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Question about Einstein's train and lightning

  1. Sep 13, 2005 #1
    Einstein wrote about a long train that experienced bolts of lighting hitting on both ends. Einstein tells us that a midway observer on the ground would see both bolts at the same time, but the midway observer on the train would have moved off from the same ground point because of the time it takes the flash to travel.

    Now if you look at this problem using the Lorentz transform, it is not true that momentum of the train is important to the Galilean observer aboard the train, writing for the transform C+V and C-V?

    This problem would not occur for the ground observer, but there is still the question if the bolts came straight down on both ends of the train, how does the bolt now instantly travel at right angles to the ground to meet at the midpoint of the tracks?
  2. jcsd
  3. Sep 13, 2005 #2

    Doc Al

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    Staff: Mentor

    The lightning bolts strike the ends of the train simultaneously according to the ground observers. And since, again according to the ground observers, the train moves during the time it takes light to travel to the midpoint observer, the light from each lightning bolt cannot reach the midway observer at the same time. From this one can deduce (by appealing to the principle that the speed of light is the same for all observers) that according to the train observers the light flashes were not simultaneous.

    I'm not sure what you are talking about here, since you mention Lorentz transforms and Galilean observers in the same breath. According to ground observers, the speed of the train is V and the speed of light is C. Thus according to ground observers the rate at which the light reaches the midpoint is C+V for one flash and C-V for the other.

    Think of the lightning bolts as flashes of light that occur at the ends of the train. The light emanates outward from each flash. (Just like turning on a light bulb.) The bolt doesn't travel to the midpoint, just the light from the bolt. (You can see a lightning bolt strike someplace miles away because the light travels from the bolt to your eyes. That doesn't mean the lightning bolt strikes you!)
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