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.

— Relativity Simply Explained

G. J. Whitrow points out in his book The Structure and Evolution of the Universe, a very simple explanation [for the Michelson–Morley experiment] would immediately occurred to everyone: the earth doesn't move!

— Relativity Simply Explained

The speed of light in an unobtainable limit; when this is reached the formula becomes 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!

— Relativity Simply Explained

FitzGerald's theory was put into elegant mathematical form by the Dutch physicist Hendrick Antoon Lorentz, who had independently thought of the same explanation. ...The theory came to be known as the Lorentz-FitzGerald contraction theory.

— Relativity Simply Explained

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.

— Relativity Simply Explained

There were many other experiments that had created a highly unsatisfactory state of affairs with respect to theory about electromagnetic phenomenoa. If the Michelson-Morley test had never been made, the special theory would still have been formulated.

— Relativity Simply Explained

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.

— Relativity Simply Explained

Classical physics—the physics of Isaac Newton—made clear that if you are on a uniformly [non-accelerating] moving object, say a train car that is closed on all sides so you cannot see the scenery as you go by, there is no mechanical experiment by which you can prove that you are moving. ...If you toss a ball straight up in the air, it comes straight down again. This is exactly what would happen if you standing still.

— Relativity Simply Explained

The special theory of relativity carries the classical relativity of Newton forward another step. It says that in addition to being unable to detect the train's motion by a mechanical experiment, it is also impossible to detect its motion by an optical experiment.

— Relativity Simply Explained

Imagine two spaceships, A and B. There is nothing in the cosmos except these two ships. They move toward each other at uniform speed. ...To speak of an absolute motion of either ship is to say something that has no meaning. There is only one reality: a relative motion that brings the ships together at uniform speed.

— Relativity Simply Explained

It is not possible to measure uniform motion in any absolute way.

— Relativity Simply Explained

In the special theory of relativity, the speed of light becomes... a new absolute. ...Regardless of the motion of its source, light always moves through space with the same constant speed.

— Relativity Simply Explained

There is no absolute time throughout the universe by which absolute simultaneity can be measured. Absolute simultaneity of distant events is a meaningless concept.

— Relativity Simply Explained

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

— Relativity Simply Explained

If two spaceships are in relative motion, an observer on each ship will measure the other ship as contracted slightly in the direction of its motion. ...The theory does not say that each ship is shorter than the other; it says that astronauts on each ship measure the other ship as shorter.

— Relativity Simply Explained

All three variables—length, time, mass—are covered by the same Lorentz contraction [ .

— Relativity Simply Explained

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.

— Relativity Simply Explained