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I Train Thought Experiment

  1. Dec 20, 2016 #1
    Hey all,

    I was re-reading my quantum mechanics books and I noticed that different books have different scenarios for the Einstein Train thought experiment. And I was just wondering if I understood it correctly.

    Scenario 1:
    A person inside a moving train flashes a light beam that goes in both directions. The person inside the train would see the light hit the front and back of the train at the same time (assuming they are going at a constant speed).

    The person outside would see the light hit the back of the train first since the light traveling towards the front has to catch up to the moving train.

    Scenario 2:
    An observer standing on a platform observes a lightning strike hitting both sides of a train at the same time. The person inside the train observes the light hitting the front of the train and then the back of the train since the light has to catch up to the back of the train.

    From what I understand is that the light hits at the same time if the light flash happens in the observer reference frame and that it occurs at different times if it is outside the observer reference frame. Or does it have to do with the fact that in scenario 1 that the light started in the middle of the train as oppose to scenario 2 where it occurred in both the front and the back and went towards the middle of the train. (But wouldn't the distance the light travels in both scenarios be the same? Assuming the same train length)?
  2. jcsd
  3. Dec 20, 2016 #2


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    An event or sequence of events doesn't happen in a reference frame. A reference frame is a particular view of those events. The point of these thought experiments is to show that simultaneity is not an absolute property of two events: they are simultaneous when viewed from one frame, but not from another. The conclusion, therefore, is that there is no such thing as (absolute) simultaneity. Simultaneity is relative.

    The distance travelled by light from one event to another depends on the frame of reference. This is true in all physics. There is certainly not an absolute distance between two events.

    For example, you may be sitting at rest in your chair, but to someone outside the Earth, you are moving at ##1,600 km/h## or thereabouts. In one frame (your frame) you didn't travel at all between sitting down and standing up again an hour later. But, to someone else in a different reference frame, you moved ##1,600 km## in that time.
  4. Dec 20, 2016 #3

    Mister T

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    In one scenario the events are simultaneous in the train's rest frame, in the other they are simultaneous in the platform's rest frame.

    Since the two frames are equivalent, the two scenarios are equivalent. :woot:
  5. Dec 21, 2016 #4
    Ah, that makes sense, thank you both for your help! :)
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