October 06, 2006

Zeitgeist String Era

Bert Schroer has written a very interesting essay (the kind I've been hoping to write some day ;) String theory and the  crisis in particle physics. A historical overview of quantum theory is also given, back from the 1920's. Making use of the clarity of style of the author I give a few quotes of his text as a warm-up for a potential reader.  Mainly from the introduction, to avoid mentioning any names of people involved ;).

"For the present particle theorist to be successful it is not sufficient to propose an interesting idea via written publication and oral presentation, but he also should try to build or find a community around this idea. The best protection of a theoretical proposal against profound criticism and thus securing its longtime survival is to be able to create a community around it. If such a situation can be maintained over a sufficiently long time it develops a life of its own because no member of the community wants to find himself in a situation where he has spend the most productive years on a failed project. In such a situation intellectual honesty gives way to an ever increasing unwillingness and finally a loss of critical abilities as a result of self-delusion."

"I would like to argue that these developments have been looming in string theory for a long time and the recent anthropic manifesto [1] (L. Susskind, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design) (which apparently led to a schism within the string community) is only the extreme tip of an iceberg. Since there has been ample criticism of this anthropic viewpoint (even within the string theory community), my critical essay will be directed to the metaphoric aspect by which string theory has deepened the post standard model crisis of particle physics. Since in my view the continuation of the present path could jeopardize the future research of fundamental physics for many generations, the style of presentation will occasionally be somewhat polemic."

"An age old problem of QFT which resisted all attempts to solve it is the problem of existence of models i.e. whether there really exist a QFT behind the Lagrangian name and perturbative expressions. Since there are convincing arguments that perturbative series do not converge (they are at best asymptotic expressions) this is a very serious and (for realistic models) unsolved problems. The problem that particle physics most successful theory of QED is also its mathematically most fragile has not gone away. In this sense QFT has a very precarious status very different from any other area of physics in particular from QM. This is very annoying and in order to not to undermine the confidence of newcomers in QFT the prescribed terminology is to simply use the word ”defined” or ”exists” in case some consistency arguments (usually related in some way to perturbation theory) have been checked."

"These problems become even worse in theories as string theory (which in the eyes of string protagonists are supposed to supersede QFT). In this case one faces in addition to the existence problem the conceptual difficulty of not having been able to extract characterizing principles from ad hoc recipes. Unlike in renormalizable QFT there exists up to date no n-th order proof that string theory is free of renormalization parameters."

"Since there was no operator formalism in which the underlying ideas (invariance, unitarity, crossing, maximal analyticity) could have been implemented,  the problem of constructing a crossing symmetric, unitary, maximally analytic S-matrix was ill-formulated. Tinkering with properties of Beta functions and their representation in terms of Gamma functions, Veneziano was able to construct the first model for an apparent crossing symmetric elastic scattering amplitude. His proposal did not satisfy unitarity, but the realization that his on-shell prescription allowed an auxiliary field theoretic description in terms of a (off-shell) two-dimensional conformal field operators theory and that it also admitted an auxiliary presentation in terms of the canonical quantization of a classical relativistic Nambu-Goto string Lagrangian contributed significantly to its theoretical attraction. It also nourished the hope that the model can be unitarized in a later stage. Its main popularity it however enjoyed among strong interaction phenomenologists who actually (for reasons which nowadays hardly anybody remembers) liked the idea of satisfying crossing already with infinite towers of intermediate particle states, duality = crossing among with (infinitly many) one-particle without the participations without the participation of the multi-particle scattering continua as would be required for an S-matrix coming from QFT. In conjunction with the phenomenological use of ideas of Regge poles, the emerging trajectory pictures (mass versus spin) had a certain phenomenological charm, and although infinite particle towers cause some field theoretic headache, there was no reason to reject it as a phenomenological proposal which captures some aspects of strong interactions and to worry about those conceptual problems later."

