Einsteins General Relativity
The general theory of relativity is founded on a set of field-equations, formulated by Albert Einstein in 1915, which deal with the gravitational force-field. The bold idea of Einstein was to regard the gravitational force as a property of space-time. Thus, General Relativity is essentially a geometrization of gravitation and its language is the mathematics of differential 4-dimensional geometry - three dimensions for space and one for time. General Relativity saw its advent as an extension of Special Relativity, also owed to Einstein, which is based on the following two assumptions: (an inertial system is a frame of reference in which the velocity of a body is constant unless it is influenced by forces)
- The laws of physics are the same in all inertial systems and no preferred inertial system exists.
- The speed of light in free space is the same in all inertial systems.
A consequence of the first assumption (the Principle of Relativity) is that one cannot by any means detect whether an inertial system is intrinsically at rest or is moving with a constant velocity - only the relative motion of systems can be detected. A drawback of Special Relativity is that, in principle, it is not applicable to bodies that are accelerated. However, in practice, this is no problem unless the body is under the influence of a strong gravitational field - much stronger than here on Earth. Nevertheless, in order to take also acceleration into account, Einstein made another assumption (the Principle of Equivalence) which can be expressed as follows:
- The laws of physics in an inertial system in which there is a uniform gravitational field - are the same as in a uniformly accelerated system in which there is no gravitational field.
The assumptions of Special Relativity as well as the Principle of Equivalence are well-confirmed by experiments. Einstein went one step further and proposed that the Principle of Equivalence also applies to non-uniform gravitational fields. Such fields are encountered throughout cosmos with its mixture of huge and small concentrations of matter (stars, galaxies, etc.) and (seemingly) empty space void of matter. General Relativity thus was born.
Now, non-uniform gravitational fields are hard to handle mathematically. By means of various mathematical transformation laws these fields can be "imitated" by fields of less complexity. In order to ease matters the cosmologists utilize a certain class of space-time metrics which are invariant under the transformations. Through this metric, geometry and gravitation can be linked in such a way that a gravitational field corresponds to a curvature of space-time - i.e. the presence of matter causes space-time to warp in a region near it. In some sense, matter is absent in General Relativity - its role taken over by the curvature of space-time.
No new theory will ever gain any acceptance in the scientific communities unless it is able to make predictions that can be tested. The testable predictions of General Relativity are rather limited in number but have had a crucial impact on our understanding of the cosmos - prediction and discovery of the gravitational redshift, prediction and discovery of the deflection of light in gravitational fields and time-dilatation in gravitational fields have changed the outlook of humanity completely.
