The KITP in Santa Barbara is having a conference in honor of its 25th anniversary on the topic of “The Future of Physics”. Some of yesterday’s talks are already online. I’ve been watching Weinberg’s talk on “Where do we Stand?” this morning (a commenter also wrote in a little while ago while I was watching to recommend it). Weinberg gives a good summary of the present state of conventional wisdom about particle theory. He goes over the standard arguments that the standard model should be thought of as an effective low energy theory, and that doing so explains many of its features, with the two big exceptions of the scale of the vacuum energy and the electroweak symmetry breaking scale.
He promotes his “prediction” of the cosmological constant, and recalls that supersymmetry is the standard way of dealing with the low electroweak scale or hierarchy problem. But he then explains the problems with all known ways of breaking supersymmety, concluding that “no satisfactory theory of supersymmetry exists, where supersymmetry breaking is accounted for in the framework of particle physics.”
As for the “Landscape”, he notes that Gross hates it, says that “I don’t love it”, that it’s a disappointment, but one that we may have to get over. He makes some extensive comments about string theory, saying that it has had a history of advances leading to momentary optimism, but ultimately disappointment, with the bottom line that after 20 years we understand string theory much better but are no closer to contact with physics. He ends his comments about string theory with a rather weird remark that maybe it is wrong to look for a “guiding principle” behind string theory, that all there is to it is that it is the only way of extending the standard model to include gravity in 4d. I guess he is implying that string theory is not a fundamental beautiful theory, but, like S-matrix theory just a general framework imposed by consistency.
In general, Weinberg sounded to me old, tired and discouraged. Like just about all the leaders in the field, he refuses to publicly acknowledge the obvious possibility that the explanation for why string theory doesn’t predict anything or have any known fundamental principles is that it is just a wrong idea. He’s so discouraged about string theory that he has stopped working on it himself for the last fifteen years, but doesn’t have the energy or optimism to envisage any alternatives. He ends his talk with some real downers, one of which he calls the “LHC nightmare”, that the LHC will just see a single new scalar particle and nothing else. The second nightmare is that observations of the CMB will never see anything that tells us more about the early universe, just a 1/f spectrum, no evidence of the effects of gravitational waves.
All in all Weinberg ended up not giving a very optimistic view of the “Future of Physics”, but something closer to John Horgan’s argument about the “End of Physics”.
Tomorrow there will be a panel on “Field Theory and Mathematics” which should be interesting. Also, Witten will be talking on the “Future of String Theory”. It will be interesting to see if he is any more optimistic than Weinberg, and more specifically if he’ll come down on the Gross (“I hate it”) or Weinberg (“I don’t love it, but maybe it’s right”) side of the Landscape issue.
In Mlodinow’s book “Feynman’s Rainbow”, in chapter 13 he described an encounter he had with Feynman when he tried to bring up the topic of string theory. It was kind of funny, with Feynman saying things like:
“…. This whole discussion is pointless! It’s getting on my nerves! I told you – I don’t want to talk about string theory!”
“My take? My take is that you hit a dry spell, and now you’re scrambling, trying to find something to work on!”
“What’s wrong is coming to me to talk about string theory!”
This encounter happened in late 1981, when hardly anybody really cared about string theory other than guys like John Schwarz and Michael Green.
Yes … sad, but true, even this desperate antidote of physical models that needed less than ten dimensions could not save the great man.
JC,
OK, I do not know what Feynman did after he stopped coming to office; but his final office blackboard had stuff about the Baxter model and other 2-D exactly solvable models.
Arun,
There was a book “Feynman’s Rainbow” by Leonard Mlodinow which mentioned the same folklore in the last chapter on p. 166 (in the hardcover version)
” … He was now weak, in pain, and often depressed. But physics still brought him vigor. He continued to teach a course on quantum chromodynamics. And, in his last months of life, he finally decided to learn string theory. Murray taught him, in a private ‘seminar’ they held each week.”
I don’t personally know Mlodinow nor do I know any of his “sources” for this particular piece of folklore about Feynman finally giving in and learning string theory. By the time I heard about this particular piece of folklore, it may very well have been a 2nd or 3rd hand account that took on a “life of it’s own”. Without mentioning any particular names, my sources were former colleagues who were in the particle theory group at Caltech around the time of Feynman’s death. At the time, I just took their word for it.
“From some folklore stories I vaguely recall, allegedly Richard Feynman finally gave in and decided to learn string theory in the last few months of his life in 1987-1988.”
As far as I know, that is not true. You might ask Sandeep Trivedi about it.
Lubos intoned:
I apologize, but your possibilities 1,2 are just plain stupidities especially if you think that one of them is likely. Both of them are silly. What you’re saying that contradicts the results of 20,000 papers or so. We know that a theory that unifies these things exists, and it is most likely unique because its consequences for any question that we have been able to answer were unique.
Obviously, arguing by paper volume, “reductio ad cellulosum”, is the stupid thing. There were no doubt 20k papers by the LMs of Lorentz’ day stating why the deformable electron “was posited by God to make the ether perfect”. And it’s not at all obvious that GR and QM are “unified” because their individual structures are utterly different (at the moment). You must find an encompassing structure to make such a statement, and neither you nor any other stringers, have.
