## Glossary term: Theory of General Relativity

**Description:** (The theory of) general relativity, published in 1915, is Albert Einstein's theory linking space, time, and gravity. In that theory, gravity is not an ordinary force. Instead, a mass or other source of gravity will distort space and time in its vicinity. This distortion changes how bodies in free fall move. For instance, when a planet orbits the Sun, it's not because of an attractive force – it's because the planet is taking the straightest possible path through space and time (or more precisely: through "spacetime", since in Einstein's theory, there is no unique way of separating space from time). The core equations of general relativity, known as Einstein's equations, provide a direct link between spacetime geometry and the mass and similar properties of the matter contained in that spacetime (specifically the energy density or, equivalently, the mass density, and also pressure). Paraphrasing John Wheeler, by these equations, matter tells spacetime how to curve, while spacetime tells matter how to move. Einstein's theory predicts corrections to the orbits of astronomical objects, which can be observed both in the Solar System – most prominently in the orbit of Mercury – and much more clearly in binary neutron stars, where two very compact objects orbit each other. Its predictions for the influence of gravity on clocks play a role in satellite navigation systems. The theory also predicts new phenomena, which have become an integral part of astrophysics: the deflection of light by mass, observable as the so-called gravitational lensing effect; black holes as the ultra-compact end states of certain stars and central ingredients of galaxies, and gravitational waves as a way of gaining information about, among other things, merging black holes. General relativity is also at the foundation of the cosmological models for expanding universes.

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