Quantum Gravity Necessary ? ∗

Quantum gravity presents something of a unique puzzle for the philosophy of science. For in a very real sense, there is no such thing as quantum gravity. Despite near unanimous agreement among physicists that a quantum theory of gravitation is needed to reconcile the contradictions between general relativity and quantum mechanics, there are no pressing empirical issues that require this resolution—the regime in which one would expect to observe a conflict between the claims of general relativity and quantum mechanics is at the Planck scale. Thus the question naturally arises “Why quantize gravity?” Are there other issues that compel us to seek a quantum theory of gravity? The standard response is intimately connected with a desire for theoretical unification. Quantum field theory successfully describes the physical world on small length scales at low “particle” density. General relativity is a successful theory of large length scales where individual features of particular objects are swamped by their mass-energy properties. It is natural to seek a unified theory that captures these successful features and yet is somehow a “fundamental” theory of both regimes. But why should the resulting theory involve a quantized gravitational field? There is clearly something wrong with the general relativistic treatment of matter fields as classical. Very well. Let us stipulate that an acceptable theory of gravitation will take due note of the quantum nature of the fields to which it couples. Now what? Are we thus compelled to treat the gravitational field itself quantum mechanically? There are a number of arguments urging the necessity of a full quantum gravity—i.e., a theory of gravity that treats the metric itself as a quantum field. There are, as well, a number of proposals for how we should go about producing this theory. I will not here be concerned to articulate the panoply of attempts to quantize gravitation theory (nor to elaborate the many prob-