  Taking the quantization of electromagnetism as the paradigm, we show how this
procedure cannot work for Einstein gravity. However, it does work for conformal
gravity, a fourth-order derivative, renormalizable theory of gravity that
Bender and Mannheim have shown to be ghost free. We show that in any gravity
theory gravity cannot be quantized canonically. Rather, because of an interplay
between the zero-point energy of gravity and that of its matter source, gravity
is quantized purely by its coupling to a quantized matter source, with gravity
being intrinsically quantum mechanical. Treating the zero-point energy this way
provides a solution to the cosmological constant problem. With the
gravitational zero-point energy issue having been ignored in standard Einstein
gravity, it is not possible to solve the cosmological constant problem in
standard gravity, since without the zero-point contribution gravity does not
know where the zero of energy is.