"But this never happened; the dual model became a phenomenological orphan and after its string-theoretic completion, by the time it was presented as a TOE (this time including gravity), the conceptual problem behind the crossing-duality relation was forgotten, and a chance to understand something about conceptual relations of string theory to QFT was (presumably permanently) lost. Particle physics these days is generally not done by individuals but by members of big groups, and when these big caravans have passed by a problem, it will remain in the desert. A reinvestigation (naturally with improved mathematical tool and grater conceptual insight) could be detrimental to the career of somebody who does not enjoy the security of a community."

"In its new string theoretical setting its old phenomenological flaw of containing a spin=2 particle was converted into the ”virtue” of the presence of a graviton. The new message was the suggestion that string theory (as a result of the presence of spin two and the apparent absence of perturbative ultraviolet divergencies) should be given the status of a fundamental theory at an energy scale of the gravitational Planck mass ∼ 10^19 GeV i.e. as a true theory of everything (TOE), including gravity. Keeping in mind that the frontiers of fundamental theoretical physics (and in particular of particle physics) are by their very nature a quite speculative subject, one should not be surprised about the highly speculative radical aspects of this proposals; we know from history that some of our most successful theories originated as speculative conjectures. What is however worrisome about this episode is rather its uncritical reception. After all there is no precedent in the history of physics of a phenomenologically conceived idea for laboratory energies to became miraculously transmuted into a theory of everything by just sliding the energy scale upward through 15 orders of magnitudes and changing the terminology without a change in its mathematical-conceptual setting."

Finally, from Conclusions:
"In this essay I emphasized that, as recent progress already forshadows, the issue of QG will not be decided in an Armargeddon between ST and LQG, but QFT will enter as a forceful player once it has conceptually solidified the ground from where exploratory jumps into the blue yonder including a return ticket can be undertaken."

"The problem is not that there are no other games in town, but rather that there are no bright young players who take the risk of jeopardizing their career by learning and expanding the sophisticated rules for playing other games."

By the way, I wrote my (modest) thesis on five point Veneziano phenomenology.
Now, probably back to break (due to ailing health).

September 23, 2006

Having Break

This blog continues to be inactive for some time.

May 17, 2006

Preons: Generation Number from Space Dimension

The preon model, ie. that quarks and leptons and some other particles would be composites of more elementary constituents, has been under consideration for some thirty years. These days only really few people seem to give any attention to preons. On the other hand, the motivation of the preons has been in grand unification schemes, and even in heterotic superstring theories.

Any preon model would need some novel features with respect to QCD: like understanding why the masses of quarks and leptons are very much smaller than their inverse sizes. O.W. Greenberg proposes in a recent paper A Schematic Model of Generations a possible solution to this and other problems of preon models. He introduces for the preon interaction the group [SU(3) x SU(3)]b. The preons of his model are

  • 6x6 of [SU(3)xSU(3)]b hexon: spin ½ color, lepton, flavor, helicity,
  • 3x3 generon: spin 0, [SU(3)xSU(3)]b degrees of freedom, and
  • [SU(3)xSU(3)]b bindon: gauge boson
The author requires the generations to be gauge group single-centered singlets. He finds three composites that meet the requirements hg-g- , hggg- , and hgggg. For the first generation the antigeneron repulsion leads to linear quasi-one dimensional configuration, and for the second generation the (anti)generon repulsion to triangular quasi-two dimensional configuration, and for the third generation to tetrahedral quasi-three dimensional configuration. As simple as that!

The author associates the dimensionless factor L/a, where L is the large linear scale of the generations and a is small linear scale, with the mass ratios between the generations, so that the mass scales of the successive generations are proportional to La2, L2a, and L3, respectively. He finds L/a to be about 100.

Generation mixing, dark matter (gg-) and some generic predictions of preon models, like higher spin particles, are also considered. But the author postpones the discussion of the weak bosons and the Higgs mesons or their substitutes to a later paper. As well as a few other questions.

May 03, 2006

Towards the ILC?

I have been quite confused, suppose like many others, of the developments in string theory getting acceptance by prominent physicists for the
landscape ideology. Therefore it felt good to read the news about one of the best known string "Wunderkinds" Brian Greene going to address the Congressional Research and Development Caucus Advisory Committee in DC on May 9, 2006. His talk is titled "Reaching for Einstein's Dream: The Quest for the Deepest Laws of the Universe."

