General relativity

Abstract

‘General relativity’ explains Einstein's general theory of relativity, which incorporates gravity and acceleration into special relativity. Galileo discovered the universality of freefall, but Einstein explained the phenomenon using the equivalency principle, which states that all behaviour under gravity is identical to behaviour under acceleration. The equivalency principle makes it easy to predict the effects of gravity in special relativity problems, for example in the twin paradox. The effects of gravitational fields also bend light, giving rise to gravitational lensing. Einstein posited that near to gravitating bodies, space, and therefore time, was distorted. This means that matter — and light — no longer travels in a straight line, but in a geodesic curve. This curvature is usually finite. However, if a mass is condensed to a region of zero volume and infinite density the curvature becomes infinite, and a black hole is formed. General relativity can predict the existence of gravitational waves, which have been observed originating from quasars. How does this theory apply to the universe? The cosmological principle states that space can either be flat, or have negative or positive curvature. Universal curvature comes from energy density, but there is not enough visible matter to cause the curvature we observe. This points to the existence of dark matter and dark energy.