[anindha]

Yes, I should have been clearer. Running slower compared to a stationary observer.

Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames.

There is a famous experiment with three synchronized atomic clocks. Two were flown in opposite directions around the globe the other stationary. The eastward flown clock lost 59ns and the westward flown clock gained 273ns, both relative to the stationary clock. The reason is that the clock flown eastward is travelling faster since it is in the direction of the earth's rotation. Measurements are consistent with the theory of relativity.

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

[soVeryTired]

> Your last point doesn't explain why or how time dilation occurs, just that we know that the speed of light is a constant to all reference frames.

But that's what the light clock experiment [1] is all about. The two observers see the light moving at the same speed, but one observer thinks the light takes a longer path than the other observer does. If both observations are equally valid, the inevitable conclusion is time dilation.

[1] http://www.emc2-explained.info/The-Light-Clock/#.WBxO8vnLdPA