This week in Honolulu there’s a major particle physics conference going on, a joint meeting of the Division of Particles and Fields of the APS, and the Japanese Physical Society, called the Joint Meeting of Pacific Region Particle Physics Communities. Slides from the talks have started to appear here.
The conference is huge, with hundreds of talks (for some reason, attending a conference in Hawaii seems appealing to many people), and I haven’t had time to look at more than a few of them. Barry Barish gave a talk on international cooperation in HEP, Dave Schmitz one about the status of MiniBoone, which is “in the endgame” of a blind analysis of their neutrino experiment, with the black box containing their results to be opened in the not too distant future.
Lots of talks about string theory, including a plenary talk by Polchinski partly about AdS/CFT and attempts to use it to get information about QCD, partly about the landscape and string vacua. There was also a remarkable talk by Wati Taylor entitled Can String Theory Make Predictions for Particle Physics? Taylor begins by noting that “If we could do experiments at greater than 1019Gev, answer would probably be Yes”. “Probably” is different than the usual claims about this… His summary of the current state of string theory and particle physics goes like this:
- String theory need not make predictions for particle physics below 100 TeV
- We can’t define string theory yet
- The number of suspected solutions is enormous, and growing fast
- Nonetheless, constraints on low-energy physics correlated between calculable corners of the landscape may lead to predictions
- If not, probably need major conceptual breakthrough to have any possibility of predictivity for low-energy particle physics
- Raison d’etre for string theory: quantum gravity
I don’t know why he chose 100 TeV here, presumably just because it is probably an upper-bound on the likely energy scales particle physicists will be able to explore during the lifetimes of anyone now living. He could just as well have picked a much higher number. The only hope he sees for getting any kind of prediction using current versions of string theory is by finding correlations between things like numbers of generations and gauge groups when you examine large numbers of string vacua (this is similar to the conclusion reached by Michael Dine, described here). In work with Michael Douglas, he has found no evidence for this. Taylor also explains that the standard 10500 number often given for the number of string vacua seems to be a dramatic underestimate, and that it is even quite possible that the number is infinite when one takes into account non-geometric compactifications. Fundamentally, his conclusion seems to be that there is only a vanishingly small hope remaining of getting any predictions about particle physics out of string theory, so it has to be sold purely as a theory of quantum gravity, unless a miracle happens.
Taylor does make the case that string theory has found potential uses not in unification, but in studying strongly coupled gauge theory (AdS/CFT) and in suggesting new structures to try out in model-building. But at this point, he characterizes low energy physics predictions from string theory as unlikely, their appearance would just be an “unexpected bonus”. So, I guess the answer to the question of his title is basically “No”. Despite this, he does end by advertising the String Vacuum Project and listing the 17 prominent theorists who are asking the NSF to fund it.
Taylor has an excellent summary of the state of string theory on his webpage at
http://web.mit.edu/physics/facultyandstaff/faculty/washington_taylor.html
He starts off by saying “String theory is currently the most promising candidate for a framework in which to understand quantum gravity. It is still not possible, however, to define string theory in a space-time background compatible with the physics we see around us, and string theory cannot yet be used to make specific predictions”.
Peter,
(slightly offtopic)
Was there ever any “landscape” type thing in the days of supergravity, before string theory (ie. late 70’s and early 80’s)?
(I looked up some old supergravity review papers the other day, and couldn’t find much resembling the “landscape” silliness).
Due to a happy cumulation of circunstances, including or starting from the fact of the mass of the tau lepton being 1.78 GeV, we happen to have one of the outsiders of physicsforums giving a talk during a paralell session there. He went to Polchinski’s, and did a brief comment here
I agree that string theory is currently the most promising candidate for a framework in which to understand quantum gravity; it keeps promising and promising, but never delivers.
One of the speakers is blogging from the conference here:
http://www.physicsforums.com/showthread.php?t=140402
Kea and Alejandro,
If people want to discuss this speaker’s talk, they’re encouraged to do so with him over at PhysicsForums.
JC,
No, there never was anything like the Landscape back in the 70s or 80s. If anyone had suggested working on classes of models that complicated and unpredictive I doubt anyone would have taken them seriously.
No, there never was anything like the Landscape back in
I don’t think people knew how, or had the motivation, to study non-susy vacua back then. Maybe the thrust in that direction came from the definitive astronomical evidence that the CC is not zero.
After the talk by Washington Taylor , one of the questioners started out her query with “I noticed that you avoided using the “A” word …”
Of course my mind leapt to the possibilities. Antimatter? Axion? Anomaly? Asymmetry? Atheism? Astronomy? Angular momentum? Algebra? Accuracy? Adultery? Ab initio? Aardvark?
Apparently the locals knew what was being talked about and the author, dared by the questioner, used the word “anthropic” in his response. I guess that that’s a bit of a dirty word among the stringers. This (parallel) talk was well attended with standing room only.
The Polchinski (plenary) talk had two questions. The first was a request to comment on Susskind’s alleged suggestion that we could be at the end of physics in that we are at the “end of the reductionist paradigm”. The speaker wisely avoided commenting on Susskind, but said he himself does not have a paradigm, and that serious cosmologists have been worrying about this for >30 years, that is, what is it that allows life.
