David Goss wrote to tell me that Physics Web has an article about physics weblogs. The theorists quoted are Sean Carroll, Paul Cook and Dave Bacon.

There’s an article by Michael Green in the latest Nature Physics reporting on the recent 23rd Solvay conference (mentioned here and here). Green notes that “Much of the discussion focused on string theory” but that “the structure of string theory is still so badly understood that it does not yet deserve to be described as a ‘theory’ at all — it is more a ‘work in progress’.” He describes the discussion at Solvay about anthropic explanations of the CC as “lively”, but that Polchinski et. al. have yet to carry the day: “there is a strong body of opinion that holds that it may be premature to decide which parameters are environmental within string theory, as the structure of the theory is still poorly understood and will surely hold further surprises.”

The same issue of Nature has an essay by Lawrence Krauss entitled Anthropic Fever where he starts by explaining the standard Landscape story that fundamental physics is really an “environmental science”, but then goes on to write:

*But I have been wondering whether things might actually be much worse. It could be that many different combinations of laws could allow life to form, and that it is a pure accident, not favoured by any particular probabilistic explanation, that the constants of nature result in the combinations we experience in our Universe. Or, it could be that the mathematical formalism is so complex that the ground states of the theory might not be mathematically determinable, even in principle.*

*Whether or not nature is ultimately ‘undecidable’ in this strong sense, these ideas point to a possible future for particle physics that is very different from the past. Fundamental physics might not be restricted by any underlying grand mathematical structure that would ‘explain’ why the Universe is the way it is. It’s a possibility that I hope will be wrong, but it’s a possibility nonetheless.*

This kind of worry seems to me completely misplaced. There’s not a shred of evidence for it, and the only reason people have been engaging in this kind of speculation is that string theory led them down this sorry path. Finding a trackless swampland at the end of the path doesn’t mean that physics is destined to spend the rest of eternity mucking about in a swamp, it is far more likely that a wrong turn was taken quite a ways back.

Last night a write-up by Nati Seiberg of his rapporteur talk at Solvay appeared at the arXiv, entitled Emergent Spacetime. It does a very good job of laying out the reasons that people often say that string theory suggests that our standard concept of “space” needs to be revised, that perhaps space is an “emergent” concept. Personally I don’t think the different reasons that Seiberg lays out add up to a very convincing case. For one thing, string theorists have been trying to come up with a new “stringy” version of space for twenty years now, without much success at all. They are still far from anything like a consistent proposal of what this new idea about space will be, and the various evidence given by Seiberg is rather incoherent, leading to different kinds of generalizations of space, not pointing to any one of them in particular.

The past two days there was a conference at Harvard on Black holes, topological strings, and invariants of holomorphic submanifolds, which also included some lectures in memory of Raoul Bott by Sir Michael Atiyah, S. T. Yau, and Dan Freed. Lubos has reports on two talks there, one by Robbert Dijkgraaf about a hoped for “Universal Wave Function”, a Hartle-Hawking kind of wave-function that would give the amplitude for the universe to be in various parts of the Landscape. The second was by Frederik Denef who spoke on “D-brane ground states, multicentered black holes, DT/GW correspondence, and the OSV conjecture [or: why OSV is probably right].” Lubos reports on a conversation with Frederik about his two forthcoming papers with Michael Douglas on computational complexity and the Landscape. It seems that what Denef-Douglas show is that, even if everything one would like to calculate is in principle calculable, the problem of identifying a specific string theory background realizing anthropically small values of the CC is NP-hard. This means that in practice you’ll never be able to do what landscapeologists would like to do: use the observed values of the CC and maybe some other standard model parameters to identify a tolerably small number of backgrounds, then use the properties of these backgrounds to make predictions. I believe it is this possibility that Krauss is alluding to in his quote above about how the “ground states of the theory might not be mathematically determinable, even in principle.”

**Update**: The last paragraph has been modified to better reflect reality. In its initial version I had assumed from Lubos’s blog entry that these computational complexity issues had been what Frederik’s talk was about.

