41. Peter Woit: What will you do if string theory is wrong?

My advice to you is to stay as far away from the string wars as possible. Getting tangled up with such discussions will soak up your time and make you feel dirty afterwards.

I have to confess (although it is painful) - about 5 years ago I was mostly reading Peter Woit?s blog, wanted to debunk string theory and was really embarassed by Lubos Motl?s rants on Woit :-) But sooner or later any person who is reading physics related blog starts trying to figure out what is the physical content of the post, what is the information it contains, since rants are rants, emotions are emotions, but we are really interested in CONTENT, aren?t we?

Anyone who stays on in high energy physics eventually comes to such a realization. Unfortunately, I’ve seen a number of prospective graduate students, who spent their undergraduate days as avid readers of Woit’s blog, and whose perspective on high energy physics is now so hopelessly divorced from reality that the best one can do is smile and nod one’s head pleasantly and say, “I hear the condensed matter group has openings.”

Fascinating stuff on the 3D Ising model. All I know of string theory is what I learned in Barton Zwiebach’s class for undergraduates, plus some bits and pieces I’ve picked up stochastically since, so this sort of thing is always exciting.

And since I never pass up an opportunity to make my name associated with stupid questions, here goes:

A while back, I was browsing the papers of Hartnoll, Kovtun et al. (arXiv: 0704.1160, 0706.3215, 0801.2785), trying to figure out this business of 2D CFTs having gravity duals in AdS_3. (I see a fair number of systems at critical points in the “complex systems” stuff I do during the day, so any hints about new calculational or conceptual tools are interesting.) In $latex d = 2$ spacetime dimensions, the central charge of a CFT is equal to the “normalized entropy density” (using the conventions of Kovtun and Ritz, 2008). This means, I gather, that AdS/CFT is applicable (their main thesis is that $latex c = \tilde{c}$ holds in the $latex d \geq 3$ CFTs which admit a gravity dual). A quantum CFT with one dimension of space and one of time can be reinterpreted as a classical theory in two spatial dimensions, with the path integral becoming the classical partition function. Does this mean, then, that any classical statistical system in two spatial dimensions described by a CFT will have a gravity (plus other junk) dual? That seems suspiciously optimistic to me, which is why I think I’m missing a point somewhere.

Dear Blake

Thanks for visiting!

1) Maybe I have to specify that they are talking about classical statistical systems with 2nd order phase transition exactly at T=T_c. Only there dynamics is described by CFT.

2) The Kovtun-Ritz statement is the following: if c=tilde(c), then CFT probably admits gravity dual (in odd d, as they believe, condition c=tilde(c) is necessary and sufficient in order for CFT to have gravity dual, in even d it is only necessary condition). There are examples of CFTs which do not admit gravity dual in the paper: one such example is O(N) sigma model, where c/tilde(c) is close to one but not exactly one.

Did I answer your question?
Cheers,
Dmitry.

Dear Dmitry, thanks for your refreshing comments.

Not Even Wrong, the blog, started in March 2004 so it is not yet 5 years old although I have interacted with the owner of that blog for many years before he began with the extremely unfortunate blog.

I am very skeptical about Jacques’ general recommendation to “stay away from the string wars”.

You know, this might be said by people who have nothing to lose - they are insured against it - and who perhaps don’t care too much whether younger and future generations of this world will continue with the quality search for the basic laws of Nature or whether they will degenerate into champions of cheap, anti-scientific ideologies and slogans. “A flood can come after me,” or whatever is tne English version of this proverb. And they usually like to live in their ivory towers that are isolated from the real world.

The misinformation and defamations propagated by those “critics”, if you want me to be extremely polite here, have had and still have a huge negative impact on the future of high-energy theoretical physics, an impact that could be easily reversed if certain people cared and didn’t prefer their “clean hands”. Moreover, I don’t really think that they should really have any “clean feelings” because their passivity helps to build these very dirty pyramids of lies. In this sense, they are a part of this problem. Jacques’ inability to make his blog relevant in comparison with Woit’s blog is a problem, too. Much like all of us, they are players in the current “Troubles With Physics”.

Concerning your Ising models. It seems to me that people like Woit won’t question that there are some valid mathematical observations within string theory that may be relevant for gauge theories or the Ising model or whatever. They are just hysterically dreaming about a string theory that would die as a unifying theory of gravity and other forces, to be removed as a coherent discipline that can be legitimately studied by specialists as a compact whole relevant for physics beyond the Standard Model and beyond classical general relativity. In my opinion, one of the key properties of string theory that have been firmly established is the fact that it is completely coherent, unified, and it is really impossible to accept some parts of it and not others.

