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Can special relativity result in alternative realities?

  1. Nov 8, 2011 #1
    Take the traincar thought experiment with light emitted towards front and back of the train. Assume that the train is rigged with mirrors at the wall that will direct the light around the edges of the train wall, ceiling, and then back to the center of the train at the rooftop, where a sensor is located. The sensor is rigged to blow up the train if it receives light from both sides within some small fraction of time.

    To an observer from the perspective within the train, the light hits both sides of the sensor simultaneously and the train blows up (killing the observer). To an observer outside of the train, the light hits one sensor long before the other, so the train does not blow up, and the person inside the train survives.

    Is this right?
  2. jcsd
  3. Nov 8, 2011 #2


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    Science Advisor
    Gold Member

    Both observers must give the same result for an experiment such as "does the traincar explode or not"? The idea is that the sensors are inside the train, so they obey the rules that apply inside the train; the train explodes. To analyze it from outside, you have to boost yourself into the inside frame first to see what the sensors are seeing.
  4. Nov 8, 2011 #3
    Suppose that the sensor device is see-through and the observer is watching with a telescope that allows him to see in great detail what is going on inside that sensor. The sensor is governed by simple mechanical principles. If he does not observe the light to enter both sides of the sensor at the same time, then he will not observe those visible principles to trigger the sensor. So, if what you are saying is true, he must observe some great "action at a distance" in order to explain the spontaneous explosion of the train.

    Secondly, if what you are saying is true, then the frame of reference makes a difference for the experiment...and I thought that one of the principles of relativity is that it should not make a difference what frame of reference the calculations are done in...
  5. Nov 8, 2011 #4


    Staff: Mentor

    No, SR will never result in alternative realities.

    Your description of the scenario was a little confusing, but there are really only two questions that need to be answered to show whether or not the bomb explodes:
    1) where and when does each sensor trigger in some inertial frame
    2) how does the bomb determine if the sensors were triggered simultaneously

    No matter how you answer those two questions, the result will be the same in all inertial frames.
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