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Almost a century later, the General Theory of Relativity remains the single most influential theory in modern physics, and one of the few that almost everyone, from all walks of life, has heard of (even if they may be a little hazy about the details). Einstein’s General Theory predicted the existence of black holes many years before any evidence of such phenomena, even indirect evidence, was obtained, and was highly suggestive of an origin of the universe beginning with a Big Bang type event, although Einstein himself was highly suspicious of both of those possibilities.
The theory also predicts, or at least permits, the existence of "wormholes", tunnel-like short-cuts through space-time, and even the theoretical possibility of time travel. In fact, the Austrian-American mathematician Kurt Gödel’s elegant solution to Einstein’s field equations (assuming a uniformly spinning universe with constant uniform energy density) specifically predicts the possibility of travel back in time, although it should be said that his model of the universe does not entirely accord with our own. For now at least, these ideas remain firmly in the realm of science fiction.
Yet another theoretical prediction of the General Theory of Relativity is the existence of gravitational waves, perturbations or ripples in the fabric of space-time, caused by the motion of massive objects, that propagate throughout the universe as objects are squeezed on a subatomic scale. There has been good circumstantial evidence for these elusive waves since the 1970s, but it was only in late 2015, a full century after Einstein's theoretical predictions, that gravitational waves were definitively observed at the twin Laser Interferometer Gravitational-wave Observatory (LIGO) detectors in the USA. This potentially opens up a whole new way of looking at the universe and the Big Bang, and a whole host of new discoveries is anticipated. It is also another indication of just how robust the theory is.
The way forward for physics now rests with attempts to combine the theory of relativity (the theory of the very large, which describes one of the fundamental forces of nature, gravity) with quantum theory (the theory of the very small, which describes the other three fundamental forces, electromagnetism, the weak nuclear force and the strong nuclear force) in a unified theory of quantum gravity (or quantum theory of gravity), the so-called “theory of everything”. Candidates like superstring theory and loop quantum gravity, however, still need to overcome major formal and conceptual problems.