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About Einestien's train example

  1. Oct 30, 2012 #1

    About Einestien's train example

    I can't get what it really does with the constancy of the speed of light , what would be the difference if , instead of two bolts , we have two balls thrown at the same angle ?!

    the observer in the train would still see the front ball first

    Or we are using light because it is what determines simultaneity in our eyes , which means when I see two balls passing by me at different times , I don't have to conclude they weren't thrown simultaneously but when I see Light doing that I will ?

    I need basically to know what difference does the constancy of the speed of light make here

    thanks in advance ;
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Oct 30, 2012 #2
    Nice animation! :smile:

    You are right that the observer in the train will still see the front ball first. The difference here is that according to her, the front ball has a higher speed than the rear ball (that is, relatively to the train). But she will still think (if she measures it unbelievably precisely and uses the train as reference system) that the front ball left a tiny little bit earlier than the rear ball.

    That is easy to understand if you think of two balls shot at the train in an identical way by the impacts of the two lightning bolts: everyone agrees that the front ball left immediately after the front lightening bolt, and in her reckoning that happened just before the rear one.
    If the balls were launched at almost the speed of light, then she would think that the balls had almost the same speed relative to the train.

    Those calculations are complex. For light rays and radio waves the calculation becomes simple!
    Last edited by a moderator: Sep 25, 2014
  4. Oct 30, 2012 #3
    You mean that the observer will see the front ball first , but also she will see the speed differences so she will not say a thing about simultaneity

    But in the light example , she will see the front first , and also she will notice the same speed of light so she will say the events aren't simultaneous

    Am I right ?!

    If so , can she notice that speed of light in order to determine they are equal ?! or Are we just supposing she has proper instruments to measure it on the train , and with these instrument she will arrive at the result of non-simultaneity ?!
  5. Oct 30, 2012 #4
    Almost - with the balls, the difference is mostly due to the speed difference.
    Almost so! In fact, she can not notice that the speed of light in both directions is equal - on that point the video is slightly misleading. In fact it is the opposite: she claims that the speed of light is the same in both directions relative to the train, and she adjusts any clocks so that the clocks agree with that assumption. And the guy on the platform sets his clocks in disagreement with her clocks. Of course, after she made her clocks say that the speed of light is the same in both directions, she will also "measure" that this is so. :tongue2:

    You can read about it in Wikipedia:

    We also had long discussions about that topic, see for example (I think that there is a better link though):
    - https://www.physicsforums.com/showthread.php?t=641102
    - https://www.physicsforums.com/showthread.php?t=544366
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