Gravity

An apple falls from a tree. A planet orbits its sun. You labour your bicycle up a hill, and accelerate smoothly down the other side. All those things are down to gravity, working exactly as Isaac Newton said it did almost three and a half centuries ago: a force that tells massive objects how to move.

Newton’s universal law of gravitation, formulated in his great work of mathematical physics, the Principia, published in 1687, was the first great work of force unification in physics. It tells us that many disparate phenomena, from falling apples to orbiting planets, all occur because massive objects experience an attraction between them that follows a set formula. The size of the gravitational attraction between two bodies increases in proportion with their masses, diminishes with the square of the distance between them, and has an absolute value ultimately determined by a universal, fundamental constant of nature, the gravitational constant or “Big G”.

For all its seeming universality, however, gravity is actually by far the weakest of the four known fundamental forces of nature. The only reason it appears to be so overwhelmingly strong in our neck of the woods is the local presence of an overwhelmingly large mass, Earth, beneath our feet that attracts them, and everything else, downwards towards it. It is the presence of large clumps of mass all over the universe, that ensures it determines how the universe works on a grand scale, on the level of planets, galaxies and clusters of galaxies.

As effective as Newton’s description is for most purposes, there is something mysterious about it, however. The way gravity seems to act instantaneously over great distances, even across half the universe, bothered Newton himself no end. He thought this “so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it”.

And indeed it turns out Newton’s theory is far from the last word on gravity. Einstein’s general theory of relativity, formulated in 1916, rewrites gravity as a property not of individual bodies, but of the universe as a whole. Gravity is just geometry, the result of the curvature by massive objects of the space and time around them. The strength of the gravitational “field” at any point in space or time is just the degree to which that coordinate in space-time is invisibly curved. Massive objects fall down these curves towards each other.

That depiction may not seem any less absurd than Newton’s, but it has passed every test thrown at it so far. It means that gravity stands apart from the other three fundamental forces, electromagnetism and the strong and weak nuclear forces, which are quantum forces, described by quantum field theory and transmitted by quantum particles.

The hope is that one day gravity might be too, when in a third iteration we uncover what quantum properties of mass, energy, space and time combine to make gravity at a fundamental level. At the moment, however, the quantum particle that transmits gravity, the “graviton”, remains stubbornly hypothetical – and with it the holy grail of a quantum theory of gravity.

And gravity remains on many levels fundamentally mysterious. Why is it so weak compared with the other forces? Why does it only pull, not push? And why is the strength of “Big G” (whose value is, incidentally, notoriously difficult to pin down) so apparently finely tuned to allow life to emerge? If it were just a little less, the expansion of space would have overwhelmed the pull of gravity on matter in the newborn universe, stars and galaxies would never have formed. If if were just a little more, any wannabe stars or galaxies would quickly have collapsed in on themselves and each other, while space-time would have folded up the whole universe in a big crunch. We have a lot to thank for gravity being like it is.