The Joy of Sexual Physics

                                                                        with Dr John

 "Love is a matter of chemistry, sex is a matter of physics"


Q  Having read your article regarding sex at the speed of light my boyfriend has expressed interest in experimenting himself, against your advice. He and I often partake in threesomes with one of my girlfriends observing us. My question to you is what will happen to her and what will she see if we actually manage to attain a speed of light oscillatory motion? Will she in turn be sucked into the penis/black hole complex of which you spoke?

Marnie Brauq

A  As we have seen before, very strange things happen at speeds close to the speed of light. In the article to which you refer, we saw that if your boyfriend was oscillating on top of you at a speed close to the speed of light, then due to the relativistic theory of length contraction, his penis would get shorter and shorter the faster he went. At the speed of light, this would lead to an interesting paradox in which you would see his penis having no length at all, but would still be managing to have sex with it. And from your boyfriend’s perspective, the faster he went, the faster he would see you age, until at the speed of light you would seem to be ageing infinitely quickly. Now imagine that we add an observer (your perverted girlfriend) to this scenario, situated above you, in the plane of your boyfriend’s motion, and observing this relativistic mating ritual with great interest. What would her account of these strange events be like?

In order to accurately predict what your girlfriend would see, we must first understand another consequence of relativity called the relativistic Doppler effect. This is basically the idea that as an object approaches an observer, its light waves are compressed, shortening their apparent wavelength, and shifting their colour towards the blue end of the visible spectrum. Conversely, as the object recedes from the observer, its light waves are stretched apart, lengthening their wavelength, and shifting their colour towards the red end of the visible spectrum. Applying this theory to the question at hand, we find that while your boyfriend thrusts into you at close to the speed of light, he will be receding from your perverted girlfriend, and his complexion will be red-shifted. And as he rises out of you, he will be approaching your perverted girlfriend, and his complexion will be blue-shifted. From where your girlfriend is standing, your boyfriend’s bottom will appear to be flashing from red to blue, red to blue - a little like the light on a police car, but about 500 million times faster.

Or at least this is how it would appear if the human eye were properly adapted to observing sexual intercourse at speeds close to the speed of light. Unfortunately, however, this trait did not seem to confer a very strong selective advantage to our primitive ancestors, and so we are left with an eye that can only resolve visual stimuli to a pitiful 24 frames per second. Since relativistic sex requires hundreds of millions of oscillations per second, then to cope with this problem, the eye merges all the images of your blue-and-red-flashing-boyfriend-having-sex together into one. In the end, what your girlfriend will see is a sort of muddy greyish white smudge only vaguely resembling a homo sapien energetically participating in sexual endeavours.

You may think that this would be quite unpleasant and repulsive to watch, but to your perverted girlfriend, it will be irresistible. If it is your boyfriend’s fantasy to be getting it on with two girls at once, then he will be very happy with the relativistic theory of mass increase with speed, because this theory predicts that the faster he goes the more gravitationally attracted you and your girlfriend will be to his penile smudge. And if he actually manages to attain the speed of light oscillatory motion of which you speak, then his penis will acquire infinite gravitational attractiveness, essentially becoming a black hole. Nobody has ever actually reported achieving this, and so we don’t really know what will happen next. But we can speculate…

One possibility is that by the time your boyfriend has actually realised that his penis has turned into a black hole, he will become profoundly depressed and will be overcome by a feeling of loss. In reaction to this anticlimax, he will probably slow down considerably, causing the relativistic mass of his ex-penis to be significantly reduced. No longer constituting a black hole, this will quite possibly result in a supernova explosion, and your perverted girlfriend will actually see your boyfriend exploding inside of you. But you will need much more protection than a simple Durex condom for this explosion. I would suggest a 10cm-thick lead condom, but this will probably not fit into your vagina. Alternatively, all three of you could be quarantined from the shower of high-energy gamma rays and Cerenkov radiation that he will emit, but this would be quite difficult to do if you are having sex at the same time. You will probably die of radiation poisoning, and depending on the electron density of your boyfriend’s penis, he will be left with either a red giant or white dwarf, both of which will be useless for procreation (but personally, I would rather have a red giant).

It would certainly be a very scientifically rewarding and arousing experience to observe sex being performed at the speed of light, but reliable scientific reasoning suggests that it would also be very damaging to your health. If your boyfriend continues to pressure you, just explain to him that his penis will turn into a black hole if he doesn't leave you alone, and his enthusiasm should quickly soften.

Got a problem, or even just a question?
Pour it all out to John.
Since the end of the twentieth century, Dr John Marshall, Ph.D. Sexual Physics has been a sex and relationships writer taking the little-known sexual wisdom from the ivory tower realm of the theoretical physicist to the layperson.
Feel free to write to him at [email protected] or you can visit his webpage at

© John Marshall, 2003