Once we believe the AdS/CFT duality for that N=4 d=4 gauge theory - a theory that is (and whose siblings are) arguably relevant to understand the real world - you are in type IIB string theory where gravity automatically arises (in the same qualitative form as in general relativity) and that inevitably contains all the branes, black hole states, moduli spaces, and transitions that can get you to any place of the string theory configuration space that is studied in stringy papers. Let me say it again and differently. We understand how gravity actually arises from some quantum physics we know - gauge theory - and the details how gravity arises in quantum physics can be studied in a detailed fashion and one reconstructs the whole string theory. There is no correct way to make a “cut” dividing string theory into acceptable or inacceptable pieces. The people who propose something like that are analogous to creationists who try to divide mutations into the legitimate, small, natural ones that can occur in Nature - and the large ones that require a God. Of course, no boundary like that can ever be drawn and if you look where various creationists have drawn it, they have pretty much filled the whole continuum, proving that there is no boundary. :-)

One more comment, about the paper that made Woit publish this particular rant that you replied to. The author argued that if string theory is wrong, it should be studied in new kinds of departments. This is a purely sociological question. I don’t know what are the rational rules that lead people to establish departments of “Physics and Astronomy” or “Mathematics and Statistics” (I studied at a department of mathematics in physics in Prague, and this union seems absolutely natural to me, in contradiction with the irrational Woit-like bias trying to invent a huge gap between the two disciplines) and why some subdisciplines are sometimes presented as full disciplines. If string theory were proven wrong, it could obviously not be studied by physicists-specialists. Whatever would remain valuable would be recycled. Because the whole string theory is a compact theoretical structure, it really means that everything would be recycled by someone who looks at the thing (still) carefully enough. It would probably be more sensible to study it in the maths departments. But the mathematicians would still need pretty much the same physics training as string theorists need today. The structure of knowledge one has to learn to investigate string theory wouldn’t really change by such a falsification. It would still be a body of knowledge that requires a physics training, besides mathematics.

If someone found a better theory that would solve the same problems as string theory but was incompatible with it, which I view as comparably likely to a new arrival of Jesus Christ, I think that physicists who would otherwise study string theory would clearly switch to this new hypothetical theory. There’s no doubt about it. Those who discovered the new one would become famous - maybe more famous than any string theorists. Still, they would find themselves using hundreds of results in theoretical physics that have already been established. Many of them are tightly connected with string theory.

The future of HEP was put in stake time ago, when it was linked to the future of Planck scale. GUT theories get some of the blame, the scale of GUT being an step beyond of the scale of Planck. But string theories did the real mistake, branching out HEP from particle physics and driving it to be considered, both by the general public and by a large part of the scientific community, as a competitor in the arena of general relativity.
I have not doubt that HEP could induce the Planckian scales, either by Renormalisation Group up to meet the three coupling constants, or by a seesaw between the mass of the neutrino and the mass of the higgs scale. In fact it is amusing we have two or more ways to climb up, not only one. There are some different ways to climb up a mountain, but to postulate that we are already up in the peak and we only need to climb down, it is not a good approach.

In physics/0001033, one year before Woit’s criticism, I wrote “But we have been mistaken, wronged, delayed. The world has tricked, outraged, raped us. When we have been wronged, should we not to revenge?”. At that time my rant was a general one, against politics and policy of science, probably inspired in Bretch’s Galileo. But when, next year, Woit released physics/0102051 I felt I agreed. Yep, since 199x I had suffered both the new rise of string theory and the ability of stringers to change topic when I asked them to produce the particles of the standard model, and I felt that Woit was showing it to be a general problem and asking for Vendetta. But when asking who the tenured Parsbjerg could be today, I can not quote a name. Of course, not Witten, who is really the only one keeping the head on particle physics. Perhaps it is a collective thing, “the system”; thus nobody to sink a dagger in.
Nowadays I add other aspect to the claim of ST being “not even wrong”: that even if it is right (and probably some variant of string theory will be right), it has not been properly and rightly researched. Any real advance will come from particle physics, and we had come to it independently of the man-centuries wasted during the last 20 years.

Dear Mr Rivero,

it is quite incredible but you are fundamentally wrong about every single statement you make. I am amazed how someone can fail to grasp these very basic facts about physics after many years in which he apparently tried to follow physics from the distance (probably too huge a distance).

First of all, the reality of the Planck scale wasn’t invented by string theorists but it was discovered by Max Planck roughly 100 years ago. It is simply true that the Planck scale is the ultimate scale below which the usual concepts about geometry have to break down. And even the people who don’t understand any details about quantum gravity, such as the proponents of “loop quantum gravity”, but who are interested in the general field realize that the Planck scale is the ultimate scale where new rules have to take over. This fact doesn’t depend on any specific features of string theory: Planck didn’t know these features, after all. Since the time when Planck realized these things, people have studied physics at many more accessible scales.