**Update**: Polyakov also has a new preprint based on remarks at the Solvay conference, entitled Beyond Space-time. It’s a mixed bag, mostly about various ideas related to AdS/CFT, as well as the cosmological constant. He begins with critical but not completely dismissive comments about anthropic arguments:

*Another danger is to get distracted by non-dynamical anthropic arguments, which recently acquired some popularity. I find the anthropic principle irrelevant. It is unlikely to uncover fundamental ideas and equations governing the universe. But, in spite of these misanthropic remarks, I believe that in special cases anthropic arguments may be appropriate.*

At the end of the paper, he characterizes the various topics he has discussed as follows:

*As it is clear from the list of the references below, these ideas (except for the gauge/strings correspondence) did not attract any attention. Perhaps they don’t deserve it. My best hope, however, is that some of them may serve as small building blocks of the future theory, the vague contours of which we can
discern at the horizon.*

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I read Seiberg’s article in disgust: statements like “general covariance is a gauge symmetry” as if that were a complete argument for anything. He also claims to have some radical things to say, but I didn’t spot them. What were they?

Kea,

I found Seiberg’s argument that “some gauge theories have duals without gauge symmetry, so gauge symmetry is not fundamental. Since general covariance is a gauge symmetry it is not fundamental also” very unconvincing. But in any case, if you are going to abandon space, you don’t even know what you mean by “general covariance” anymore, so you are abandoning that too.

Not sure which he thought were the radical claims. Perhaps his two points at the end:

1. Maybe every quantum mechanical system is dual to some string theory, so you shouldn’t ask “what is string theory”, but ask “Which string theories have macroscopic dimensions” (where, presumably, the answer is “all sorts of them”…l)

2. If you can’t think about what it means to be smaller than the Planck length, you have to give up the idea of reductionism as involving more and more fundamental laws that operate at smaller and smaller distances.

Hi Peter,

Small correction: my talk was not about the computational complexity or any other feature of the landscape, but titled “D-brane ground states, multicentered black holes, DT/GW correspondence, and the OSV

conjecture [or: why OSV is probably right]”. After all, it was a conference about black holes, topological strings and invariants of holomorphic submanifolds, and indeed, I do have a life outside the landscape 🙂

As far as the computational complexity paper is concerned, stay tuned, it will appear soon…

Hi Frederik,

Sorry about that, will correct. I was relying on Lubos’s blog, didn’t realize he was reporting on a conversation with you, not your talk.

From Peter:

I wouldn’t be so quick to assume this. See Joe Henson’s new review

The causal set approach to quantum gravity(gr-qc/0601121). Also see John Stachel’s recent paper, Structure, Individuality and Quantum Gravity (gr-qc/0507078).It is quite an admission for Michael Green to assert that “the structure of string theory is still so badly understood that it does not yet deserve to be described as a ‘theory’ at all — it is more a ‘work in progress’”, given that he and John Schwarz have been working on string theory for over 30 years.

Kea – agreed. I just stop reading when such a statement is made completely out of context.

-drl

I predict that none of the spacetime destroying features of string theory that are described in Seiberg’s talk will be met with the derision that LQG’s doubly special relativity, etc., receive.

Speaking of LQG…someone is proposing what I guess is a concrete test of quantum gravity within that framework:

http://physicsweb.org/articles/news/10/2/2/1

I don’t know how reliable this news is, or if I’m misinterpreting, but it seems to suggest that some LQG folks are proposing looking for a humanly-observable feature of BH formation that could possibly falsify their theory…meaning it’s bona fide physics, at least potentially.

There are also some initial (negative) results for chaotic quantum foam models using quasar halos.

DB, if that article is right, all they have to do is observe a brief dimming during the final flash of a dying star. Since there are 10^12 stars in our galaxy, with an average lifetime of 10,000 million years, about 100 must die each year in the Milky Way alone. Automatic CCD observation programs (of the kind used by Perlmutter to observe the spectrum of very distant supernovae) could be adapted to observe these flashes. This is very interesting.

It’s excellent news that at last LQG is being taken seriously by some people. (Sorry Peter if this is off topic…)

The number of visible massive star collapses in our galaxy is less than one per hundred years, not one hundred per year.

Thanks for the link and some of your exposition on your blog, Christine. Perhaps it’s best to continue discussion over there so as to not get too far off topic.

