Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

GR experiment of light observance

  1. Jun 13, 2012 #1
    My question might appear simple to a few. As the light observance of Einstein's experiment comes into proposition, I am having a difficult TIME understanding. (No pun intended). Let us start with the basic idea...as I'm sure there are several variations.

    A man stands in the middle of a train car with two observers in the box equidistant from him. He has a lighter in his hand. As he flicks the lighter, light travels in both directions to each observer and reaches them at the same time...from his point of view.

    Before I move to my actual question, I must digress. The light is said to reach each observer at the same time from the center perspective. This is where things get a little complex in my mind. Since light speed is a constant and we cannot add or subtract the speed of the train in one direction or the other, it would be logical that from said perspective, this light reaches each person at the same observed time (from our initial central point....not to be redundant). Now we add a second observer. This one is outside the train. When the center observer and the outsider are perfectly lined up and the light is struck the person outside is said to observe light reaching each of the other two at different times. This is the basis for Einstein's GR Theory...no? There are different realities occurring simultaneously based on perspective. But if the speed of the train and thusly those on it cannot be added or subtracted the the speed of light, then how can the outside observer see something different than the one in the middle of the train car.

    If we treat the car as a vacuum and disregard the idea of ether...which is insane....I know...then we come to the same conclusion. But if we do not....then I still come to the same conclusion. I haven't done the experiment with baseballs, seeing as these objects have relative mass. Under such circumstances the experiment makes sense. I have plotted the graph and done the simple math. But I haven't been able to reconcile this idea even with the given experimental data.

    Maybe I'm not Einstein.
  2. jcsd
  3. Jun 13, 2012 #2
    He assumes the light arrives at both observers based on the assumption that the speed of light is equal in both directions.
    It is quite simple really. Light goes at the same speed relative to the observer onboard the train and relative to the observer on the track side. To the track side observer, the back of the train is going towards the light wavefront and so arrives at the back sooner than the light going towards the front which has to chase after the front of the train which is going away from the light wavefront.
    It was part of the analysis of SR. GR is SR generalised to include gravity and this thought experiment does not relate directly to the gravitational aspect.
    If you do the thought experiment with baseballs and assume Newtonian velocity addition then the balls will arrive simultaneously at both ends from everyone's perspective. If you use the correct relativistic velocity addition to calculate the velocity of the balls then the relativity of simultaneity is applicable to balls and light.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook