Tuesday, July 26, 2005

Epistle to the Boetians

Fundamental physics seems to have hit a bit of a flat spot, with Quantum Gravity being the most obvious target out there and proving rather obdurate. String Theory is what Sean Carroll called the "Microsoft" of quantum gravity theories, with a death grip on many prominent university faculties, while Loop Quantum Gravity plays the role of Apple. For reasons not entirely clear to me, the dispute between the theories has taken on some of the rancor and vitriol one expects in English or Sociology departments rather than physics. Each theory has found a few eloquent public expositors, but with little of the give and take one saw in the great physics controversies of the past - over relativity, quantum mechanics, the big bang, or even Regge pole theory.

String Theory plays its Microsoft role to the hilt, rarely even deigning to admit the existence of competitors, aside from a few more or less rabid attack dogs. LQG, though, has produced a number of intelligently argued criticisms and critiques, mostly by Lee Smolin and Carlo Rovelli - two central figures in the theory's development. Unfortunately, STists never seem to address these critiques.

Smolin has now produced a sort of "Epistle to the Boetians," addressed mainly to ST types, published in "their" section of the ArXiv, arguing for the necessity of what's called background independence.

hep-th/0507235
It's 46 pages long, (with hardly any equations) so I will only give a couple of excerpts
Now here is my thesis, which it is the task of this essay to support:

The reason that we do not have a fundamental formulation of string theory, from which it might be possible to resolve the challenge posed by the landscape, is that it has been so far developed as a background dependent theory. This is despite there being compelling arguments that a fundamental theory must be background independent. Whether string theory turns out to describe nature or not, there are now few alternatives but to approach the problems of unification and quantum
gravity from a background independent perspective
.
One of the core elements of his arguement is that only a relational, background free theory can be free of boundary conditions or other things outside the domain of the theory. This seems true to me, but every physical theory we've had to date is of this form, so maybe that's all we should expect.

A more specific technical critique
Some string theorists have also claimed that string theory does not need a background independent formulation, because the fact that string perturbation theory is, in principle, defined on many different backgrounds is sufficient. This assertion rests on exaggeration and misunderstanding. FIrst, string perturbation theory is so far only defined on stationary backgrounds that have timelike killing fields. But this is a measure zero [set, I guess] of solutions
to the Einstein equations. It is, however, difficult to believe that a consistent string perturbation theory can be defined on generic solutions to the Einstein equations because, in the absence of timelike killing fields, one cannot have spacetime supersymmetry, without which the spectrum will generally contain a tachyon.
I have only begun to digest this paper, and am not competent to comment on the key issues, but it would be nice if some STer would make a careful, reasoned, and, ideally, collegial response to Smolin. I fear hell will freeze over first.

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