Quantum Gravity Without Strings

A new paper (hep-th/0604120) by Fotini Markopoulou and another (gr-qc/0602037), recently revised by Gary Horowitz and Joe Polchinski, explore some quantum gravity without strings. Markopoulou is strongly associated with Loop Quantum Gravity, though that's not her theme here. Polchinski, by contrast, is a card carrying member of the jihadist wing of the string theory mafia, while Jacobson has feet in both camps but seems to be more concerned with GR and black holes.

Fotini Markopoulou's paper probably has aspects to offend most of the prejudices of our favorite string jihad button man, since she is female and propounds a theory that is discrete, spin foamy, and background independent. Her abstract:
We review quantum causal histories starting with their interpretations as a quantum field theory on a causal set and a quantum geometry. We discuss the difficulties that background independent theories based on quantum geometry encounter in deriving general relativity as the low energy limit. We then suggest that general relativity should be viewed as a strictly effective theory coming from a fundamental theory with no geometric degrees of freedom. The basic idea is that an effective theory is characterized by effective coherent degrees of freedom and their interactions. Having formulated the pre-geometric background independent theory as a quantum information theoretic processor, we are able to use the method of noiseless subsystems to extract such coherent (protected) excitations. We follow the consequences, in particular, the implications of effective locality and time.


Horowitz and Polchinski start with:
Assertion: Hidden within every non-Abelian gauge theory, even within
the weak and strong nuclear interactions, is a theory of quantum gravity.
So is this really a "We don't need no stinkin strings" paper from the original D-brane-iac himself? Well, not exactly.

J&P start with the idea of a composite graviton built of two spin-one bosons and the famous Weinberg-Witten no-go theorem that forbids such. It turns out that this no-go theorem has an inconspicuous hidden assumption, like some other no-go theorems, and that this hidden assumption (that the graviton propagates in the same spacetime as the vector bosons) can be relaxed via holography. From this and some other assumptions they get gravity, and, oh by the way, strings.

Both papers are contributions to the forthcoming CUP book Towards Quantum Gravity, edited by Danielle Oriti.

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