Yet More News

26 Responses to Yet More News

1. Lee Smolin says:

   October 24, 2014 at 2:53 pm

   Dear Peter,

   If the conferees want an answer to the question “Are there any plausible alternatives to string/M-theory as a fundamental theory of physics?” they ought to invite some of the people contributing to the development of such alternatives and have an honest examination of the question. They have plenty to choose from, they could start with the speakers at Loops 13, which 200 people attended: http://www.pirsa.org/C13029.

   Fifteen or twenty years ago it was possible to say that string theory was more promising than its alternatives, and indeed 15 years ago I switched my research from LQG to M theory. But I think that any objective evaluation of the evidence has to credit much more substantial progress has taken place since concerning the alternatives; especially spin foam models, but also other background independent approaches including shape dynamics, CDT, group field theory and tensor models, etc.

   To support this I would point to the fact that much more is known from spin foam models about the challenges it faced 15 years ago, including substantial recent results on the emergence of GR in the semiclassical limit, the entropy and temperature of generic black holes, the elimination of cosmological and black hole singularities, finiteness, etc.

   So, I would urge my string theory friends who think they know the answer to the question to have another look. There is a whole generation of brilliant young theorists working on alternatives to string theory you should meet!

   Thanks,

   Lee
2. Marty Tysanner says:

   October 24, 2014 at 6:11 pm

   The Horgan/Shears/Tong debate was interesting, but I found the some of the reasoning (especially by Tong) to be wanting. A few examples:

   1. Mathematical consistency as a pillar of science (experiment being the other): Tong argued the internal consistency of string theory should be taken as significant evidence for it even without experimental evidence. The obvious problem he ignored is that mathematical/logical inference is only useful in physics if the starting physical assumptions are correct. Even if string theory were completely consistent mathematically, to justify his argument Tong would need to show that its underlying assumptions are uniquely implied by observations, a task that isn’t humanly possible. (One can’t exhaust all possible alternatives — that requires unlimited creativity and time to propose and carefully work out all possible theories and their consequences.) Experimental evidence of deduced consequences of his favored theory is mandatory to help validate its initial assumptions — “pure thought” doesn’t cut it.

   2. Tong argues for the proposal of atoms in the 18th century as an example of “pure thought” that only later was shown true experimentally. Atoms were proposed as a possible way of explaining concrete observations — the idea was driven by unexplained observations, not “pure thought.” Is Tong suggesting that any newly proposed model that attempts to explain new phenomena, but only later is supported observationally, supports a conjecture that his favorite quantum gravity theory will someday have concrete observational evidence specific to that theory?

   3. Dark energy. The moderator tried to point out the distinction between excellent observational evidence of accelerated expansion of the Universe, and the theoretical nature of conclusions that it is due to dark energy. Tong insisted the acceleration must be caused by dark energy because GR predicts the acceleration if dark energy is inserted (i.e., as a cosmological constant in Einstein’s equations); therefore “dark energy” is not just a theoretical conclusion. He overlooks that, while observations are consistent with a c.c., they don’t demonstrate (yet, at least) that the acceleration is uniform everywhere and that therefore dark energy is the only plausible cause. The Lambda-CDM “concordance model” hasn’t yet attained an observational status that makes it unassailable…
3. Art Brown says:

   October 24, 2014 at 6:52 pm

   Thanks much for the link to the Festival Albertine video. I had hoped that the topic of “style” might form a bridge across the chasm between hi-level math and the rest of us, but that organizing principle damped out in the first half hour or so (to re-emerge, it is true, in the final question on the math “lab” vs solo work).

   I was surprised that Dr. Villani appeared to downplay the role of intuition in guiding his work; I had thought that quite important, and look forward to his book to learn more. (I suppose that also qualifies as “style”, so maybe the topic was more successful that I at first thought.)

   Two trivial notes:
   1) Was that a large spider lapel pin I glimpsed from time to time on Dr. Villani’s jacket?
   2) Microphone technology seems stuck in the 20th century: if you aren’t within licking distance, it’s useless.
4. Yatima says:

   October 25, 2014 at 6:30 am

   Unrelated and Classical: In the area of “Amazingly Correct Pop Science”, Kip Thorne has been tasked by Warner Brothers to come up with a visually correct Black Hole. Disregard the default “the feels” music:

   Interstellar – Building A Black Hole – Official Warner Bros.
5. amused says:

   October 25, 2014 at 9:49 am

   I expect that alchemy had a good degree of `mathematical consistency’ too, back in the day when heavyweights such as Isaac Newton and Robert Boyle were working intensely on it.
6. David says:

