kr75 said:
I am a novice in relativity as of now and I'm still learning about it. Currently reading Feynman's lectures and an example has confused me. So here it is,
Consider a spaceship moving at a velocity u. If a light source is placed equidistant from two detectors at opposite ends, one in front of it and the other behind it (with respect to the direction of motion). Then for a person inside the spaceship, a ray of light reaches both detectors at the same time but for a person observing from outside the spaceship the ray of light takes more time to reach the detector in front of it than the one behind it (as the detector in front is seemingly moving away from the source while the one behind is moving towards it).
I do not fully understand this concept. I'd be glad if someone could explain it to me
For the person inside the spaceship, the reason the ray of light reaches both detectors
at the same time is purely and only because the person has defined
the same time to be when the two rays hit the two detectors. Let's back up a bit. In order for the spaceman to determine when the light rays hit the two detectors, he must have timing devices installed in the two detectors that record the times when the rays hit them. He performs his experiment and then he goes to one detector and sees what time is recorded on it. He goes to the other detector and sees what time is recorded on it. Chances are, the first time he does this, they will not record exactly the same time. So what he does is tweak the time on one of them by whatever amount the difference in the two readings was and then he repeats the experiment, and whatayahknow, the times are now the same.
Now what about the guy outside the spaceship. He can't do the same experiment, because he's not on the spaceship so he sets up his own detectors with timers that he adjusts by the same technique. The only problem is that he needs a bunch more detectors because he doesn't know where the spaceship will be when it flies by. And he also needs to set up all the timers so that no matter where the light rays start from, each pair of detectors at equal distances from the light source will have to read the same time.
Now there is one more added complication. He needs to have camcorders placed at every detector because he doesn't just want to know when the light rays get to each detector, he also wants to know which two of his detectors were adjacent to the two detectors on the spaceship when the rays hit them.
It will take him awhile to do this but when he gets done he is ready for the spaceship to come flying by. When it does, and the light source emits the two rays in opposite directions, this will be recorded by whichever camcorder is closest to the light source. We'll call this camcorder 'S' (for source). Then as the rays travel outwards in both directions, each pair of his detectors that are equal distant from the first one will record equal times, all the way out to the end. However, since the spaceship is moving while this is happening, the detector at the rear of the spaceship will see the light ray first and it will be recorded on a stationary camcorder nearby. We'll call this camcorder 'R' (for rear). Later on, the other ray will eventually catch up to the detector at the front of the spaceship and this will be recorded by another camcorder which we will call 'F' (for front).
Now it should be obvious that the time on 'R' will be earlier than the time on 'F' even though the times on the spaceships two detectors will read the same time as each other, but not the same times as what the stationary detectors record. Each camcorder records both the time on the stationary detector and the time on the adjacent spaceship detector but only two camcorders capture the events of the light rays hitting the spaceships detectors. And although they will record the spaceship's two detectors having the same time on them, the two stationary detectors will have different times on them.
Does this help you understand what's going on?
