I am trying to understand the idea that the speed of light is constant for each observer regardless of their motion. But I don't understand how come. I also don't understand how this was measured.
Let's say there is a distance from point A to point B and the way to measure it would be to use a stopwatch (at point A) that snapshots the time when the laser beam was shot and another stop watch (at point B) where it was received. That I understand.
Now, let's say point B is dynamic and it is moving away from point A. Of course it would take a little more time to reach that point B; because during that period of time when the laser was shot and the time when it reached point B, point B moved a little bit already, which means it would take extra time for the laser beam to cover that gap. This experiment still would show that the speed of light is the same.
Let's say that the moving point B is a train wagon and the point B is actually it's front side. Let's say there is another point, point C which is the wagon's back side. The laser beam that would hit point B, would also go through the point C first. Also, let's say there is another stopwatch that tracks the moment when the laser beam passes through that point C. It makes me think that since the distance increases (the wagon is moving) then it would take more time for the laser beam to reach the point B. Which means that for the observer (who sits in that wagon) it would seem that light travels slower!
When it is said that the speed of light is constant for each observer, is it meant the objective light speed is the constant or the perceived by the observer one? If it is the latter then how come it is happening? If it is happening, then there is a mistake in my thought chain.
Best Answer
In order to understand special relativity you must remove from your mind the idea of objective velocity, or time of an event or duration or distance. By doing this, Einstein was able to formulate a consistent theory in which light would always be observed (calculated) to be travelling at the same velocity ($c$) by any observer. There is no point in space that can be considered still in objective terms but only still relative to an observer in the same "inertial frame", i.e. who is traveling at the same velocity.
Light (in a vacuum) is always observed to be traveling at one velocity ($c$) in any experiment, whether it is in your own inertial frame or in another which is moving at high speed relative to yours. This is of course counter-intuitive. In Galilean (common sense) relativity a bullet shot from the front of a fast moving train would have a velocity relative to the track that is the sum of those of the train relative to the track and the bullet relative to the train. That is not the case when a beam of light is fired from the front of a fast-moving spaceship. An observer who is still (not moving with the spaceship) would calculate the light velocity to be $c$, the same as the observer on the spaceship.
To resolve this apparent paradox it is necessary to make use of time dilation, distance contraction, and lastly the non-simultaneity of events separated by distance in the direction of travel in a moving inertial frame. That would include that, clocks at the front and back of a long, fast-moving spaceship, which have been synchronised by a traveller on the spaceship will not be considereded to be synchronised by an observer at rest.
It should be mentioned that the train and the bullet in the above example are not exempt from the special theory of relativity, but their velocities would be such that the relativistic effects on them would be extremely small, and probably too small to measure by any conceivable experiment.