Quantum Gravity at Scientific American

Scientific American is doing a good job this month of putting out stories related to quantum gravity that actually make sense, steering clear of the multiverse and other pseudo-science. This month’s magazine has a very nice article by Steven Carlip about quantum gravity in 2+1 dimensions. For a more technical introduction to the subject, Carlip’s book and review article are good places to start.

I haven’t seen the new May issue yet, but from their web-site, it seems that their cover story on new ideas about “A Unified Physics” is what looks to be an interesting article from Zvi Bern, Lance Dixon and David Kosower: Quantum “Graviton” Particles May Resemble Ordinary Particles of Force, summarized as

Maybe unifying the forces of nature isn’t quite as hard as physicists thought it would be.

I’m curious to see the full article, but I assume it’s about the intriguing work on amplitudes of recent years that has shown that supergravity theories have fewer divergences than people thought, for reasons that are still unclear. There’s presumably some new symmetry structure here, and understanding it may offer a way around the old argument that “you can’t put quantum mechanics and general relativity together, the quantum fluctuations at short distances are just too violent.” Maybe you don’t need strings, M-theory, the multiverse, and all the other baggage theorists have been weighed down by for the last quarter-century. There has been quite a bit of discussion about this topic here, the earliest posting is this one from 2005. For another take on how these ideas might lead to a new way to handle quantum gravity, the latest visionary talk by Nima Arkani-Hamed from last week at the Simons Center is available here.

Also at Scientific American, George Musser has been producing some interesting blog entries on these topics. There’s a video here about the Carlip piece, a story about Darth Vader and the Emperor Palpatine that was discussed here, and a recent nice explanation of work on higher spin theories here.

Update: Also in the May issue, from Davide Castelvecchi, there is a shorter article, Is Supersymmetry Dead? with summary

The grand scheme, a stepping-stone to string theory, is still high on physicists’ wish lists. But if no solid evidence surfaces soon, it could begin to have a serious PR problem.

Peskin is still a believer though:

“It is the next step up toward the ultimate view of the world, where we make everything symmetric and beautiful,” says Michael Peskin, a theorist at SLAC National Accelerator Laboratory…

Many are still hopeful. “There are still very viable ways of building supersymmetry models,” Peskin says. Expecting to see new physics after just a year of data taking was unrealistic, says Joseph Lykken, a theorist on the CMS team.

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14 Responses to Quantum Gravity at Scientific American

  1. Vince says:

    I’m not, by any stretch of the imagination, an expert at any of this, but I heard it said that supergravity still needs a “UV completion” even if it is finite/renormalizable. And string theory is supposed to provide that UV completion. Is this necessary?

    Also, is this allegedly finite supergravity theory a supergravity theory that may describe our universe? That is, is this supergravity theory based on 4D spacetime, etc.?

  2. Proudmemberofthecult says:

    @ Vince

    There are two issues here:

    - does N=8 supergravity make sense as a perturbative quantum field theory? (is it finite or renormalizable)
    - what is the complete non-perturbative structure of this theory?

    Bern et.al. have something to say about the first question. The second question is very interesting as it has relevance for black hole physics and many other things. If your interest is in question 2, then it seems at present you need superstring theory to give a well-defined answer. However, we do not know the (full) answer to the second question even for Yang-Mills theories! As far as *perturbative* finiteness is concerned superstring theory is irrelevant.

    Moreover, if N=8 would make sense perturbatively (be finite or renormalizable), there’s no reason why it shouldn’t make sense also non-perturbatively. This however is very much a handwaving argument.

  3. Peter Woit says:

    Vince,

    Yes, this is 4d supergravity. In principle it could describe our universe. It doesn’t obviously provide an interesting unified theory though (in the sense that it doesn’t explain the standard model or make solid LHC predictions). What’s interesting about this though is that it raises the possibility that there are new ways of dealing with QG and making unified theories, you don’t need completely new, different degrees of freedom.

    I’ve repeatedly seen the argument made that “OK, maybe supergravity is alright perturbatively, but there’s problem X non-perturbatively, which is solved by string theory”. But we know little about supergravity non-perturbatively, and definitely don’t have a viable non-perturbative string theory, so any arguments of this kind seem to me less than solid. But, again, even if N=8 supergravity has some problem as a viable unified theory non-perturbatively, the interesting thing is that its unexpected perturbative consistency raises all sorts of new possibilities, opening back up an area of research that had been considered closed off.

