Relativistic sex

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.

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17 thoughts on “Relativistic sex

  1. Let’s see… As far as i know, the pole moves toward the barn as fast as the barn comes to the pole, so i don’t see why just the pole should shrink. As both of them shrink by the same factor they will fit.
    In fact, about sex, i find more probable the quantum quid, so when you gets the stance don’t have the moment and when you gets the moment don’t have a stance. That’s why Heisenberg dies a virgin.

    P.S. If we cant have relative sex, nor quantum sex… should we have Newtonian?
    Hmmm… attraction based on mass… Two boobs pulling more than a carriage…
    *mumble, mumble*

  2. Poor Heisenberg…! :) Anyway, regarding the pole and the barn problem, the asymmetry is a false issue here. From the point of view of the running guy, the barn shrinks, the pole remains long. And from the point of view of a guy sitting at the gate of the barn, it is the pole that shrinks, the barn does not. The asymmetry is there… sorry, at least in special relativity. So, you have two options: (a) state that the theory is inconsistent (then you’ll have to fight with me) XD (b) state that the theory is wrong (then you’ll have to fight with the experiments)…

  3. Hm… Erynus, I have the feeling that you don’t really understand relativity… I may recommend some readings to you. Try http://en.wikipedia.org/wiki/Ladder_paradox

    To javirl: you are right, girls do not have a back door. So, what comes inside has to go outside again… or stay forever. The penis has to stop when it reaches the bottom, at a certain moment it will be at rest from the reference frame of the girl, so they both coincide: the penis fills the vagina entirely.

  4. Hypazia, I agree with you. Still there are funny issues. For example, how does the perceived length of the penis change as the movement goes. Intuitively, let us assume that the cycle starts when it is completely outside, at the border. Then, it starts accelerating. So, it goes inside faster and faster, so the girl feels it shorter and shorter. Is it possible that she may feel it “stationary”? I mean, that the penetration length does not increase, at some moment?

  5. In fact i think that relativity is wrong. I hope to porve it someday.
    Saying that the “relative point of view “of someone changes the events sounds a lot like quantum physics to me, as “if you open the box you collapse the cat (and hence you kill him)”.
    I think that things happen no matter how we perceive it. And it’s our lack of information what gives us wrong feelings.

    As far as i deny the “absolute lightspeed” postulate, all that comes later is a nonsense.

  6. Erynus, you have an epistemological problem: either you think relativity is nonsense or that it is false. Can’t be both. It is nonsense, it gives a peculiar vision of the world, but it has no inconsistencies. It gives clear cut predictions for all experiments in its area of validity. And all the predictions have come true… Still, it can be false, in the sense that a new theory may explain what it does and more…

    About QM, the issue is different. From my point of view, it does not give clear cut predictions in some experiments, because it depends on an incomplete concept: measurement. We have different interpretations: Copenhague, many-worlds, decoherence, etc.

  7. Well, define “clear cut experiments”. I can make up an alternate explanation for the effects predicts by relativity. I don’t deny that relativity can works but i doubt about the root of the events, the explanation it gives.
    If the universe works depnding on the observers point of view it whould be merely anthropocentric. With no humans, events won’t happen. That’s a nonsense to me.

  8. Hm… that’s a misconception. Special relativity is a fully realistic theory: events are out there, whether somebody is observing or not. The analogy can be made (not my idea, from Taylor and Wheeler, in “Spacetime physics”) with two teams of surveyors, making the map of a country. The first team defines North with respect to the stars, the second with respect to the magnetic pole. Their results disagree a little bit, and this can be annoying for them. Yet, they measure the same physics, and they can check, for example, in the fact that the distance between two cities is the same in both maps. In relativity you have something analogue to the “distance” between two events in space-time, which is called the interval. So, different points of view give different measurements, sure, but from the same underlying reality. This underlying reality is the essence of the theory of relativity, despite its name… Einstein himself proposed (I’ve heard) to call it “theory of invariants”, he disliked the soft philosophical resonances the theory had acquired.

  9. The distance between two cities only will be the same if both use the same scale. And according with SR there isn’t an “absolute scale”. Making a map from scratch using different references will gives different maps. If you reads a map and don’t know where the north is, you can’t use it.
    Newton’s Mechanic worked.. for a bit, and gives “clear cut predictions” except for when not.
    What i’m trying to say is that it works but i’m not sure if the “why” it points is the real “why”.

  10. No, SR doesn’t say anything about absolute scales. There is none in Newtonian mechanics, there is none in any physical theory. But let us assume both surveyors use “meters” as unit. And we assume that people using different reference frames, still use the same units. Only they differ in the measurements.

    About the whys… did newtonian mechanics give a why? did aristotelian? Relativity has some nicer “why”s, like, for example, making the universe “local”: what you do here can’t affect the other extreme of the universe “immediately”, it takes some time for the perturbation to reach there. For me, that’s intuitively clear.

  11. Erynus, when you ask for the why of a theory, you’re thinking in terms of a “lower” theory that explains it. If a theory is “the last”, there is nothing below… it has to be explained in its own terms. Why is the speed of light unsurmountable? No answer, it is like this, it is the basic postulate. Why is the straight line the shortest between two points in Euclidean geometry? No reason, it is simply.

    • I don’t mean “why” that way. I mean the explanations. Every theory says that things are this way because this o that. And that are the “whys” i’m talking about.
      Relativity says that a mobile moving faster than another covers a distance in less time cause for the faster one, the distance is “contracted”.
      This statement sounds weird for me, cause is obvious to me that a mobile moving faster covers a distance in less time cause…well… moves faster.
      And the postulate that light speed is a constant, loses value as light gets slowed by the medium.

  12. i’ll think on it.
    Now i head home too much hours of pretending doing work.
    i’ll be back.
    But, if you use a polar map as a mercator you’ll get lost. even if you use meters as unit.

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