General relativity
General relativity (GR) or General relativity theory (GRT) is the theory of gravitation published by Albert Einstein in 1915. The conceptual core of general relativity, from which its other consequences largely follow, is the Principle of Equivalence, which describes gravitation and acceleration as different perspectives of the same thing, and which was originally stated by Einstein in 1907 as:
- We shall therefore assume the complete physical equivalence of a gravitational field and the corresponding acceleration of the reference frame. This assumption extends the principle of relativity to the case of uniformly accelerated motion of the reference frame.
This principle explains the experimental observation that inertial and gravitational mass are equivalent. Moreover, the principle implies that some frames of reference must obey a non-Euclidean geometry: that spacetime is curved (by matter and energy), and gravity can be seen purely as a result of this geometry. This then yields many predictions such as gravitational redshifts and light bent around stars, black holes, time slowed by gravitational fields, and slightly modified laws of gravitation. However, it should be noted that the equivalence principle does not rigorously uniquely determine the field equations of curved spacetime, and so the latter are now considered to be pre-eminent.
The modifications to Isaac Newton's Law of Gravity produced the first great theoretical success of general relativity: the correct prediction of the precession of the perihelion of Mercury's orbit. Many other quantitative predictions of general relativity have since been confirmed by astronomical observations, and the theory is now considered well established, although alternatives are still occasionally proposed. However, a continuing unsolved challenge of modern physics is the question of how to correctly combine general relativity with quantum mechanics, thus applying it also to the smallest scales of time and space.