
Originally Posted by
chinglu
In the T/E experiment, both observers are equidistant to the light sources when they are fired.
Also, Einstein suggested in the train frame, the train observer moves toward the light.
Let's take a look.
Now in reality (considered with reference to the railway embankment) he is hastening towards the beam of light coming from B, whilst he is riding on ahead of the beam of light coming from A. Hence the observer will see the beam of light emitted from B earlier than he will see that emitted from A. Observers who take the railway train as their reference-body must therefore come to the conclusion that the lightning flash B took place earlier than the lightning flash A. We thus arrive at the important result:
http://www.bartleby.com/173/9.html
Let's look at the "Now in reality". There is no reality that can be applied from the E to the T as suggested in Einstein's passage above. That would imply an absolute frame. But, carefully, Einstein claims the E frame imposes reality onto the T frame. That is not SR.
No, there is no absolute frame implied at all. What he
does insist is that "local events" must be agreed upon between the two frames. IOW, the train frame and the embankment frame have to agree as to the Train observer's position with respect to the embankment when the two light flashes reach him, and as to the embankment observer's position with repect to the train when the flashes reach him.
For things to be otherwise would violate the principle that the laws of physics are constant across frames.
For instance, we can put recording devices on the embankment and train that record exactly what part f the other frame is exactly opposite them whne the light reaches them (train devices record what part of the embankment is opposite them and embankment devices record what part of the train is opposite them).
If you bring these records back together They have to agree. If train device 10 records that is is opposite embankment device 25 when the light reaches them both, then embankment device 25 has to record that it is opposte train device 10 at that same moment. (here we assume that the physical distance between the two devices as they pass is infinitesimally small enough not to effect the outcome)
The fact that the embankment observer is the one that sees the flashes simultaneously was just a arbitrary choice. This does not "impose its reality" onto the train frame any more than the train frame "imposes its reality" unto the embankment frame.
It's like having two people, one facing West and the other East. The first, from his view point, says that Chicago is to the right of Dallas, while the second says that, from his viewpoint, Chicago is to the left of Dallas.
Neither imposes his view on the other, but they can use their viewpoint to determine what the other sees.
This is what happens in the train experiment, the embankment observer uses what he sees, in respect to the embankment frame, to figure out what the train observer sees in his.