni and Aya Peleg published a case report in The Canadian Family Physician journal reporting their observation of sexual intercourse as potential treatment for intractable hiccups.

Sex is good for lots of things - now it seems we can add hiccup cure to the list. That's cool, but it begs the question as to why (and if) sex has anything to do with hiccups.

Now, I consider myself to be a skeptic as a rule. But I also try to be open minded, so I've been withholding judgement on these particular cures until I could see further data. As it happens, I came down with a heavy duty case of hiccups a few days ago . . .

Naturally, I thought I would try one of the cures myself. The massage thing seemed a bit involved and messy, so I went with the alternative. Considering the fact that I was in a rush to try it before the hiccups ended on their own and I didn't want the confounding complication of involving anyone else in the experiment, I went solo.

It worked perfectly. At the climactic moment, my hiccups ceased.

As a result, I was inspired to see if any physicists had taken a look at hiccups and whether they had anything useful to say about the phenomenon.

It turns out that in 1995 W. A. Whitelaw of the University of Calgary, along with Parisians J.-Ph. Derenne of the Groupe hospitalier de la Pitié-Salpêtrière and J. Caban of the Hopital St. Antoine published a paper in the physics journal Chaos titled Hiccups as a Dynamical Disease."= They concluded that hiccups are produced by a central pattern generator (CPG). A CPG is a neuron circuit that generates a signal, which causes an action that in turn stimulates another signal, and the pattern repeats, sometimes indefinitely. Similar circuits apparently handle numerous other repetitive actions such as breathing and walking

The Hiccup Generator as a "Black Box"

Just what all the components are in the hiccup CPG isn't entirely clear. What's more, it doesn't really matter. Instead the researchers treated the hiccup CPG as a black box. To an engineer or scientist, a black box is a system that's studied in terms of what it does, rather than what it's made of. In other words, the physicists studied the behavior of the biological system that causes hiccups without worrying too much about the individual pieces that go into it. The work led to some interesting insights, including the fact that the rhythms of hiccups seem to be tied to breathing rates and heartbeats, but it didn't do much in the way of offering any new cures.

As I see it, the most important aspect of the research is the simplified perspective on hiccups. We have a hiccup black box in our bodies that normally is in the 'off' state. Any number of disturbances can turn it on: eating too quickly, coughing, drinking a hot liquid, drinking a cold liquid, a sudden shock, a sneeze, acid reflux, or even (though, thankfully, rarely) tumors, renal failure, or chemotherapy.

Many causes of hiccups (that aren't related to diseases, anyway) involve a chemical or physical shock that kicks the hiccup black box out of its resting state and into its annoying active state.

A simple way to generally illustrate this sort of thing is to imagine a bunch of kids playing soccer (football for those of you outside the US) at the bottom of a valley. When one of the kids kicks the ball hard enough, they might knock it over the ridge of the valley wall and into a neighboring valley. If the valley next door is not as deep, the kids over there will soon kick the ball back over the ridge to the soccer game. How long that takes depends in part on the height of the ridge between the valleys, and in part on the random chance that some kid kicks the ball hard enough to clear the hill.

We see lots of situations like this in physics; an electron in its lowest orbit in a hydrogen atom can absorb a photon and get kicked into a higher orbit; an atom possessing a characteristic called spin can be flipped from one orientation along a magnetic field to the opposite orientation (this is critical for magnetic resonance imaging); and some types of glass that radically change state when heated in certain ways (a technology based on these glasses may eventually lead to novel data storage chips), to name just a few of the countless examples.

Often in physics we see systems that have been knocked from their ground states (the states they naturally prefer to be in) to higher states, which spontaneously drop back some random amount of time later. If you don't feel like waiting, hitting a system with another shock that's similar to the one that bumped it out of its ground state often knocks it back. In the case of an electron in a higher orbit around its atom, this is called stimulated emission. It takes a photon to get the electron up there in the first place, and another photon can induce the electron to fall back to the ground state immediately instead of randomly.

Hiccups work essentially the same way - a shock to your system bumps the hiccup CPG into its active state. Simply waiting will often be enough that the bout stops on its own as the CPG randomly returns to its resting state. But if you're impatient, any number of hiccup cures that rely on physical or chemical shocks to your system may do the trick immediately.

Trying to scare the hiccups out of someone is obviously a physical shock. The spoon full of sugar cure is a chemical shock to your mouth, throat, and stomach. Holding your breath, breathing into a paper bag, and related asphyxiating cures cause a chemical shock through a relatively rapid build up of carbon dioxide. I could keep going down the list, but as far as I know just about every folk cure involves the equivalent of stimulated emission to kick your CPG to its resting state.

The interesting thing about Fesmire's digital massage is that he is taking advantage of the fact that while we know very little about what's inside the hiccup black box, we know about one thing in there - the vagus nerve. (Remember, the vagus nerve stimulator implant is the only FDA approved hiccup cure.)

The other thing we know about the vagus nerve is that it's involved in orgasm. This was shown in recent studies with paraplegic women who had lost sensation in their lower bodies as the result of back injuries. The startling outcome of the experiments was that they could still experience orgasm from stimulation of their genitals. The researchers believe that the orgasms must involve the vagus nerve because it's the only intact nerve pathway back to their brain stems.

Some folks might prefer Fesmire's massage, but I'm guessing that most people would choose the orgasm stimulation to tickle their vagus nerves and kick the hiccup CPG back to its resting mode.

Do Your Part for Science

As most scientists will tell you, anecdotal evidence is pretty unreliable. And even though I experienced the cure myself, I'm willing to accept the possibility that the success was coincidental. The Pelegs’ case study adds to the evidence, but that's still only two tests.

We need more data. I'm willing to try again, but I don't get hiccups very often.

I'm hoping that you will help test the cure. The next time you get hiccups, and have enough time and privacy to do the experiment, have an orgasm (alone or with a friend) and write to me to let me know whether or not it cured the problem.

I'll compile the data and report back as soon as we have a clear answer one way or the other.

You can post your results in the comment section of this post or email me the results at "BuzzSkyline at gmail dot com."

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Woman on Top: Closing the Feedback Loop

When I was on Tiffany Granath's show a few weeks ago, we took several questions from listeners who called in. A few of the topics lay at the very edge of the domain of physics as it applies to sex, but most were excellent questions that I was pretty comfortable dealing with.

One fellow in particular said that he and his wife have a good sex life, but she's only fully satisfied if she's on top when they make love. He was wondering why that is and what he could do to add some variety without neglecting her needs.

My guess (given the caveat that I was working with a minimum of data) was that he should take into account his wife's sexual feedback loop.

In physics, engineering, and other sciences, we often think of experimental systems as being open loops or closed loops.

An open loop system is one that has a control, which is also known as an input (think of a volume knob on your radio, or the handle on your water spigot), and an output (the radio volume or the amount of water flowing through your garden hose), but no feedback. That is, the person adjusting the radio volume is deaf and cannot hear when the sound level is correct, or the garden hose extends around a corner and you can't tell how much water is pouring out of it.

Alternatively, a closed system sends some information about the output back to the input. In other words, you turn the knob on the radio until the volume is correct, and then you either stop turning or turn it back a bit. By watching the spray coming from your sprinkler, you know whether you have turned the spigot handle as far as you need to in order to water your yard. In either case you're using information about the output to adjust the input.

Open loop systems work fine for lots of applications, and are particularly handy if you just want to turn something all the way up or entirely off. (In electronics, a common jargon for open loop amplifiers is to say that they "go to the rails," which means they can either put out the lowest voltage or the highest voltage that the power supply can handle, but they don't provide any intermediate voltages.)

If you need some reasonable amount of control over an output, you must have feedback. Sexual response can be considered one of nature's closed feedback loops. The input of of sensual contact leads to pleasurable signals passed through the nerves to the brain. In order to work well, information about the pleasurable signals have to make it back to adjust the sensual contact.

If you're masturbating, you don't need any help figuring it out - you just do what the feedback from your nerves tells you feels good. When you're making love with another person, feedback is a lot trickier. You can't share your partner's sensations directly, so you have to rely on secondary clues - by observing the way they're moving or the sounds they.remaking. The loop is more or less closed, but the feedback is relatively tenuous.

A woman who is on top during intercourse, however, can take advantage of her own strong sensory feedback to ensure that the right spot is being stimulated in the right way.

That is, it may not be the woman-on-top position itself that satisfied the caller's wife. It may instead be an issue of closing her feedback loop.

There are numerous ways for the caller and his wife to attempt to get the same result while making love in other positions. For one thing, he could work harder to interpret his wife's responses to his actions. Studying her movements or the sounds she makes during sex may strengthen the feedback enough to close the loop. Of course, it's important for the woman to broadcast her pleasure as much as possible as well. It can be very difficult to satisfy a woman who is too shy to communicate what she needs and enjoys.

Simply holding relatively still while she sets the pace may be enough to help the caller out. It's possible to accomplish this even in the traditional missionary position, if the man supports himself a bit as the woman thrusts her hips rhythmically. Placing a pillow under the woman's buttocks to raise her hips may make this easier to accomplish. It's worth experimenting with other sexual positions - any position that limits the man's motion while leaving his partner free to take charge will shift the focus and the feedback into the woman's control.

Another possibility is to encourage her help out by stimulating her clitoris or nipples to let her strengthen the sensory feedback loop herself, regardless of the sexual position they are using.

One advantage to focusing on your partner's feeback loop is that it often comes at the expense of your own feedback. That can help slow things down if you tend to finish sooner than your partner would like.

You'll get similar advice from traditional sex therapists and experts, but they usually talk about things like communication and sensitivity to your partner's needs. That's all good, but personally, I feel it's easier to think in terms of feedback loops. Of course, I'm just a physics nerd, and I tend to consider sex in terms of the little diagram you see here. If you click the picture, you can visit the Wikipedia entry that explains (in engineering jargon) the meaning of the components in the schematic.
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Physics, Sex, and Comics

I wasn't surprised to learn that other people have already noticed the intimate connection between sex and physics, but I was amazed to see how well Randall Munroe portrays the connection in comic strip form.

If you click the image here, you can see one of Munroe's takes on the intersection of passion, sex and physics.

Some of my other favorites include

Angular Momentum
and
The Romatic Drama Equation

In truth, only some of Munroe's comics are about physics and sex. Many of them touch on computer programming, math, or random topics that interest him, like this interesting supermarket prank.

It's funny stuff. Check out the rest of the 'toons to see what I mean.

Thanks to my good friend Davide the science writer for letting me know about xkcd.com.
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Part 2 of The Physics Guide to Hooking Up: Why It's Better to Pursue than to be Pursued, or the Trouble with Rule 6

I'm sorry to burst the bubble of any Disney fans out there, but Prince Charming almost certainly lived more happily ever after than Snow White did.

I'm saying this horrible thing because a famous mathematical puzzle known as the Stable Marriage Problem shows that a person who pursues a mate is almost always more satisfied with their spouse than a person who is pursued.

This is a particularly important fact for women who adhere to The Rules, because physics and math suggest that rule number 6 of the top ten rules for women appears to be very, very wrong.


Skip to the Tip in this week's post.

The first analysis of the Stable Marriage Problem was described in a 1962 paper by mathematician David Gale and economist Lloyd Shapley. They were attempting to determine if a set of 100 men and 100 women could pair up in marriages in a way that no one could find a better mate in the bunch who would have them.

They way they set up the problem goes like this . . .

Each man ranks all 100 women from their first choice of potential partner to their last. The women all do the same for the men. Because the reasons one person finds another attractive is often mysterious, Gale and Shapley selected each person's ranking of potential mates at random. As a result, no two rankings were alike and one person's top choice would likely be farther down on any other person's list. Once everyone has their ranked list, the marriage game begins.

Probably because the paper was written way back in 1962, pairings among men and women occur when a man proposes and a woman accepts. To begin with, the first man proposes marriage to the woman at the top of his list. Because it's early in the game and this is the only marriage proposal the woman has gotten, she accepts (remember, it's just a simple model).

Once the first pairing is out of the way, the second man proposes to the top woman on his list. Assuming she's not engaged to the first man, she accepts. If, however, she happens to be the fiancé of the first man, the woman looks at the ranking of her two suitors and chooses to go with the one she ranked highest.

The game continues with each man in turn proposing to women in the order that he ranked them. As he goes down the list, a woman will accept his proposal if she is either unattached or engaged to a man who she ranks lower than him. Any man who has been thrown over for a higher ranking fellow eventually goes back down his list looking for a woman who is single or prefers him to her current fiancé.

Gale and Shapley found that there are always stable solutions to the problem (usually many solutions, in fact), regardless of the number of people involved. Stability in this case means that once everything is sorted out, a man who checks out all the other couples in the group would not find a woman he prefers over his own fiancé who also ranks him higher than her fiancé.

Mathematically speaking, that's a pretty interesting result. But it's not terribly useful or informative for real people like you and me. The truly fascinating revelation, in my opinion, is that something very surprising comes out of the study if you consider the relative satisfaction of men and women in the model. Specifically, if you look at the ranking of the women who the men ended up with, most men got engaged to a woman who was high on their list. Women, on the other hand, were stuck with men who ranked relatively low on their lists.

To put hard numbers on it, in an expanded a study of 1000 stable solutions to the problem when it included 512 couples, men on average hooked up with women who ranked 8th on their respective lists, while women were engaged to men who ranked an average of 80th. That's a huge discrepancy. Bear in mind that the only difference between men and women in the mathematical model is that men always proposed and women only accepted or rejected proposals.

You'd be right to take all this with a grain of salt. Mating in real life is a much more complicated affair. Even a slight modification of the problem, such as adding the potential for degrees of inherent beauty among the men and women, can radically change the numbers of stable solutions and the average degree of satisfaction. (Some realistic details can actually make the problem so complex that it's essentially unsolvable.) Nevertheless, in general when only men made proposals they were much better off than the women.

The world has changed a lot since '62. Back then, the Stable Marriage Problem didn't have a lot of relevance to the actual complexities of dating and mating. These days, there is one situation that pretty closely approximates the bare-bones problem that Gale and Shapley studied - online dating services.

When you join Yahoo Personals or some other matching service, you post your profile and often your picture. You then have a choice; you can sit and wait for invitations (for dates usually, rather than marriage) to come rolling in, or you can check out the profiles of other people and decide who you would like to contact.

If you passively wait for someone to write to you, you mimic the behavior of the women in the Stable Marriage Problem. That is, you sit on your hands waiting for an email or an instant message from a suitor, then you check out their profile and either accept or reject them.

If instead you take the initiative, you act like the men in the Stable Marriage Problem. You perform some sort of ranking and choose the person you want to contact most from all the people who have posted profiles. If the first person you write to rejects you, you are forced to move farther down in your list of possibilities.

According to the solutions of the Stable Marriage Problem, if you take the initiative in asking out the people you're most attracted to you will meet much more desirable people through online dating services than you would if you wait for someone to contact you.

This runs completely counter to rule number six of the top ten rules for women, which reads "When considering whether to use personal ads or other dating services, you should place the ad and let men respond to you."

Don't get me wrong, I think many of The Rules work just fine. Most of the rules will help a woman play a man like a starving trout hooked on a line. But if you ignore rule six and take the initiative in luring a mate, you dramatically increase your odds of landing a trophy catch rather than some loser you'll want to heave back into the pond.
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Book Review: Sex in Space

It turns out that my family has a tenuous connection to tabloid-headlining astronaut Lisa Nowak. Apparently, she's a few years younger than my uncle and they went to middle school together in the suburbs of Maryland's upscale Montgomery County.

According to my family's lore, one day my uncle was using crutches because of a leg injury, and Lisa kicked one crutch out from under him. He wasn't hurt as a result of the alleged assault, and knowing my uncle I'm pretty sure Lisa had a good reason to do it, assuming it's true (over years of retelling, we tend to embellish and distort stories like this in my clan).

My uncle and Lisa met up again years later when they were both flying in the US Navy. I don't think she attacked or threatened him the second time they crossed paths, but it's possible that they were both wearing diapers (my uncle has applied for astronaut slots on occasion, I'm guessing he might have had to wear the diapers as part of the flight testing process).

The Lisa Nowak love triangle naturally led me to wonder about the status of sex in space. Lisa never flew on a shuttle mission with the astronaut who was the object of her affections, so it seems unlikely that she had a chance to do a zero-g tango. The question is: have any other space travelers attempted sexual relations in orbit or beyond?

There's more to it than simple titillation. President Bush has declared travel to Mars and the establishment of a lunar base to be official goals of our space program. In either case, humans will spend extended periods in low gravity environments. Sex is an important part of human interactions. Whether or not astronauts have attempted to make love during past missions, it's hard to imagine that at least some of them won't try it during excursions lasting months to years.

This raises several concerns. We don't know whether prophylactics will work properly in space. We can't be certain that we can conceive children in low-g. And if we can, we have no idea what effect it would have on the fetus. Is gravity necessary for fetal development, or will space children suffer birth defects? Assuming the lack of Earth-like gravity itself is not a problem, will we find ways to protect sperm, ova and fetuses, not to mention astronaut parents, from the increased levels of radiation in extraterrestrial environments?

Fortunately, Laura Woodmansee has taken time to investigate the latest wisdom on all these issues and more, and compiled them in a very tasteful book entitled Sex in Space. Woodmansee is a science journalist who specializes in covering the space program. Two of her other books, Women Astronauts and Women in Space: Cool Careers on the Final Frontier specifically focus on the female astronaut contingent.

Although Sex in Space is a brief 136 pages long, Woodmansee covers topics such as whether or not anyone has had sex in space (the official answer is 'no, but the extensive hours that people have spent in space in the past 50 years and the numerous opportunities available to them suggests that there's a strong possibilty that the true answer is 'a few times'), how they might make love if given the chance, the effect of low-g on astronaut libidos, and the future potential for honeymoon trips to space.

OK, I confess, I turned first to Chapter 2 - How to make love in space. Woodmanse includes several instructional diagrams of possible positions, and brings up issues I never thought of - like just how sloppy space sex is likely to be. But once I finished that portion and went back to read the rest of the book, I found there was plenty to learn about space sex that never would have crossed my mind without Woodmansee's guidance.

One thing that didn't surprise me in reading Woodmansee's book is that NASA has not conducted any official studies of sex between humans in space. Large, formal institutions don't deal with sex well, as Nowak's troubles seem to confirm. In my opinion, however, turning a blind eye to a natural and important part of human behavior is nothing short of irresponsible, particularly if they seriously mean to put people into space for long periods.

Even a simple mission to Mars and back is going to take years. During that time, it's highly likely that some astronauts will experiment with sex. Besides, sexual intimacy is probably a good way to maintain a happy and cohesive crew, provided the whole thing is carefully thought out. After all, they will likely spend most of their time cooped up in a craft about the size of a school bus (at best). The intrepid explorers are going to need all the stress relief they can get.

Ideally, I think NASA administrators and scientists should read Woodmansee's book, and then get to work designing a comprehensive study of sex in space. At the very least, it would be a powerful rebuttal to the concerns of critics who feel that the International Space Station is a waste of time and money that could be better spent on unmanned and robotic missions. Robots can do just about everything humans can do in space except help us to anticipate the various aspects of low-g sex and conception.

Whether we like it or not, sex is going to be among the most important issues we will face if we are ever to truly to break free of Earth's gravitational bonds and move out into the vast galaxy that surrounds us. So NASA might as well face the facts and start investigating the science of sex is space.
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Those ain't just your daddy's genes (and maybe not your momma's)

You probably get half of your genes from your mother and half from your father, but it's possible that you got some of your genes from someone - or something - else.

A new model proposed by Jeong-Man Park of Rice University in Houston and his colleague Michael Deem (the same guy working on the HIV vaccination scheme I mentioned a few posts back) suggests that much a significant portion of our DNA was donated by viruses and bacteria that infected our ancestors over the ages. Although the chances are slim, it's possible that some of your DNA comes from microbes that infected your mother or father.

Park and Deem were led to the conclusion as they sought a theoretical answer to the question of why evolution proceeded fairly slowy for 2.5 billion years, as simple multi-cellular organisms developed, and then raced ahead for the next billion years to produce you and me and Brad and Angelina.

The answer may be horizontal gene transfer (HGT). When it was first proposed as a mechanism for bacteria to trade chunks of DNA and effectively adapt without reproducing, the idea of HGT was very controversial. By looking at common sections of DNA in species that should not be related, many scientists have come to the conclusion that we must be exhanging DNA through HGT. In fact, it seems to be at least as important for evolution as the passing on of mutations through sexual reproduction. Among other things, it appears that our immune systems arose from a gene transfer that must have occurred about 400 million years ago.

Park and Deem presented their model of HGT enhanced evolution in a paper published in the journal Physical Review Letters in January. In addition, Deem and Jun Sun (also of Rice University) presented a paper at this week's APS March meeting that shows how genes consist of modular chunks that lead to various traits, rather than having the genetic information spread throughout your genes. This modularity could be handy when it comes to swapping useful blocks of DNA.


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A More Modest Security Scanner

At the Physics of Sex blog, we're huge supporters of freedom and tolerance. But part of ensuring those prescious commodities includes protecting personal privacy. Recently, some airports have installed backscatter x-ray scanners that see through clothing, revealing weapons in the very rare case that someone tries sneaking something on board, while giving security staff a gander at the most intimate details of the bodies of terrorists and innocents alike. You can see some examples in this Google image search. In addition, although the risk is low, you have to get at least a small x-ray dose to suffer the indignity.








Tomorrow morning at the APS March Meeting in Denver, Panu Helisto of the Finnish research company VTT will describe a new imaging system that measures some of the heat-radiation your body emits all the time. It is inherently unable to reveal personal details because it simply lacks resolution to produce a picture of anything smaller than several inches across. And yet it measures terahertz radiation (a type of radiation that's somewhere between infrared light and radio waves) that passes through all but the heaviest clothing, to provide enough detail to pick out the shapes of most knives, guns, and other dangerous stuff. Check out the pictures here that Helisto and colleagues at VTT and NIST made with a microbolometer. The shot on the right is a microbolometer image of the guy in the photo on the left. Looks like he's packin' some heat.

The system will be built of detectors called microbolometers that heat up and change electrical properties when light radiation is focused on them. They were initially developed as parts of antennas for imaging faint radiation from space. A bolometer-based radio antenna measured echoes of the Big Bang that started the universe running, and earned a Nobel Prize for Arno Penzias and Robert Wilson in 1978.

Coincidentally, their antenna lacked resolution to produce pictures of small objects like Uranus, and Helisto's system lacks resolution to reveal details of your a. . .

I'm not going to say it, but I bet you can figure out what I was going to type. I'm not being modest, it's just too lousy a joke. (Feel free to use it though, if you need a bad joke for the pub.)

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An innovative HIV vaccination scheme

I'm currently in Denver looking for Physics of Sex topics at the year's largest gathering of physicists, the American Physical Society's annual March meeting.

Yesterday I saw a presentation by physicist Michael Deem of Rice University. He applies the math of physics (including things like field theory) to look at all kinds of things in biology and medicine.

One of the papers he presented at the conference analyzed the ways that HIV manages to evade the immune response. His research suggests an intriguing vaccination technique that could cope with the ability of HIV to rapidly evolve in the human body.

One of the problems with viruses like HIV is that it mutates after infection and produces of several different virus strains. Your body's immune system develops T-cells to fight each of the strains, but tends to focus on just one variety. That means you are pretty good at fending off only one strain, while the rest of the strains run amock.

Deem's analysis of HIV suggests that once vaccines against the disease are developed, similar problems would arise if we tried to vaccinate against more than one strain at a time with a single shot containing a blend of vaccines - that is, only one of the vaccine varieties would take effect.

In order to counteract the problem, Deem proposes that future HIV vaccines should be given with several shots simultaneously injected at different locations around the body. The reason is that T-cells are produced in the lymph nodes located primarily near your joints (behind your jaw, under your armpits, etc.). Introducing different vaccines near different joints induces lymph nodes at one location to concentrate on fighting one particular viral strain, while leaving other strains to other lymph nodes.

When HIV vaccines are finally developed, a person at risk might get a shot in each shoulder, one near each hip, and maybe even at the knees or elbows.

In the meantime, Deem thinks the scheme could help in the prevention of dengue fever. Apparently there are vaccines for several strains of dengue fever, but getting the shot for one strain prevents the others from taking effect, and can increase the risk of developing life-threatening dengue hemorrhagic fever. Deem is hoping to get some medical studies started to see of his multi-shot vaccination scheme works against dengue fever, and eventually against HIV.
Read the rest of the post . . .

Growler Interview

Listen to an excerpt from the interview with Tiffany.




Read the rest of the post . . .

Tiffany Granath and the Growler

In case you missed it, Tiffany Granath of PlayBoy Radio was a hoot when she interviewed me this afternoon. (I'll post a few snippets of the show on my iTunes podcast in a day or two.) We talked about everything from the reasons humans have sex to the the fluid mechanics of blood flow during arousal to the physics of the nervous system.

Even better, we took a few moments to discuss a sexual technique that I call the Growler. If you listened in, you heard it first on Tiffany's Afternoon Advice.

A Growler is a low frequency hummer. If you've never heard of a hummer, it's just oral sex, except that the person performing it hums as they work. The extra vibration adds spice to the experience, whether you're doing to it a man or a woman.

So, why is it a Growler? Your nerves transmit signals in a way that limits the vibrations you can feel in your hands, feet, genitals, and basically any other body part besides your ears, to a maximum of 500 hertz or so. That's about the pitch of the A note above middle C on the piano. To give a good hummer, you should stick to pitches somewhere lower on the scale.

But as you lower your pitch, the sound turns into more of a purr or growl than a hum, hence the name "Growler." You should play around with the tone to find the right note. I can tell you that it feels great, and women seem to love getting growlers too (perhaps even more than we guys do).

Thanks to Tiffany for letting me explain it on her show!
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The HPV Vaccine

If someone had asked me a few months ago whether there could possibly be an objection to a vaccine that could prevent cancer in thousands of people each year, I would have confidently answered "Absolutely not."

And yet, here we are, facing debates over the morality, cost, and efficacy of an FDA approved HPV vaccine. Human papillomaviruses cause more than ten thousand cases of cervical cancers and four thousand deaths annually in the US alone. The numbers are much worse in developing countries where sex education is inadequate, screening is rare, and cancer treatments are prohibitively expensive.

The new vaccine, which is being actively (and clumsily) marketed by Merck pharmaceuticals, appears to effectively prevent certain virus strains that are responsible for two thirds of HPV-related cancer cases.

I'm sure that no one will be surprised to learn that I support HPV vaccinations. Anything that extends life and reduces suffering gets my support. The fact that it also makes sex safer only strengthens my conviction.

If you've been following the public debate, you've probably noticed that the connection to sex is one of the chief objections that some vocal opponents to the vaccine point to.

One opponent, conservative Jill Stanek, focuses on the immoral origins of HPV cases. "[Like smoking,] HPV is also the consequence of a destructive behavior, sex outside of marriage."

Besides that fact that sex outside of marriage is both widespread and healthy (if you take reasonable precautions), Jill has a pretty odd idea about how HPV is transmitted. I can't imagine that many viruses can distinguish between sex inside and outside of marriage. If your partner or husband or wife carries the virus, you may be exposed during sex.

I suppose a woman could search for a mate who claims to be a virgin, and then take him at his word. Or she could simply get the vaccine and protect herself.

But the biggest issue I see is that vaccines like the Merck HPV vaccine don't just protect the individual, they protect the community as a whole. A Stanford University study from 2004 showed that HPV-related cancers could be reduced by 64% as a result of a vaccination program targeting prepubescent girls. The benefits extend beyond vaccinated girls. Both men and unvaccinated women will be safer as a result of herd immunity, even if as few as 40% of young girls are immunized. (Of course, the greatest protection will go to the vaccinated girls.)

Abstinence-based objections to HPV vaccinations strike me as both unrealistic and antisocial. It's not just about you, Jill Stanek, it's about protecting society as a whole. Stanek points out that an HPV vaccine will not protect us from other STDs. Such reasoning is so absurd that I can't imagine where to start. Were Small Pox vaccinations a bad idea because they didn't also prevent polio?

Another, more frivolous and short sighted objection is the fact that Merck will make a bundle off of the vaccine. I don't trust the altruism of major corporations (they're set up to make money, not save the world), and I certainly think we should look closely at any drug they provide. That's why we have the FDA. But the lack of profit in vaccinations is one reason we are facing a potential crisis the next time a highly-contagious and virulent flu hits our shores. In this case, it looks as though an HPV vaccine will lead both to profits and improved societal health.

Overall, the Stanford study concludes that the additional cost of the vaccine will raise average lifetime medical expenses of people in the US by $245, or about 0.6%, while saving thousands of lives every year. How can a compassionate person possibly object?

Think of all the additional people who will be alive to hear the abstinence messages that folks like Stanek promote.
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Buzz Skyline on Playboy Radio, Feb. 26

Mark your calendars!

I will be a guest on Playboy Radio's Afternoon Advice with Tiffany Granath on February 26 at 1:00PM PST (4:00PM EST).

The show is on SIRIUS radio's Playboy Radio channel 198. You can call in toll free to ask questions at 1-877-205-9796.

I'm looking forward to hearing from you on the air.

A few months ago, I never would have imagined that physics could be a topic on channel like Playboy Radio. I'm thrilled!

Now, I wonder if I could convince Physics World to include a centerfold model just to bring the whole thing full circle.

Even better, maybe we could raise enough of a public outcry to get science journalist Karen Hopkin to resurrect her Studmuffins of Science wall calendar.


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Valentine's Day Physics

Science is probably the farthest thing from your mind as you make plans for Valentine's Day next week. But for this popular holiday dedicated to romance, it occurred to me that you should keep a few of the suggestions from earlier Physics of Sex posts in mind.

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Listen to the podcast with Text-to-Speech roboreaders Kate and Paul.

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If you're a regular reader, suggestion number 5 is a new one that you haven't seen here yet. The rest are taken from earlier posts.


1. Opt for a low fat dinner. Fat from your meal rapidly moves into your blood, making it sticky, thick and more difficult for your heart to pump around. Reduced blood flow dampens erectile vigor (in the genitals of both men and women), and can reduce lubrication in women. So skip the foie gras on the 14th. Salads and other low fat foods are sexier for your Valentine’s Day dinner. See the entry Pumped Up and Ready for Love, part 2 for more information.

2. Tune your bed and body for better sex. Different beds have different rhythms: firm beds are better for faster sex, and soft beds are better for slower loving. For the most versatility, start with a firm bed and add pillows or thick comforters to slow things down. If you want to take even more control of the pace, experiment with sexual positions. You will find that various positions often encourage distinct natural rhythms. See Sexual Rhythms for more details.

3. Mix it up for sensory bliss. The sensory cells that respond to touch, temperature and other information tune out sensations that don’t change much. (That’s why you may forget about the sunglasses resting on top of your head, for example.) So mix things up in bed – change how and where you touch your lover to keep the sensory cells firing and the excitement levels up. See Sex and Sensibility, part 1

4. Keep going longer with sensory repetition. If you or your lover suffer from premature ejaculation, you may be able to stave off the inevitable with the start-stop method. (The method is essentially the opposite of the suggestion above.) Just as the sensory cells and nerves in your scalp soon forget about the sunglasses stowed there, repeatedly taking a man to the brink of orgasm and stopping briefly makes the sensory system less responsive, and can help him last longer in bed. See Sex and Sensibility, part 1

5. Hum a low pitched tune. Human ears can detect high frequencies, but the nerves in the rest of your body can’t register vibrations much over 500 hertz (roughly the B note above middle C on the piano). So if you give the gift of a hummer this Valentine’s Day, keep the pitch low for the best effect. The details of this suggestion will be in the upcoming post Sex and Sensibility, part2.

6. If you still need to find a Valentine's Day date, try looking the physics way. I can't guarantee results, but researchers have found that some approaches are better than others when it comes to cruising for mates. (Valentine's Day is a week away, so you still have one more weekend to try it out.) The details are in last week's post The Physics Guide to Hooking Up.

Before Valentine's Day gets here, check out other Physics of Sex suggestions in the entry Skip to the Tips.


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Warning: E. Coli Bacteria Can Swim Upstream

Here's a bit of disquieting research news - the bacteria E. coli tend to swim upstream in flowing liquids. The revelation, which appears in a paper to be published Monday, February 5 in the journal Physical Review Letters, could explain how the bacteria manage to make their way far up the urinary tract to cause pyelonephritis, a particularly nasty kidney infection.

Yale University researchers Jane Hill, Jonathan McMurry and Hur Koser collaborated with Ozge Kalkanci of Bogazici University in Istanbul on the work, which they believe is the first observation of the natural tendency of bacteria to swim upstream.

The researchers discovered the phenomenon by filming E. coli being swept along in tiny channels filled with flowing liquid. The bacteria tended to swim to their left (when viewed from above) as they were washed downstream. Eventually, their leftward swimming caused them to move toward the side of the channel, where they promptly turned around to swim back upstream. You can watch the behavior yourself in a video supplement to the paper that the researchers recorded. (The crosshairs in the video highlight the path of one of the bacteria.)

It seems that the leftward swimming and upstream migration result from the mechanical design of the bacteria. E. coli are propelled by whip-like flagella that push the bacteria along by rotating counterclockwise. Their cell bodies rotate clockwise in response to the torque of their twisting flagalla. The combination of motions cause E. coli to swim to the left when they encounter a surface in slowly moving or still fluid - an effect that has been observed in earlier studies.

The surprising result in the recent experiment, however, is that the motions also cause the bacteria to face upstream when they are submersed in a liquid flowing rapidly along a surface, in a manner that the researchers say is "much like a weather vane orienting into the wind."

The researchers suggest that the behavior could explain the incidence of infections in patients fitted with catheters, and could be the cause of the biofilms that form inside some plumbing systems. They even speculate that leaving a running hose in contact with the ground could lead to bacterial migrations out of the dirt and into the toilet tanks and water heater inside your house.

You might wonder what's the Physics of Sex connection to E. coli paddling upstream. Well, if you consider where the largest population of E. coli is in the human body, then you and your partner have yet another excellent reason to use condoms if you happen to practice anal sex. You wouldn't want to give those nasty fellas a chance to swim up anyone's urethra, would you?
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The Physics Guide to Hooking Up

Another Saturday night is just around the corner, and you're looking forward to cruising for action at the local hot spots.

How'd it work out for you last time? Did you hook up with your dream guy or gal, or did you strike out? Either way, it's possible that you could improve your odds by applying the physics published in the journal Physical Review Letters a few years ago.

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Listen to the podcast with Text-to-Speech roboreaders Kate and Paul.

Or, if you're in a rush, Skip to the Tip in this week's post.

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In 2002, a group of Spanish and Brazilian physicists looked at two types of search strategy that might be employed by such things as predators in search of prey, bees in search of flowers, or other creatures (like you) in search of mates. They found that the searchers could dramatically improve their odds by tailoring their strategies depending on the distribution and motion of their targets. Theoretically, you should be able to improve your odds of finding that special someone as well.


When you go out on the town looking for love, you have at least two options. For one thing, you could pick a bar and settle in for the night, while doing your best to mingle as you work to attract or seduce someone.

This type of search strategy is called a Brownian random walk. You just bounce around to search randomly for a love connection in some small area, such as the dancefloor of your favorite bar. Eventually, you might drift to another nearby establishment. But in any case, you don't cover a lot of ground over the course of the night.

Alternatively, you could bar hop - drop in on a bar, work the room, and then if there's nothing promising, dash to another bar to do it again.

This second type of strategy is called a Lévy flight search. Lévy flights involve poking around in one location, and then zipping off to poke around somewhere else. Lots of creatures use Lévy flights for searching large areas, when there are sparse distributions of what ever it is that they're after. Bees often hunt for pollen rich flowers this way, and there's a good chance that you look for your lost keys with a similar search pattern. (You might check the dresser, skip down to look through the desk, pop over to the closet to check your coat, etc.)

As the research team ran their simulations, they found that when the targets were relatively stationary and far apart, searchers increased their odds of success by performing Lévy flights from place to place. Picture, for example, groups of eligible singles nestled at bars around town, with the bars far enough apart that you have to drive or walk a long way to get from one place to the next.

If instead, the targets moved around a lot or there were many of them packed in a large area, then searchers were more successful when they avoided Lévy flights and just flitted around randomly in a small area.

The first scenario sounds a lot like the club scene in most major cities, and the second scenario is more like the flowing crowds at Carnival in Rio or Mardi Gras in New Orleans.

At first glance it seems like the best bet is simply to zip from bar to bar with a series of Lévy flights, so long as you're cruising in town. But if you're at a big event with lots of available singles around, you should stay in one place.

Unfortunately, things aren't always so easy. If you're making lots of Lévy flights to search the clubs, and your targets are making frequent Lévy flights as well, then the chances are you're going to miss many of your potential love connections while in transit. When targets are highly mobile the physics model suggests that a searcher, like you, should pick one place and stay there to wait for your potential mate to make a Lévy flight right into your lap. In fact, the faster your targets are moving the less you should stray from your barstool.

In some cases it makes sense to evaluate the traffic flow and adjust your strategy throughout the course of the night. That would've worked best for me when I was in college and I used to head with my buddies into town to meet girls. Early in the evenings, clusters of women would seem to be roaming everywhere. And we were roaming too, occasionally flirting as we were going along. In retrospect we could probably have met more women if we'd settled somewhere and waited for them to come to us.

But as the night progressed and the alcohol kicked in the girls tended to travel less, and many of them eventually took up residence at various bars. That would've been the time to zip from bar to bar looking to hook up. Of course, by then we were usually pretty tipsy too and probably in no shape to walk or drive very far.

If I had it to do over again, I would spend the early part of the evening sipping mild drinks and sodas while attempting to charm the girls who were cruising through one of my favorite hangouts. Then I could've turned to Lévy flight searches from bar to bar, once the women slowed down.

The physicists who ran the search simulation weren't specifically thinking of the singles scene, they also considered things like the relative sizes of searchers and targets. For large creatures in search of small targets - like foxes hunting for rabbits - Lévy flights usually work best. For small searchers on the prowl for large targets - like parasites hoping to latch onto passing horses - it's best to sit tight and wait.

For humans, who are all roughly the same size (from a physics point of view) relative size isn't an issue. We only have to worry about the relative motion and the distribution of the people we'd like to meet.

So the next time you head out to the club scene to hook up, stop to take stock of the situation. If the kind of people you're after are bar hopping, you should stop in one establishment and mingle. If there's not much traffic in and out of the bars, then consider Lévy flight searches. And if you're going to Mardi Gras, pick a location, sit tight, and wait for the prospective mates to come to you.

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Book Review: The Science of Orgasm

The Science of Orgasm by Barry R. Komisaruk, Carlos Beyer-Flores, and Beverly Whipple has got to be one of the best science books on sex that you can buy, if you can deal with wading hip deep through medical and biological research jargon.

I learned something new on just about every page, and each fascinating factoid and phenomenon - from the horrific sexual behavior that can result from certain types of brain damage to the question of whether or not orgasms are good for your health - is backed up with citations from top research journals and institutes. The authors themselves are responsible for a significant amount of the original research in the book. It seems pretty clear that that they know what they're taking about.

Still, you have to be awfully determined to plow through passages like this, "The participation of the adrenal cortex as a source of steroids capable of maintaining sexual response in women after bilateral oophorectomy has often been suggested" (page 179).

It can't hurt to have browsers open to Gray's Anatomy (the medical text, not the TV show) and Wikipedia as you make your way through the book, just to keep up with the lingo.

It's interesting that the publisher would choose cover art that resembles a plain brown wrapper, as if you're going to buy a copy to read under the covers while your mom thinks you're sleeping in. No matter how sexy the topic, doctor-speak is hardly a turn on. (Although, I'm sure it works for someone.)

If you read nothing else in this book, I highly recommend the brief section addressing the biological function of the female orgasm (pages 10-15). I have never seen a more coherent and compelling argument that orgasms in women serve some vital, if only partially understood, purpose. It's an excellent counterpoint to arguments claiming that orgasms are critical rewards to induce men to mate, but in women are only evolutionary accidents.

I'm not going to go into details here, but basically the authors point out that women seem to have at least some specialized anatomy that lets them experience types of orgasms that have no male equivalent. (The authors even invented a device to give women orgasms by stimulating only the cervix, which is an exclusively female body part.) The female-only orgasms can't be something left over from male anatomy, they conclude, if they can't exist in male bodies.

I plan to explain things more fully in a future post, but if you can't wait and you think reading the Journal of the American Medical Association is a good way to pass the day, pick up a copy of The Science of Orgasm. If nothing else, it's a good addition to your sexual science reference shelf.
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Is the Select Comfort air mattress good for sex? A PoS experiment

If you're eagerly anticipating part 2 of Sex and Sensibility, we'll have that for you next week.

In the meantime, we decided to send two of our writers, Buzz Skyline and Martica, into the field to do an experiment inspired by a portion of the Physics of Sex post Sexual Rhythms.

Specifically, we wanted them to see what physics could tell us about the Select Comfort brand adjustable bed. We hoped they would learn enough to help you determine if it's the best bed for your love life.

You can listen to roboreader Heather interviewing Martica and Buzz in our latest podcast, or read the transcript below, to find out what (if anything) the Sleep Number bed has to offer for sex.

Skip to the tip in this week's post, if you're in a rush.


In any case, please take part in The Great Physics of Sex Bed Test. We want you to test out your bed and send us the data so that we can figure out, once and for all, what type of bed is best for sex. But don't do it for us. Do it for yourself. Do for the world. Do it for science.

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TRANSCRIPT OF THE SELECT COMFORT BED TEST INTERVIEW

Heather: Welcome to the Physics of Sex podcast. My name is Heather.

If you've been listening to our past episodes, you know we usually give a little lecture about a fascinating aspect of physics, as it applies to your love life. But this week, we decided to try something different.

We sent two of our writers into the field to do a few experiments on an unusual kind of bed, in order to find out how it might affect your sex life. Here to report on what they found out about the Select Comfort sleep number bed, are Martica. . .

Martica: Hi Heather.

Heather: . . . and Buzz Skyline.

Buzz: Hi Heather.

Heather: So guys, you ventured out to test a bed. Did you do what I think you must have done? How'd you keep from being arrested?

Martica: Well, we didn’t actually do anything that's not appropriate in a mattress store. We actually just went and jumped on the bed.

Buzz: Well, we sat and bounced on the bed.

Martica: Exactly.

Buzz: We didn’t stand on the bed.

Heather: OK, well tell us about the bed.

Martica: It’s a Select Comfort Sleep Number bed, which [allows you to] change the firmness of the mattress.

Buzz: You can change the amount of air inside the mattress.

Martica: Right.

Buzz: And they call it the firmness.

Martica: It’s a big air mattress with a pump and a little remote control which you can use to pump the air in or let the air out and that changes the firmness, what they call the firmness, of the mattress.

Buzz: And it basically is just inflating this bladder that’s inside the bed instead of springs.

Heather: So this is the bed Lindsay Wagner promotes on TV. Great. How did you do the experiment?

Martica: We sat on the bed and bounced up and down.

Buzz: At different numbers. We set it for different numbers and we took turns bouncing on it. Martica bounced on it a few times at different settings and I bounced on it at a few different settings, a few different sleep number settings, and the last time . . .

Martica: We sat next to each other and bounced up and down at the same time, which is actually really hard – but fun. (laughs)

Heather: What sort of results did you expect?

Buzz: So, what we thought would happen was we assumed the sleep number really was what you call firmness, which on a spring would basically be the spring constant. It would tell how strong the springs are. And that means that as you turned it up it should increase your resonance frequency- the frequency that you bounce on the bed. And as you turn it down, it should decrease your resonance frequency. And so we started out with Martica on the bed, and what we found was no matter what sleep number we chose, she bounced at about the same rate.

Martica: I was trying! (laughs)

Buzz: It wasn’t her fault. It was obviously the physics. And so we thought there must be something wrong. So I sat on the bed and tried it for several numbers and I also bounced at almost exactly the same rate every time.

Martica: But a little bit more because you’re a little bit heavier than I am.

Buzz: Yeah, I bounced slower than Martica did because I weigh about 50 pounds more. So you would expect it to be a little slower, and it was a little slower, but by the same amount every time than Martica’s bounce was.

Martica: No matter whether the bed was really, really, really firm and full of air or really, really, really soft and the balloon was almost flat.

Buzz: So basically that means that whatever the sleep number tells you, it doesn’t really tell you the firmness in the same way that a bed is firm, it doesn’t tell you how firm the springs are. It’s changing something else.

Heather: All right then, what's going on?

Martica: So what it actually is that’s changing is the damping of the bed. It’s like having a giant pillow that you can compress and make it firmer or make it softer.

Buzz: Yeah. So if you were to open the bed and look at the bag, you would either see that it was completely full at a hundred percent, or a hundred sleep number, but usually it was kind of floppy.

