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Length contraction illusion

  1. Mar 26, 2009 #1
    In SR is length contraction physically real, or is it just and illusion brought on by high speed motion.
  2. jcsd
  3. Mar 26, 2009 #2
    What do you mean by "real" and "illusion"? :)
  4. Mar 26, 2009 #3
    Does the length physically shrink relative to another observer, or does it just appear too. For example, when an object is moving close to the speed of light one can see around corners, it is not that the corners have changed, it is merely an appearance caused by high speed motion. I might be wrong about this though.
  5. Mar 26, 2009 #4
    Lorentz contraction means that if you simultaneously(clocks must be synchronized) measure positions of two edges of a moving rod (for example) then the distance between them is shorter than the distance measured by a guy moving with this rod (with respect to this guy the rod is at rest). I don't know how "real" or "illusive" this is to you =)... no tensions appear in a moving rod due to its movement with constant velocity.
    In fact, if you look at something moving close to the speed of light, your eye will catch photons that arrive simultaneously, but they were not radiated simultaneously. For instance, a fast moving sphere will appear to be not contracted, but rotated.
  6. Mar 26, 2009 #5


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    Hi Forestman! :smile:

    muons created at the top of the Earth's atmosphere and travelling near the speed of light should decay before they reach the ground, but they don't

    this is usually described from the point of view of a stationary observer on Earth: time dilation means that the muon is ageing less, and so doesn't get "old" enough to decay

    but we can also look at it from the point of view of the muon: the muon regards the atmosphere as Lorentz-contracted (by the same factor as the time-dilation for the first observer), and so it has less far to travel and it eaches the ground before it decays.

    So, for the muon, the length-contraction of the atmosphere is an effect with real consequences :smile:
  7. Mar 26, 2009 #6
    As pointed out by tiny-tim the effects of length contraction and time dilation are "real" enough to have observable/measureable effects. And so too is the increase in relativistivc momentum (or mass if you prefer) of fast moving particles....as in particle accelerators.

    That doesn't mean general relativity is infallible but so far experiment/observation/measurement verifies theory where we have been able to make such tests.
  8. Mar 26, 2009 #7
    In recent experiments in QED using the muon, Brookhaven National Laboratory stored muons in a strong focusing ring with the relativistic gamma = about 29.3 [see http://accelconf.web.cern.ch/accelconf/e02/PAPERS/TUZGB001.pdf] [Broken]. In our restframe, time dilation increased the lifetime by this factor, and the distance traveled around the ring was about L = beta-gamma-c-tau, where gamma is the increase in distance as observed in our rest frame. In the muon's rest frame, the muons passed about 29.3 times more quadrupoles than it would have if there were no length contraction.
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  9. Mar 26, 2009 #8
    It's the result of perceiving the non simultaneous emission of light from the end coordinates simultaneously! This allows for expansion also.
    It's a perception thing.

    It's also a calculation thing, when a moving observer assumes he isn't moving.

    If you carefully read SR, it will inform you that the Lorentz transformation equations apply to coordinates, not objects.
  10. Mar 26, 2009 #9
    It sounds like an explanation of the ladder or barn-pole paradox might be instructive.

    Wikipedia has it http://en.wikipedia.org/wiki/Barn-pole_paradox" [Broken].

    My answer would be that length contraction is as real and as illusionary as any phenomenon. (For instance, your desk seems quite solid but look at it really really close and it is mostly empty space with only the occasional atom in it. Then look at the atom and really it is mostly empty space with a nucleus in the middle and a few electrons buzzing around it. So is the solidity real or an illusion?)


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  11. Mar 26, 2009 #10
    From Rindler, Relativity – Special, General and Cosmological. 2nd ed. Page62.

    This length contraction is no illusion, no mere accident of measurement or convention.
    It is real in every sense. A moving rod is really short! It could really be pushed into a hole at rest in the lab into which it would not fit if it were not moving and shrunk. (See Section 3.4.) Whatever physical forces of attraction and repulsion are responsible for holding together the constituent elementary particles of the body when the body is at rest, will all change in accordance with the laws of relativistic mechanics when the body is in motion relative to an inertial frame, in such a way as to produce the shortening in that frame.

  12. Mar 27, 2009 #11


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    Hi neopolitan! :smile:
    I like that! :biggrin:
    I haven't seen Rindler, but I don't think that can be correct: a long moving thing can't fit through a shorter stationary hole unless it goes through at an angle (which is exactly how you'd do it at normal speeds anyway :wink:)

    neopolitan's quote from wikipedia …
    … goes into this in detail.

    btw, the version of the paradox I prefer is the http://en.wikipedia.org/wiki/Barn-pole_paradox#Relativistic_trains_passing", but with the addition of two barriers at the same height that sweep vertically down so as to just miss the front and back of the train … the driver sees the front barrier as always lower than the back one. :smile:
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  13. Mar 27, 2009 #12
    Hello tiny-tim

    The quote from Rindler was copied and pasted so there is no mistake in transcriprion. i am in no position to contradict either of you but it is disturbing that you and he do not agree. When i get home I will reread Rindler's section on length contractioon to see if it reads differently in context. However in an earlier book by Rindler, a Dover publication i have at home, he proposes an experiment in which rods moving relative to each other are directly compared and would be found to differ in length. He does however say that this has never been carried out due to the technical difficulties involved. Later if need be i will give the full paragraph.

    Other authors i have read also use words to the effect of "length contraction is ""real"" in every sense" but this of course depends upon ones interpretation of real.

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