There was never any non-dynamical freedom in string/M-theory found ever. If something can be adjusted about your theory, you can always view it as a choice of the environment, and in principle, you can create another environment *physically* within the Universe with the first environment.
What?? Was there a point in this?
The “mystery” of M-theory in 11 dimensions
Yada yada yada,
We all know this mantra.
So we distinguish the “big M-theory” that contains all insights that string theorists ever studied, from “narrow M-theory” which is the UV completion of 11-dimensional supergravity. The latter is more or less understood and defined. The former is well-defined by hundreds of “patches” which beautifully fit together – the “transition functions” are the dualities – but we would prefer to describe M-theory “without patches”, as a single whole – to reveal the rules that illuminate immediately why all these patches belong to the structure. This is what the question “What is string theory?” means.
So! You should call it “U-theory” for “unworldliness”:
U = (F – ma) + (Rmn + 8piTmn) + (2+2-4) + … = 0
There it lies.
I think that you are completely wrong if you say that you are “not spending of huge amount of your time” with string theory. You ARE spending a huge amount of time, even more than me, by these philosophical worthless speculations and attacks. You are just not spending this time efficiently. If you spent the same amount of time with learning string theory, you would have known it better than me.
And thank God for it.
No it wasn’t … String theory is what killed him.
And I thought it was cancer. Live and learn!
From some folklore stories I vaguely recall, allegedly Richard Feynman finally gave in and decided to learn string theory in the last few months of his life in 1987-1988.
For people who don’t believe in string theory, they would see it as hype and propaganda. For somebody who does believe in string theory, they would not see it as hype and propaganda. It’s a matter of a difference of opinion for different people.
As long as there isn’t a death penalty for holding a particular opinion on some topic of interest, there’s always going to be a spectrum of different opinions on a particular topic.
There’s some folks today who don’t even believe in renormalization. Even more extreme are some folks who today still don’t believe in quantum mechanics, for whatever strange reasons they have.
Though I do agree with Lubos and others that a dearth of impressive new results (to spark a new string revolution) and the anthropic stuff in today’s string theory, has curtailed the optimism of some people in the field in recent years. The anthropic stuff has certainly curtailed my optimism about string theory these days. I certainly would want to see some impressive new results to be optimistic about string theory again, like how optimistic I felt about the field during the mid-late 1980’s and mid-late 1990’s. There has to be a better way of doing things without invoking the anthropic principle.
I’m sure John Schwarz must have had a lot of faith and optimism about string theory during the latter 1970’s and early 1980’s when only a handful of people were working actively on string theory, as well as dealing with Feynman’s sarcasm and jokes directed towards him. I don’t know if I could have maintained an optimistic spirit about string theory if I was in Schwarz’s shoes during those days.
There is no hype and propaganda about string theory. People just try to explain why it is a fascinating and unique theory. In the middle 1980s or middle 1990s, people were more excited and the progress was faster, and therefore the comments about string theory were more optimistic. Today it’s closer to the opposite. As Witten pointed out on the KITP conference, even string theory *enthusiasts* underestimate how rich and powerful string theory is – so speaking about hype and propaganda is not a reasonable description of reality.
I get the sense most of the criticisms of string theory on this weblog is largely about the hype and propaganda surrounding string theory over the last 20 years. The discussions about specific technical details about string theory and field theory seem to be more neutral and less controversial.
It seems like advocacy of any particular strong viewpoint is always going to be a hotly debated, regardless of what the subject is. One just has to see the sort of heated discussions on the *.advocacy newsgroups in the comp.* hierarchy, or the sort of stuff that goes on in debating clubs and societies on any university campus.
“The real LHC nightmare will be if it sees charged particles at 70 GeV and a scalar at 115 GeV.”
Well, as far as this sentence goes, it could be a confirmation of the LEP’s 115 GeV Higgs, plus – for example – selectrons at 70 GeV that were missed by LEP II, because of some reasons mysterious to me. That would be a cool victory rather than a nightmare. ๐
The real LHC nightmare will be if it sees charged particles at 70 GeV and a scalar at 115 GeV. THis is two doublets higgs but not SUSY, and besides it should raise doubts about the statistical methodololgy of LEP2
Hey Peter!
Not just Gross. I am also saying that the question “What is string theory?” is the most important one.
But you severely misunderstand this question. This question is not meant to say that “string theory probably does not exist” or “the theory is probably ambiguous” or even “string theory is probably just a dream and hope”.
On the contrary. We already know a lot of very well-defined things about “something” and all these things fit together beautifully and exhibit a lot of quantitative agreements, miracles, uniqueness. But we still do not really know “why” all these things work so well, but we know pretty certainly that an answer exists – a scheme that organizes all the insights about string/M-theory that we have already gathered in a very coherent way.
