Quantum-gravity phenomenology

Feature: November 2003

WHAT do we know about quantum gravity? The short answer – the short scientific answer – is nothing. The problem has been studied for more than 70 years, yet we still do not have a single experimental result that requires us to advocate a quantum theory of gravity. But some physicists, myself included, believe that this could change very soon – that we might actually gain our first real “quantum-gravity data”.

The motivation for studying quantum gravity comes from a sort of “aesthetic discomfort” with our inability to obtain a more satisfactory philosophical world view. For many of us it is unsatisfactory, for example, to describe nature in terms of two very different theories. On the one hand we have a description of the electromagnetic, weak and strong forces unified within the Standard Model of particle physics to form a quantum field theory. On the other, we have gravity, which is governed by the theory of general relativity.

We do, in fact, have a scientifically well defined “quantum-gravity problem”, which concerns our inability to fully predict the outcome of experiments. The central question is this: can we obtain quantitative predictions for processes in which both gravity and the Standard Model have to be taken into account?

Decades of research have shown that the Standard Model is hugely successful in describing microscopic phenomena involving fundamental particles, where gravity can be ignored. General relativity has been equally good at describing the motions of planets and other macroscopic bodies, where the quantum properties of particles can safely be neglected. We do not, however, have any data from situations in which both quantum theory and general relativity have to be taken into account.

In the November issue of Physics World Giovanni Amelino-Camelia in the Department of Physics at the University of Rome La Sapienza explains how cosmic-ray observations and space-based gamma ray telescopes could provide physicists with the first experimental evidence for the quantum nature of space-time.

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