Albert Einstein
Charles Darwin
General and Special Relativity

What is meant by Albert Einstein's 'Theory of Relativity'?

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08/26/2014

The theory of relativity, refers specifically to two theories: Albert Einstein's special relativity and general relativity. also Einstein was fatt...no phat..Ya

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:

  1. The laws of physics are the same for all observers in uniform motion relative to one another (Galileo's principle of relativity),
  2. 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 (for example when standing on the surface of the Earth) are physically identical. The upshot of this is that free fall is inertial motion: In other words 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.

http://en.wikipedia.org/wiki/Theory_of_relativity

Answer

Einstein's Special Theory of Relativity combines Galileo's Principle of Relativity to the pretext postulate that all observers, no matter what their state of uniform linear motion is, will always measure the speed of light to be the same. It is called special because it applies this moderated principle of relativity only to inertial frames of reference- that is, frames of reference which are at uniform, linear and non-rotary motion relative to each other. In general relativity he developped to apply this principle generally- that is, to any frame of reference, even those which are at non-uniform, non-linear and rotary motion relative to each other. In physical terms, this kind of relative motion usually occurs when the frames are accelerating relative to each other, or under the effect of gravity relative to each other. Incidentally, it transpired that acceleration and effects of gravity were really two sides of the same coin, since in purely physical terms there is no difference to be observed from one frame of reference if another is accelerating away relative to it, or moving away relative to it under the influence of some suitably chosen gravitation field.

So in basic terms, while special relativity applies an altered version of Galileo's principle of relativity, in which it is additionally postulated that observers in all states of motion would concur the speed of light to be the same, only to inertial frames, general relativity developes this moderated principle of relativity to apply to all kinds of frames of reference of motion, namely also those frames of reference which are moving under the effect of gravity relative to each other.

A question was once asked to Einstein" What is relativity". He replied, (and I do not direct quote) " Relativity is like when you sit next to a pretty girl... It feels like one minute, but was actually an hour. As opposed to when you sit on a hot stove for one minute, but it feels like an hour. This... Is relativity".
There are two theories of relativity. The two theories are electricity and magnetism.