e 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.