Theory of relativity
The theory of relativity, or simply relativity, refers specifically to two theories: Albert Einstein's special relativity and general relativity.
The term "relativity" was coined by Max Planck in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity.
Special relativity
For a more detailed treatment, see Special relativity.
Special relativity is a theory of the structure of spacetime. The reason we have three space dimensions and one time dimension, or better, space-time, could lie in the way gravity works. One of the foundations of Poincaré’s Theory is the fact that gravity can affect time. 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:
- That observers, when in uniform motion relative to one another, have no way of determining whether one of them is "stationary" (Galileo's principle of relativity),
- That the speed of light in a vacuum is the same for all observers, regardless of their motion or of the motion of the source of the light.
The resultant theory has many surprising consequences. Some of these are:
- Time dilation: Moving clocks tick slower than an observer's "stationary" clock.
- Length contraction: Objects are shorter along the direction in which they are moving.
- Relativity of simultaneity: two events that appear simultaneous to an observer A will not be simultaneous to an observer B if B is moving with respect to A.
- E=mc²: energy and mass are equivalent and interchangeable.
The defining feature of special relativity is the replacement of the Galilean transformations of classical mechanics by the Lorentz transformations. (See Maxwell's equations of electromagnetism and introduction to special relativity).
General relativity
For a more detailed treatment, see General relativity.
General relativity is 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. The upshot of this is that free fall is inertial motion. This is incompatible with classical mechanics and special relativity because in those theories intertially moving objects cannot accelerate with respect to each other, but objects in free fall do so. To resolve this difficulty Einstein first proposed the spacetime is curved, and in 1915 devised the Einstein field equations which relate the curvature of spacetime with the mass, energy, and momentum within it.
Technically, general relativity is a metric theory of gravitation whose defining feature is its use of the Einstein field equations. The solutions of the field equations are metric tensors which define the topology of the spacetime and how objects move intertially.
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.
- A rotating mass "drags along" the space time continuum around it.