[PairTheBoard]

[ QUOTE ]
In a nutshell, this is what goes on.

The underlying structure of the theory of relativity all comes back to this basic idea: people moving at constant speed should always agree on what the laws of physics are. In particular, for the purposes of this argument they should agree on Maxwell's equations. Now Maxwell's equations predict a certain velocity for light in vacuum. Thus observers moving at different speeds should agree on the speed of light. Experiments done in two different inertial frames should be the same, even if they are moving at different speeds.

How does lead to time dilation? The easiest way (in my opinion) to see this is through the light clock. Imagine that I have a clock that works as follows. I have two mirrors parallel to each other and bounce a pulse of light between them. I can use the regular "ticks" of the pulse hitting the top mirror as a timing mechanism. Now let's imagine that somebody watches one of these light clocks whiz by at high speed. The path that the light takes is no longer a straight line up and down, but two diagonal lines; after hitting the top mirror, the light travels horizontally as well to hit the bottom mirror. These lines are longer, and so it takes more time per tick from the perspective of the outside observer because the light is still going the same speed (this is crucial). You can work out that the amount of time extra taken is just what Lorentz transformations would tell you.

But a person travelling with the light clock won't notice this. He'll still think that the clock is going at its normal speed. This, in essence, is time dilation. The person perceives each tick of the clock as taking less time than an outside observer. And this has to be more than just a quirky feature of light clocks, otherwise we could come up with an experiment to determine the absolute motion of the moving clock, which doesn't jibe with the axiom that physics should look the same in all frames moving at constant velocity. So the conclusion is that time itself moves slower for a moving person.

Try Googling "light clock;" I bet you'll find some diagrams that will demonstrate geometrically some of the things I'm talking about.

[/ QUOTE ]

Nice. But why doesn't the person traveling conclude the same thing about the stationary light clock? From the traveler' perspective doesn't the stationary clock appear to have it's light bouncing diagonally?

PairTheBoard