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Evidence for length contraction

  1. Oct 7, 2003 #1
    would some be able to give me an example that shows some evidence of length contraction?

    i have looked at the muons example, but it seem to be dependent on the time dilation effect, rather than being evidence of length contraction.

    thanks for any help.:)

    much appreciated.

    btw, feel free to move this thread, if it isnt in its relevant location.:smile:
  2. jcsd
  3. Oct 7, 2003 #2


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    If I run real fast for a long time, I will get thinner!

  4. Oct 7, 2003 #3

    hmm...maybe i should try that
  5. Oct 7, 2003 #4


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    Time dilation and length contraction occur concurrently. They are, in effect, two ways to describe the same effect. The Lorentz transformations are the "cause" of both effects.

    In the case of the muon, you have two choices: you can consider the muon's clock running more slowly than a stationary muon's clock on earth, due to its high velocity.

    You can also think that, to the muon, the earth's atmosphere is length-contracted, making it not so thick and easier to penetrate.

    - Warren
  6. Oct 18, 2006 #5
    I know, I'm a bit late to answer!
    A clear example is doppler red shift of stars light.
  7. Oct 18, 2006 #6
    Could you explain how that is evidence for length contraction?
    The redshift of starlight is a combination of the regular doppler effect adjusted for relativistic time dilation and gravitational time dilation.
    Last edited: Oct 19, 2006
  8. Oct 18, 2006 #7


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    There is no direct experimental evidence for length contraction, as far as I know. That is, no one has managed to measure the length of a fast-moving object, by measuring the locations of its ends simultaneously in the laboratory reference frame, and get a result that differs from the non-relativistic expectation.

    As one might expect, the technical obstacles are rather large. Either you have to make a macroscopically sized object move very very fast, and then figure out how to measure it while it's whizzing past you, or you have to be able to make very very precise measurements on somewhat more slowly moving objects.

    I don't consider this to be a problem for the validity of relativity, because length contraction is so tightly bound with other phenomena that we can verify. It sure would be nice to do this, though!
  9. Oct 18, 2006 #8
    Please have a critical look at
    arXiv.org > physics > physics/0507016
    Search for(Help | Advanced search)
    All papers Titles Authors Abstracts Full text Help pages
    Physics, abstract
    From: Bernhard Rothenstein [view email]
    Date: Sun, 3 Jul 2005 18:04:26 GMT (224kb)

    Length measurement of a moving rod by a single observer without assumptions concerning its magnitude
    Authors: Bernhard Rothenstein, Ioan Damian
    Subj-class: Physics Education

    We extend the results presented by Weinstein concerning the measurement of the length of a moving rod by a single observer, without making assumptions concerning the distance between the moving rod and the observer who measures its length.
    Full-text: PDF only

    where I try to show that there is a lenght measurement procedure which can lead to length contraction but to lenght dilation as well.
  10. Oct 18, 2006 #9
    I don't think anyone's ever moved a macroscopic man-made object at anywhere near relativistic speeds.

    There is one possibility, though:

    The planet Mercury. Mercury is close enough to the sun to show relativistic effects when viewed from earth. The time dilation has been proven; but I don't think anyone's done any research on a corresponding length contraction.
  11. Oct 18, 2006 #10

    Meir Achuz

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    This is the best answer to date until the penultimate sentence.
    The measured length of a moving object depends on how you measure it.
    If you use a pre-relativistic (Galilean) notion that an equal time measurement is appropriate, then you get an equal time measurement.
    With a light cone measurement, as is fashionable in parton physics or photography, a moving sphere remains a sphere. If you look at a moving sphere, you see a sphere. Some people would consider this a measurement. ("Seein is believin.") The only unambiguous measurement of the length of an object as an intrinsic propert is in its rest system, if you don't look at it from a rotated angle.
  12. Oct 19, 2006 #11
    c = nu*lambda. The frequency nu decreases because of time contraction, so the wavelenght lambda must increase to keep c at the same value.
    This is actually evidence (IMO) for lenght dilation (lambda), instead of contraction, but the principle is the same.
    Of course gravitational redshift must also be taken into account, but, after having considered it, what remains is the doppler effect.
  13. Oct 19, 2006 #12
    It would make more sense (to me) out of SR if you did have length dilation, and time dilation when talking about things moving faster than our perferred frame of reference, then the change to the meters per second in the speed of light would be the same for both the meter and the second keeping the speed of light proportional relative to the moving observer.

    You, using length dilation, could think of the muon as increasing in size, which would also explain why the earths atmosphere appears to be length contracted to the muon.
  14. Oct 19, 2006 #13
    If you have a long straight current carrying wire in frame S and you move to a frame S' which is moving parallel to S then the wire will be come charged. This is a direct effect of length contraction.

  15. Oct 20, 2006 #14

    Meir Achuz

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    This effect is because (rho,j) transform as a 4-vector. It has never been observed experimentally as "evidence".
  16. Oct 20, 2006 #15

    Meir Achuz

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    Although the thread has lengthened, the short answer ot your question is that I do believe the equations, but there is no evidence of length contraction.
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