- #1

MalachiK

- 137

- 4

I’m reading Einstein’s book on SR and GR and I’ve reached a discussion of simultaneity that involves an observer attempting to decide if lighting strikes that occur ahead and behind of a moving train happen at the same time. This is how I understand the argument so far; due to the finite speed of light that has the same value in all reference frames, the information about the event that takes place ahead of the train will reach the observer in a shorter time that the information about the event that happens behind the train. As a result of this, it is not possible to reason that the simultaneous arrival of light from both events corresponds to both events happening at the same time in the rest frame of the train track. This idea is then developed to illustrate that events that are simultaneous in one intertial frame are not simultaneous in another. This all makes sense to me.

As part of his exposition, Einstein asks the reader to think about how they could determine the simultaneity of such lightning strike events, and given the constand speed of light it is made clear that this cannot be done simply by reference to the light that arrives from the events.

However, what if there were clocks placed as the sites of the lightning strikes? These clocks could be synchronised in the rest frame of the track. Wouldn’t the light / information from the lightning strike then arrive at the observer along with information about the state of the clock at the instance that the strike happened? Wouldn’t this then allow the moving observer to determine the simultaneity (or lack there of) in the rest frame of the track?

I suspect that I’m missing something profound about relativity here, and maybe I’m thinking about measuring the wrong time or something. I was sure that this sort of question must come up all the time, but I can’t find a satisfying answer.

Thanks for any replies.