Heather: So the sleep number doesn't matter for sex? You just set it wherever you want and things don't change?

Buzz: Well we did find that there was a change as we turned the sleep number up or down.

Martica: Right, it got harder to bounce on.

Buzz: It was more . . .

Martica: It took more energy because you’re bouncing on a less inflated balloon, basically.

Buzz: You are tuning something. You’re not tuning the spring constant, but we were tuning the damping. It made it harder to maintain a bounce because it was . . . it wasn’t changing the spring constant, but it was sucking energy out of the bed, it was damping the energy. It meant that we had to work harder to bounce when you turned the sleep number down, which corresponds to turning up the damping but not changing the spring constant.

Heather: Do people who actually buy the bed think about this sort of thing?

Buzz: In fact, we had the same question. So I went back when the store was closing and there weren’t any customers to distract the salesman, and I asked him that very question. And this is what he said.

(Mall noise)

Buzz: And so the feature we’re doing is a question of whether or not some beds are better for a love life than other beds.

Salesman: Uh huh.

Buzz: And we’ve gone around and we’ve measured basically the resonance frequency of various beds and . . .

Salesman: Uh huh.

Buzz: . . . things like that to try to understand how they’re different.

Salesman: Huh.

Buzz: So I was wondering – do people ever take that into consideration? When they talk to you when they are about to buy a bed, is that something that ever comes up?

Salesman: Nah, it never comes up. Maybe, maybe they think about it, or maybe they talk to each other about it.

Buzz: So it’s not like a waterbed store where you know what they’re there for . . .

Salesman: Right.

Buzz: This is more for . . .

Salesman: I’ve never, never heard anybody say anything like that.

Buzz: How long have you been sellin’ the beds?

Salesman: Since November.

Buzz: Oh OK, so you haven’t been doing it too long. So some people may think that . . . But if they did there’s not . . .

Salesman: I’ve slept on one. I’ve slept on one for about six years. But my wife and I, we don’t talk about it.

Buzz: You don’t adjust the pressure . . .

Salesman: No.

Buzz: . . . or anything?

Salesman: No no. Nope.

Buzz: Oh OK. (laughs) Alright. Well I know they’re strange questions.

Salesman: No no no. It’s all part of research.

Buzz: Exactly.

Salesman: I’m doing my part for science.

Buzz: OK. Alright.

(Mall sounds fade.)

Heather: So, having put the Sleep Number bed through it's paces, what are your recommendations?

Martica: Well if you already have a sleep number bed then you can turn it up for sex, if you want to make sure you’re getting the most bounce for the energy you put in.

Buzz: With a regular mattress, you can add damping. You can add something like comforters or pillows or something. You have to add additional material to make it softer to make it more comfortable or to change the rhythms of sex. Whereas with the Sleep Number bed you could potentially, with just this one adjustment, quickly go from what’s comfortable for sleeping to what’s comfortable for sex and back again.


Heather: Do you have plans for follow up experiments?

Martica: We plan to go to a mattress discounter store where there will likely be more mattresses with springs inside of them rather than air inside of them, and jump up and down on the beds there as much as they’ll let us. And ask some of the same questions. If maybe people who buy spring beds are looking to see how their sex lives will be changed by these beds. And if we go to a waterbed store, maybe people are even more interested in sex when they come into a waterbed store looking to buy a waterbed.

Buzz: It would be a completely different experience. And we could also consider alternative sleeping surfaces, like futons.

Heather: That's great. You've certainly given us something to think about. Thanks for stopping by.

Martica: Thanks Heather. It was great to talk to you.

Buzz: Bye.

Heather: After the select comfort adventure that Martica and Buzz went on, it occurred to us to ask our listeners and readers, to tell us about your beds. What type of bed do you sleep on? Is yours good for sex? Is it fast or slow? Do you prefer lots of damping or just a little?

We'll post instructions on our website, ThePhysicsOfSex.org, to let you know how to send us some data. We'll analyze it and hopefully report back with the results in a few weeks.

Thanks for listening to the Physics of Sex podcast. I'm Heather.

Download us next time for the second segment of last week's show, part two of Sex and Sensibility, the physics of the nervous system.

Bye for now.

END OF TRANSCRIPT

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The Great Physics of Sex Bed Test

It's this easy.

Send us an email or write in the comments section the information you collect by following these instructions.

1. Tell us what kind of bed you have (spring mattress, waterbed, space-age foam, futon, air mattress, etc.)

2. Estimate the damping on your bed - is it plush (like a pillow top bed), moderately padded, or a plain mattress?

3. Measure your bed's resonance frequency by

a. getting a watch with a second hand

b. sitting on the bed and bouncing at the rate that feels most natural

c. timing how long it takes to bounce 25 times

d. It couldn't hurt to do the experiment a few times, or even get your partner or friends to try it. Be sure to note both the time it takes to bounce 25 times and the weight of the person bouncing. We'll need both numbers to calculate your bed's spring constant.

4. Tell us whether your bed is good for sex on a scale of 1 to 5, where five is Nirvana and one is like doing your taxes.

5. Send the information to us in an email to BuzzSkyline@gmail.com, or paste it into the comments section for this blog entry.

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The Physics of Sex Cited Among the Best of Science Blogging

The Physics of Sex post Sexual Rhythms is included in a brand new anthology of the 50 best science blog posts of 2006.

The Open Laboratory: The Best Writing on Science Blogs 2006 was edited by Bora Zivkovic (aka Coturnix of Seed Media's Scienceblogs.com), who is probably one of the hardest working folks in the blogosphere.

Bora chose the entries with the help of a select group of the top science bloggers around. The whole process is detailed on Bora's web page.

Buy the book and find out what wild and wonderful things are out there under the heading of science blogging. (No one at The Physics of Sex is getting a penny for this endorsement, BTW. We just like to encourage initiative like Bora's.)
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Skip to the Tips

One of the great things about pondering the Physics of Sex is that it naturally leads to tips to enhance your sex life. Several of the posts that you can read here have a physics inspired suggestion or two in them.

Some of the suggestions - such as how to choose a good bed for sex - include information that you may not be able to find anywhere else.

For those of you who don't have time to read the full posts or listen to the podcasts, we have made a list of the tips that lets you jump straight to the good stuff.

You'll see this little icon next to italicized text to highlight the Physics of Sex Tips in each post.

Check out the tips below.

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The Physics of Sex Tips (so far . . .)

-> Find the best bed for your sexual style. (from Sexual Rhythms)

-> Take control of your vibe with damping. (from Sexual Rhythms)

-> Choose the right lube. (from Slip, Slide or Stick)

-> Find out how the pinch technique can enhance your erection . . . or how it can be adapted to women. (from Pumped Up and Ready for Love, Part 1)

-> How you should adjust your smoking habits to reduce the negative impact on your sexual function. (from Pumped Up and Ready for Love, Part 2)

-> How losing weight can improve your circulation and make you better in bed. (from Pumped Up and Ready for Love, Part 2)

-> A safer alternative to the dangerous practice of erotic asphyxiation. (from Pumped Up and Ready for Love, Part 2)

-> How minding the negative feedback in your sensory receptors can help you make sex more intense. (from Sex and Sensibility, Part 1)

-> How the "start-stop method" helps you manipulate your sensory receptors to get past premature ejaculation. (from Sex and Sensibility, Part 1)

-> Two ways the physics of sensory receptors can add an extra tingle to sex. (from Sex and Sensibility, Part 1)
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Part 1 of Sex and Sensibility: the Physics of the Nervous System

Sensation – it’s not all that sex is about, but it’s a lot. Sure, intimacy is important, and so are trust and communication. But when it comes down to it, one of the best things about sex is that it feels good.

(Listen to the podcast with Text-to-Speech roboreaders Heather and Graham.)

You might wonder why the touch of a fingertip, lips or tongue that is barely detectable on one region of your body causes a surge of pleasure somewhere else. And what is it that makes a caress, pinch, slap, or tickle feel just right at one moment and completely wrong at another? There’s more to it than simple physics, but a look at the nervous system through the eyes of a physicist can help you get a handle on the sources of your sexual pleasure. . .

In order to enjoy sexual sensations your body needs to do at least three things. First it must detect the sensations. Next it has to send information about them to your brain. And finally, it must interpret those sensations as pleasurable. The three components in your body that accomplish these tasks - sensory receptors, nerves, and specialized regions of your brain - are portions of your overall nervous system.

These parts of your body are responsible for much more than making sex pleasurable, of course. The nervous system controls the movement of muscles, both voluntary ones such as your arms and legs and involuntary ones including your intestine and heart. It also monitors the status of your organs and generates thoughts and emotions. In a way, the nervous system is the part of the body that makes us who we are. After all, a person who loses a limb or has a heart, eye, or even face transplant is still the same person.

In effect all the parts of someone’s body could be replaced, but as long as the lump at the end of your spinal cord - which we call the brain - remains intact most of us would feel the essential person that is you is still here. Death, in fact, is defined as the cessation of activity in the brain, regardless of the condition of the rest of the body.

When you have sex or fall in love, revel in a moment of ecstasy or sink into the depths of despair it’s your nervous system that is experiencing all these events. The rest of you - including the bones, muscle, fat, and organs - is only a compilation of components in the vehicle that carries your nervous system around and lets it enjoy the world.

In this week’s edition of The Physics of Sex, we’ll start at the beginning - by looking at the structures in your body that first respond to events around it.

All sensations, sexual or otherwise, start with a sensory receptor. Rods and cones in the retina of your eye react to light, tiny hairs deep inside your ear detect sound, chemical receptors in your nose and on your tongue reveal smells and flavors, and receptors in your skin alert you to cold, heat, pressure and pain. In each case a receptor converts a stimulus into a signal that nerves transmit to other parts of your body.

To a physicist, a sensory receptor is a kind of transducer. Transducers take incoming signals of one type and change them to a convenient form that is easier to transmit or interpret.

Manmade transducers often convert information into electrical signals. A microphone, for example, changes sound into electrical waves before it sends it over metal wires. A speaker, in turn, is a transducer that converts the electrical signal back into something you can hear.

Long before humans thought to build transducers, nature had already discovered essentially the same trick. For example, when cold receptors in skin are exposed to low temperatures they move chemicals around to produce a small electrical voltage.

Heat receptors do the same thing when exposed to elevated temperatures, and receptors on the tongue and in your nose produce voltages in response to chemicals. Mechanical receptors, located primarily in your skin and muscles, produce electrical signals when they are squeezed, stretched, bumped or shaken.

Regardless of their particular sensitivity, all sensory receptor cells are really just small bags of electrically charged fluid. The bags are made of membranes of fatty cells, (much like the soap micelles described in the Physics of Sex chapter on lubrication).

Tiny chemical motors called ion pumps are embedded in the membrane. The pumps move electrically charged ions in and out of the cell. Ions that have excess electrons are negatively charged and ions with missing electrons are positive. The pumps push more of the positive ions out of the cell than into it, so the fluid inside becomes charged from the excess of negative ions that stay behind. The charge is small, roughly 70 thousandths of a volt, which is about twenty times smaller than the charge on a fresh flashlight battery.

When a sensory cell is triggered, pores open up in the cell membrane. This lets positively charged ions flow back into the cell, which makes the electrical voltage inside surge upward. If the stimulus is too mild, the voltage only goes up a tiny bit and the ion channels shut down again to let the pumps restore the cell to its negative resting voltage.

For a larger stimulus, above what is called the triggering threshold, the flow of charge is large enough that it causes more and more of the cell's other ion channels to open, leading to a dramatic voltage change and the cell fires an electrical pulse.

Triggering a sensory cell to fire is a bit like setting off an avalanche on a snow bank. A whisper may not get the snow to break loose, but a shout or a handclap can sometimes be a large enough trigger to send a wall of snow careening down a mountain. Similarly, when some ion channels open up in a sensory cell membrane they induce others to open as well. If too few open up to begin with, they all shut down in a fraction of a second. But if enough open up initially, they trigger an avalanche of ion channels to open.

Mathematically, a sensory cell’s response to stimulation is known as positive feedback. It occurs whenever a stimulus causes an effect that in turn increases the stimulus. Positive feedback typically leads to dramatic events, such as stock market crashes and orgasms, in addition to sensory cell bursts.

Once the flow of positive ions into a sensory cell raises the voltage to 10 millivolts or so, then no more positive ions can squeeze in and the ion channels clamp down. In the same way, an avalanche ceases after all the snow has slid down a valley wall. At this point the sensory cell and the snow filled valley are in their refractory states, which means that they cannot respond to stimulation, at least for a while. In a sensory cell the ion pumps charge the fluid back to a negative voltage and the cell is ready to fire again. Valleys prone to avalanches, however, must wait for snow fall to build up before they can go off once more.

The whole process takes a few thousands of a second in a sensory cell. If the stimulus remains after the cell has completed its cycle, it will fire another pulse.

The positive feedback in a sensory cell ensures that any stimulation, regardless of its strength, will lead to the same electrical burst, provided that the stimulation is above the triggering threshold. (Anything below the threshold - too gentle a touch, too quiet a sound, etc. - will simply go unnoticed.) Nevertheless, it's clear that we can tell the difference between a light touch and a heavy touch. Sensory cells reflect the intensity of a given stimulation by firing a train of electrical pulses. The more intense the stimulus, the more rapid the pulse train. In cases of extreme stimulation, the sensory cells fire immediately upon recovering from their refractory period.

Over time, a receptor exposed to an unchanging stimulus will gradually cease to respond. If you were to record the pulses coming out of a touch receptor, you would see that when it is first stimulated by a firm touch it emits a rapid string of pulses. After a few moments the pulses slow. At this point, the receptor has adapted to the stimulation. Removing the stimulus would then lead to another string of rapid pulses, which again taper off over time.

It is this sort of desensitization that allows you to ignore the touch of clothing against your skin, forget that you have your sunglasses resting on top of your head, and gradually come to tolerate the temperature of a hot shower.

Adaptation to an unchanging sensation can be thought of as a kind of short-term cellular memory. Once a sensory receptor adapts to a stimulus, it acts as though it can’t remember a time when the stimulus wasn’t there. Taking it away is a shock, but the cell soon gets used to the absence of stimulation as the new status quo, and loses all memory of the earlier stimulus. In contrast to the positive feedback avalanche of a firing sensory cell, sensory adaptation is a form of negative feedback.

Despite the term’s pessimistic-sounding name, negative feedback is a good thing most of the time. If a radio is too loud, you turn it down. If it is too quiet, you turn it up. Eventually you'll find a comfortable volume. This is negative feedback. Anything that must remain stable over time needs negative feedback, whether it's your weight, body temperature, emotional state, or even your bank balance.

There must be some flexibility in negative feedback systems to adjust for new situations. For example, there was likely a time when you were young that your appropriate weight was fifty pounds and negative feedback helped you stay close to that number, at least for a while. In order to grow, your body had to allow your weight to increase over the course of months and years. Most negative feedback systems operate this way - working to reduce sudden changes while adapting to gradual shifts. It is as if negative feedback systems have short-term memory and long-term amnesia. In sensory cells, the long-term amnesia results in raising or lowering the cell’s triggering threshold.

