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
Space an time are linked. So essentially what Einstein was saying is that unless two events occupy the same space they do not occupy the same time. That is to say, they cannot be *absolutly* simultaneous, though they may appear to be so locally.
As you move through space you also move through time, meaning that if we were moving in reference to the first event (or point) and not traveling perfectly perpendicular to both events, then we would see the events as slightly staggered, where as someone standing still might see them as simultaneous
There’s also a mistake I think you’re possibly making regarding light. The speed of light in a vaccume (“C”) is the same regardless of the reference point of the observer.
In otherwords if you were on a train traveling towards a light source then the light from that source would appear to be moving towards you at a speed of C. If you were then to reverse course and travel away from the light source the light would also be moving towards you at C.
This is only really relevant to your specific example, the light from the flashes would reach your eye at different times because they occupy different spaces, but your movement relative to the light is irrelevant.
I probably fucked that up somehow, but that’s basically what I remember from AP physics
Edit: a word
Prior to this, there was an understanding that there is a “universal clock”. Meaning there could be no dispute over what happened in the past, what is “now”, and what is future.
The idea of seeing light flashing might appear trivial, but consider that the implication of this thought experiment is that past, present and future are observer dependent. Different observers would NOT agree on what sequence of events occurred in which order. Some event that you think happened already could be an event in someone else’s future.
This generates profound ideas on determinism – ie since you already “saw it”, then essentially you can predict the future for the other observer? So is there an observer that already knows everything that will ever happen? Is the universe deterministic?
The train thought experiment is highlighting the fact that light acts funny at high speed. And it acts funny because of a specific property
You see, the observation of light is based on the stream of photons hitting your eye (or some other object). If the object is moving toward the observer, then the photons are hitting the observer faster and the waves of light are crashing at a higher speed. But functionally there is no difference between red light from a source moving away from us, white light from a source stationary, or blue light from a source moving towards us.
We observe the wave as moving the same speed, regardless of whether we are standing still or travelling at 100,000,000 meters per second. But that wave is stretched or compressed into a different color of form of light.
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.
Einstein made a wild claim and then used the thought experiment to explore that claim: what if the speed of light is the same for everyone, including those who are moving (i.e. a different frame of reference). *No, really, what if everyone in all frames* (no matter how fast they were moving relative to each other) *could measure the speed of light and it was exactly c (for everyone)*? The human part is only that the human is the observer in each frame of reference. This was at a time when the idea that the speed of light was constant had just come into vogue (earlier the speed of light was thought of similar to the speed of sound, based on the belief in the luminiferous aether).
The result of the thought experiment had some pretty insane implications (relativity, space-time dilation), but you had to accept them if you held to the belief that c is a real constant. In some ways it helped socialize the concept of relativity (which was really the first spooky science, where reality no longer matched human’s day to day experiences).
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