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Simultaneity Problem

  1. Jun 6, 2008 #1
    I'm having trouble understanding the simultaneity experiment with Alice on the railroad car, and Bob observing from the ground. If 2 lightning bolts hit the front and back of Alices car at the same time in Bob's frame, it won't be instantaneous in Alices due to simultaneity. My question is simple, if instead of 2 separate lightning bolts, we use 1 laser pointed directly at Bob and Alice at t=0, where both persons have 2 beam spliters to send some of the light to the left and some to the right in both frames to the front and back of Alices car where she has two detectors that will emit flashes once light is detected, how can simultaneity still hold? If for any reason Bob and Alice disagree about the simultaneity of flashes at the end of her railroad car, then in at least one frame, the right or left moving light went slower or faster than the other which is obsurd.
     
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  3. Jun 6, 2008 #2

    Doc Al

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    Disagreement about simultaneity doesn't imply that the light goes slower in one frame, just that it has a different distance to travel to the end of the car. For train observers, the two beams of light are shot off from the middle at the same time and travel the same distance to reach the ends of the car. So they reach the ends simultaneously.

    But for ground observers, one end of the train is moving away from the light while the other is moving towards the light. Of course the light beams arrive at different times.
     
  4. Jun 6, 2008 #3
    any more absurd than the idea that both observers calculate the speed of light to be c even though one of them is moving?

    it is the assumption that the speed of light is constant for all observers that leads to the conclusion that there is a loss of simultaneity. you have forgotten to take the observers motion into account.
     
    Last edited: Jun 6, 2008
  5. Jun 6, 2008 #4
    Let me rephrase the experiement. Bob holds the only beam splitters and at t = 0 he has split the light and it has reached the two ends of the railroad car at the same time in his frame. Alice would say in her frame, they were not simultaneous, and I would tell Alice that she's wrong because they came from a point source.
     
  6. Jun 6, 2008 #5

    Doc Al

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    Bob is on the ground. Why would he say the light beams reach the two ends of railroad car at the same time? (The train is moving.)
    I think you have Alice and Bob mixed up.
     
  7. Jun 6, 2008 #6
    both observers will see the light move at c. alice would therefore not expect to see the light hit front and back at the same time. why does that seem like a contradiction to you?
     
    Last edited: Jun 6, 2008
  8. Jun 6, 2008 #7
    Bob can set up the experiement in a way that ensure both beams arrive the ends of the railroad car at the same time in his frame if he knows her speed and associated contracted length. If her railroad car lenght is 10m at here speed he could set the experiement up such that when Alice is directly in front of him, the split photons are exactly 5m apart.
     
  9. Jun 6, 2008 #8

    paw

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    It doesn't matter that the light came from a point source. Only the events of light hitting the detectors are in question. Those events are simultaneous in Bobs frame and not simultaneous in Alices frame. Alice is just as entitled to say Bob is wrong as Bob is to say Alice is wrong.

    In fact, if you forget the detectors and focus on the beam splitter Bob will say the light was emitted forward and backward at the same time. Alice would say the light was emitted forward first and backward second. Alice would still be as free to say Bob is wrong as Bob is to say Alice is wrong.
     
  10. Jun 6, 2008 #9
    Bob will say the light was emitted forward and backward at the same time. Alice would say the light was emitted forward first and backward second.

    say what? 2 events that occur at the same place and at the same time will do so for all observers regardless of their velocity.
     
  11. Jun 6, 2008 #10
    I dont think thats true. Relativity is based upon the premise that events actually happen. You can't have some one dead in one frame and alive in another (for all time) like Shrogengers cat. It's only there relative times and locations that vary and are relative. The point in this experiment is taking the different locations out the argument by forcing them to be coincident at one place and one time. If Alice says they arrive at different times, she would be forced to explain to the physics community how a laser can emit S and P light at different times from the same source. That just doesnt happen.
     
    Last edited: Jun 6, 2008
  12. Jun 6, 2008 #11

    Doc Al

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    Sure. But in that case the light beams did not get sent out from the center of the railway car (according to either observer). So why is it a surprise that they reach the ends of the car at different times according to Alice?

    (I thought you wanted the light beams to be sent from the center of the car, just as Bob passes Alice. Now you're describing a different set up.)
     
  13. Jun 6, 2008 #12
    Yes I changed the experiement because the first one was incorrectly stated and didn't correctly address my issue. This new set up ensures that the two split photons arrive at Alices detectors simultaneously in Bob's frame, and that they came from a point source.
     
  14. Jun 6, 2008 #13
    who is moving and who is stationary?
     
  15. Jun 6, 2008 #14

    Doc Al

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    The "events"--the emitting and receiving of the light--really do happen.
    Sure.
    Which you failed to do.
    Alice surely agrees that the light beams were emitted at the same time. And she observes them reach the ends of the car at different times. No mystery here!
     
  16. Jun 6, 2008 #15
    Alice is moving, Bob is stationary.
     
  17. Jun 6, 2008 #16

    Doc Al

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    Right. So what's the problem? Ask yourself: Where along the train is the emitter when the photons are emitted?
     
  18. Jun 6, 2008 #17
    Having the two photons emitted at the same time from the same location is one event, not two.
     
  19. Jun 6, 2008 #18
    slightly forward of the train.
     
  20. Jun 6, 2008 #19

    paw

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    That's my point exactly. It's the events of light striking the detectors that counts. Bob claims those events are simultaneous. Alice claims they are not. Both Alice and Bob are free to claim they are right. There is no absolute definition of who is right.

    Instead of detectors lets clone Bob and have Bob1 at the front end of the train and Bob2 at the back of the train. I'll (P) take Bobs place at the center of the car. Now let's make the laser lethal and see what happens.

    In my frame I see Bob1 and Bob2 die at the same time. Alice sees Bob1 die first and Bob2 dies second. Both Alice and I agree both Bobs are dead. We simply diagree on who died first. I did not claim that either 'Bob' was alive and dead at the same time.

    You're neglecting the beam splitter. The light started out as one source but was split into two sources. After splitting they are two independant signals and that's what counts.
     
  21. Jun 6, 2008 #20
    The only thing Alice sees are flashes once the light hits her two detectors. You don't tell her anything about whether or not they came from a point source. All you tell her is that the flashes were simultaneous in Bob's frame and then ask her the question. She'll tell you they arrived at different times.
     
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