Negative feedback helps explain why it’s important to mix things up in bed. Pinching a nipple may be pleasurable for a few moments, but the sensation will fade if it's not varied and the sensory cells have a chance to adapt to the pinch. On the other hand, simply releasing the pressure on a nipple can be pleasurable as the sensory cells react to the change in status.

Varying the type and location of stimulation on a penis or clittoris during oral sex, and switching sexual positions from time to time during lovemaking, take advantage of the short-term memory of sensory receptors.

By focusing on one portion of an erogenous zone, you allow the sensory receptors in other places to adapt to the lack of stimulation, which makes them particularly sensitive when it’s their turn.

A commonly repeated myth claims that women who use vibrators will eventually lose sensation in their genitals. While it’s true that extended vibrator play in a session can lead to numbness, the effect is only a temporary resetting of the triggering threshold of the mechanical sensory receptors. It is certainly possible that intense vibration can rupture cells, but we all have plenty of pain sensors in our genitals that produce clear signals to warn of tissue damage. The intense agony of misusing a vibrator would make most women stop well shy of doing themselves any permanent injury. Thanks to the dense packing of sensory receptors in our erogenous zones, the risks that come with using vibrators are low and the orgasmic benefits are very high. In fact, many women find that using a vibrator helps them learn to orgasm more easily, even when they are not using powered toys.

Sensory adaptation also provides ways to alleviate problems with premature ejaculation in men. One method for increasing a man’s staying power involves repeatedly stimulating him to bring him to the brink of orgasm. Stopping just short of ejaculation and waiting a few moments, and then repeating the procedure several times, raises the triggering threshold of the sensory receptors in the penis to the point where a hair-trigger man can maintain self-control for longer and longer periods. Sex therapists often call this the stop-start method.

It may seem paradoxical that sensory adaptation can enhance sexual sensations under one set of circumstances, and desensitize a man’s penis under another. The key to the different benefits lies in the timing. Consider a hot shower, for example. As you step under the showerhead the steaming water might be barely tolerable. After a minute or so, the pain subsides as your temperature sensors set their pain threshold higher. Provided you remain under the shower, or move away for only brief periods, the water will still feel hot but won’t hurt. If you step out of the shower for several minutes or neglect to run water on your back for a while, the water will sting again when it hits the regions of your skin that have readapted to cooler temperatures.

Temperature sensors in your skin adapt more slowly than touch sensors, so you only have to wait a few seconds for a nipple, penis or clitoris to become re-sensitized after removing its stimulation. Similarly, in helping a man to stave off premature ejaculation it’s important not to entirely cease stimulating him for too long as he nears orgasm. Just ramp it back briefly. Otherwise you may end up speeding things along rather than slowing them down.

There are other ways to manipulate sensory receptors. You can use an ice cube to set temperature thresholds in your skin very low, say by cooling down a nipple, and then sucking on it to give your lover a sensory roller coaster ride. You can achieve a similar effect chemically with oils and lubricants that make your skin feel hot or icy cold (sometimes in rapid succession). Cooling and heating liquids and lubes contain compounds such as camphor and menthol that cause skin sensors to overreact to small temperature changes. If your partner breathes gently onto skin exposed to such a chemical, the warm breath will feel oddly hot, and if your lover blows vigorously on your skin, the rapidly moving air will feel surprisingly cold. In each case their breath is actually very close to body temperature, but the slight difference in skin temperature is amplified by the effect of camphor or menthol on the sensory receptors.

Once a sensory cell has fired in response to a stimulus, the signal is passed to nerve cells. But that is a topic that will have to wait for now. To learn about the physics of the nerves that connect to your sensory cells, be sure to come back next week to read part 2 of Sex and Sensibility: the Physics of the Nervous System.
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Part 2 of Pumped Up and Ready for Love: Sex and Fluid Physics

More ways that your experiences during arousal and sex result from changes in your blood flow, and how you can use fluid physics to make it even better.

Listen to the podcast with roboreaders Audrey and Paul.


Circulation's Chemical Connection

Last week, we talked about several ways to increase the volume of blood in your erectile tissues. Generally, the methods involved inhibiting the blood flow out of your groin through the veins - with selectively applied pressure or sexual aids such as cock rings and penis pumps. Another way to enhance your penile or clitoral erection is to improve the flow of blood in towards your genitals.

A simple way to do that is with the use of drugs that relax the muscles that constrict your genital arteries. The relaxed muscles allow the arteries to expand in diameter.

According to the laws of fluid physics, pressure in a tube carrying a flowing liquid will be higher where the tube diameter is larger, and lower where it is narrower.

(Incidentally, that's what happens in an arterial aneurysm. A defect in an artery leads to a bubble in the arterial wall. Because the diameter is larger in the expanded bubble, the pressure increases and leads to a growing aneurysm. It's a vicious cycle that can eventually result in a disastrous pop, or other, equally unpleasant complication.)

The expansion of the diameter of healthy genital arteries, raises the pressure in your erectile tissue, compressing the outgoing veins and increasing the volume of the erectile reservoir.

Urologist Giles Brindley famously demonstrated the approach by injecting his own penis with the muscle relaxant Phentolamine. He displayed the resulting erection, while making a presentation at a medical convention in 1983. Many men faced with erectile dysfunction began injecting their penis's with muscle relaxant, in the years following Brindley's display.

More recently, a growing number of men have opted for drugs such as Viagra and Cialis, which treat erectile dysfunction in a very different way.

The drugs don't affect arteries themselves, instead they change the chemical signals that control the arterial muscles. Although you rely on nerves for movement in most of your body, they don't directly control erectile tissue. Instead, nerves leading into your groin trigger the release of chemicals that cause the muscles in the walls of genital arteries to relax. This allows them to expand in diameter.

As your arousal subsides, enzymes in your erectile tissue break down the chemicals that relaxed the muscles in your erectile arteries. Viagra and related drugs block the enzymes. As a result, the drugs keep the levels of muscle dilating chemicals high, and the blood vessels stay open. In essence, the drugs expand the arterial vessels that lead to the genitals, rather than directly constricting the flow out through the veins, as cock rings do.

The reason that you may have heard that Viagra is not considered an aphrodisiac, even though many people think of it that way, is that it can't lead to erectile tissue engorgement unless your body produces the chemical to relax your genital arteries in the first place. Viagra can't cause arousal, as a true aphrodisiac would; it only works if you are already aroused and your body produces the initial muscle-relaxing chemical.

The enzymes in your genitals that break down the dilating chemicals are unique to your erectile tissue. They're slightly different from the enzymes in other parts of your body. Viagra is designed to block the enzyme that turns off your arousal response. But no drug is perfect. It also mildly blocks the enzymes in the tissue of your retina, which is why some people experience changes in their vision while on the drug. The various side effects of Viagra and other erectile drugs are mostly related to the fact that developing precisely targeted drugs is very difficult.

Viagra should work for women as well as men, leading to enhanced genital engorgement and the improved vaginal lubrication that comes with better blood flow. Unfortunately, recent studies have shown that because arousal in women is more complex than it is in men, most female test subjects found that Viagra did little to improve their sexual experience.

Circulatory Troubles

Anything that keeps the arteries from dilating can hamper sexual response. Arteries hardened by age or disease can't expand to allow the additional pressure and blood supply required for erectile tissues to swell. The nicotine in cigarettes, causes the muscles of the arteries to clamp down and reduce blood flow, particularly in the extremities and genitals. If you must smoke, try not to do it just before having sex, or else the nicotine will work against the artery-dilating chemicals accompanying arousal. Besides, you'll smell better when it comes time for that first kiss.

Even fatty foods can dampen sexual response. High concentrations of dissolved fat make blood more sticky and viscous. Thickened blood flows poorly through your arteries, which means that there's less pressure than there should be by the time blood makes it to your groin. Losing weight by cutting back on fatty foods helps blood to flow better and can increase erectile vigor, which means larger erections and more fully engorged clittorises and labia as well as smaller, sexier waistlines.

The combination of rich food and cigarettes is particularly devastating to blood flow, creating a double whammy on your sexual function. Thick, fatty blood squeezing through nicotine narrowed arteries puts an extra strain on the heart, which is why heart attacks may be more likely to strike as you enjoy a cigarette after a big rich meal.

Assume the Position

Circulatory fluid flow, of course, is responsible for more of the sexual experience than mere genital mechanics. Flushing in your cheeks comes with increased blood flow as small vessels in the skin expand in response to things like overheating, embarrassment, or arousal. In fact, rouge and lipstick may owe their sexiness in part to the fact that they mimic the facial flush that accompanies sexual excitement.

Changes in circulation can also affect the sensations you feel during some activities by modulating the amount of oxygen that gets to your brain. The lightheadedness that comes with riding a roller coaster is in part due to the forces you experience during the ride, which push blood up toward your brain or down to your feet.

People who practice erotic asphyxiation attempt to heighten their orgasmic sensations by reducing oxygen in their brain. Often they achieve the effect through partial strangulation to slow blood flow. It's a highly dangerous activity, and leads to many unfortunate deaths every year, particularly among young men.

There are, however, some much less dangerous sexual techniques that create similar sensations, because they involve positions that modify blood flow without strangulation or asphyxiation.

Your circulatory system is designed to operate best when you're standing up, lying down, or somewhere in between. Veins have tiny valves in them that work against gravity. When you stand, the valves prevent the blood in your veins from backing up into your legs, but the valves only work in one direction. If you stand on your head, you'll feel an increase in the blood pressure in your face and head as the valves in your veins become useless and gravity takes over. Although there's more blood in your upper body when you're upside down, it doesn't flow as well. Your heart will work harder to keep your blood moving, but you will probably experience some lightheadedness due to the reduction of oxygen in your brain.

There are some, rather athletic sexual positions that involve one partner essentially standing on their head to reduce blood flow in the brain, but you can get the same effect by simply hanging your head off the edge of the bed during sex. A slightly less effective method is to stand and bend deeply at the waist while your partner enters from behind or stimulates you, manually or orally. In either position, the longer you do it and the lower your head in comparison to your torso, the more intense the sensation. Of course, you may end up with a throbbing headache when it's all over.

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Now you know the basic fluid physics important for sex. Whether or not you experiment with sex toys and methods for modifying the blood flow in your body, be sure to take a moment to appreciate the importance of fluid physics in your sexual activities.

The Physics of Sex will be on vacation for about two weeks, so expect our next episode, "Sex and Sensibility: The Nervous system," in the second week of January.

In the meantime, take a moment to leave your comments and questions about Physics and Sex. Or email us at buzzskyline@gmail.com. If we use your comment in a future column, we'll send you a free Physics of Sex coffee cup from our CaféPress store.
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Part 1 of Pumped Up and Ready for Love: Sex and Fluid Physics

Much of what you experience during arousal and sex results from changes in the way your circulatory system moves your blood around. Your heart races and your cheeks flush as excitement builds. Sooner or later, if all goes well, swelling of the erectile tissue in your groin will lead to the erection of your clitoris or your penis, as the case may be. Generally, your body handles all the blood flow issues automatically. Still, a look at the physics of fluids reveals that there are many things you can do to take control of your blood flow during arousal.

Listen to the podcast with roboreaders Audrey and Paul.

Despite the fact that male and female genitalia look very different from the outside, they operate in essentially the same way, from the point of view of fluid physics. Both the penis and the clitoris are built of erectile tissue that becomes engorged with blood during sexual arousal. The chief difference between the two is that while most of the engorged portion of the penis is visible on the exterior of the male body, the exposed clitoris is only a small part of the erectile structure in a woman's genitals. In fact, women have roughly the same amount of erectile tissue as men. It's just hidden from view behind the clitoris and extends down on either side of the vaginal opening.

In either case, a physicist could describe genitals in terms of a simplified model consisting of a reservoir connected to an incoming tube, which supplies fluid, and an outgoing tube, which drains the fluid away. Your body adjusts the amount of fluid in the reservoir by regulating the relative flow, in through the source and out through the drain. It's similar to the way you can control the amount of water in your kitchen basin by turning the faucet handle.

Arteries are the blood vessel analogue of the kitchen tap. Instead of water, of course, they deliver blood provided by the pumping of your heart. They are stretchy tubes wrapped in muscle tissue. The muscles control blood flow through arteries by contracting and closing them down somewhat to slow the flow, or relaxing to let the vessels dilate and pass lots of blood.

Veins, like arteries, are also stretchy tubes. Although they don't have the sort of muscle layers that surround arteries. Instead, veins passively expand or contract as blood pressure rises or falls.

Most of the time, the muscles wrapping the genital arteries squeeze down to restrict blood flow, and the erectile tissue is relatively empty. During arousal, the arterial muscles relax to open up the tap and let lots of blood through to begin the process that leads to an erection. If there were enough blood flow, this would be sufficient to do the job without making any changes in the outward flow of blood through your veins.

In your genitals, however, the situation is a little more complicated. The erectile tissue reservoir is really more like a sponge than a kitchen basin. The large arteries that supply blood to your groin branch off into smaller and smaller vessels that are embedded in the spongy erectile tissue. Veins also fan out through the tissue, collecting blood and returning it to the large veins that lead back out.

When the arteries dilate during arousal, the swelling erectile tissue puts pressure on the veins. This forces them to collapse slightly and reduces the rate of the blood flowing out. The whole process amounts to opening the taps while simultaneously closing off the drain somewhat.

If things were to continue in this way, you would eventually have a problem as increasing pressure shut off the veins altogether and the blood flow stopped. Fortunately, there's a natural safety mechanism. Erectile tissue can only expand so far before it's fully engorged. At that point the blood still has room to force its way out through the veins.

In physics terms, the change in your erectile tissue as you become aroused is called a transient state. When you are not aroused, your genitals are in an unchanging steady state. Your genitals are also in a steady state when they are fully engorged. During a transient period, the flow of blood into your groin is different from the flow out as the erectile tissue becomes engorged. During a steady state, fluid physics requires the flow and out to be the same. In fact, the total flow of blood through your groin in either the aroused or relaxed state is just about the same, only the amount of blood loitering in the erectile reservoir is different between the two states.

It's a bit like using a damn to turn a valley into a lake. Diverting a river into the valley begins filling the reservoir. Closing the flood gates in the dam stops the flow of water out of the valley and downstream. Once the valley is filled up, the total amount of water flowing into the lake and spilling over the dam at the other end is the same that always flowed down the river. However, a lot more water is stored in the dammed up reservoir, just as more blood is stored in your erectile tissue when it's engorged.

You could rely solely on your body's natural processes to achieve a penile or clitoral erection, but many people enhance their experience with various sexual devices and techniques. In terms of the lake metaphor, most methods either widen the river flowing into the reservoir or raise the dam higher at the reservoir's outlet.

One of the most common sexual aids is the cock ring. It is a constriction placed at the base of the penis that effectively raises the dam holding more blood in your groin reservoir.

Cock rings squeeze down on the outside of a penis. They compress both arteries and veins, which causes them to collapse a little, restricting blood flow through the penis. This might lead you to wonder why they should enhance an erection. After all, if they squeeze down on both the arteries and veins you could imagine that a ring would reduce the blood flowing in as much as it reduces blood flowing out.

