Ok, I'm not a physicist, so I don't know if my question is silly (probably yes), but there is something in special relativity that I can't understand and I would really like it to be clarified.
If two observers are experiencing different velocities, then they will experience time differently. Ok. But, since there is no absolute referential, ever, how can I tell which one is moving "away" from the other in order to know which one is experiencing time faster than the other? I'll try to be more specific. Let's get the classic experiment of the light clock where a beam of light is moving up and down between two mirrors, and counting time at each passage. Now let's get an exact copy of that mirror and put it side by side with the first one. Now let's start moving the second clock horizontally to the right, away from the first one. In order for the speed of light to be the same to all observers, the moving light clock should experience time slower than an observer attached to the first clock. Ok, got it. Very logical. But here's my doubt: if the entire universe is composed of just these two watches, how can I tell that the second mirror is the one that is moving away from the first to the right, and not the first mirror that is moving away from the second to the left? It's impossible to tell. So, how can I know which one of the clocks is the one "moving away" in order to define which one is experiencing time dilation?
I know the answer must be silly because I simply can't find it. Any help?
Best Answer
I'm not sure what you mean by this, but I'm confident you mean something wrong. Everyone "experiences time" at exactly the rate of 1 second per second.
Alice says she is standing still and Bob is moving rightward. Bob says he is standing still and Alice is moving leftward. These are two equally valid descriptions of the same reality, just as when Alice (facing west) says that the North Pole is to the right, while Bob (facing east) says that the North Pole is to the left.
Alice describes it one way, Bob describes it another way, and that's okay. There's no need for a referee here, and as you seem to understand, there's no such thing as the One True Description.
I don't know what you mean by "experiencing time dilation", but the answer to what you seem to be asking is that:
according to Alice's description of the Universe, Bob's clock is ticking slower than Alice's
according to Bob's description of the Universe Alice's clock is ticking slower than Bob's.
Both descriptions are correct (just as Bob and Alice are both correct when one says the North Pole lies to the left and one says the North Pole lies to the right). Once again, there is no One True Description.