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Two light sources in train

  1. May 13, 2015 #1
    I am standing in the middle of a train. There are two lightsources attached to the train. One in the front, and one in the back. Both aimed at me, both with the same distance from my position in the train. No matter how fast the train is moving, i will always stay equally distanced to them. The lights are connected with equally long cables to a switch i am holding. Assuming that the lights are identical. The train is always moving at a constant velocity in a inertial referance frame.

    If i flip the switch, will i see the to lights flash at the same time, regardless of my velocity?
     
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  3. May 13, 2015 #2

    ghwellsjr

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    Yes, as long as the train has been at a constant velocity for a sufficient length of time for any transients to die down.
     
  4. May 13, 2015 #3

    pervect

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    Yes - this is a variant of Einstein's "train experiment". See for instance http://www.bartleby.com/173/9.html, or any of the large number of PF posts on "the relativity of simultaneity". According to you, with equal length cables, the lights will flash at the same time. They will not, in general, flash at the same time in other frames of references, though. If you had multiple trains moving at different rates, each train would have it's own notion of "simultaneity" based on equal-length cables. The cables actually complicate the thought experiment, Einstein's definition of simultaneity is very similar to "equal length cables", but he actually used signals propagating through a vacuum rather than signals propagating through a cable to define simultaneous events.

    What makes the cable results equal to the vacuum results is the principle of relativity, the theoretical assumption that the laws of physics that determine how signals propagate down a cable appear the same in all reference frames.
     
  5. May 13, 2015 #4
    For instance accelerated frames of references?
     
  6. May 13, 2015 #5

    ghwellsjr

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    I disagree. It's Einstein's second postulate not his first postulate that establishes that the propagation of signals through cables are the same in all reference frames. In fact, that is part of the definition of a reference frame in Special Relativity.

    Without that second postulate (or its equivalent), the round trip time for both paths will be equal in all inertial cases but you cannot conclude that the one way propagation times are equal in all inertial cases.
     
  7. May 13, 2015 #6

    Ibix

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    Yes. Also, other frames in motion at constant speed with respect to you.
     
  8. May 13, 2015 #7

    ghwellsjr

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    All bets are off with accelerated frames because there is no standard definition for them and there is no standard method to establish simultaneity. Pervect's comment is true even for inertial reference frames.
     
  9. May 13, 2015 #8
    So i will not see the two lights flash at the same time, regardless of my velocity?

    Im totally confused now. First yes, then no?
     
  10. May 13, 2015 #9

    Nugatory

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    You will unambiguously, no wiggle room, no dispute at all, see both flashes at the same time if the train does not change its speed between the moment when you press the switch and when the flashes of light reach your eyes.
     
  11. May 13, 2015 #10
    Thats what i thought.

    Will I also see both flashes at the same time regardless of the inertial velocity? (i.e the velocity before i flip the switch)
     
  12. May 13, 2015 #11

    Ibix

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    You can always consider yourself at rest, as long as you don't accelerate. So you will always see the flashes at the same time and be able to deduce that they were emitted at the same time.

    All other observers will agree that you see the flashes at the same time, but will not agree with your deduction about the simultaneity of the emissions. For them, one of the pulses has to chase you while you move towards the other one. For the reception to be simultaneous, the emission must have been non-simultaneous.
     
  13. May 13, 2015 #12

    Nugatory

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    Yes. One thinking about this problem is that as far as you are concerned the train is at rest while everything outside the train (platform, rails, people watching from outside the train) are moving backwards at some constant speed. It doesn't matter what they're doing, your lights and wires and electrical flashes will all behave the same way.

    BTW, the equivalent fo this experiment has been done many times, by taking advantage of the fact that the surface of the earth is moving in different directions and speeds at different times of day and different times of year, because the earth is both rotating on its axis and moving around the sun.

    [Edit: I should add, to be perfectly clear, that you "see" the flashes when they hit your eyes. That's not the exact same time that the flashes left the lights at thend of the train]
     
    Last edited: May 13, 2015
  14. May 13, 2015 #13

    ghwellsjr

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    I already gave you the answer to this question in my first post.

    Keep in mind that the issue of "did the lights flash at the same time?" and "did you see the lights flash at the same time?" or two entirely different kinds. You were asking about the second issue for which the answer is yes for inertial motion. You don't need to establish a frame of reference for that to be true--it is true experimentally every time it has been tried. The first issue is a matter of establishing the definition of simultaneity for distant events for which the one-way propagation of signals needs to be stipulated because it cannot be measured or determined by experiment.
     
  15. May 13, 2015 #14

    ghwellsjr

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    That is not a "deduction", it's a stipulation or a definition or a postulate. It's part of the establishment of an inertial frame of reference according to Einstein's Special Relativity. It's not something that we can deduce from experiment.
     
  16. May 13, 2015 #15

    Ibix

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    I'm not sure I completely agree with you there. Certainly I'm using the assumption of the isotropy of light speed. However, I then deduce the simultaneity or not of the flashes from the arrival time of the flashes and that assumption (or any other convention I choose to use).

    So, while I completely agree that I am basing my deduction on an assumption, it's still a deduction.
     
  17. May 13, 2015 #16
    What if the light sources are no longer attached to the train, but attached to the train station. Lets imagine that the train passes the two lights in such a way that they both are triggered to flash when one of the lights is in front of me, and one behind me, both equally distanced from me. Will i then see the light in front of the train first?
    I think the question boils down to: Does the velocity of the light sources matter?
     
  18. May 13, 2015 #17

    Ibix

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    It makes no difference to the arrival times whether the light sources are attached to the train or not. You (on the train) will see Doppler shifted light (so the pulse coming from behind you will be red-shifted while the one from in front will be blue shifted), which the person on the platform will not. In your original setup the platform observer saw Doppler shifted light whereas you did not. That's the only difference.
     
  19. May 13, 2015 #18

    A.T.

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    http://en.wikipedia.org/wiki/Special_relativity#Postulates
    • The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." (from the preface).[1] That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source.
     
  20. May 13, 2015 #19

    ghwellsjr

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    They are the same assumption. One is not a deduction of the other one.
     
  21. May 13, 2015 #20

    Dale

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    For the question you are asking, no it does not matter.
     
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