Fluid physics provides the solution to the cock ring puzzle. Here's how -

Whenever a liquid flows through a tube, it experiences some friction that opposes its motion. The amount of friction depends in part on how thick the fluid is. Physicists call this the viscosity. Water flows easily because its viscosity is low. Slow moving molasses is highly viscous, and blood has a viscosity somewhere between water and molasses.

When your heart pumps blood into your arteries, the resistance to the flowing blood causes the pressure to decrease the farther it moves away from your heart. (Other things affect the pressure as well, but we will worry about those later.) Narrow tubes, such as capillaries, resist flow more than wide arteries and veins.

The blood that flows through your groin passes through many tiny capillaries. If you were to measure the blood pressure on its travels from the arteries to the veins , you would find a significant pressure drop. Because the pressure in the out-going veins is so much lower than in the in-coming arteries, they are easier to pinch closed and they collapse more than arteries do under the same force.

You don't need a cock ring to test the effect. If you wrap a piece of string around one of your fingers, you will find that the finger tip swells and turns purplish-red as the string squeezes the veins down but the arteries remain more open.

Of course, a ring that's too tight could potentially shut down your veins entirely, leading to a stagnant pool of blood in the penis. This can cause permanent tissue damage as the cells run out of the oxygen that your blood provides. Most cock rings, however, include snaps or other fasteners that allow you to release them if things get out of hand.

You can also enhance an erection by pinching the base of the penis - with your thumb on the upper side, and your fingers pressing under the testicles just in front of the perineum. The pressure compresses the veins leading out of the penis just as a cock ring does. Although your hand will be in the way for some activities, and the effect will subside immediately upon releasing pressure, it can temporarily enhance girth and pleasure for a man, particularly during oral sex.

Unfortunately, the structure of a woman's genitals means that there are no feminine equivalents of cock rings, although the clamps that some people apply to their clittoris alters blood flow and often causes engorgement in a small region.

The penile pinch, however, can be adapted to women. If you spread your fingers in a V shape and place them on either side of the clitoris, you can restrict venous blood flow, and enhance clitoral size and sensation by pressing and gently squeezing your fingers together. Just as is the case with the penile pinch, this can be particularly pleasant for your female partner when you are performing oral sex.

Tensed muscles put pressure on blood vessels running through muscle tissue, much as a ring squeezes veins in a penis. When weightlifters and other athletes strain to perform an exercise, the veins near the surface of their skin stand out as the muscles squeeze the blood out of deep muscle veins. The extra engorgement of the labia, clitoris and penis just before orgasm comes in part from similar muscular tension, primarily due to straining in the abdomen, buttocks, thighs, and calves.

Vacuum pumps, including both penis pumps and clitoral pumps, also aid in erectile tissue engorgement, but they operate on an entirely different principle from cock rings. They reduce the air pressure outside of the organ rather than compressing veins inside. Normally, the atmosphere pushes on us from all directions with a force of about fifteen pounds per square inch. You don't notice the pressure because the fluids inside your body press outward and balance the air pressure on your skin. If you could selectively reduce the pressure around part of your body it would bulge outward because the pressure inside of you pushing out would no longer be balanced by the atmospheric pressure pushing back in.

A vacuum pump allows you to reduce the air pressure surrounding your penis or clitoris by surrounding it with a chamber that seals to the skin of your groin. You can then reduce the pressure by sucking the air out of the chamber with a pump.

Because the air pressure on the rest of your body remains unchanged, it's effectively the atmosphere pushing on the rest of you that forces blood into your genitals.

Unlike cock rings, which require least some arousal to function, pumps can inflate a penis or clitoris even in the absence of any natural erectile response. Once a penis is engorged with a pump, a cock ring can help maintain the erection.

Pumps can cause injuries if things are taken too far. The high pressure difference between the inside of the blood vessels and the vacuum in the pump may lead to ruptured blood vessels and permanent vein damage. They are generally safe, however, if you follow the manufacturer's instructions.

There are many other things that can affect blood flow in your genitals, including drugs, diet, and even your choice of sexual positions. We'll address several of them in next week's episode - Part two of Pumped Up and Ready for Love: Sex and Fluid Physics.
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Slip, Slide, or Stick: Friction and Lubrication

Your hand slipping over your lover's back, metal pieces sliding past each other in a car engine, and even blood flowing in your blood vessels all involve friction. A little friction is a good thing in the bedroom, but a whole industry has developed to provide products to keep sexual friction under control.

Listen to the podcast with roboreaders Audrey and Paul.

The amount of friction between two surfaces depends on the roughness of the materials, their chemical composition, and the force pressing the materials together. A lubricant is any substance that reduces friction. Some lubricants are powders, such as graphite, but the lubricants involved in sex are generally liquids, gels, and creams.

Lubricants for sex, which are often called personal lubricants or simply lubes, come in a dizzying array of varieties. There are water-based, silicone-based, and petroleum-based lubes. Some lubes include fragrances; others are edible and offer an array of succulent flavors. Many lubes feature additives such as pigments and dyes, anesthetics, moisturizers, preservatives, and chemicals that warm or cool skin.

How do you know which to choose? The Physics of friction and lubrication can help you figure out the best lube for you and your partner, no matter what activity you have in mind.

Natural Lubricants

The human body is a complex machine, with hundreds of moving parts. Like all machines, it needs lubricants. Elbows, knees, ankles and the rest of your joints would eventually grind themselves to dust if they lacked lubrication. Tears lubricate your eyeballs, saliva lubricates your mouth and throat, and your skin is constantly moistened with sweat and oils to keep it supple as millions of skin cells jostle against each other.

The natural lubricants most important for sex are saliva, vaginal fluids, and male pre-ejaculate.

Saliva consists primarily of water and mucus. Mucus is made of long proteins called mucins, which are coated in compounds related to sugars. The mucins bind to water to make saliva slippery.

The slipperiness of saliva mucus makes it a convenient lubricant for oral sex, hand jobs, and anal sex. Unfortunately the high proportion of water in saliva makes it dry out quickly as the water evaporates.

Vaginal fluids are also packed with mucins to ease penetration and protect vaginal linings from germs. In addition, vaginal fluids include various acids to provide the right chemical environment for sperm, and sugars that nourish sperm swimming through the vaginal fluid toward the ovaries.

Women produce vaginal fluids when they become sexually aroused. The amount varies depending on their age, health, and the timing of their menstrual cycle. Smoking just before sex can reduce fluid production by diminishing blood flow to the vaginal lining. Antihistamines and other drugs can reduce natural lubrication as well. All women need a little lubrication help from time to time, and even the most abundantly lubricating women may need to supplement their vaginal fluid with saliva or artificial lubricants - for extended love sessions.

Men produce some lubricant as well. The Cowper's gland near the prostate secretes a small amount of slippery fluid commonly called pre-ejaculate or pre-cum. Some sex experts believe it helps to lubricate the head of the penis before penetrating a woman's vagina, but most men only produce a few drops and it is often ill timed for insertion. It's more likely that the Cowper's fluid prepares the urethra for the passage of sperm by adjusting acidity, clearing out any traces of urine, and lining the urethra with a slippery, sugar-rich energy source to get the sperm swimming.

There is some lubrication in the anus, but nowhere near enough for most anal sex activities. Generally, you're going to need to bring along some artificial lube for anal play.

Water-Based Lubes

Pure water is an excellent lubricant - sometimes. If you've ever slipped on a wet tile floor, or nearly broken your neck stepping into a tub, you know how slick water can be. Many lubes consist primarily of water.

Unfortunately, water can sometimes dramatically increase friction instead. For example, people may lick a finger to add friction before turning a page in a magazine. Slightly moistening your hands will give you a better grip when trying to take the lid off of a jar, provided you don't make your hands too wet.

When you step into a tub, you usually experience a little of both water's lubrication and its tackiness. After initially sliding over the bottom of a filled tub, your feet will suddenly gain traction, giving you much more grip than you would have had if the tub were totally dry.

The two radically different properties of water stem from the attraction that water molecules have for each other and for some other materials. Materials that attract water are called hydrophilic, or water loving, and materials that are not attracted to water are hydrophobic, or water hating. Water beads up on a freshly polished car because the polish is hydrophobic and repels water. Rain droplets spread out on a car that needs waxing because the old oxidized polish is hydrophilic and attracts water.

Water is a liquid because the attraction between the molecules is too weak to turn it into a solid and they slip and slide over each other. It's strong enough, however, to bind water into small droplets when it falls as rain or runs down a window pane.

When your foot slips as you're stepping into a tub, it's because there's a thick layer of water between your foot and the porcelain. The mild attraction between the water molecules makes them act a bit like tiny marbles, with very low friction.

As you put weight on your foot, you squeeze most of the water out of the way, until there is a very thin layer between your skin and the tub. In some places, the layer is only a few molecules thick. Because your skin and the surface of the tub are slightly water-loving, the molecules are attracted to both. The slight attraction the water molecules have for the tub and your foot combine to give you traction.

It's because of the dual lubricating and adhesive properties of water that making love in a pool tub may seem like a good idea, but rarely turns out well. Your skin slides easily over your lover's skin, as long as the contact is light and there is a lot of water between the two of you. Once your skin presses together, you lose water's lubricating properties and the adhesion takes over, which can make vaginal and anal penetration particularly rough experiences.

But it's possible to exploit the forces between molecules to ensure that water stays slippery. That's what's going on in water-based lubes. These types of lubricants work in one of two ways; either by ensuring that the water molecules clump together so that you are less likely to get a thin adhesive layer, or by reducing the water molecules' attraction to each other and other hydrophilic materials. Some lubes have ingredients that do both.

Mixing in glycerin is one way to make water molecules clump together and form a good liquid lubricant. Glycerin is a small molecule that's hydrophilic in two places. As a result, water can attach to each side of a glycerin molecule. Another glycerin then attaches to the water, and so on. Eventually long molecular strings will form. If you could see the mixture through a powerful enough microscope, you would see that the strings tangle up like spaghetti. They slip and slide, like a plate of heavily buttered pasta noodles. To the naked eye, the result is a clear liquid that is much thicker and slicker than water.

The binding between the glycerin and water is weak enough that water molecules can break free of the mixture. They may then evaporate or get absorbed into your skin, which means that lubes relying on glycerin to hold water molecules together will slowly dry out. The glycerin molecules that have lost their water will be mildly attracted to your skin, which makes the lube get sticky as it dries. Adding a little water will restore the glycerin lube's slipperiness.

Glycerin is related to the sugar glucose. If you taste some, you will see that glycerin lubes are sweet. Like sugar, glycerin is a good energy source and sometimes serves as a nutrient for microbes. Women may find that glycerin lubes foster yeast infections. If you or your lover suffers from frequent vaginal infections, look for glycerin-free water-based lubes. There are several other molecules that can hold water together as glycerin does, without feeding populations of vaginal bacteria and fungi.

Another way to make water stay slippery is by mixing it in a gel. Gel lubricants get their jelly-like consistency from long molecules of protein rather than short glycerin molecules. The proteins in gels have many places along their lengths that attract water. Chemical treatments or heat cause the long molecules to crosslink, which means that they connect to each other in some places.

It's like tying pieces of string together at random places to make a loose, three-dimensional web. Water molecules get trapped in the web at the hydrophilic points along the protein molecules. Food gelatins, like Jello, trap water the same way.

The more places that the molecules are connected in a crosslinked gel, the more rigid and jelly-like the gel will be. Like glycerin lubes, water-based gels may dry out in time. Because they're more complex than liquids, with water trapped in a net of crosslinked proteins, you cannot rejuvenate them as well by simply adding water. It's better to add fresh gel if it gets too dry.

Some lubes rely on chemicals called surfactants that reduce the attraction between water molecules. Instead of ensuring that there is a thick layer of water between your skin and your lover's skin, surfactants make thin layers of water less adhesive. They're generally medium length molecules, longer than glycerin and shorter than most proteins.

Surfactant molecules each have a hydrophilic connection at one end. Surfactant molecules link up with water, effectively making the molecules larger and more bulky. This keeps them farther apart. The attraction that water molecules feel for each other gets much weaker if they are even slightly separated. The reduced attraction also reduces the adhesion of the surfactant-water mix.

Many gel and liquid lubes include surfactants to make the water in them ultra slippery.

Water-based lubes are safe for use with latex condoms and diaphragms, as well as all sex toys. They wash off easily with nothing more than warm water. Of course, that means they rinse away too readily for making love in the bath.

Oil-based Lubes

Some lubes don't include any water at all. Lubricants based on vegetable oils and petroleum products are often very slick and long lasting. Vegetable oils common in lubes include olive, sesame, and palm oils, to name just a few of the many possible varieties. Most petroleum-based lubes are varying grades of petroleum jelly, with Vaseline being the best known brand.

Both plant oils and petroleum lubricants are made of hydrocarbon chains, long strings of carbon atoms with hydrogen atoms attached to the sides. The texture of petroleum-based lubes is determined primarily by the lengths of the carbon chains.

Molecules made of chains ten to fifteen carbon atoms long form mineral oils and light watery lubricants. Longer chains are heavier and clump together to form jellies like Vaseline. Still longer chains result in paraffin wax.

Vegetable oils are a little more complicated. They're also made of hydrocarbon chains. However, they typically consist of multiple chains linked together by a glycerin molecule. Like petroleum products, heavier oils usually are made of longer chains. In addition, light, watery oils can be made to solidify by changing the number of hydrogen atoms attached to the chains, through a process known as hydrogenation. Margarine and shortening are made from light vegetable oils that have been hydrogenated.

Hydrocarbon chains that make up oils are highly hydrophobic. If you've ever made salad dressing with oil and vinegar (which is mostly water) you've seen how hard it is to mix the two.

The molecules in oils and petroleum products don't attract each other or your skin very strongly, which is why they're slippery.

You might think oils and petroleum jellies would be easier to clean off of your body if they're not as strongly attracted to your skin as is water, but that's clearly not the case. One reason it's harder to remove hydrocarbon lubes is that their large molecules don't evaporate very well. If you get water on your skin, just wait a while and it'll dry all by itself. Oils and petroleum products will stick around for ages with little or no sign of evaporation, which is good for long lovemaking sessions, but not so great for the post-coital clean up.

The fact that the lubes are hydrophobic also means that you cannot simply rinse them off with water. To remove the lubes you'll need to wash with soap.

Many men prefer oils and petroleum products over water-based lubes for masturbation. Some people feel that they are better suited for anal sex because the petroleum jellies in particular are heavier and last longer. Hydrophobic vegetable oils and petroleum lubes work well in the tub or pool because they won't rinse away.

Unfortunately, all oils and petroleum products dissolve latex, and should never be used in combination with latex condoms, diaphragms, and latex sex toys. It's also generally a bad idea to use them for vaginal sex because the soap necessary to clean the lubes away removes the protective vaginal mucous as well. This leaves the delicate membranes open to infection.

Silicone-based Lubes

Some of the newest lubes on the market are silicones. Silicone molecules have essentially the same structure as petroleum except that the long carbon chains are replaced by chains of alternating silicon and oxygen atoms. Like oils and petroleum products they are hydrophobic, long lasting lubes. They're just as slippery as oils but will not dissolve latex. They wash off with soapy water, making them less than ideal as vaginal lubricants but much better alternatives for anal sex when latex condoms are involved.

