- This page is about the scientific concept of relativity, for philosophical or sociological theories about relativity see Relativism.
The theory of relativity , or simply relativity, refers specifically to two theories of Albert Einstein: special relativity and general relativity. However, "relativity" can also refer to Galilean relativity.
The term "theory of relativity" was coined by Max Planck in 1908 to emphasize how special relativity (and later, general relativity) uses the principle of relativity.
Special relativity is 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.
The resultant theory has many surprising consequences. Some of these are:
- Time dilation: Moving clocks are measured to tick more slowly than an observer's "stationary" clock.
- Length contraction: Objects are measured to be shortened in the direction that they are moving with respect to the observer.
- 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.
- Mass-energy equivalence: , energy and mass are equivalent and transmutable.
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 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
(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:
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 inertially.
References and links
See the special relativity references
and the general relativity references
. For information on the silent film produced on this subject, see The Einstein Theory of Relativity