The Director of DOE's Office of Science, Raymond L. Orbach, will present the new High Energy Physics Advisory Panel publication "Discovering the Quantum Universe," a companion volume to the "Quantum Universe." The new document explains the outstanding discovery opportunities at the Large Hadron Collider and the proposed International Linear Collider. Both publications will be available at the meeting and can be downloaded at http://interactions.org/quantumuniverse/. As Lubos Motl has recently shown the need for the ILC is well indicated ;-)

Though the LHC is beginning operations next year, I understand the above activities as necessary preparations to go experimentally beyond the Standard Model.

(Sent by mobile mail)

April 20, 2006

Inventors in the Landscape

Bert Schellekens has written a note The Landscape "avant la lettre" on the history of string theory landscape as he has experienced it. Whatever one's opinion of string theory and the landscape in particular, nightmare or brightware, it is pretty safe to say that most of us have had such periods as Schellekens, at some time, perhaps with less grandiose problems. In the golden 1960's, George Zweig must have been very frustrated.

After some leisurely thinking, I accept, for the moment, the landscape may be a logical possibility after all. It would imply a radically new (less attractive, fuzzy) paradigm for fundamental physics. It may even have some support from the arguments of no boundary initial condition theory, as discussed by S. Hawking and T. Hertog. Look also here and here.

I quote parts of Schellekens' paper, first from the introduction:

    "In 2003 L. Susskind published his paper entitled “The Anthropic Landscape of String Theory” [1], which I read with great pleasure. The reason was that, many years before, I had come to the conclusion that everything we knew about String Theory was pointing towards an “anthropic landscape” of vacua. I had advocated this idea consistently during many years, on the basis of far less evidence than we have today. It seemed obvious to me, but the response I got was frustrating."

    "I have considered many times writing a paper about my ideas, but I could not bring myself to write something that seemed so obvious. It would have been a paper with many words an no formulas, and it was not at all clear where to publish it. There were no blogs, no homepages, no “arxives”, and no obvious journal to send it to. And furthermore the hard evidence was not available yet. One valid objection was that we did not know enough about string theory yet to make any claim about the existence of a large number of non-supersymmetric, stable vacua. Today that is still an often-heard objection. My feeling about that was exactly what was written in [1], “if we find one such vacuum we are going to find a huge number of them”. To me that was clearly the message String Theory was sending us already in 1986, but most people preferred to ignore it, although of course everyone agreed that the number of supersymmetric vacua was huge."

    "It is difficult and dangerous to claim with the benefit of hindsight that one arrived at some conclusion at a particular time. After 1986 it took some time to understand that all four-dimensional string vacua that were proliferating quickly had moduli, and that this was an important problem. It took some time to appreciate that they were all related to each other, and could be thought of as ground states of one theory, the Heterotic string. And of course there were five string theories, not just one. The most common attitude was to ignore the others and assume that one day we might know what was wrong with them. This was also my point of view until 1994, but uniqueness of the underlying theory was anyhow not the most crucial part of the argument. Apart from that issue, I was already defending a point of view quite similar to the one expressed in the 1998 speech during my time at CERN, which in any case means before 1992. I had discussions about that with many people and encountered a lot of resistance, and I do not recall anyone wholeheartedly agreeing with me. It is therefore somewhat strange that after 2003 some people started telling me “this is what I have always been saying”."

    "Most people associate the Landscape with an anthropic solution to the cosmological constant problem. For me that was not the crucial issue. Even if we came to the conclusion that the cosmological constant was not anthropically tuned after all, I would still expect an anthropic landscape for the structure and parameters of the standard model. String Theory was the first and so far only theory that made the question about uniqueness of the standard model unavoidable. Most, if not all, other attempts to “derive” the structure of the Standard Model involve a new layer of gauge theories, for example composite models or GUTs. Then on inevitably runs once again into the same problem one tries to solve, namely an essential non-uniqueness. The most promising candidate, SO(10) Grand Unification, requires an unexplained triplication of families and has a large number of parameters, even if one takes this choice of gauge group for granted. The only hope for uniqueness is a theory that itself has a chance of being unique, namely a theory of gravity. Such a theory, String Theory, was explored during the past decades, and it gives a very clear answer: there is no unique ground state, but a landscape of vacua. It is in my opinion the only answer that makes sense, and the fact that this answer came out of String Theory is a sign that we are on the right track. I see this as a fundamental result that may even survive if String Theory turns out to be incorrect, or if String Theory is just the tip of an iceberg."