  4. Speculative says:

    I’m no expert so I’m only speaking from a standpoint of analyzing possible reactions, but if N=8 Supergravity is finite, it probably won’t dampen string theory research. In my opinion it will probably be a boon to ST because supergravity is a low energy limit of ST. Correct me if I’m wrong, but if a supergravity theory is finite, wouldn’t the ST UV completion also be finite? Besides string theory does still hold the promise of a unified theory in a way that sugra doesn’t.

    I’m not making any judgements on the merits of the theories themselves, I just think this will be a more likely reaction among the HET/String community.

  5. Peter Woit says:

    Speculative,

    If N=8 supergravity is finite, that doesn’t fix any problem with string theory that anyone was worried about, quite the opposite. It removes the whole “ultraviolet completion” motivation for string theory, since it says that N=8 supergravity already is “ultraviolet complete”. Yes, N=8 supergravity does not now provide a viable unified model, but the interesting thing here is that we may be learning something new which could affect our understanding of the possibilities for such models. I go on far too much about this, but I think it has been clear for a while that string theory is a failure on the unification front. The possibilities inherent in string theory for unification have been investigated to death for more than a quarter century, leading to the dismal current state of affairs.

    At this point though, having the main motivation for string theory removed isn’t likely to convince many string theorists to head for the lifeboats. If the landscape didn’t do that for someone, there’s no reason not to believe they’re not willing to go down with the ship.

  6. Giotis says:

    Peter,

    I didn’t know that you are promoting N=8 supergravity these days. This is a pleasant surprise from someone who don’t believe in SUSY.

  7. Peter Woit says:

    Giotis,

    I “don’t believe in SUSY” in the sense that I don’t believe that the standard version of SUSY is a symmetry that appears in nature. It’s not completely mathematically compelling, one doesn’t see it explicitly, and breaking it, even spontaneously, leads to a useless mess. However, there are some very interesting mathematical structures involved in SUSY, which likely do have something to do with the real world. An example of the kind of thing I have in mind is Witten’s use of “twisted N=2 SUSY” to produce topological quantum theories. Maybe there’s some other way of “twisting” SUSY to get something more interesting.

    N=8 supergravity doesn’t give a viable unified theory, but, if it shows that there is a class of QFT’s which give quantized gravity without non-renormalizability problems, I’d love to know why this happens, and how large the class of such QFTs is.

  8. SA says:

    I am surprised that you have not discussed the recent article from Kane, attempting to shift the perception of string theory towards it being a theoretical framework, like vanilla QFT and not necessarily a theory of particle physics.

  9. Peter Woit says:

    SA,

    I’ve discussed Kane’s claims quite a lot here, probably more than is a good idea since no one takes them seriously. The “string theory is just a framework, like QFT” excuse for failure has been around a long time, also often discussed here.

  10. Giotis says:

    OK Peter but the thing is that you are facing a contradiction here. All these novel approaches (like those in Musser’s posts) which apparently you find very interesting are more or less related to String theory research. They are investigated and promoted by String theorists working within the general context of String theory and become important due to that.

    Amazingly String theory incorporates every promising idea in high energy theoretical physics and soon you won’t be able to talk about anything without referring to the general String theoretic framework.

    In fact this general framework would become so large that practically would be synonymous to high energy theoretical physics.

    This ship is unsinkable…

  11. Peter Woit says:

    Giotis,

    “This ship is unsinkable…”

    I don’t doubt that whatever idea has some success, string theorists will insist it is “string theory”, since there will be some connection of some sort between it and at least one of the tens of thousands of “string theory” papers written over the last 30 years. At this point, Strings 20XX mostly has nothing much to do with string theory. Where this was going was clear back at the time of the following quote, way back in 2005

    “Most string theorists are very arrogant,” says Seiberg with a smile. “If there is something [beyond string theory], we will call it string theory.”

    Whatever. If a great discovery is made that gets us beyond the SM that will be wonderful, no matter how many string theorists will insist on saying it is “string theory” to make themselves feel better about the wasted last quarter-century.

  12. Aleksandar Mikovic says:

    String theory is not the only known theory which provides an UV completion of quantum General Relativity. Spin foam models of quantum gravity are another example, and it has been recently shown that the EPRL-FK class of spin foam models has representatives whose classical limit is General Relativity, see
    arXiv:1104.1384, Effective action and semiclassical limit of spin foam models,
    A. Mikovic and M. Vojinovic,
    Journal-ref: Class. Quant. Grav. 28, 225004 (2011).

  13. sheepdip1 says:

    > I haven’t seen the new May issue yet, but from their web-site, it seems that their cover story on new ideas about “A Unified Physics” is what looks to be an interesting article from Zvi Bern, Lance Dixon and David Kosower:
    The article is out now:
    http://www.scientificamerican.com/article.cfm?id=search-for-new-physics

    Any comments?

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