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Lenght contraction

  1. Sep 8, 2011 #1
    Regarding the observer riding on the middle of the train sees the light strike first the fwd first and then the aft. Will it be safe to say that for the observer to see the light strikes them both at the same time to adjust the tail of the train by shortening the train so that light can arrive at the same time..... shortening the path of light

    any opinion on this..
  2. jcsd
  3. Sep 8, 2011 #2
    Hi agmateo, could you clarify your test setup a little more? Are you talking about two lights--one located at the front of the moving train and one located at the back of the moving train--and the observer is positioned at the middle of the moving train. Now, are you saying that both lights flash at the same instant in the inertial frame of the moving train? Or do they flash simultaneously in the reference frame of the ground?

    Or are you talking about a single light located at the middle of the train that the observer flashes--then the observer watches for the reflections from the aft and the front?

    (Sorry, I'm a little slow about following setup details.)
  4. Sep 8, 2011 #3
    A lightning strikes the back and front of moving train v=c and the observer riding is in the middle of the train. He see thee light strikes first the front and later the back of the train. To correct this observation we should shortening the train equal to the length of contraction computed by the formula in order for the light to arrive at the same time to the eye of the observer on the train. in other word, make the path light shorter for the back lightning strike. Is that the essence of simultaneity.
  5. Sep 8, 2011 #4
    Were the lightning strikes simultaneous in the inertial frame of the moving train? Or did you mean they were simultaneous in the reference frame of the ground? Also, I don't understand the train moving at the speed of light. Is that what you meant by v=c?

    Did you mean two lightning strikes?

    First, we must establish in which inertial frame the two strikes are simultaneous. Then we deal with the travel times and distances for the light flashes. Also, I'm not sure what you mean by "to correct this observation." I get the impression you may feel that one of the observations is somehow incorrect. A pair of events may not be simultaneous for two different reference frames, but that does not mean that one of the observations is incorrect. They are both correct, even though the pair of events are not simultaneous in both reference frames. That's the way nature works, and special relativity theory clarifies the situation for us.

    When I think of simultaneity, I'm generally not thinking about whether the light flashes arrive at the observer's position simultaneously. I'm more interested in which inertial frame the two flash events are simultaneous. Then, if you draw a space-time diagram that includes the initial flash events, light paths, and observer, the situation is pretty clear.
    Last edited: Sep 8, 2011
  6. Sep 8, 2011 #5
    The inertial reference frame is the moving train at a velocity approaching light... because if train is moving at velocity of light light cannot catch the train time becomes infinity.. the observer cannot see the flash of light..

    The length of contraction is an assumption to correct or make the light back catches with the light at the fwd end so that the observer on the train can see the light flashers simultaneously or Einstein called it simultaneity.

    If shorter the distance your time also dilates automatically...This is practical explanation of relativity without those complex mathematical equation that is simple to understand.
  7. Sep 8, 2011 #6
    No. If the entire experiment along with simultaineous events is taking place within the moving train reference frame, then there are no relativistic effects at all to worry about. There is no length contraction or time dilation at all for the observer who is just making observations within his own rest frame of reference.

    I have a hunch I am slow on the uptake so far as understanding the experiment you intend to present. (sorry for my ineptness--someone else here probably understands your setup)
  8. Sep 8, 2011 #7
  9. Sep 8, 2011 #8
    We are correcting the observation of the observer in the middle of the train... without correction he saw first the flash at the front and later the back.. The observer outside the train see both occur at the same time..The law of physic must be equal to all inertia reference frame..You need length contraction in observer in the train to make it equal to the observation of observer outside the train..now the observer in the train see both light at the front and back arrive at the same time because you contract(shorten) the train.
  10. Sep 9, 2011 #9


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    While length contraction does play a part in this scenario, it actually works to ensure that the observer on the train does not see the flashes at the same time.

    It works like this:

    As the train goes past the observer standing by the tracks, he measures its length as being length contracted. Thus he measures a shorter length for the train than the observer sitting in the train does. and it looks like this, with the length contracted train just fitting between the points where the lightning strikes (indicated by the red dots).


    Now, as I mentioned above, the train observer measures the train length as normal. However, he measures the train tracks as length contracted. Thus for him, the distance between the point of the tracks where the lightning strikes is shorter than the length o fthe train. This means that the ends of the train and the strike points cannot line up all at the same time. The front of the train reaches its strike point first and then the rear of the train reaches its.

    So for the train observer, events happen like this:


    With the front strike happening first, and then the rear strike.

    There is no way that length contraction can "correct" things so that both observers see the flash at the same time.
    Last edited by a moderator: Apr 26, 2017
  11. Sep 9, 2011 #10
    If there is no length contraction how can you say the

    "law of physics applies to all reference frame whether in the train or outside the train."

    What shown in the demonstration your demo and my demo is show the mistakes of the classical physics ........that why there is special relativity otherwise what is your solution.

    How can you make that both observer (in the train and in the ground) see the flash of light of front and end to came at these same time ........simultaneity ?????? this is the idea of Einstein.

    The demo of correction is not shown... maybe it can be made by expert cartographer..

    again I will repeat the law of physics applies to all reference frames....

    To satisfy this statement...... I will shorten the length of train at the back so that the light flashes will reach the observer at the middle and that of the front at same time...

    I will cheat the time and the length to do it.....

    If you do not agree with me .....please explain how a train become shorter in the view of the observer.how can you connect it to the statement..

    The law of physics is correct to all reference frame...
  12. Sep 9, 2011 #11
    There is another way to do it make the speed of light faster at the back end so that it catches the front flash at the same time.

    But we will violate Einstein second principles:

    The speed of light is a constant under any circumstances...

    so that we sacrifice the length and time so correct this mistakes.
  13. Sep 9, 2011 #12
    nice animation , they are really helpful in grasping and visualizing relativity effects.

    I save these gif file and watch them frame by frame on a player that has option to play framewise.
  14. Sep 9, 2011 #13
  15. Sep 9, 2011 #14
    Great animations, Janus!
    Last edited by a moderator: Apr 26, 2017
  16. Sep 9, 2011 #15


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    I did not say that there wasn't length contraction, In fact,I said there was and my animation shows it. What I said was that length contraction will not allow both observers to see the light flashes simultaneously
    No, Einstein was showing that the observers cannot see the flashes at the same time, and thus cannot conclude that the lightning strikes occured at the same time
    Which is exactly what my animation shows
    The train has to become shorter as from the tracks in order for the laws of physics to be the same in both frames. Namely so that both observers measure the speed of light as having the same value relative to themselves.

    You don't seem to grasp what the statement: "The laws of physics are the same in all reference frames" actually means.
  17. Sep 9, 2011 #16
    Thank you very much JANUS, I have no objection. We can explain relativioty in class at the most simple term without using an equation .. by simple drawing..
  18. Sep 11, 2011 #17
    How important or pertinent is it to distinguish the contraction seen by the stationary observer as the measurement of a physical shortening of length vs a measurement of the light arriving from the front of the train having been emitted sooner than the light from the back - the image of the train being comprised of "newer" light at the back and "older" light in the front from a more previous position?

    This would cause the train to appear to be contracted somewhat because path lengths of light seen at the same time from front and back parts of the train would have to be such that the train front path was emitted "earlier" than the back - at a time before its "present position"...

    Asked another way, is the apparent contraction caused by seeing the train's front end from an earlier time of emission relative to the back end sufficient to account for all the measured contraction?
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