Remember the paradox of the pole and the barn? It’s a classical paradox in special relativity, I read it in Taylor and Wheeler’s book so many years back (the world was young, the mills were white… ainssss). OK, returning. Consider a guy running with a horizontal pole traversing a barn which has a front and a back gate, both open. Let the barn rest-length be exactly the same as the rest-length of the barn.
Now, let’s speed things up. The guy with the pole runs through the barn at a speed close to that of light. From his point of view, the barn is coming towards him, so its length must shrink. Ehm… OK. Now, let’s stand by the barn, at rest with respect to it. The guy is coming so fast… the pole is shrinking from our point of view.
The question comes now: does the pole fit inside the barn or not? Well. The solution is not easy, but it is everywhere on the web. If you know it, continue reading. If you don’t, fight with it for a while…
So, spoiler alert! I assume that you’ve realized what is the answer.
Short story: simultaneity. The events of the front of the pole going through the front door and the rear of the pole going through the back door are connected via a space-like interval, so there can be no causal connection between them. Therefore, they may happen in one order to one observer, and in the reverse order to another observer. This is indeed what happens, so the question “does the pole fit in the barn?” is not frame-independent.
OK, now for the twist. Imagine a couple, boy and girl for simplicity. They’re going to make love. Let us assume that the penis fits in the vagina exactly, when measured at rest. Assume that the velocity of the penentration is close to the speed of light (come on, a little bit of imagination!). Now, girls have no “back door” so… how is the paradox solved? From his point of view, the vagina is too short, the penis doesn’t fit. From her point of view, the penis is too short, it will never reach the bottom…
What happens, really?
Bonus: for some extra fun, check the top 10 reasons which forbid relativistic sex.
Physics Napkins 