To anyone who has ever fallen off a bike or slipped over in the recent icy weather, gravity seems like a pretty strong force. It’s what prevents the Earth hurtling out of its cosy orbit and off into space, and governs the motions of galaxies. However, physics tells us that our intuitive grasp of this force is wrong. Just think about jumping into the air—with only your puny muscles, you are overcoming the gravity of the entire planet.
In fact, gravity is incredibly weak. There are three other forces which are considered ‘fundamental’, which describe completely different interactions: the electromagnetic force, which sticks magnets to fridges and binds atoms together; the strong nuclear force, which holds the nuclei of atoms together; and the weak nuclear force, which is responsible for certain types of radioactivity. The strongest of these (unsurprisingly, given its name) is the strong force, which is 1039—one with 39 zeros after it—more potent than gravity. (It would take the world’s fastest computers over a billion times the age of the Universe to count this high!) A proton a metre away from another proton would be repelled by the electromagnetic force one trillion trillion trillon times more strongly than it would be attracted by gravity. (For a slightly off-the-wall exploration of these forces, try this article life-forms made of quarks.)
The reason that gravity seems so important in the Universe is that most things are electrically neutral on average, so we don’t see the electromagnetic force acting over large scales. The other two fundamental forces act over the very tiny distances inside the atomic nucleus, and are undetectable in everyday life.
There is a complication, of course: physics is never simple. As particles are imbued with higher and higher energies in particle accelerators, the relative strengths of the fundamental forces begin to change and, not only that, but they meld together into one indistinguishable ‘superforce’. Current particle accelerators probe the ‘electroweak scale’, the energy at which the electromagnetic and weak forces start to behave as the same force. This has an energy of about 103 GeV or a trillion electron-volts (an eV is the energy given to an electron if it passes through a potential of one volt—we would need seventy billion AA batteries to get an electron to the electroweak scale!). The electromagnetic and weak forces get stronger as energies increase, while the strong force becomes less powerful. This has led physicists to predict that the forces ‘unify’ at high energies, becoming one ‘grand unified’ force, at energies which are probably far out of reach of current particle colliders.
The next important energy scale after that is the ‘Planck scale’, the energy where our separate theories of quantum mechanics and gravity break down, and we have to look for a new theory to describe gravity. This energy, about 1018 GeV, is the energy at which gravity’s effects are hypothesised to become comparable in strength to the other forces.
We don’t understand why there is such a massive difference between these scales. Why should the other three forces be important at relatively low energies, while gravity is so pathetic until ridiculously high energies? We can’t investigate the Planck scale directly because this would take a particle accelerator larger than our Solar System (making $8 bn for the LHC look like pocket change), so we have to rely on theories which predict effects at lower energies as well.
Of course the LHC will be the best place to look for an explanation to this problem. One of the most exciting theories sounds like it comes from the depths of science fiction: hidden extra dimensions. Far from being plucked from the imagination of some obscure writer, this is actually a credible possibility. The basic idea is that just like a three-dimensional hair looks like a one-dimensional line from a distance, we could have extra dimensions curled up so small that we can’t see them in our everyday life.
One suggestion, known as M-theory, tells us we live on a four-dimensional (three space dimensions and one of time) hyperspace, known as a ‘brane’, in an eleven-dimensional universe, as though trapped in the 2D pages of a 3D book. As well as seeming bizarre, this suggestion has profound implications for gravity.
In the last years of the millennium, scientists Nima Arkani-Hamed, Savas Dimopoulous and Gia Dvali, as well as Lisa Randall and Raman Sundrum, came up with a way to explain the seeming weakness of gravity by suggesting that whilst the strong, weak and electromagnetic forces are confined, along with us, to our 4D sheet, gravity leaks into all dimensions, meaning that we see it as a weaker force. If you imagine that the strong, weak and electromagnetic forces are just shared between four dimensions, while gravity is spread over eleven, you can begin to understand why gravity seems weaker. So how could we test this in our Universe? Well, in a normal, 4D world, gravity weakens with distance squared, according to Newton’s universal law of gravitation from back in 1687. This means that an object twice as far away would feel a force four—two squared—times weaker. In a 5D world, gravity would weaken with distance cubed (so that the object twice as far away feels an eighth of the force), and so on. If we can probe gravity at distances smaller than the size of these curled-up dimensions, we might catch it misbehaving.
As with most things in physics, extra dimensions are not the only way to solve this problem. Two other big contenders are ‘supersymmetry’, where every particle has a more massive ‘superpartner’, and ‘technicolour’ theory, which boldly suggests that the much-hyped Higgs boson doesn’t exist. Evidence for one of these theories would be another step towards the long sought-after ‘theory of everything’ which, if it exists, would concisely explain all the forces and particles in the Universe. The LHC will be searching hard for strange effects that point us one way or another. However no-one really knows for sure what we are going to find, and some scientists are secretly hoping for something totally unexpected to make their lives that little bit more interesting: weak though it may be, gravity could be the force which shatters physics as we know it.
