Where did all the gravity go?1 November 1999
As Albert Einstein knew all too well, gravity is difficult. Even though Einstein's theory of general relativity revolutionized our understanding of gravity, he was not able to solve the biggest puzzle of all: how gravity is related to the other fundamental forces.
Now, a new theory proposed in Physical Review Letters1 offers an explanation for why the gravitational force seems to remain so aloof from the other forces of nature.
In addition to gravity, there are three of these: electromagnetic force and two forces that explain the properties of atomic nuclei, prosaically called the strong and weak nuclear forces. Every event in the Universe, it is believed, stems from the interplay of one or more of these forces. One of the central suppositions of modern physics is that all four forces are related, and were in fact once all one and the same. These forces are thought to have become distinct one after another, like species splitting from a common ancestor, when the Universe was created during the Big Bang.
There is already a theory to explain how the weakest two forces – electromagnetism and the weak nuclear force – went their separate ways. This 'electroweak' theory, developed in the 1960s, predicted the existence of new particles which were susequently seen in high-energy particle physics experiments. And there are promising ideas for unifying electroweak theory and the strong nuclear force – among them the idea of 'supersymmetry', for which tentative experimental evidence was announced earlier this year.
Each of the four forces is thought to have gained its unique identity at different times during the very earliest moments of the Big Bang, when the Universe was still tiny and unimaginably hot. To test unification theories, physicists have to find ways of recreating these tremendously energetic conditions. At present they are barely able to reach the energies needed to test supersymmetry – the current generation of particle colliders isn't quite powerful enough.
But the energies required to reunite gravity with the other forces seem, according to conventional theories, to be way beyond physicists' wildest dreams. This huge disparity between the energy scale of electroweak unification and that of gravitational unification – a factor of something like ten thousand trillion – is bizarre. Worse still, it makes it hard to develop an all-encompassing theory that can unify gravity with the other forces and still explain the relatively negligible distinction between electromagnetism and the weak force. Physicists call this the 'hierarchy problem'.
Now Lisa Randall of Princeton University, New Jersey, and Raman Sundrum of Boston University, Boston, Massachusetts, have proposed a bold resolution to the hierarchy problem. They suggest that space-time is not four-dimensional, as in Einstein's theory, but has an extra dimension.
This in itself is not so revolutionary – physicists routinely posit extra dimensions in unification theories. They suppose that the extra dimensions are tightly curled up so as to be invisible at any length scales we can probe, just as, at a distance, a hosepipe is indistinguishable from a one-dimensional line.
But the extra dimension of Randall and Sundrum is special, because it is here, they say, that gravity truly resides – in the form of 'gravitons', the particles that are hypothesized to mediate gravitational attraction just as photons (particles of light) act as the 'carrier' of the electromagnetic force. The problem, they say, is not that the energy scale appropriate to gravitons is really so huge, but that they barely penetrate into our four dimensions. In five-dimensional reality, the difference between the energy scales of gravitational and electroweak unification is just a factor of ten.
Perhaps the most exciting feature of this suggestion is that, as a result of its circumvention of the hierarchy problem, it should be testable at much lower energies than other attempts to unify gravity. Specifically, the theory predicts the existence of new particles, which should be accessible by the Large Hadron Collider currently being built at the European particle-physics laboratory CERN in Geneva. So we can anticipate the rare treat of a real test of the exotic dreams of high-energy physicists.