Loop quantum gravity

Feature: November 2003

Loop gravity combines general relativity and quantum theory but it leaves no room for space as we know it – only networks of loops that turn space–time into spinfoam.

General relativity and quantum theory have profoundly changed our view of the world. Furthermore, both theories have been verified to extraordinary accuracy in the last several decades. Loop quantum gravity takes this novel view of the world seriously, by incorporating the notions of space and time from general relativity directly into quantum field theory. The theory that results is radically different from conventional quantum field theory. Not only does it provide a precise mathematical picture of quantum space and time, but it also offers a solution to long-standing problems such as the thermodynamics of black holes and the physics of the big bang.

The most appealing aspect of loop quantum gravity is that it predicts that space is not infinitely divisible, but that it has a granular structure. The size of these elementary “quanta of space” can be computed explicitly within the theory, in an analogous way to the energy levels of the hydrogen atom. In the last 50 years or so, many approaches to constructing a quantum theory of gravity have been explored, but only two have reached a full mathematical description of the quantum properties of the gravitational field: loop gravity and string theory.

The last decade has seen major advances in both loop gravity and string theory, but it is important to stress that both theories harbour unresolved issues. More importantly, neither of them has been tested experimentally. There is hope that direct experimental support might come soon, but for the moment either theory could be right, partially right or simply wrong. However, the fact that we have two well developed, tentative theories of quantum gravity is very encouraging. We are not completely in the dark, nor lost in a multitude of alternative theories.

In the November issue of Physics World Carlo Rovelli explains how quantum gravity offers us a fascinating glimpse of the fundamental structure of nature.

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