2013-09-27 17:15:56: Jon Bain here…. and Mattingly begins!
James promises brevity and chattiness…
2013-09-27 17:18:15: We think GR and QM are “just right”. But: we don’t think we can have both at once.
– Both claim unlimited domains, but can’t have c-numbers and q-numbers at once.
2013-09-27 17:21:08: 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
2013-09-27 17:24:44: Do we give up? James say: “no”.
– mathematical consistency is not an issue…
– Heisenberg isn’t an issue… (audience agrees by show of hands)
2013-09-27 17:30:00: Contradicted by experiments? James argues at some length, …. “no”.
2013-09-27 17:34:02: 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…)
2013-09-27 17:35:29: 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.
2013-09-27 17:37:16: 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”.
2013-09-27 17:38:52: Historical analogy: progressive modifications of Maxwell’s theory by Lorentz and others that culminated with Einstein. (Ptolemaic fine tinkering, as another example…)
2013-09-27 17:41:26: 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.
2013-09-27 17:46:15: 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).
2013-09-27 17:48:39: Basic tenets of stochastic gravity:
– assume a complete micro theory incorporating gravity
– assume it’s a quantum system
It must be open. (And a couple other characteristics that flew by me…)
One direction is “kinetic theory”…
2013-09-27 17:51:53: Two other approaches: hydrodynamic and condensed matter.
James: Intriguing ideas but not well-developed as yet.
Virtues of general method:
– it’s a *method* and not speculative ontology.
– it holds promise of making contact with other approaches
– it’s worked in the past
2013-09-27 17:52:34: limitations of the method:
– no guarantees
– progress is slow
– problems of interpretation
2013-09-27 17:54:23: James is unclear what the role for philosophers is here. There are good philosophers, and there are good physicists, but are there good philosopher-physicists out there?
2013-09-27 18:01:03: Discussion:
Audience: Philosophers can explain what “beauty” in a final theory could be like.
James: Really good art historians become physicists, not philosophers.
Audience: Conceptual work can be contributed by philosophers.
James: But who can do it when it’s coupled technically with the physics?