   October 25, 2014 at 12:05 pm

   Dear Peter,

   Can we find the slides or videos of the workshop of string cosmology and inflation at Princeton?
7. Peter Woit says:

   October 25, 2014 at 12:07 pm

   David,
   Sometimes someone posts slides and/or videos of talks at these PCTS workshops after they’re over, sometimes not. It’s still going on, so, wait and see. Most likely they would be posted at the workshop website
   http://pcts.princeton.edu/pcts/StringOpen2014/StringOpen2014.html
8. Jesper says:

   October 25, 2014 at 5:27 pm

   Dear Lee

   could you provide links/references to the recent results you mention on emergence of GR in a semi-classical limit?

   Thanks
9. Lee Smolin says:

   October 25, 2014 at 6:36 pm

   Dear Jester,

   There are many papers which discuss the emergence of GR from spin foam models; here is a small selection of them, in no particular order.

   http://arxiv.org/abs/1108.2258, http://arxiv.org/abs/1304.5626, http://arxiv.org/abs/1109.6538, http://arxiv.org/abs/1308.4063, http://arxiv.org/abs/0809.2280, http://arxiv.org/abs/0902.1170

   Thanks, Lee
10. vmarko says:

    October 26, 2014 at 10:03 am

    Jesper, Lee,

    Just for completeness, there are also some criticisms regarding the way GR emerges from spin foam models (and ways to get around that). See for example

    arXiv:1307.5352, arXiv:1104.1384, arXiv:1111.2865, arXiv:1201.2187.
11. Bob says:

    October 26, 2014 at 1:03 pm

    Dear Lee,

    Thanks for those paper references. They look interesting. I have a question for you. LQG, spin foams, etc., are characterized as a quantum theory of pure gravity, without matter and matter interactions. Can you discuss the ways in which Loop Quantum Gravity addresses the criticisms that quantum gravity needs to unify gravity with the other interactions because gravity cannot be decoupled at Planck scales from all other interactions (if there are any) at Standard Model, and higher, energy scales?
    Thanks very much!
12. Oregonator says:

    October 26, 2014 at 2:03 pm

    I can’t make sense out of Nash’s 4th order gravity equation. The Lagrangian is that of conformal gravity except that he has replaced the 1/3 by a 1/2. Why ?
13. Lee Smolin says:

    October 26, 2014 at 2:16 pm

    Dear Bob,

    Actually there is a moderate sized literature on matter coupling and unification within LQG. To summarize, let me emphasize that there are several different questions.

    1) It is just not true that “LQG, spin foams, etc., are characterized as a quantum theory of pure gravity, without matter and matter interactions.” From very early in its development, it was worked out in detail how to include gauge fields, fermions and scalars in LQG. For example, see Thomas Thiemann’s book.

    2) Extension to supergravity is also straightforward, at least for N=1, including D=11. See papers by Yi Ling and myself as well as recent papers by Thiemann and collaborators,

    3) Hence, even if LQG doesn’t constrain the matter or gauge field content of what it is coupled to, LQG provides a framework in which matter gauge fields and gravity are all coupled together.

    4) There are issues that need to be addressed with fermion doubling which are the subject of work in progress with Jacob Barnett.

    5) You can ask in addition whether, “quantum gravity needs to unify gravity with the other interactions”. This from a LQG point of view is an open question.

    6) But you can ask if LQG gives a compelling framework for unifying the different interactions? Here the answer appears to be yes. The simplest possible extension of the theory, gotten by extending the gauge group of gravity, which is the Lorentz group, to a larger group, G, yields a natural extension of Einstein-Yang Mills. This comes about from a built in spontaneous breaking of the gauge group G to the product of the Lorentz group with a compact factor H, which becomes the Yang-Mills gauge group. See arXiv:0712.0977, older papers by Peldan and later papers by Krasnov et al. See also arXiv:1212.5246 which explores implications for a unification of the electroweak interactions with gravity.

    7) Going beyond all these solid results are indications that the chiral fermions of the standard model may emerge from topological excitations of spin network states, arXiv:hep-th/0603022.

    Clearly there is much still to do in this direction, but these are sufficient to assure us that LQG provides a framework within which to describe matter and gauge fields coupled to quantum gravity which can suggest hypotheses as to further unification.