    "I expect that the String Theory Landscape will acquire an important place in science history. Of course its ultimate fate depends on the correctness of String Theory, and the unexpectedly huge size of the landscape is making it a lot harder to convince ourselves of that. But String Theory won’t be correct without the landscape being correct. And if that is true, it would be one the most fundamental discoveries one can make. It implies that we would know the answer to Einstein’s question if the creator of our Universe had any choice: indeed, we would know all the choices. This insight is probably the most important one we have obtained from string theory so far. It should be remembered that in 1984 this would have been completely unthinkable. Unlike the other main result we hope to get out of String Theory, consistent Quantum Gravity, the landscape emerged against everyone’s initial expectations and wishes. It is a revolution that is unfolding so slowly that few people even recognize it as such. But nevertheless, discovering that our standard model is just one entity in a huge landscape, and hence cannot be completely derived from first principles, is a paradigm shift for our field."

From the original talk, translated into English:

    "This line of thought fits in very well with a series of insights that pointed out our modest place in the cosmos. Our planet is not the center of the solar system, our sun is just one of many stars and not even a very special one, and the same is true for our galaxy. It seems natural to assume that also our universe, including the quarks, leptons and interactions we observe is just one of many possibilities."

    "This way of thinking has important consequences. If indeed our universe, including its laws of physics and the entire Standard Model is just one of many possibilities, this implies that there are limits to what we can compute. The properties of the quarks and leptons, their interactions and the parameters of the Standard Model (or at least part of them) were fixed at the birth of our universe, when a choice was made out of the many possibilities. We will never be able to compute that choice, because it could just as well have been different."

    "I have the impression that many of my colleagues believe or hope that this will ultimately not be the case. They hope to find a kind of mathematical formula that has only one solution. That single solution should then correspond to our four-dimensional world, including all quarks, leptons and the four basic forces. Also the values of the nineteen (or more) parameters, such as the masses of all particles, should then ultimately emerge as the outcome of a mathematical computation."

The final paragraph:

    "The foregoing was a sketch of a possible end of the story. It is the end that given the current state of affairs seems the most desirable to me, but in the end only hard results matter. Nature will probably not care much about my wishes. Despite the word “end” in the title it was not at all my intention to suggest that the end will be reached soon. On the contrary, it will take many decades of work to produce a complete map of String Theory. I am looking forward to an exciting continuation of this adventure."

[1] L. Susskind, “The anthropic landscape of string theory” arXiv:hep-th/0302219.

Finally, remember that some string and other theorists retire sooner, some later ;-)

April 04, 2006

Cosmological Constant and the String Landscape at Solvay 2005

This is a rapporteur's talk at the 23rd Solvay Conference by Joe Polchinski, the article is dated March 31, 2006. (I feel it is the best paper of the year until now.) The author divides theories of the cosmological constant (CC) into two groups: value fixed by theory and value adjustable like in string theory landscape. The central issue in the report is to discuss the extent to which physics is predictable. Polchinski sees three major questions in the CC: why it is not large, not zero, but comparable to the matter density now. He focuses mainly in the first question: "this is hard enough!"

The fixed value theories are discussed comparing the problem to Lamb shift (main topic in the 1947 Shelter Island conference). The author leads the reader through the Lamb shift in the presence of external gravitons discussion to short and long distance modifications to gravity. There is no solution visible in those directions.

