Re: How active is research on other quantum gravity theories than loops or strings

I.Vecchi wrote:
> EvT ha scritto:
>
> > According to some physicists (for instance John Baez
> > and Peter Woit), both string theory and loop quantum
> > gravity have not made much progress recently.
> >
> > How active are other approaches like noncommutative
> > geometry, euclidean quantum gravity, discrete
> > approaches (Lorentzian, Regge calculus, ...), twistor
> > theory, topos theory, supergravity, Ads/CFT, emerging
> > properties (Robert Laughlin)...?
> >
>
> Very interesting question, to which I would like to append my own.
>
> Is there (or better, on the basis of the current knowledge can one make
> an educated guess about) a common issue underlying the diffuculties of
> the different theories?

There are two fundamentally different kinds of approaches, continuous
ones and discrete ones. I'd say that for any kind of discrete theory,
the biggest problem is showing that the classical limit of smooth
space-time can be recovered. While for continuous models, the biggest
problem is most likely the parametrization of the dynamical degrees of
freedom in a diffeomorphism invariant way (or equivalently, the
solution of constraints imposed by diffeomorphism invariance). Now,
these are technical problems. In general, there are always problems
with interpretation and recovering known physics in the limits of
various combinations of G, c, and hbar being small.

> And here is another volley.
> In 1984 Wald wrote ([1]) "... consider a state of matter where , with
> probability 1/2, all the matter is located in a certain region O1 of
> spacetime and , with probability 1/2, the matter is located in a region
> O2 disjoint from O1 ... Suppose now that we make a measurement of the
> location of the matter. We then find the matter to be entirely in O1 or
> in O2 ... after we have resolved the quantum state of the matter by
> this measurement , then the gravitational field must change in a
> discontinuous , acausal manner ... These difficulties apparently can be
> avoided only by treating the space time metric in a probabilistic
> fashion , i.e. by quantising the gravitational field".
>
>
> My further questions are :
> Is Wald's formulation of the problem still considered appropriate?

I see nothing wrong with Wald's thought experiment, extrapolating from
what we actually know.

> If yes, which of the currently fashionable theories of gravity provide
> a concrete answer to the issue above?

None as far as I know.

> Are there explicit (possibly simplified) models available?
> Is the dearth of relevant experimental results due only to the fact
> that we don't know how to put planets or stars into the quantum state
> described by Wald and that with smaller objects the effects are too
> small to measure?

Yes. The basic problem is that gravity is so many orders of magnitude
weaker than all the other forces. Direct experiments in a regime in
which neither gravity nor quantum mechanics can be neglected are rather
unlikely in the foreseeable future. The best we can hope for is some
sort of indirect evidence from astronomical observations or very large
particle colliders (like the LHC) where, hypothetically, miniature
black holes can be created. Either is yet to be seen (or conclusively
recognized).

> [1] R.M. Wald "General Relativity", 14.1

Igor

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