    Lee
14. Noboru Nakanishi says:

    October 26, 2014 at 9:40 pm

    Dear Peter,

    SUSY is an unnatural symmetry when gravity is taken into account, because the local version of translation group already include Lorentz group. Indeed, in the Einstein gravity, the local Lorentz group is completely disconnected from general coordinate transformation group. This situation cannot be resolved by considering supergravity, in which the Poincare symmetry becomes internal symmetry and therefore quantum supergravity necessarily becomes inconsistent with SUSY in the sense that the anticommutator between two supersymmetry generators cannot yield the translation generator.
    I think it more natural to supersymmetrize the local Lorentz symmetry alone. Since its Lie algebra is sl(2, C), its natural “super”extension is the orthosymplectic superalgebra osp(N,2; C).
15. Marc says:

    October 27, 2014 at 1:33 am

    Kaku still keeps on spreading the message (to the public, unfortunately) that string theory is the only game in town.
    https://www.youtube.com/watch?v=fV1eAAvAQVc (4:50)
16. Aleksandar Mikovic says:

    October 27, 2014 at 5:46 am

    Dear all,
    The exact statement regarding the classical limit of a spin-foam model is that it is given by the area-Regge theory (see arxiv:1104.1384) whose geometry is different from the the usual Regge discretization of General Relativity (these geometries are known as the twisted geometries, see arxiv:1308.0040). A natural way to solve this problem, as well as the problem of coupling of fermionic matter to a spin-foam model, is to introduce explicitly the edge lengths into a spin-foam model, which can be done generalizing spin-foam models by replacing the symmetry group (Lorentz group) with a 2-group (Poincare 2-group), see arxiv:1110.4694 and arxiv:1302.5564.
17. Bob Teller says:

    October 27, 2014 at 11:52 am

    Kaku discussing the LHC and its latest discovery, “The Higgs boson is included in string theory.”
18. Shantanu says:

    October 27, 2014 at 5:47 pm

    Peter, something OT
    A talk by Tom Kibble on history of EW symmetry breaking
    https://cds.cern.ch/record/1956423?ln=en
    First time I have hard a talk by Kibble
19. David Metzler says:

    October 27, 2014 at 6:40 pm

    Roughly on-topic: I notice that my recent Phi Beta Kappa newsletter has a quote from member Brian Greene: “The boldness of asking deep questions may require unforeseen flexibility if we are to accept the answers.” One may reasonably ask just how much flexibility is appropriate, however.
20. Peter Woit says:

    October 27, 2014 at 7:24 pm

    David Metzler,
    Google tells me that quote is from Brian’s 1990s “The Elegant Universe”, and in context refers to the unintuitive nature of quantum mechanics and relativity. The “unforeseen flexibility” there isn’t the dubious flexibility about what’s science and what isn’t currently in vogue among some string theorists.
21. Cormac says:

    October 28, 2014 at 4:45 pm

    Hi Peter, I too enjoyed the Gates article, thought he hit the nail on the head.
    How did you manage to give a link to a PW article without password issues? I had an article in the September issue of PW myself, but couldn’t show it to colleagues in the US, am I missing something?
    Regards, Cormac
22. QG says:

    October 28, 2014 at 7:17 pm

    Yet More News
    Posted on October 24, 2014 by woit
    On the other coast, today and tomorrow at Princeton there will be a workshop on string cosmology and inflation. They have a list of questions to be addressed, including

    Are there any plausible alternatives to string/M-theory as a fundamental theory of physics?
    …
    Does string theory make any cosmological predictions? Does it exclude anything?
    ^

    it’s October 28, 2014 and the workshop was Oct 24.

    has anyone seen this Princeton workshop on string cosmology and inflation? what is this workshop’s answer to the question

    Are there any plausible alternatives to string/M-theory as a fundamental theory of physics?
    …
    Does string theory make any cosmological predictions? Does it exclude anything?

    regards
23. Peter Woit says:

    October 28, 2014 at 7:41 pm

    Cormac,

    Not sure where I saw a link to the Gates article. I couldn’t actually find it on Physics World. No special knowledge about linking to them…
24. David Metzler says:

    October 29, 2014 at 12:57 am

    Peter—I appreciate the clarification on the Greene quotation, it sounds much more reasonable now. It’s a shame that it is so hard to convey to the general public the difference between the rigorously tested but counterintuitive notions of quantum mechanics and relativity, on the one hand, and the dubious claims of (some) string theorists, on the other.
25. Hendrik says:

    November 3, 2014 at 3:46 pm

    Some recent hype that may be worth noticing:
    http://www.eurekalert.org/pub_releases/2014-11/uosc-sft110314.php
26. Peter Woit says:

    November 3, 2014 at 4:11 pm

    Thanks Hendrik,
    That required its own blog posting…

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