From my interpretation of it, it seems to be just a quirk of how humans see things; namely that light from the thing being seen must hit a person’s eye(s).
Given this, it would make sense that M’ saw flash B first, since the light from B has to travel less distance to M”s eyes than the light from flash A (because M’ is traveling right at a high speed). While with M, the two flashes were equal distance from him *and* he was stationary, so the light from both flashes has the same distance to cover to meet his eyes, making it appear simultaneous.
Am I misunderstanding something?
In: Physics
It’s not just some human quirk, the same thing happens regardless of whether there’s a human there or not. It’s a property of relativity. Because light moves at the same speed for everyone regardless of their frame of reference, the order of the events is relative to each observer.
Let’s use another example. It’s basically the same, except that instead of a light shining, the person on the train throws two baseballs at the exact same time with the exact same speed. If the train is moving at say 50mph relative to the ground, the speed of the baseballs are relative to the observer. The guy on the train might see both of them moving away from him at say 10mph. The guy on the platform will see the one moving towards the front of the train going at 60mph and the one moving to the back at 40mph (50mph +/-10mph depending on the direction). Both of them will see the baseballs hit their ends of the train at the same time.
With light, that doesn’t happen. Both people will see light moving at the speed of light, C, relative to them. So the situation is basically the same for the person who’s on the train. The light is moving away from him in both directions at C, so it hits the front and back at the same time. For the person on the platform it’s different though. They don’t see the light moving at C +/- the speed of the train like they did with the baseball. By the time the light hits the back of the train, the back is closer to the where the light came from and so it takes less time. The front meanwhile has moved further away, so the light has travel longer before it reaches it.
It’s not a matter of just perception though. The order both of them see these events happen are both equally valid. If you had sensors that recorded the exact moment they detected the light, and you had one pair inside the train and another pair sitting still outside the train at the exact spots where the light hits the front and the back of the train (and they can just phase through the train or something), they would record detecting the light at different times regardless. Both pairs of sensors will be in the exact same location when they go off, but because they’re moving relative to one another they experience the events differently regardless. It’ll be negligible because the train is moving incredibly slowly relative to the speed of light, but it will be different if you measure with enough precision. And if you go into space or something and start hitting relativistic speeds (sufficient fractions of the speed of the light), you’ll find the results are a lot more drastic and obvious.
It’s significant in that it demonstrates how the order of the events happen at different, but equally correct, times depending on the relative velocities of the observers.
It can go even further too. Add a second train. It’s going in the same direction as the first, but twice as fast. As soon as they’re perfectly lined up next to each other, the light goes off. Relative to this train, the first one is moving away at 50mph now, so they’ll see the opposite happen. The light will hit the front of the first train before it hits the back.
Now you have three different observers with three different accounts of what happened, and all of them are correct.
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