Skip to main content
Quantum

Quantum

Welcome to quantum gravity

01 Nov 2003

Quantum theory and general relativity will only be unified when theory meets experiment

Physics in the 20th century was built on two great revolutions: the general theory of relativity and quantum mechanics. These two theories have profoundly changed the way we think about space, time and the meaning of reality, and both have been verified to extraordinary precision. However, the two theories are also completely incompatible with one another.


Three of the four known forces in nature – the electromagnetic, weak and strong interactions – are described by quantum field theories. These theories, which make up the highly successful Standard Model of particle physics, explain fundamental interactions in terms of the exchange of field particles between elementary matter particles. Gravity, on the other hand, does not fit into this framework. Einstein’s elegant description of gravity is classical, and gravitational forces result from the curvature of the space-time continuum.

But there is something deeply unsettling about this whole picture. Ever since Maxwell unified electricity and magnetism with a single set of equations, finding a general theory that can describe everything that we observe in the physical world has been one of the primary goals in theoretical physics. A unified description of the electromagnetic and weak interactions was achieved in the 1960s, but a true theory of quantum gravity would be a giant step towards this goal. Moreover, a theory of quantum gravity is needed to understand what happens in circumstances when both gravitational and quantum effects are large – such as in the very early universe.

Strings and loops

There are two obvious routes to quantum gravity. The first, and most beaten path so far, is to formulate general relativity into a quantum field theory in which the gravitational force is carried by the exchange of gravitons. The problem is that gravitons carry mass and energy, which are the source of the gravitational field in the first place. This leads to infinities that render calculations meaningless.

One way round this problem is to replace the idea of point-like particles with infinitesimal 1D strings. In “Superstrings” Leonard Susskind describes how string theory is not only our best bet for a theory of quantum gravity at present, it naturally incorporates the other three forces of nature as well. A major problem is the fact that string theory seems to offer too many, rather than too few, possibilities for a theory of quantum gravity – see “The string-theory landscape”. However, developments such as D-branes and M-theory are helping theorists get a better handle on what the theory actually means. Meanwhile, Gerard ‘t Hooft, Nobel laureate and sometime collaborator with Susskind, is pioneering a new deterministic approach to quantum theory (see article on p12, print version only).

Mastering string theory is about understanding higher dimensions. The theory lives in 10 dimensions, six of which are compactified into strange spaces that conceal the fundamental properties of particles. But do not be put off – Susskind can only visualize three dimensions too.

String theory is based on conventional quantum mechanics and assumes that space-time is a fixed background on which particles move and interact. The second route to quantum gravity, however, starts with general relativity and involves completely rewriting quantum theory. In loop quantum gravity there is no such thing as space – only fields. In “Loop quantum gravity” Carlo Rovelli explains how he arrived at a background-independent quantum field theory by treating the gravitational field in terms of closed lines or loops.

A remarkable prediction of loop gravity is that space-time is quantized in elementary “grains” at scales of about 1.6 x 10–35 m – the Planck length. However, like string theory, loop gravity is still a long way from making concrete predictions about the mass of fundamental particles and the strength of the different interactions between them.

Theory and experiment

Will we ever be able to detect or measure the effects of quantum gravity? In order to do this we need to study processes in which both quantum effects and gravity play a role. By definition, this is the point at which both general relativity and quantum theory break down: the Planck scale.

A particle accelerator that could probe energies at the Planck scale – about 1028 eV – would need to be as big as the universe itself. In “Quantum-gravity phenomenology” Giovanni Amelino-Camelia describes a more practical approach – the emerging field of quantum-gravity phenomenology. For instance, if space-time is quantized then photons with different energies should travel at slightly different speeds, and space-based observatories such as GLAST should be able to detect these differences in observations of gamma-ray bursts. Similar effects might also be observable in cosmic rays.

Despite all this progress, there is still some way to go. Indeed, the “grand unification” of the strong and electroweak interactions has still to be achieved. Quantum theory and general relativity will only truly be unified when theory finally agrees with experiment – an event that will define the path of physics in the 21st century.

Related events

Copyright © 2024 by IOP Publishing Ltd and individual contributors