The Joy of Sexual Physics

                                                                        with Dr John

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


Q  Your recent article regarding sex at the speed of light has caused me great concern, for if my boyfriend were to ejaculate whilst thrusting into me at the speed of light, then would not his semen be travelling faster than the speed of light? Could this superluminal semen travel back in time to fertilise me before he put on his condom? And does this thought experiment disprove the theory of relativity?


A  Your thought experiment does disprove one theory of relativity, and that is Galilean relativity. Contrary to popular belief, it was Galileo and not Einstein who invented the theory of relativity. Galileo proposed that it was relative velocities that mattered, and not velocities measured relative to the Earth, as Aristotle had previously suggested.

The average speed of ejaculation has been measured to be approximately 45 km/hr (just below the allowable speed limit in most suburban areas). This is the average speed of the ejected semen relative to the ejaculatory penis, and can be increased by training the Kegel muscles (by pretending to withhold your urine). Anyway, according to Galilean relativity, if one is to ejaculate whilst thrusting inwards at the speed of light, then the relative speed of their ejaculatory semen will be equal to the sum of the two speeds, namely the speed of light plus 45 km/hr. In other words, Galilean relativity says that the semen will be travelling faster than the speed of light. Einstein’s special theory of relativity showed this to be impossible.

By assuming that the speed of light is the same in all inertial reference frames, Einstein showed the speed of light to be the cosmic speed limit. Actually, only massless particles can reach this limit, because an infinite amount of energy is needed to accelerate a massive object (and a penis is a massive object) up to the speed of light. It is possible to persist with the calculations at the speed of light, but this invariably leads to paradoxes, such as a penis having no apparent length, and therefore semen travelling through a penis with no apparent length. According to the semen, time stops, and space contracts down to two dimensions. Obviously, it is more realistic to consider the scenario of ejaculation at speeds arbitrarily close to the speed of light.

At such speeds, the relativistic velocity addition formula applies. Suppose your boyfriend has been training his Kegel muscles and he can achieve a speed of ejaculation of 2% the speed of light. Then, if he is to ejaculate while thrusting inwards at 99% the speed of light, his semen will be travelling not at 101% the speed of light as common sense would suggest, but rather at 99.04% the speed of light. This relativistic semen will then be decelerated to about 94.2% the speed of light as it escapes the gravitational pull of the penis.

An average ejaculation produces approximately two teaspoons of semen (this amount decreases with age, and increases with time since last ejaculation). Anyway, two teaspoons of semen travelling at 94.2% the speed of light will create enormous air resistance, which will heat up the semen in the same fashion as a spaceship re-entering the Earth's atmosphere. The semen will burst into flames almost instantaneously, creating deafening sonic booms in its wake.

Meanwhile, two teaspoons of flaming semen will generate enormous impact forces, sufficient to rip straight through the structural integrity of an extra-strength Durex condom. But you will have much greater concerns than an unwanted pregnancy. The relativistic flaming semen will pierce a small hole straight through your lower torso, just like a speeding bullet, only incinerating the surrounding tissue as it passes through. Relativistic ejaculation brings true meaning to the question, "Is that your gun in your pocket, or are you just happy to see me?" Well it's not a gun baby… it's a rocket launcher!

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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