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Evidence of length contraction

  1. Aug 22, 2004 #1
    Hello all.

    I've recently had something of an epiphany regarding relativity. One of those moments when things become clear and it all makes sense. However, having not actually performed any of the experiments which may or may not support the whole thing, I have a question. Is there evidence of length contraction? If so, what is it? Are there any alternative explanations for it?

    Basically it all started making sense in the most amazing way (and it's really far simpler than I had expected), but my understanding of it requires the acceptance of length contraction, which, as I said, I have not personally checked out much.
  2. jcsd
  3. Aug 22, 2004 #2
    No human has ever been able to actually see length contraction for himself. There is plenty of evidence for time dilation, otherwise short-lived fast-moving particles could not reach the distances they are routinely seen to reach. But in the rest frames of those particles, there is no time dilation. According to their frames, how then do they reach farther than they could otherwise? The solution is length contraction: they move for a small time (after which they decay), but the distance they travel (or technically, the distance that the rest of the world zooms by) is contracted in their frame. Does this help?
  4. Aug 22, 2004 #3
    Unfortunately not. I am aware of the muon thing, but unfortunately it seems that without corroborating evidence from the other side of the matter, why not just assume they are travelling very fast? I know it doesn't fit, but it sure is the simple solution.

    Okay, how about this. Can someone explain to me the mechanism by which length contraction occurs?
  5. Aug 22, 2004 #4
    Thanks to another poster, of another time :smile:

    Last edited: Aug 22, 2004
  6. Aug 22, 2004 #5
    Indeed it doesn't fit. When we measure their speed, we find it is below c. In their own frames, they must see the universe as travelling backwards at the same speed - otherwise you'd have a situation where A says B is moving at a speed v_a, and B says A is moving at speed v_b > v_a. This is unacceptable.
    I'm not sure what sort of mechanism you have in mind. Try http://aci.mta.ca/Courses/Physics/4701/EText/LengthContraction.html and the link therein on time dilation to see how it works. However, as far as we know, there is no deeper mechanism to length contraction; it is simply a derivable result of the constancy of c postulate.
  7. Aug 22, 2004 #6


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    It doesn't "occur", per se...

    Geometrically, it's closely analogous to this:

    Suppose you have a needle resting on a table and you're looking at it from the edge of the table. (So you see it from the side, not above)

    The apparent length of the needle depends on the angle you look; if someone rotates the needle, it would appear to change size.

    The "angle" here is analogous to relative velocity.
  8. Aug 27, 2004 #7
    So if the subject is moving at a constant distance from the observer, such as in an arc?
  9. Aug 28, 2004 #8


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