The Joy of Sexual Physics

                                                                        with Dr John

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


Q  My boyfriend, Terence, is an adventurous physics student always looking for new ways to improve our sex life. Recently he has been talking about Einstein and relativity, suggesting that we should try having sex at the speed of light or faster. But I am worried. If we were to engage in consensual sex at faster than the speed of light, would I go back in time, so that he would end up having sex with a 5 year old girl? And what are the potential legal implications of this? 
A  The area of physics which deals with speeds comparable to the speed of light is called relativity. Unfortunately for this question, Einsteinís special theory of relativity says that it is absolutely impossible for any information or matter to travel faster than the speed of light. This is backed up by a significant amount of experimental data and mathematical rigour. However, we can use relativity as a model to see what happens at speeds close to that of light. 

As appropriate for any physical investigation, we must first make reasonable assumptions and simplifications to the real situation, so that our calculations become achievable, but still retain reliability. We will assume that your boyfriend in his aroused state is a rectangular box with a generous 9-inch cylinder protruding from about halfway along his length. Your boyfriend is on top of you, and is oscillating up and down, reaching a speed infinitesimally close to the speed of light for most of his upwards and downwards motions. With these simplifications in place, we can now treat your boyfriend as an inertial system, from which we can calculate the consequences of relativistic events. 

One thing relativity tells us about is length contractions. This is the idea that the measured distance between two points depends on the frame of reference from which they are viewed. By convention, we define the proper length of an object (such as a penis) to be the length of the object measured by someone not moving with respect to it. Relativity then says that someone who is moving with respect to this object will always measure its length to be shorter than its proper length. So while your boyfriendís penis is happily travelling along at close to the speed of light during intercourse, although he will perceive its normal 9-inches, from your frame of reference you will perceive it to have almost no length at all. This leads to an interesting paradox - you will see his penis having almost no length at all, but are somehow still managing to have sex with it. 

So from your point of view, the faster he goes, the shorter his penis will get. But what he sees will be quite different. This is where relativistic time dilations come into it. Time dilation is basically the idea that in a moving frame of reference, all physical, chemical and biological processes slow down relative to a stationary frame of reference. So as your boyfriend thrusts in and out of you, while only taking a fraction of a nanosecond for each oscillation from his point of view; since you are in a stationary frame of reference, this time interval may be significantly longer for you. From your boyfriendís point of view, the faster he goes, the faster you will age. Theoretically, if he was actually thrusting in and out at a speed infinitesimally close to the speed of light, then from his point of view, you would be aging extremely quickly, and would be long dead before he finally shot his load (legally, he would be committing necrophilia). 

So far we have only been considering sex at sub-light velocities. However, we can speculate that there may be a loophole somewhere in relativity, and that this loophole might make sex at superluminal velocities attainable for brief moments. If so, then under certain conditions, it would be theoretically possible for your boyfriend to climax shortly before you actually start having sex with him (an extreme case of premature ejaculation!). Your boyfriend and his schlong, in other words, would assume for brief periods the characteristics of tachyons. I will admit that tachyons remain hypothetical, but then so do black holes, and who really doubts their existence anymore? 

But I should warn you that there might be even more severe complications with this scenario. If we extend the general trend of length contraction (from your point of reference), then your boyfriendís penis will gradually appear to get shorter and shorter as he speeds up, until at the speed of light it will appear to have no length at all (size does matter). But beyond the speed of light, things get even stranger, and your boyfriendís penis will actually attain imaginary length (that is, it will have a length equal to the square root of a negative number). And from the relativistic prediction of mass increase with speed, this imaginary oscillating penis will attain beyond infinite mass, essentially becoming a black hole. This penis/black hole complex will generate an intense explosion, attracting anything and everything towards it through the curvature generated in space-time. Ultimately, you and your boyfriend will be inescapably pulled into the centre of this penis/black hole, ripped apart into your constituent atoms, and regurgitated out of the penis in the form of electromagnetic radiation. 

It would be interesting to see how sex could be performed under such circumstances, but my guess is that it couldnít be. In response to your original question, you would not go back in time, and he would not end up having sex with a 5 year old girl; but you have good reason to be worried! Donít let your boyfriend pressure you into doing anything you donít want to do. 

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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