It is not possible to measure uniform motion in any absolute way. — Relativity Simply Explained

"Jules Henry Poincaré... who anticipated many aspects of relativity theory, once put it this way... Suppose... everything in the universe became a thousand times larger than before. ...would you be able to tell that anything had changed? Is there an experiment you could perform that would prove you had altered in size? No, said Poincaré... "Larger" means larger in relation to something else. ...Size, then, is relative. There is no absolute way to measure an object."

"The speed of light in an unobtainable limit; when this is reached the formula becomes \scriptstyle \sqrt{1-\frac{c^2}{c^2}} which reduces to 0. ...In other words, if an object could obtain the speed of light, it would have no length at all in the direction of its motion!"

"Lorentz made an important addition to his original theory. He introduced changes in time. Clocks, he said, would be slowed down by the ether wind, and in just such a way as to make the velocity of light always measure 299,800 meters per second."

"Einstein, following the steps of Ernst Mach, took a bolder view. The reason Michelson and Morley were unable to detect an ether wind, Einstein said, is simple: There is no ether wind. He did not say that there was no ether; only... [that the ether] is of no value in measuring uniform motion."

"If an astronaut traveled as fast as light his clock would stop completely."

"The speed of light can never be reached. If it were reached, the outside observer would find that the ship had shrunk to zero length, had acquired an infinite mass, and was exerting an infinite force with its rocket motors. Astronauts inside the ship would observe no changes in themselves, but they would find the cosmos hurtling backward with the speed of light, cosmic time at a standstill, every star flattened to a disk and infinitely massive."