David Wallace: Quantum gravity at low energies

Quantum gravity at low energies or Yes, Virginia, there really is a cosmological constant problem
“Quantum gravity” is usually defined as the program of reconciling classical general relativity with quantum mechanics, so that a ‘quantum theory of gravity’ is any theory that reduces to general relativity and the standard model of particle physics in appropriate limits. And it is often claimed that (a) we have no such theory at present, only a variety of research programs; (b) there is no, or virtually no, empirical data to inform the search for such a theory. I will argue that both claims are false. We have a perfectly viable quantum theory of gravity – the ordinary quantum-field-theoretic version of general relativity, defined and interpreted in modern ways – and there is actually quite a lot of evidence – comprising a large part of astrophysics and cosmology – that is in support of that theory. I will develop some implications of this attitude to quantum gravity: in particular, I will argue that what high-energy physicists mean by ‘quantum gravity’ is a high-energy completion of our existing quantum theory of gravity, and that the cosmological constant problem is best understood as a Kuhnian anomaly in that theory