In the adjustable scenario, several possibilities are mentioned: unimodular gravity, non-propagating four-form field strengths, scalar potentials with many minima, rolling scalar with nearly flat potential, spacetime wormholes, self-tuning, and explicit tuning (one or more free parameters). Among the mechanisms leading to the observed CC value the following are discussed: the Hartle-Hawking wavefunction favoring the smallest positive value of CC, the de Sitter entropy suggesting that the HH wavefunction has some statistical interpretation in terms of the system exploring all possible states, and the Coleman-de Lucchia amplitude for tunneling from positive to negative CC vanishing for some parameter range keeping the universe in the state of smallest positive energy density. Polchinski considers all these tantalizing in the same way as supersymmetry is as a solution for the CC problem. These mechanism would work in an empty universe. Next, I quote "In the course of trying to find selection mechanisms, one is struck by the fact that, while it is difficult to select for a single vacuum of small cosmological constant, it is extremely easy to identify mechanisms that will populate all possible vacua — either sequentially in time, as branches of the wavefunction of the universe, or as different patches in an enormous spatial volume. Indeed, this last mechanism is difficult to evade, if the many vacua are metastable: inflation and tunneling, two robust physical processes, will inevitably populate them all.

But this is all that is needed! Any observer in such a theory will see a cosmological constant that is unnaturally small; that is, it must be much smaller than the matter and energy densities over an extended period of the history of the universe. The existence of any complex structures requires that there be many ‘cycles’ and many ‘bits’: the lifetime of the universe must be large in units of the fundamental time scale, and there must be many degrees of freedom in interaction. A large negative cosmological constant forces the universe to collapse to too soon; a large positive cosmological constant causes all matter to disperse. This is of course the argument made precise by Weinberg, here in a rather minimal and prior-free form.

Thus we meet the anthropic principle. Of course, the anthropic principle is in some sense a tautology: we must live where we can live. There is no avoiding the fact that anthropic selection must operate. The real question is, is there any scientific reason to expect that some additional selection mechanism is operating?

If there is a selection mechanism, it must be rather special. It must evade the general difficulties outlined above, and it must select a value that is almost exactly the same as that selected by the anthropic principle, differing by one order of magnitude out of 120. Occam’s razor would suggest that two such mechanisms be replaced by one — the unavoidable, tautological, one. Thus, we should seriously consider the possibility that there is no other selection mechanism significantly constraining the cosmological constant. Equally, we should not stop searching for such a further principle, but I think one must admit that the strongest reason for expecting to find it is not a scientific argument but a psychological one: we wish fundamental theory to be as predictive as we have long assumed it would be."

That's correct, next comes a discussion of the string landscape. And a chapter "What is String Theory?" This is very interesting, of course. But the best I can do is to recommend everybody to read Polchinski's talk! Final quote from the end: "Let me close with a quotation from Dirac: One must be prepared to follow up the consequences of theory, and feel that one just has to accept the consequences no matter where they lead. And a paraphrase: One should take seriously all solutions of one’s equations. Of course, his issue was a factor of two, and ours is a factor of 10^500."

March 09, 2006

Surface Paradigm for Dark Energy

There has been indications that the dark energy and dark matter have their origin in general relativity, with some extensions. T.Padmanabhan discusses dark energy as the Mystery of the Millennium (an optimist would hope a decade or two). Dark matter discussion is given in the references of this paper.

The author assumes the dynamics of gravity follows from an approach which uses only the surface term of the Hilbert action for the continuum. Gravity has thermodynamic origin in this approach. The surface term's variation is related to entropy TdS and the matter term's variation gives PdV and dE terms. The entire variation is equivalent to the known equality between these. The main point I quote and condense:

Within the true theory of quantum gravity, measurable quantities should be the fluctuations in the vacuum energy and not the absolute value of the vacuum energy. The cosmological constant is most likely a low energy relic of quantum gravitational effects, arising from the surface term alone. This procedure is applicable to a large class of theories, including Gauss-Bonnet type actions in higher dimensions (superstrings). This suggests that the mechanism for ignoring the bulk cosmological constant is likely to survive quantum gravitational corrections which are likely to bring in additional, higher derivative, terms to the action. In a universe with two length scales L_Lambda (proportional to the Hubble radius) and L_p, the vacuum fluctuations will contribute an energy density of the correct order of magnitude r_DE =√rho_IR rho_UV, according to the author.
Addendum (more refs.)