Best explanation of special relativity for physics and science students?

Answer 1

The fundamental basis for Relativity is that the speed of light is constant from any frame of reference. If I measure the speed of light very accurately from two different frames of reference, say, my arm chair here on earth, and from a space ship moving away from earth at 90 percent of the speed of light (relative to earth), I will get the same measurement for the speed of light. But for that to be possible, something else has to be variable, and that something is time. My clock will seem to run normally (to me, as I move with it) in any frame of reference, but if I place two identical clocks, one next to my arm chair and the other in the ship, they will show different elapsed times when the ship returns. The clock that was aboard the ship will show that less time has passed, so it must have run slower relative to the clock in the earth-bound rest frame. There are other consequent changes to an object that moves at relativistic speed (insignificant until about half light speed, after which they increase rapidly): Since distance can be measured in terms of time of travel, and the on-board ship's clock is ticking more slowly, the distance from point A to point B that I measure from on board the ship will be less than that measured from my alter-ego's position of rest. The change applies not only to distance traveled but to the very object that gets me there, so length decreases as velocity increases. At relativistic speeds, the Universe is actually shorter (but only along the line of travel). On the ship, all operations procede normally, including chemical and nuclear reactions. But viewed from an external rest frame, they would appear to be running slower. So, for a given amount of reaction mass expended to accelerate the ship, it will seem (from on-board the ship) to take progressively more fuel expenditure to increase the ship's velocity by a given percentage as that velocity becomes greater. As the ship accelerates at relativistic speeds, it's mass therefore increases in inverse proportion to its linear forshortening and proportionally to the dilation of time. One other way of looking at it is to say that as velocity increases, space and time change places to a degree. The physical dimension of Length is exchanged for time. This occurs when anything moves in relation to something else, but the effect is negligible at conventional speeds.