James:  This is not a theory of quantum gravity.  This is a proposal for how to move towards a theory of quantum gravity in incremental steps.

Audience input:  This is not an attempt to define a fundamental theory.

James:  The incremental extensions may intersect other approaches at various points (condensed matter approach for instance).

Enter stochastic gravity:  add a term corresponding to a heat bath (?) to the semi-classical Einstein equation.  Use this term to probe the nature of further corrections.

Audience input:  The extra term represents quantum fluctuations of the stress-energy tensor.

Historical analogy:  progressive modifications of Maxwell’s theory by Lorentz and others that culminated with Einstein.  (Ptolemaic fine tinkering, as another example…)

A conservative extension of SCG.  In Virginia, you buy old houses, tear them down, and build new ones (apparently).

No theory of quantum gravity here; i.e., it’s not an end state.  (No membranes vibrating in various ways.)

Just a way of generating progressive modification of theories we have:  the “Lorentz strategy”.

The *real* issue with SCG is that we’ve thrown out all the interesting quantum stuff.

So what’s left?  Revolutionary stuff?  James says… “no”.  All we have to do is extend SCG by putting back in all the interesting quantum stuff.

Why not stick with semi-classical gravity (SCG) then?

James sez: It’s *probably* not empirically adequate (and refers to problems with the back reaction of matter on the metric).

And:  What does it mean to define the expectation value of the stress-energy tensor operator (since it involves undefined products of operators)?  (But Wald has a prescription for this malady…)

Contradicted by experiments? James argues at some length, …. “no”.

Do we give up?  James say: “no”.

– mathematical consistency is not an issue…

– Heisenberg isn’t an issue… (audience agrees by show of hands)

GR and QM are “framework” theories. Why not hybridize? Enter semi-classical gravity.

End of story?  James says “no”.

Recall problems with semi-classical gravity:

– mathematical inconsistency

– Heisenberg uncertainty principle

– contradicted by experiment

– violates unification principles

We think GR and QM are “just right”. But: we don’t think we can have both at once.

Why?

– Both claim unlimited domains, but can’t have c-numbers and q-numbers at once.