What is theory of relativity?

Answer 1

There are two theories of relativity.

1. Special Relativity (Einstein)

2. General Relativity (Einstein)

Einstein's theory of relativity posits that time is a dimension in addition to space, and that space and time have relative properties to one another. This is not commonly observed because it is only obviously apparent as an object approaches the speed of light (a constant and ultimate property in the theory.)

Albert Einstein's special relativity and general relativity.Special relativity: a theory of the structure of spacetime. It was introduced in Albert Einstein's 1905 paper "On the Electrodynamics of Moving Bodies". Special relativity is based on two postulates which are contradictory in classical mechanics:

• The laws of physics are the same for all observers in uniform motion relative to one another (Galileo's principle of relativity),
• The speed of light in a vacuum is the same for all observers, regardless of their relative motion or of the motion of the source of the light.

General relativity: a theory of gravitation developed by Einstein in the years 1907-1915. The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field are physically identical (for example when standing on the surface of the Earth). The upshot of this is that free fall is inertial motion. An object in free fall is falling because that is how objects move when there is no force being exerted on them, instead of this being due to the force of gravity as is the case in classical mechanics. This is incompatible with classical mechanics and special relativity because in those theories inertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed that spacetime is curved. In 1915, he devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it. Some of the consequences of general relativity are:

• Time goes slower at lower gravitational potentials. This is called gravitational time dilation.
• Orbits precess in a way unexpected in Newton's theory of gravity. (This has been observed in the orbit of Mercury and in binary pulsars).
• Even rays of light (which are weightless) bend in the presence of a gravitational field.
• The Universe is expanding, and the far parts of it are moving away from us faster than the speed of light. This does not contradict the theory of special relativity, since it is space itself that is expanding.
• Frame-dragging, in which a rotating mass "drags along" the space time around it.