Peter Woit replied:

This kind of worry seems to me completely misplaced. There’s not a shred of evidence for it, and the only reason people have been engaging in this kind of speculation is that string theory led them down this sorry path. Finding a trackless swampland at the end of the path doesn’t mean that physics is destined to spend the rest of eternity mucking about in a swamp, it is far more likely that a wrong turn was taken quite a ways back.His statement is also false, because we do have a good idea that

‘many different combinations of laws’WILL NOT‘allow life to form’… so‘the value of the constants’ARE‘favoured by any particular probabilistic explanation.So there is no evidence for anything that was quoted, and fundamental physics is therefore, more-likely to be restricted by an

‘underlying grand mathematical structure that would ‘explain’ why the Universe is the way it is.And don’t even tell me that my points aren’t relevant.

Please take discussion about conditions under which life forms to some other more appropriate forum. Virtually all the discussion along these lines that people keep posting here has little to do with the topics I’m posting about, and is just adding to the noise level.

” (Sorry Peter if this is off topic…)”

Dear Peter or Lubos or Urs,

Given we have some prediction for LQG of a dying star, what would string theories’ prediction be? Would it be similar to what LQG predicts or would it be different, and if different, how different? (see below)

We have LQG’s prediction. Does string theory predict a naked singularity as classical GR does, or does it predict something different? Even if string theory has problems with SUSY and landscape, could it offer a prediction regarding singularities as LQG has done?

Speaking of LQG…someone is proposing what I guess is a concrete test of quantum gravity within that framework:

http://physicsweb.org/articles/news/10/2/2/1

“However, in the final stages of a star’s collapse, the curvature of space-time becomes so large that classical general relativity theory no longer holds and quantum-gravity effects should take over. By applying the techniques of loop quantum gravity — a leading candidate for a quantum theory of gravity — Joshi and co-workers calculated that a dying star does not form a naked singularity but has all its mass thrown away in a flash instead. This burst has a characteristic signature: the star dims briefly before it rapidly radiates away to produce extreme energy gamma rays, cosmic rays and neutrinos. If this fingerprint were observed by astronomers, it might provide the first true observational test for quantum gravity.”

dan,

For the 1000th time: string theory predicts nothing at all about this, or about anything else.

As for the idea that LQG makes real predictions in this context, I think this is very unlikely.

The paper in question required a subscription, which I do not have.

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PRLTAO000096000003031302000001&idtype=cvips&gifs=Yes

Perhaps prediction is too stringent, but perhaps “qualitive expectations consistent with known string theory” is a more specific term in the scenario described in the paper, would it differ or be similar to what LQG predicts, or would it match what GR predicts (a naked singularity)

Lubos actually responded to this post on his weblog

“string theory predicts extra dimensions in the form of Kaluza-Klein modes of known particles; excited strings at very large energies; specific patterns of black hole evaporation; Lorentz-invariant physics; topology changing transitions near the Planck scale; forces different from gravity, and possibly particles associated with supersymmetry and grand unification.”

— not that i’m trying to start a flamewar or anything.

There is a new paper by Craig J. Hogan posted in the arXiv: Nuclear Astrophysics of Worlds in the String Landscape. The readers of astro-ph are mainly astrophysicists. I would say that a good fraction of them do not have any idea on the current debate on this landscape issue! I did not read the paper so I do not know whether the author includes a balanced comment on the fact that the landscape issue is quite an open problem. (Or, to some, a kind of dead end for string theory).

Christine just posted a pretty good reading list for Quantum Gravity.

Since the topic is “Various Somewhat Related Links”, and she didn’t mention it, I will

http://christinedantas.blogspot.com/2006/02/basic-curriculum-for-quantum-gravity.html

Re the two forthcoming papers by Denef and Douglas mentioned in Peter’s log entry, this evening Douglas posted

http://arxiv.org/abs/hep-th/0602072

Computational complexity of the landscape I

Frederik Denef (KU Leuven), Michael R. Douglas (Rutgers and IHES)

53 pp, 2 figures

“We study the computational complexity of the physical problem of finding vacua of string theory which agree with data, such as the cosmological constant, and show that such problems are typically NP hard. In particular, we prove that in the Bousso-Polchinski model, the problem is NP complete. We discuss the issues this raises and the possibility that, even if we were to find compelling evidence that some vacuum of string theory describes our universe, we might never be able to find that vacuum explicitly. …”