There are also water-based lubes that replace glycerin with dimethicone, a silicone molecule that can link water molecules into long slippery just as glycerin does. They're good lubrication alternatives for vaginal intercourse if you want to avoid glycerin. Like all water-based lubes, those that include dimethicone wash off easily with water and are no good in the tub or pool. They're safe to use in conjunction with latex products, but will still damage silicone sex toys.

Emulsions and Creams

Although water and oil don't mix as a rule, there's a way to almost make them get together, which leads to another type of lubricant - emulsions. Surfactants do the trick. You've already seen that surfactant molecules that are hydrophilic on one end can make water more slippery. If the other end of the surfactant molecule is hydrophobic, then it can help get water and oil molecules close together, even if they don't actually mix.

Creams and many creamy lotions are emulsions. If you mix a surfactant in water and then add oil, the surfactant molecules will surround oil droplets with their water-hating ends pointed in, toward the oil, and their water-loving ends pointed outward. The surfactants create tiny balloons of oil in the water called micelles. The oil and water still don't actually mix, but the micelles act like large dissolved particles in the water. Mayonnaise is a common emulsion of vegetable oil and water, with a bit of egg white mixed in to act as an emulsifying surfactant.

All emulsions, including most lubes advertised as creams or lotions, have either oil or silicone mixed with water. Be sure to check the ingredients before using an emulsion in combination with latex or silicone prophylactics and toys; all the same precautions apply for emulsions as do for simple oils, petroleum jellies, and silicones.




To summarize. . .

- Water-based lubes are usually safe with latex condoms as well as silicone and rubber, provided they don't have any oils or silicone surfactants. (Check the label to be certain.) They're also easy to clean up.

- Oil and petroleum lubes are super slick and long lasting, but destroy latex condoms and toys, and are hard to wash off.

- And finally, silicone lubes are also very slick and long lasting, but don't harm latex products. Although they can be as tough to clean as oils, and will damage silicone toys

The options for sexual lubes are vast. So which is best for you? It's hard to say, but I can't think of a better way to answer the question than buying several types and spending a night trying them out. It'll be a slippery, sloppy session of sexual fun, and in the end I hope you'll have a new appreciation for the physics of friction and lubrication.

Next time . . . Pumped Up and Ready for Love: Fluid Physics and Sex
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The Physics of Sex podcast listings

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My Odeo Channel (odeo/b83238c710d143b5)

Podcast Alley
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Sexual Rhythms

The swaying of breasts, the menstrual cycle, and hip thrusts are just a few of the important oscillations in our sex lives. Physics provides a simple and powerful description of rhythmic motion and cycles, and can help you get the most out of oscillations, from choosing the best bed for your sexual pleasure, to enjoying and exploiting the natural rhythms of your body parts during sex.

Listen to the podcast with roboreaders Audrey and Paul.

Seasons shift through the course of a year, the ocean tides ebb and flow every day, and your mood may swing with the periodic changes in the chemicals in your brain, but the most common types of oscillators are mechanical - a tree bending back and forth in a breeze, a string bowed on a violin, or a couple making love.

Mechanical oscillators work by transferring energy between two forms - kinetic and potential. Anything that moves has kinetic energy. Stored energy is potential, as in the case of a ball poised to roll down a mountain. The energy in oscillators is sometimes kinetic, sometimes potential, and usually a bit of each.

When a playground swing moves back and forth it's briefly stationary at the highest point in its arc. For an instant it has no kinetic energy. Because it is higher above the ground than at other times in its motion, it has a maximum amount of potential energy, just like a ball on the verge of rolling down a hill. When the swing descends, it speeds up as its potential energy is converted to kinetic. The swing is moving fastest, with the most kinetic and least potential energy, at the lowest point of the arc. As it swings up in the other direction, the kinetic energy is converted back into potential.

Similarly, when you make love on your bed - at least when one partner is bouncing up and down on top - you rhythmically compress the mattress springs. The springs have lots of potential energy when they are compressed, but as the springs extend and push you upward, the potential energy is converted into the kinetic energy of your moving bodies.

The rate that the energy flows back and forth in an oscillator is its resonance frequency. The frequency of an oscillator is measured in hertz, which is the number of oscillations in a second. A clock ticks at one hertz , or once per second; your heart can beat at two hertz or more during heavy exercise and sex; and middle C on a piano is a 440 262 hertz vibration in air, which we hear as a musical note.

All oscillators have at least one resonance frequency. Many, such as violin strings, have several resonances. The distinctive sound of an instrument has a lot to do with the many resonances that are produced along with every note; it's the combination of resonances that ensures that a violin and a piano produce rich and distinct sounds even when they are playing the same note.

One resonance, however, is usually more important than the rest - it's the fundamental resonance. In the case of a musical instrument, the fundamental is the note a musician is playing. When a violinist chooses to play middle C, for example, he or she presses on a string to ensure that its fundamental resonance occurs at the 440 hertz frequency we identify with the note. Other, lesser resonances are called harmonics.

Lovers on a bed form an oscillator with a fundamental resonance and a spectrum of harmonics. Just as a gifted musician can make sweet music by adroitly manipulating an instrument's resonances, lovers can add to their resonant bliss in the bedroom by understanding and controlling their bed's oscillations.

When you sit on a bed, you'll sink into the mattress until you reach an equilibrium position. At that point, the force of gravity pulling you down is balanced by the force of the bed springs pushing you up. If you start to bounce up and down, you'll find that there's a certain frequency that allows you to get a big steady bounce. That's the bed's fundamental resonance. Most beds resonate at frequencies of a few hertz.

By rhythmically bouncing on the bed, you're doing what physicists call driving the oscillator. It's easiest to drive an oscillator at its resonance frequency. At frequencies just below or above resonance, it takes much more force to get a big bounce. If you start out very slowly, you'll probably end up oscillating well below your bed's fundamental resonance and won't bounce on the bed much at all.

Increasing your speed can bring you into resonance, allowing you to achieve large bounces with seemingly little effort. Once you're moving too quickly , you may pass the resonance. As a result, you'll end up working against the bed's rhythmic sweet spot. If that happens you will have to exert much more force to get a good bounce at the same time that you're trying to move quickly. The chances are, you'll be rapidly exhausted. If you stick close to the resonance, on the other hand, you get maximum motion for minimal energy input, which helps you keep going longer before you wear out.

Many things can affect a bed's dynamics, but it's the mattress' firmness that plays the greatest part in determining the resonance frequency. Firmer mattresses have higher resonances than soft mattresses. If you visit a mattress store, you can check this for yourself. Sit on several beds with different firmness and bounce on each. You'll find that the frequency varies from a very firm to very soft mattress.

Beds get their bounce from springs, and springs have resonance frequencies that depend both on their firmness, which physicists call the spring constant, and the mass on top of the bed. That means the resonance frequency will be different for you than it will be if you have someone in bed with you. In fact, if you and your partner are about the same weight, the resonance frequency will be roughly half two-thirds as fast with the two of you close together on the bed as it will be with just one of you. (To be more precise, it will be 1/(the square root of 2) or about 0.7 times slower.)

Determining a bed's resonant frequency is only part of the issue. After all, you may not be content within the confines of one rhythm. Fortunately, there's another factor that affects bed motions. Harmonic oscillators, and beds in particular, often include a certain amount of damping, which gives you a little more leeway in choosing your own rhythms.

Shock absorbers in cars are a good example of damping. Car suspensions consist primarily of simple springs. When a car hits a bump, the springs allow the wheels to travel up or down relative to the car, maintaining contact with the road. If it weren't for shock absorbers, a car would continue to oscillate on its springs after hitting a speed bump or a pothole, leading to a nauseatingly bouncy ride. Shocks settle a car down quickly by dissipating the energy of the bounce. As a result, they reduce the resonance frequency and make the resonance less pronounced. If you push down on the bumper of a car with bad or missing shocks, you can easily get it to resonate and bounce dramatically. It is much more difficult to find the resonance frequency of a car with good shocks.

Damping has the same effect on a bed that shock absorbers have on a car - it will be harder to drive a resonance on a very damped bed, but at least you won't suffer the frustration that comes from trying to move at rates higher than resonance.

Although no commercial beds currently come with automotive-type shocks, padding in the mattress adds damping. Alternatively, you can add your own damping by spreading a thick comforter on the bed and making love on top of it. A few well-placed pillows under you can increase damping too.

The bottom line is this: if you want to use the resonance to your advantage and you like it fast, choose a firm bed; if you like it slow, go with a soft bed; and for maximum flexibility, buy a firm bed but keep a few soft comforters and pillows around to dampen the resonance to suit your mood.

If you have the soul of an experimental physicist, try making love on a trampoline, which has almost no damping and a powerful resonance. Then try it on a water bed, which also has little damping and strong resonance, as the water sloshes from one place to another, but at a much lower frequency than a trampoline. To round things out, make love on a squishy foam bed, like the Tempurpedic mattress, to experience lots of damping with very little resonance. It can be an exhausting and frustrating challenge.

Some people find that their favorite lover is a machine - specifically, their vibrator. Vibrators get their buzz from an electrically powered oscillator.

In battery-powered models, vibrations generally come from an electric motor attached to a rotating disk, with its weight placed off-center. The principle is the same thing that causes unbalanced washing machines - which are some, other, people's favorite lovers - > to buck violently when more of the laundry is on one side of the washer drum than the other. The faster the motor turns, the higher the vibrator frequency.

Many of the vibrators that plug into the wall generate oscillations with a different type of electric motor called a solenoid. Instead of spinning an unbalanced weight, electricity passing through a coil of wire forces a metal slug to vibrate rapidly back and forth. The speed of vibration in a plug-in vibrator is related to the 60 hertz oscillations of the electricity in wall sockets. They are usually more powerful than battery vibes. Unfortunately, their power comes at a price - they can only vibrate at the frequency of the electricity in the wall, at a multiple of the electrical frequency, or certain fractions of the wall frequency. Unlike battery powered vibes, which can run at a spectrum of speeds, most plug in vibrators have only one, two or three speed settings.

Ideally, vibrators would also come with adjustments to increase the strength of the vibrations independently of the speed, but that is not the case with any vibrators currently on the market. This is likely due to the fact that adding a power setting would complicate vibrator design, but you can always adjust the power you feel by changing how hard you press the vibrator against your body.

Vibrators have resonances just as beds and cars do. That means that increasing the speed of a continuously variable model can either increase the power of the vibrations or decrease them, depending on whether you are approaching or passing the resonance. As a rule, the power of the vibrations will slowly increase as you turn up the speed, then reach a maximum at the resonance frequency, and slowly fall as you continue to turn it up.

Fortunately, you can gain a bit more control over your vibe with damping, just as you can use damping to adjust a bed's dynamics. Changing how tightly you grip a vibrator and where you hold it will change the amount of damping, allowing you to modulate the speed and intensity. Inserting it into your vagina or anus will also change the speed as the soft tissue touching the vibrator absorbs energy. If you listen to the pitch as the vibrator moves in and out, you can hear the speed change. Alternatively, pressing the vibrator against a soft rubber or gel can also slow it down.

Some dildos have a cavity that allows you to insert vibrators into them. The softer and heavier the dildo, the more it will dampen the vibrations and slow the resonance. Many manufacturers of plug-in vibrators offer soft sleeves and attachments to allow you to dampen oscillations in the same way.

Damping is the reason that vibrators are more comfortable when used in the anus or vagina than on a hard penis or clitoris. A rigid clitoris or penis has much less damping than softer and more enveloping anal and vaginal tissue. The vibrational motion, and resulting energy, is transmitted at full intensity to sensitive nerves in a small area where the vibrator makes contact with the rigid tissue. In the vagina and anus, the energy is distributed to more tissue and nerves, which feels less intense.

Springs, like those in a mattress, are just one classic type of oscillator. Pendulums are another. A pendulum consists of a weight at the end of a rod or string. It will swing at a rate determined by its length, regardless of the amount of weight on the end. A long pendulum swings slowly, and a short pendulum swings quickly. It's easy to adjust a pendulum's frequency by changing its length. Old fashioned clock pendulums included adjustments for fine tuning the length of a pendulum depending on whether the clock ran fast or slow; if it ran too fast you could turn a screw to lengthen the pendulum, or turn it the other way to shorten it if the clock ran slow.

Much of sex involves motions that have the characteristics of a little of both pendulums and springs. From a physics point of view, a woman's breasts are a complicated combination of springs and pendulums. Depending on whether she is lying on her back, standing, or on her hands and knees, her breasts are more like one or the other, which can have a huge effect on how they move.

Take a woman on her hands and knees, for example. Her breasts will hang down and sway as a result of her motions. Because hanging breasts are similar to pendulums, they have resonances determined primarily by the length they extend from a woman's chest. The resilience of her skin and breast tissue will also have an effect, but for gentle motion, the pendulum-like aspects are most important. Breasts will naturally swing slower if they hang farther from the chest, and swing faster if they are more compact.

It's easy to tell when hanging breasts have reached their resonance frequency, because they will swing dramatically back and forth. Increasing the frequency of the forces driving her breasts will reduce the amplitude of their motion, as they pass the resonance frequency, until they stop moving altogether.

When a woman is on her back, her breasts will tend to resemble springs more than pendulums, and will resonate at a frequency that has more to do with their resilience and mass. Heavier breasts have lower resonance frequencies, and tauter breasts (with higher spring constants) have higher resonances.

Resonant frequencies of breasts vary dramatically from one woman to another, and will vary even in a specific woman as her breast size and tissue resilience changes over time, or as she changes position from standing to lying on her back to getting up on her hands and knees.

Resonances are also the reason some women need sports bras when they exercise. It can be painful if a woman moves with rhythms close to her breasts' resonance frequency because that is when they are moving the most. Sports bras solve the problem by compressing breasts and making them, in effect, more taut. This raises the resonance frequency, hopefully beyond the frequency of jogging and other repetitive motions. Depending on a woman's cup size and the design of the bra, it may just move the resonance frequency up enough to make one exercise comfortable while making another painful.

For instance, if a woman with large breasts finds that her resonance frequency comes at a slow jog, it's possible that she will experience less motion if she sprints at a rate that drives the breasts at frequencies above resonance. Essentially, speeding up changes the bounce to a jiggle. Potentially, if she were to wear a sports bra that makes jogging comfortable it could move the frequency up to the point that things get bouncing and painful when she sprints.

Penises too have natural frequencies, which can change depending on arousal. Although the first sports bra (according to one story of their origin) was made from a pair of modified jock straps, men do not usually need extra support to prevent the sorts of resonances that plague women's breasts during exercise. A hanging, flaccid penis and testicles form short pendulums with resonance frequencies well above the frequencies of nearly any athletic activity. All jock straps do is lift the testicles up and forward to keep them from being squashed between the thighs - resonances aren't usually an issue during exercise.

An erect penis, however is like a large mass on a spring attached at the pelvis - the larger and heavier the penis, the lower the resonance frequency - and it may well resonate at rates that would interfere with many sports, but a man with an erection probably isn't in the right state of mind for jogging, basketball or soccer anyway.