The second question was about the abilitiy of a large extra dimension allowing solving of the hierarchy problem. He speculated that maybe someone in the audience knew the answer to this but he didn’t call on me by name (LOL), and mentioned the embedding of 5-D theories in 10-D.
For some mysterious reason Susskind equates the lack of predicitivity with giving up reductionism. Giving up reductionism in standard sense (reducing physics to Planck length scale) need not mean lack of predictive power. For instance, a loss of reductionism in the sense that there exists a hierarchy of scaled up variants of standard model realized as dark matter hierarchy, does not mean the loss of predictivity since fractality allows precise predictions using scaling arguments.
The additional bonus is that there is no need to continue repeating that there is no empirical input to guide the theoretician. Consider only biology: it serves as fantastic gold mine of effects if one accepts the possibility that biology is something more that mere complexity. I dare guess that the historians of physics will see the dogmatic belief in reductionism as the deepest reason for the recent crisis in theoretical physics.
A potentially useful result that is contrary to conventional wisdom of the physics community appeared in the Hawaii DPF meeting in a section on Low Energy Tests of the Standard Model.
A contribution by Goran Senjanovic entitled “Grand unification and proton decay: fact and fancy” has an abstract that stated:
“… I review the minimal grand unification based on SU(5) and SO(10) groups, with and without supersymmetry. I discuss the predictions for the proton decay and show how they depend crucially on the fermion (and sfermion) masses and mixings. …”.
Since the conventional view has been for years that proton decay experimental observations have ruled out SU(5) GUT,
and since I saw no full copy of Senjanovic’s Hawaii contribution,
I looked up his arXiv postings, and found hep-ph/0204311 by Borut Bajc, Pavel Fileviez Perez, and Goran Senjanovic with an abstract that stated:
“… We systematically study proton decay in the minimal supersymmetric SU(5) grand unified theory. We find that although the available parameter space of soft masses and mixings is quite constrained, the theory is still in accord with experiment. …”.
A couple of years later, coauthor of hep-ph/0204311, Pavel Fileviez Perez, wrote a paper (with Ilja Dorsner) at hep-ph/0410198 whose abstract stated:
“… We investigate model independent upper bounds on total proton lifetime in the context of Grand Unified Theories with the Standard Model matter content. … Our result implies that a large class of non-supersymmetric Grand Unified models, with typical values alpha_GUT = [about] 1/39, still satisfies experimental constraints on proton lifetime. …”.
In an even more recent paper, hep-ph/0601023, Pran Nath and Pavel Fileviez Perez say in section 5.6:
“… In this section we discuss the possibility of finding an upper bound on the total proton decay lifetime … one may focus on the gauge d = 6 contributions since all other contributions can be set to zero in searching for upper limits …
any non-supersymmetric theory with alpha_GUT = 1/39 is eliminated if its unifying scale is bellow 4.9 x 10^13 GeV regardless of the exact form of the Yukawa sector of the theory.
Further, a majority of non-supersymmetric extensions of the Georgi-Glashow SU(5) model yield a GUT scale which is slightly above 10^14 GeV.
Hence, as far as the experimental limits on proton decay are concerned, these extensions still represent viable scenarios of models beyond the SM. …
For example in a minimal non-supersymmetric GUT based on SU(5) the upper bound on the total proton decay lifetime is …[less than or equal to]… 1.4 x 10^36 years …”.
If non-supersymmetric SU(5) and SO(10) GUT models ARE consistent with experimental observations,
and
if N=8 supergravity might indeed be finite ( see the thread in this blog citing the UCLA workshop on 11-15 December 2006 described at http://www.physics.ucla.edu/tep/workshops/supergravity/index.htm )
then
maybe they could be useful in building a physically realistic unified theory/model.
Tony Smith
http://www.valdostamuseum.org/hamsmith/
PS – The proton decay ideas of Senjanovic, Perez, et al seem to be independent of the ideas of Adarkar, Krishnaswamy, Menon, Sreekantan, Hayashi, Ito, Kawakami, Miyake, and Uchihori expressed in hep-ex/0008074 that experimental backgrounds in proton decay experiments may have been incorrectly defined, and that different (and possibly realistic) background definitions would render experimental results to be consistent with SU(5) and SO(10) etc GUTs. If Adarkar et al are also correct, then the case for such GUTs is even stronger.
PPS – The UCLA workshop abstract says in part: “… The intimate connection of N=8 supergravity to N=4 super-Yang-Mills theory will also be discussed. …”.
As is well known, quaternionic structure is key to the special properties of N=4 super-Yang-Mills theory,
and
N=8 supergravity has octonionic structure that may be useful in studying finiteness of it and related physics models.
“The first was a request to comment on Susskind’s alleged suggestion that we could be at the end of physics in that we are at the “end of the reductionist paradigm”.
It’s not the end of physics, just the end of Susskind.
100 Tev may come from the ultra high energy cosmic rays. their energy can be as high as 10^{19} electron volts, or above, which means a center-of-mass energy 100 Tev when they hit on protons in the air.