General relativity

Einstein’s general theory of relativity can be summed up in just 12 words: “Space-time tells matter how to move; matter tells space-time how to curve”.

But this short description from the physicist John Wheeler hides a more complex and profound truth. Besides quantum theory, general relativity is one of two pillars of modern physics – our working theory of gravity and of the very large, of planets, galaxies and the universe as a whole. It is an extension of Einstein’s special theory of relativity – but such a massive one that it took him 10 years, from 1905 to 1915, to get from one to the other.

Special relativity tells us motion warps space and time. Einstein’s central coup with general relativity was to combine that with a principle noted over three centuries earlier by Galileo: that falling objects accelerated at the same rate regardless of their mass. Famously, a feather and a hammer dropped from the Leaning Tower of Pisa will hit the ground at the same time, once you discount air resistance. (During the Apollo 15 lunar landing in 1971, astronaut David Scott confirmed this principle on the airless moon.)

Following Galileo, Isaac Newton showed that this could only be true if an odd coincidence held: inertial mass, which quantifies a body’s resistance to acceleration, must always equal gravitational mass, which quantifies a body’s response to gravity. There is no obvious reason why this should be so, yet no experiment has ever prised these two quantities apart.

In the same way that he had used light’s constant speed to construct the special theory of relativity, Einstein declared this a principle of nature: the equivalence principle. Armed with this and a new conception of space and time as an interwoven “space-time”, you can construct a picture in which gravity is just a form of acceleration. Massive objects bend space-time around them, making things appear to accelerate towards them.

That explains why we feel a downwards pull towards Earth and why Earth orbits the sun. Although gravity is dominant over large cosmic scales and near very large masses like planets or stars, it’s actually by far the weakest of the four known forces of nature – and the only one not explained by quantum theory.

Quantum theory and general relativity in fact don’t get along at all. The two theories generally work at very different scales, so that’s not a huge problem. But it prevents us from understanding what happened in the very earliest instants of the big bang, for example, when the universe was very small and gravity was very strong. And in another situation where the two clash – at the event horizon of a black hole – unresolvable paradoxes emerge.

The great hope is that some “theory of everything” might one day unify quantum theory and general relativity – although attempts such as string theory and loop quantum gravity have so far failed to come up with the goods.

Meanwhile general relativity has never been found lacking. Its prediction that very dense agglomerations of mass could warp space-time so much that not even light could escape from it has proved to be true. We now call these objects “black holes”, can photograph the “event horizons” that surround them, and are pretty confident there’s one to be found at the centre of every massive galaxy.

But perhaps the biggest triumph of general relativity came in 2015, with the discovery of gravitational waves – ripples in space-time caused by the movement of very massive objects. The signal of two black holes spiralling together and merging was a triumph of painstaking, patient detective work by the Advanced LIGO experiment.