Conversations on Quantum Gravity

Things for many years now have been going badly for string theory on the public relations front. Today the Economist has Physics seeks the future: Bye, bye, little Susy, where one finds out that:

But, no Susy, no string theory. And, 13 years after the LHC opened, no sparticles have shown up. Even two as-yet-unexplained results announced earlier this year (one from the LHC and one from a smaller machine) offer no evidence directly supporting Susy. Many physicists thus worry they have been on a wild-goose chase…
Without Susy, string theory thus looks pretty-much dead as a theory of everything. Which, if true, clears the field for non-string theories of everything.

Unfortunately for the public understanding of science, this is followed by

But at the moment the bookies’ favourite for unifying relativity and the Standard Model is something called “entropic gravity”… in the past five years, Brian Swingle of Harvard University and Sean Carroll of the California Institute of Technology have begun building models of what Dr Verlinde’s ideas might mean in practice, using ideas from quantum information theory.

For something much more anecdotal, on Saturday night I was having dinner outside in a hut during a rainstorm on the Upper East Side (having fled an aborted Central Park concert), and started talking to a couple seated nearby. When informed I taught math and did physics, one of them recommended Carlo Rovelli’s new book to me, and said he hoped I wasn’t doing string theory. Luckily I could reassure him about that.

This morning I found out about Conversations on Quantum Gravity, a fascinating book published by Cambridge that appeared online today, hard copies for sale in November. It consists of interviews about quantum gravity put together by Dutch string theorist Jay Armas, starting in 2011. The scale of this project is immense: there are 37 interviews, most of them rather long and detailed, making up a book of 716 pages. What I’m writing here is based on a day’s worth skimming of the book. I’ll likely go back again and look more carefully at parts of it.

Roughly half the interviewees are string theorists, with the author making a concerted effort to also include non-string theory approaches to quantum gravity. I made the mistake of starting off by reading some of the string theorist interviews, which was rather depressing. By the end of the day, after making my way through about 20 long interviews with string theorists, with few exceptions the story they were telling was one I’m all too familiar with. It’s roughly

We don’t actually know what string theory is, just that it’s a “framework” that encompasses QFT and much more. We can’t predict anything with it now and don’t see any plausible way of predicting anything in the future, but the theory is a successful theory of quantum gravity, unlike our competition. There is no good reason for people to be working on anything else.

For example, here’s Cumrun Vafa:

If a young student asks you what approach to quantum gravity they should work on, what would your answer be?

There is no question that string theory is the right framework to understand quantum gravity. By this I mean that it is closer to the truth than any other existent theory.

Is it worth exploring other approaches?

Well . . . certainly being close-minded is not good. We should be open to other developments. But the fact that there exist other subjects does not justify exploring them if they are not on equal footing with string theory.

and here’s Edward Witten:

Due to the lack of experimental data, there exist a plethora of different approaches to quantising gravity. Which of these approaches, in your opinion, is closer to a true description of nature and why?

I would say your premise is a little misleading. String theory is the only idea about quantum gravity with any substance. One sign is that where critics have had interesting ideas (non-commutative geometry, black hole entropy, twistor theory) they have tended to be absorbed as part of string theory.

and David Gross:

So you don’t think that other approaches like loop quantum gravity have . . .

Loop quantum gravity is total BS. I mean, it’s really not worth discussing it. Don’t put that in the book. But, it really isn’t.

Luckily Armas doesn’t take up Gross on the suggestion that loop quantum gravity is not worth discussing, interviewing quite a few people who are working on research programs that have grown out of it. I got much more out of these interviews, which were very different in tone and content than the ones with string theorists. Many of them gave a very clear account of the technical problems these approaches have encountered, referring to very specific well-defined models and calculations. Instead of the triumphalist claims and vague speculation of the string theorists there was a careful explanation of exactly what they were trying to do and the problems they were trying to overcome.

There’s a huge amount worth reading in these interviews, perhaps I’ll later add some more pointers. A couple specific examples that occur to me right now are Steve Carlip’s careful discussion of the quantization of the toy model of 2+1 dimensional gravity, and Lee Smolin’s very personal account of his frustration at the reception of his book “The Trouble With Physics”.

If your institution is paying Cambridge for access, you should take advantage of this now and take a look. Congratulations to Jay Armas for bringing us this material.

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