But the reason why cutting-edge physics no longer studies phenomena below 100 GeV is that these phenomena have been understood. Some people still study old-fashioned aspects of QCD or whatever and it is interesting in some ways but it is simply not cutting-edge physics. On the other hand, it is not true that string theory studies the Planck scale all the time. There is a lot of scales in it, and especially when one looks at string phenomenology, there are all types of scales one can derive from the underlying equations. Of course, the physics at scales below 100 GeV must agree with what we already know.

Second of all, string theorists have never “branched” anything from particle physics. String theory is at the very heart of particle physics, at the very core of 90+ percent of valuable results that have been found in high-energy theoretical physics in the recent decades, and it has been holding this status for more than quarter a century. There don’t seem to be any hints that this situation will change, at least not in a foreseeable future. Anyone who was led to believe anything else is just an irrelevant brainwashed layman. As Jacques tells us, this category clearly includes many people who are nominally physics grad students (unusable for theoretical physics) but it doesn’t change the fact that these people - just like you - have no clue whatsoever about the actual status of high-energy physics and string theory in it.

The statement that string theory includes general relativity is not a result of some “marketing strategy” as you indicate. Instead, it is a fact about Nature and it is clearly a critical fact. You may dislike it and write silly comments about it but it is the only thing you can do against the facts of Nature. Moreover, it is not just string theorists who study gravity. Gravity is simply the “next force” in particle physics that is (or was) waiting to be fully understood and incorporated into the picture of particle physics because others have already been included. Not only every string theorist, but every good phenomenologist also cares about gravity as described by general relativity and its generalizations. Whoever thinks that physicists who try to work on new things can ignore or should ignore gravity and general relativity is, once again, a brainwashed irrelevant layman, if I have to be extremely polite here.

The approach that you say “is not good” is the standard top-down approach that has led to roughly 1/2 of the key results in the history of physics, including special and general relativity. Einstein didn’t try to refine kinematics of spacetime at higher and higher energies, to follow the bottom-up approach. He directly looked at the top and derived how things have to behave when velocities are directly around the speed of light itself: he only used very general postulates, logical reasoning, and consistency constraints. The effects of special relativity at lower velocities are very small and, indeed, we must “climb down” to determine what they are. The same thing applies to general relativity. Its father(s) didn’t try to refine Newton’s theory at increasingly strong gravitational fields. They directly wrote the correct equations that hold all the way up to black holes with the strongest gravitational fields one can have. It was also a top-down approach. The top-down approach of string theory is absolutely analogous.

Your comments that you have been raped (like Galileo) is incomprehensible to me but your comments that one must tell you a list of masses of particles reflects your complete misunderstanding of the state of affairs. At this moment, this calculation is simply impossible - both in string theory and, even more obviously, outside string theory. Only crackpots claim otherwise. Your insisting that someone must answer questions that simply cannot be answered at the present level of knowledge shows that you are an unfortunate, uninformed layman. But the fact that some questions cannot be answered - by string theory or anything else - doesn’t mean that nothing can be studied. A lot of things can be studied very well and a lot of questions can be answered and are being answered. That’s what scientists are doing. They are answering questions that can be answered, not questions that can’t.

Your comment that string theory “wasn’t properly researched” even if it is right is an amazing example of an outsider’s collapsed mental abilities - a person who can only produce ad hominem attacks but cannot ever do anything useful. If you think that you can do research in physics - or even better than the real physicists, right? - why don’t you do it instead of the defamations and crackpot preprints of yours? Science is investigated in the way that is allowed given the finite brain and other resources available on this planet with 6+ billions of people.

Cheers
Lubos

Hi Lumo! Funny you mention the masses. It is true that some of my crackpotty papers are worried about masses but it is not a general pattern: some others are worried about charges, and some other about gauge forces. But here in the post I told about situations when “asked them to produce the particles of the standard model”. Not masses.
That was in the period 1993-1998; perhaps I was unfortunate about the sample of graduate students I asked to.
Phenomena below 100 GeV have been understood? Well, I would say that most of them have been understood by calculus, and some of them have been understood as coincidences. The number of the later ones is, to my taste, embarrasingly high; but mileage may vary. Let me recall some, from my preprints: put appart, if you wish, Koide’s and de Vries’s observations, for which I am only humble reporter. But let me name Z0 decay, which depends only of SU(2)xU(1), happens to have the same rate that J/Psi and that the neutral pion, the later ones come from the anomaly and are related to QCD. Or the mass of the pion itself, which comes from QCD, but is a few percents off from the mass of the muon, which comes from yukawa coupling to the Higgs (and then from the Electroweak scale, unrelated to QCD). Think of the pion as a QCD string… the number of different QCD “neutral pions”, based on quark content, is 9+4, almost equal -some U(1) to be ruled out- to he number of neutral fermionic degrees of freedom, 12. The number of different “charged pions” for +1 charge, is 3*2. The number of charged +1 fermionic degrees of freedom is 6. The number of charged -2/3 diquarks is also 6, as well as the number of charged +1/3 diquarks. So the QCD string reproduces the standard model content.

