|Albert Einstein (1879 - 1955) developed the theories of Special Relativity and General Relativity. Both demand that, to understand them, we discard familiar concepts about time and space.
Special relativity is the simpler theory of the two. It became obvious at the turn of the century that something was wrong with our ideas about space and time, when a series of experiments proved that the speed of light in air never changes, no matter whether the source of the light or the observer of the light are moving. This is not the case with something like sound. The measured speed of sound emitted by a moving car is equal to the normal speed of sound plus the speed of the car. However, a person inside the car measures just the normal speed of sound.
If we used light in this experiment rather than sound, then both we and the person in the car would measure the speed of light as being the same. This paradox was resolved by Einstein, who developed his theory using two axioms, or postulates. These were that the speed of light is always the same, regardless of the speed of the source or the observer, and that it is impossible to tell the difference between standing still and proceeding at a constant velocity.
The remarkable consequences of these assumptions include: (1) A person travelling close to the speed of light ages more slowly. This effect has been tested with a very accurate clock in Concordeâ€”it had lost time after travelling around the world when compared to a clock that was kept on the ground. (2) It is impossible to exceed the speed of light if you try, you just get heavier! This has been shown to be true in particle accelerators, where electrons are accelerated up to 40,000 times their normal mass. (3) Moving objects grow shorter along their direction of motion.
We do not notice these effects, as they are very small at speeds less than a few percent of the speed of light, which is 300,000 kilometres per second.
General Relativity is a far more complex theory, involving tensor mathematics. Special Relativity only describes objects travelling at a constant speed, that do not turn, accelerate or decelerate. General Relativity includes all these phenomena. It has one basic postulate; that it is impossible to tell the difference between gravity and acceleration. Thus, an observer in a box, who feels what seems to be gravity acting downwards, cannot perform any experiment that will tell him whether he is in fact on the Earth, or in space accelerating upwards at a certain rate.
The consequences of this theory include: (1) The presence of a massive body (like a planet) slows down time in its vicinity. (2) Time stops at the surface of a black hole, that ultimate producer of gravity. (3) Even light will fall into a black hole. JJ
See also gravity; space and time.Further reading H. Bondi, Relativity and Common Sense.