Some types of penis enlargement involve snipping the tendon that supports the penis, allowing it to extend farther from the pelvis. This can radically reduce the spring constant of the penis attachment, and lower the resonance frequency of the erect penis a great deal. It's hardly iron-clad evidence, but if you watch a porn movie, you may notice that two men with similar sized erections seem to have very different penile resonance frequencies. (Look for the motion when their penises are not being touched directly, but are being driven by some indirect motion, say shifting position on the bed or walking across a room in the nude.) It's possible that a fellow with a low frequency oscillation has had his penile tendon snipped.

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Bouncing beds, humming vibrators, and oscillating body parts are only a few of the ways that simple harmonic motion is important in the boudoir. In fact, your entire body is a kind of a simple harmonic oscillator. Moving from one position to another changes the portions of your body that come into play and affects the resulting resonances. The rhythm that feels natural with one partner on top is likely to be different from the rhythm when the other partner is on top, particularly if they are significantly different sizes.

Clearly, we've only touched on a fraction of the ways rhythms are important in sex. We hope it's enough to convince you to keep oscillations and resonance frequencies in mind, in order to help you enhance your sexual experience - whether you're choosing a bed, buying a vibrator, or searching for a new position.
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Why do we have sex? Part 3

In the competitive game of natural selection, the winning organism is the one whose DNA is most prevalent and longest lasting.

Listen to the podcast with roboreader Kate.

When a bacterium divides, the offspring carry the same DNA as the parent, as do the offspring in the third and fourth generations, and every generation after.

Sexually reproducing organisms lack this sort of genetic continuity. Your children will each carry genes that come in part from you and in part from their other parent. When your children breed, the portion of their DNA that you contributed will be further diluted. In the course of a few generations, there's not likely to be much of you left in your descendents. On the bright side, genetically speaking, at least your descendents and your species as a whole, can survive in the face of extreme stresses, while an asexually reproducing species could be wiped out easily. In fact, the scarcity of asexual species suggests that they hardly ever survive for long in nature.

I know what you're thinking. If two sexes are better than one in dealing with stress and mutations, wouldn't three be better still? The fact that there don't seem to be any suggests the answer is no.

Nevertheless, physicists have developed mathematical models of hypothetical creatures that breed in sets of three. (The researchers who wrote the papers examining the three-sex models, unfortunately, didn't bother to explain how these creatures would get their three-way groove on.)

Three-sex creatures and their offspring are triploid, with three complete sets of genes rather than the two of diploids like us, or the single genes of haploid amoebas and other simple animals and plants.

The numbers work out poorly for three-sexed creatures. For one thing, it's much more complex to get a reproductive trio together. The simple fact that it takes more parents to produce the next generation means that the population will grow slower than that of competing diploids just as diploids are out-bred by asexual haploids. Triploids, however, have one thing going for them - they are less susceptible to random mutations than haploids or diploids, thanks to even greater genetic redundancy than two gene animals like us. Unfortunately, they lag behind when it comes to adapting to other sorts of stress. Like so many cases of competition in nature, too much of a good thing turns out to be bad.

If triploid genes work in the same way ours do (which seems like a good guess) then each of the three genetic sequences has a genes for every trait, but it's the dominant one that wins out, or the trait ends up being a blend of all three. If the blue eye gene is dominant in a triploid population, the fact that each member of the population has three shots at getting a blue eye gene means that it is much more likely that everyone will have blue eyes.

It's easier to grasp the problem if we take it a few steps further. Imagine a population with ten sexes (and ten sets of genes per cell), and blue eye genetic dominance. Even if nine out of ten genes code for non-blue eyes, the one remaining blue eye gene wins. In cases of incomplete dominance and co-dominant genes, blending more and more versions of a certain trait leads to genetic uniformity instead of diversity, just as mixing more and more colors from a painter's palette results not in brilliant new colors, but to ever muddier shades of brown. While single-gene haploids have essentially no genetic diversity, increasing the numbers of complete genes in an organism beyond the two of diploids also leads to steadily decreasing diversity, which means less and less opportunity for evolutionary adaptation.

For most organisms, at least the larger and more structurally complex ones like humans, two-sex genetic diploids have an optimal combination of diversity, adaptability, reproductive efficiency, and resistance to genetic errors.

When humans rely solely on the tools nature provides us, we reproduce as most two-sex creatures do - a male provides semen, a female provides an ovum, and another generation begins. With a little medical and scientific intervention, however, we have lots more options.

Artificial insemination of course is one of the oldest and simplest alternatives to actual intercourse. Infertility treatments involving insemination in a Petrie dish are much like the external insemination practiced by fish and other aquatic and amphibious creatures.

While it is not triploid sex, when a woman serves as a surrogate mother for a fertilized ovum she is part of an interaction much like the three part male-female-female mating model.

Human cloning, however, is perhaps the most controversial method that may soon be among our potential reproductive options. Setting ethics aside, humans who reproduce via cloning would gain many of the asexual benefits that bacteria enjoy. Presuming that people who choose to clone are women who carry their own fetal clones in their wombs, and tend to have the same numbers of children over their breeding lifetime as other women, they would be able to increase their numbers much faster than sexual human couples could.

Men who opt to clone themselves need to seek out a woman to host the fetus in her womb, which means the process still takes a man and a woman. So that's really not an advantage numerically. In addition, the woman host would have to agree to waste precious reproductive time and effort to bear a child of no genetic relation to her. Surrogate mothers do that today for couples who can't, or choose not to, carry their own children to term, but surrogates usually require fee in exchange for the rented womb.

In a community where male-female couples and cloning women each choose to raise two children, the numbers of mating couples stays constant with each passing generation, but the numbers of clones double from one generation to the next. The clones' numbers could grow exponentially. Again, it's the males that are the reproductive liability in mating couples.

Other asexually reproducing creatures are highly susceptible to changing stresses in their environment, which favors sexual populations molded by natural selection. Humans, at least those living in more highly developed nations, tend to deal with stresses through controlling the environment and counteracting the stresses rather than through evolution.

If it's too cold, there's no need to evolve fur; we turn up the heat or put on a coat. If it's too hot, we turn up the air conditioning. We no longer adapt natural immunity to diseases; instead we develop vaccines, antibiotics, and prevention methods. As a result, people who are reasonably well suited to surviving in modern society, and who reproduce via cloning, would face few, if any, drawbacks from their asexual reproduction while gaining all the benefits.

Scientists have managed to clone many types of animals including cattle, cats, sheep, and monkeys. As of this writing, there have been no confirmed human clones produced from adult DNA. The complexity of cloning and the risks of severe birth defects mean it may be years before human cloning is perfected. But it will happen eventually.

If cloning ever becomes accepted practice, society could rapidly become populated primarily with women who reproduce by strolling down to the corner pharmacy to pick up the Acme Home Clone kit, rather than wasting time and energy looking for a male mate.

Clones won't evolve, so there is no reason for them to lose their sexual urges. Although they will have to learn to rely on lesbian sex to fulfill their needs, because men will eventually die out as sexually reproducing people lose ground to the clones.

When clones come to rule the world, sexual intercourse will be nothing more than a source of recreation, relaxation, and social bonding. Sex will be useless for procreation.

Of course, that's already true 99.99% of the time anyway.
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Why Do We Have Sex? Part 2

Although there are countless exceptions and variations when it comes to reproduction, there is one fundamental characteristic that typically distinguishes between sexual and asexual organisms - the structure of their genes.

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Most asexually reproducing plants and animals carry a single complete set of genes in their cells. In biological terms their genes are haploid, which is just a Latin word for simple.

Sexually reproducing organisms, on the other hand, are generally die-ploid, meaning they carry two complete sets of genes. Every one of your cells effectively has twice the genetic information it would take to make a single person. Your mother and father each contributed genes that encode the color of your hair and eyes, the size of your nose, the proportions of your limbs, and so on. Your mother's genes determine some of your traits, your father's genes determine others, and some of your traits are determined by both your mother's and father's genes.

Asexual reproduction has some distinct benefits. For one thing, it's fast. If a typical bacterium takes twenty minutes to split in two, it will have eight descendants in an hour, and sixty-four in two hours.

If there's sufficient food around to support a population explosion, a single germ dividing at this rate could boast more than a million offspring in seven hours, and over a billion in ten hours. It's what physicists call exponential growth.

Sexual creatures can grow their numbers exponentially as well, although not as fast as asexuals. If two populations of organisms, identical in every way except for their mode of reproduction, were to squeeze into the same ecological niche, the asexual population wins, at least in the short term.

Rabbits are the iconic example of animals that breed like, well, rabbits. Imagine a fertile valley that's settled by two colonies of ten rabbits each. One colony consists of five male and five female rabbits that reproduce once a year in the usual way. The other is a colony of ten asexual females that also reproduce once a year, but have no need of males (fortunately this is a purely hypothetical type of rabbit).

If every female rabbit can bear a litter of ten babies each season, then in one year the sexual rabbits increase their numbers by five litters - one litter from each female - resulting in fifty babies plus the ten original colony members, for a total of sixty sexual rabbits. The asexual colony has ten litters, one hundred babies, plus the original ten for a total of a hundred and ten rabbits.

Presumably, the amazons give birth only to females, while the sexual rabbits produce half male and half female babies. After a single breeding season, the females in the colony of asexual rabbits outnumber the sexual females by nearly four to one.

The all-female asexuals will swamp the sexual rabbits in a few generations. Even if there are limitations of food and water in the valley that keep the total number of rabbits in check, the asexuals efficient breeding scheme allows them to overrun the sexual rabbits in short order.

As you can see, males are the true liability in breeding populations - they eat food that could go to the girls, produce waste, and take up precious space in the colony. But they are of little help in increasing population numbers other than donating sperm, which asexuals can live without.

Why don't we see asexually reproducing rabbits, squirrels, rats, elephants, or humans in the real world? The answer lies in adaptation to stress. And we can thank males for that.

Asexual populations consist essentially of clones, with each child carrying exactly the same genetic material as its parent. If a new disease, parasite, or predator were to come along with a particular talent for attacking our asexual rabbits, the whole population could be rapidly decimated.

Sexual rabbits have a better chance of surviving in the face of stress thanks to the presence of the boys. In mating, the male and female of a species each contribute a portion of the offspring's genetic material, which means babies are always at least slightly different from their parents. Sex stirs the genetic pot, leading to combinations that may occasionally handle stresses better.

When a fox finds a valley full of bunnies, you might imagine that it eats the slowest ones first. All the asexual rabbits are equally swift because they're identical. If the fox can catch one it can catch them all.

Some of the sexual bunnies however, will be faster than others as a result of the variability that comes from male-female breeding. Pretty soon, the pressure of having a fox hanging around might lead to natural selection of fleet-footed bunnies. Of course, rabbits could deal with foxes in other ways - developing better camouflage, enhancing their wariness, or growing wickedly sharp claws. But in any case it's the sexual ones that have the potential of finding solutions, while the asexuals are doomed.

Once sexual rabbits have developed an adaptation to deal with a specific stress, you might wonder what is to prevent them from spontaneously changing reproductive tactics to become a new asexual super rabbit that can fend off a given type of threat.

Based on some physicists' models, the primary reason is that foxes, germs and parasites evolve as well. A rabbit that adapts to the stress of a certain fox causes stress for the fox by denying him food, which in turn leads to the evolution of better hunters, forcing rabbits to evolve further, and so forth. Populations of rabbits and foxes ebb and flow as each adapt to changes in the other, leading to long-term stability of predators and prey that is maintained by sexual mixing of each species' genetics.

As organisms evolve, they face lots of shifting stresses, which firmly establishes sexual reproduction as the procreation method among just about everything larger than an amoeba.

Even if there were no threat of predators, parasites or diseases, all life faces the risk of random genetic mutations. Mutations are changes that arise from errors that occur when DNA replicates, or from exposure to things like radiation and chemicals. Asexual organisms that have a single precious copy of their DNA are in deep trouble as errors accumulate. Sexual organisms gain protection through their genetic redundancy - if an error develops in a gene contributed from one of your parents, repair mechanisms in your DNA can use the genes from your other parent as a map for repairing the problem. Or the problem may be moot if a healthy gene is dominant over the flawed copy.

In short, sex provides multiple levels of genetic protection. It offers a route to adaptation through gene shuffling, ensures backup copies of genes are available, and keeps genes in good shape with DNA repair mechanisms.

Tune in next time for part three of, “Why Do We Have Sex.”

I'm Kate. Thanks for subscribing to The Physics of Sex podcast.
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Why Do We Have Sex? Part 1

If your answer is "to make babies," you're wrong. Sexual activity among humans has nothing to do with fertilization more than 99.99% of the time.

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Typical heterosexual couples make love an average of a hundred times a year. Assuming they keep up this pace most of their adult lives, they will end up having had sex as many as four thousand times.

In addition to sex with a partner, most people seek lots of sexual relief when they're all alone. Men typically learn to masturbate in adolescence and keep up the practice daily until their twenties. The pace usually slows down as men age and often dips when they enter sexual relationships, but most men will probably masturbate ten thousand times in fifty to sixty years of sexual activity. Altogether, the average man can expect to experience fifteen thousand or more orgasms over the course of his life.

Women start off masturbating at a similar age and frequency as men. Most masturbate daily until their late teens, but slow down when they reach their early twenties to about a third the rate of men. Still, they typically enjoy sexual stimulation, either alone or with a partner, for a lifetime total of five to ten thousand sexual experiences.

Despite all that sexual activity, the population in the US and most other highly industrialized nations is fairly stable. That is, there is roughly a single child born for each person in the country, which means that there is one successful pregnancy for every ten thousand sexual experiences.

Humans are unusual creatures in this regard, though hardly unique. Certain apes, dolphins and wolves are among the animals that use sexual interactions for things like pleasure, bonding, and establishing social structure. But intercourse for most other organisms is all about making babies.

Even though humans rarely have sex in order to get pregnant, it's primarily our genetic mandate to mate and bear young that is reflected in our sexual desire. Evolution ensures things that are good for the propagation of our genes bring us pleasure. For most people, and apparently many animals as well, the orgasm is the benchmark of pleasure. The fact that it produces the most enjoyable sensations and the strongest desires that we are likely to ever experience suggests that sex is effectively the most important thing we do in life, from an evolutionary point of view. Although we have found ways to separate intercourse from procreation, sexual ecstasy is nature's reward to us for continuing the species.

Sex between a male and a female, however, is not the only way for organisms to reproduce. For millions of years, when life was new on Earth, plants and animals got on just fine without intercourse. Most of them were simple single-celled organisms. At times when there were plentiful resources to keep some organism alive, rather than letting anything go to waste, or go to a competitor, the organism would simply duplicate its DNA and split into two identical copies of the original. Modern bacteria, amoebas, and many types of algae and molds continue the practice.

A look at the pros and cons of various types of reproduction can help explain why we mate the way we do instead of opting for asexual reproduction, or some other scheme altogether. For physicists who ponder reproductive strategies, the subject is similar to many other sorts of problems in physics where systems, in this case populations, naturally find the optimal solution to complex and competing demands.

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Prologue: Love is Chemistry, Sex is Physics

Discuss
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