Special Relativity did direct contact with the finiteness of lightspeed and with Lorentz transformations. General Relativity did straiht contact with newtonian gravity and Mercury trajectory. But Planck-scale string theory allows it self to do only approximate contact with GUT theories, which in turn do only approximate contact with HEP models.

Dear Alejandro,

when I wrote “masses”, I clearly meant both masses as well as coupling constants, mixing angles, and all other low-energy parameters. Masses of elementary fermions are determined by Yukawa couplings, after all.

I am telling you that these things cannot be understood by any argument based on effective physics below 100 GeV - almost by definition. It is because we know that the effective theories such as the Standard Model work equally well with different values of masses, couplings, and mixing angles. The only way how they could be determined is to study physics at shorter distances or higher energies - the internal structure of the elementary particles, if you will. That’s why physics at higher scales is so important. It is the only discipline that can tell us something about the details of effective theories that look arbitrary.

Just like the spectra of atoms could be understood once the internal architecture including electrons was taken seriously, the strucutre of masses etc. can be understood if we properly take into account their stringy/M origin. Or, if you want to be speculative, another theory that would replace string/M-theory. But you cannot possibly understand these numbers “directly”. When it is done properly, there is some fundamental physics at a higher scale - perhaps GUT scale or Planck scale - where the calculations of parameters are simpler. But they must be extended by the renormalization group flows to the bottom. Top-down approach is absolutely essential for these things and if someone wants to understand parameters of the Standard Model by the bottom-up approach, he cannot possibly succeed.

Your identification of muon with pion (or other QCD strings) reminds me of the 1930s when people talked about mesotrons - both pion and muon - because they were not yet able to distinguish very different types of particles of similar masses. Muons and pions can have comparable masses but this proximity is pure coincidence - there is nothing statistically shocking about some ratios of many masses to be relatively close to one - and detailed properties show that muons and pions are extremely different. When I was 10 years old, I also didn’t know anything about particle physics except for properties of particles listed in tables such as the mass. Well, today I know slightly more. It allows me to see that your “prophesies” about the “miracles” are pure and worthless numerology.

Best
Lubos

Dmitry,

Thanks, that helped. The O(N) sigma model lives in three dimensions; are there examples for d = 2, or does conformal symmetry preclude them?

Yes, but string theory does direct contact to quantum gravity as an effective low-energy quantum field theory, with the correct classical limit.

hep-th/0602112 makes the point that we really have two scales in cosmology, Planck’s, and lets call Einstein’s. To consider the 10^19 GeV scale more important that the 10^-3 eV scale is just an historic artifact.

Hi Blake

Oh, sorry, I belive I did not help you really, your question was deeper than my answer. Now I understand what you asked.

The answer to your question is no.

1) There are examples of CFTs which do not correspond to string theories at all (I recall LCFT - logarithmic conformal field theories but there should be much more examples around). The reason is that string theory is more constraining than CFT. For example, the sum of central charges should be zero in order for CFT to correspond to string theory (string theory experts, am I correct here?), and you clearly don’t have that in the general case.

2) Let us consider CFTs corresponding to string theories. Even though c=tilde(c) in 2D for them, they do not necesserily admit gravity dual (I was unable to recall examples though). This does not contradict Kovtun-Ritz proposal since they, as I wrote, expect the condition c=\tilde(c) to be necessary but not sufficient in even dimensions for the CFT to admit gravity dual.

3) I also wanted to emphasize that condensed matter is actually so much reacher than CFT. I should correct my statement in the previous comment: exactly at T=T_c correlation length becomes of the order of the size of the system, and condensed matter “lattice” degrees of freedom (spins in Ising model for example) admit description in terms of QFT. This QFT has generally some non-zero beta function which also may have zeros. They correspond to RG fixed points; when the theory reaches them, it can be decribed in terms of CFT. There are many examples of theories with (everywhere) non-zero beta functions. One example immediately comes in mind: take O(N) in critical dimension d=2 (theory with second order phase transtion and vector order parameter). It is asymptotically free everywhere and beta-function is always non-zero. The theory has a single phase (with non-zero mass gap).

4) Also, when you say “gravity+other junk”, remember that it is 3D gravity without dynamical degrees of freedom (basically equivalent to Chern-Simons).

Cheers,
Dmitry.