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Time dilation, length contraction

  1. Nov 2, 2007 #1


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    Has it been shown that observed length contraction that occurs due to high relativistic speeds is any different from that that appear due to the additional time it take a signal from the trailing edge of object (as opposed to the leading edge) to reach an observer? If not, then obviously a Newtonian explanation could be given for why length contraction would appear to occur.

    If not, then special relativity (as far as observation) simply is making non-simultaneous events appear to be occuring at the same time (such as treating information coming from a leading and trailing edge of an object as appearing in the same time frame).

    This would of course explain why the object itself doesn't "feel" the contraction. Thus, there would be no paradoxes, simply poor explanations.

    Also, has any experiment been done where a fixed observer watches a signal being emitted by a "very fast" rotating disk emitting signals from points tangential to the observer? If there were any differences in the speed of light from these points, they, in addition to color shift, would have slightly different arrival times? The Michelson-Morley experiment doesn't apply since the mirrors are fixed with the observer. The Sagnac experiment located the observer on the rotating disk, so it doesn't apply either.

    Thank you
  2. jcsd
  3. Nov 2, 2007 #2


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    The meaning of length contraction in SR is very clear and has nothing to do with the finite speed of light. To emphasize this, good books make it clear that the length of a moving rod must be measured by local observers which observe events taking place right in front of them in order to avoid having to worry with the fact that light propagates at a finite speed. When you find a length contracted result [itex] \Delta L = \Delta L / \gamma [/itex] using Lorentz transformations the result refers to observations made by local observers. so it's a true effect, not something due to the finite speed of light (in other words, two observers are involved in measuring the length of a moving rod). If there is a single observer, an additional effect due to the finite speed of light has to be taken into account as you mentioned, but this would be on top of the length contraction given by Lorentz transformations.

    unfortunately, many teachers do not make this clear which leads to much confusion.
  4. Nov 2, 2007 #3

    Doc Al

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    To deduce length contraction based on observations, one must of course take into account the travel time of the light.
    It turns out that for a rapidly moving object, length contraction would not be directly seen, for the reason that you point out. Instead of an apparent contraction, one sees an apparent rotation. (See: Can You See the Lorentz-Fitzgerald Contraction? Or: Penrose-Terrell Rotation.) Of course, if you take into account the fact that light from various parts of the object requires different travel times, then you will deduce the standard length contraction.
  5. Nov 3, 2007 #4


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  6. Nov 3, 2007 #5


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    Has length contraction actually been measured? It seems to me (at first thought) that many of these thought experiments are exactly that: an easy way to think about something, but not actually observed experimentally due to the nature of the thought experiments.

    Has the time dilation of the standand candles (pulsing) been shown to NOT match a simple Newtonian Doppler shift explanation of a receding source when travelling near speeds of light? (This is different from showing that sr matches the results.) If that were so the predicted periods would be different.

    Also, near the edge of the visible universe, red-shifts showing receding speeds approaching that of light. Do you get different observed relativistic effects there as opposed to those near an event horizon of a black hole? If not, then I would think the edge of the visible universe could be treated something like a singularity itself with nearly parallel diverging force lines instead of converging.
  7. Nov 3, 2007 #6


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    I looked at The Ball is Round. It makes a little mention of the difference in time it takes for light at the back of the ball to reach the observer vs. the time at the front. Thus even without sr, a ball will be elongated because at a point in time the visible back of the ball will be further back in time than the front will be.

    What is never shown is what the difference is. Perhaps what the sr transformations simply do is to produce a single time frame for the entire ball, thus the length extension for an approaching object. This is my guess and contention.

    If this is the case, then all of analogies are unnecessarily confusing. This is all something of a Doppler shift. Showing that in fact a Newtonian (light) Doppler shift of the ball or some object is different from the sr version would be instructive.
  8. Nov 3, 2007 #7


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    Relativistic Doppler Effect
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