I apologize, but your possibilities 1,2 are just plain stupidities especially if you think that one of them is likely. Both of them are silly. What you’re saying that contradicts the results of 20,000 papers or so. We know that a theory that unifies these things exists, and it is most likely unique because its consequences for any question that we have been able to answer were unique. And it is a theory whose qualitative features reproduce everything we know in the real world.
There was never any non-dynamical freedom in string/M-theory found ever. If something can be adjusted about your theory, you can always view it as a choice of the environment, and in principle, you can create another environment *physically* within the Universe with the first environment.
There is some confusion about the meaning of the phrase “M-theory”. Before M-theory in 11D was understood and defined (e.g. by the matrix model), people were dreaming about it. It was the only vacuum with more than 10 dimensions and it was reasonable to think that this is the real “mother” of all theories, and understanding physics in 11 dimensions exactly is enough to understand any other background in string/M-theory.
This is not how we view it today. The “mystery” of M-theory in 11 dimensions diminished significantly, and we view this 11-dimensional vacuum on equal footing with other, stringy vacua – e.g. heterotic strings in 10 dimensions. It’s just *another* limit of the big “theory of everything”, and it differs from most others because it has no strings and corresponding stringy perturbative expansion.
So we distinguish the “big M-theory” that contains all insights that string theorists ever studied, from “narrow M-theory” which is the UV completion of 11-dimensional supergravity. The latter is more or less understood and defined. The former is well-defined by hundreds of “patches” which beautifully fit together – the “transition functions” are the dualities – but we would prefer to describe M-theory “without patches”, as a single whole – to reveal the rules that illuminate immediately why all these patches belong to the structure. This is what the question “What is string theory?” means.
I think that you are completely wrong if you say that you are “not spending of huge amount of your time” with string theory. You ARE spending a huge amount of time, even more than me, by these philosophical worthless speculations and attacks. You are just not spending this time efficiently. If you spent the same amount of time with learning string theory, you would have known it better than me.
You are free to only learn string/M-theory once someone computes the exact Universe around us by string theory, but be sure that it will be already too late for others (and you) to become significant contributors to the subject. Sure, if you’re only want to be an (average) teacher, it is enough for you to only learn the things that have already been proved and awarded by the Nobel prize – and you can even end up with classical mechanics. But it’s not enough to become a physicist that contributed to the human knowledge. The previous sentence applies not only to string theory, but to everything. It just happens that string/M-theory is the most (and perhaps, the only) reasonable path to progress in theoretical physics.
So I understand that your statement “string/M-theory is not well-defined” was just meant as an unphysical empty emotional rhetorical statement that should not be controversial because it was meant to express no idea whatsoever. In that case, I find this statement totally OK, but I am not interested in it. Any interpretation meant to make your statement meaningful leads to the conclusion that it is bullshit, but of course if you don’t want me to study your statement seriously, I can just view it as a set of words that respect the English grammar, and everything’s fine.
Best
Lubos
Hi Lubos,
The point I was making shouldn’t be a controversial one. Listen to Gross’s talk at the KITP conference. He lists one of the big unknown questions to be answered in the future as “What is String Theory?” and describes the current situation as something like “we have discovered a mathematical structure that has a life of its own, but we don’t know what it is”.
You know very well that when people say “M-theory” you have to figure out from context exactly what they are referring to. Often they are just doing 11-d supergravity, sometimes they mean a specific proposal for what M-theory is in a specific background, like the one you mention. The problem with the version of M-theory you refer to is that it doesn’t work in a physically realistic background with 4 large and 7 small dimensions. Maybe there is such a version of M-theory, but for now its existence is just a hypothesis. I have no idea whether it exists, but from everything I’ve seen the two most likely possibilities are:
1. No such theory exists.
2. Such a theory does exist, but there are an infinite number of them, one for an infinite number of possible backgrounds, and such theories may be able to parametrize an infinite number of possible extensions of the standard model, but will have no predictive value.
Maybe I’m wrong, but it’s perfectly rational for me not to spend a huge amount of my time becoming expert on the details of such theories until you or someone else comes up with a version of M-theory that can reproduce the real world and has some predictive value.
Peter, what you say – that there is no theory – is just a piece of bullshit, and the fact that you like to fool yourself with this bullshit only proves your laziness. That’s it. There is nothing deep about it.
Take the BFSS Matrix model – I am just trying to calculate the exact black hole entropy including 1/4 out of it, for neutral black holes, and solve the model using a matrix counterpart of the state-operator correspondence. It is a super (and supersymmetric) well-defined QM model, whose basic variables are nine hermitean matrices X^i, their canonical momenta Pi^i with the obvious commutators, and 16 hermitean fermionic matrices theta^alpha transforming as the spinor, and the Hamiltonian is
H = P^- = Tr (Pi^2 – [X_i,X_j]^2 – theta.gamma^i.[X_i,theta])
Is that ill-defined for you? Physics in decompactified 11D M-theory is the large N limit. You can prove that this reproduces 11D supergravity at low energies – gravitons, multiparticle states, their correct statistics, gravitational scattering amplitudes. You can prove that this theory contains M2-branes with the correct dynamics, and you can prove, after deforming it to the pp-wave, that it also contains M5-branes with the right excitations.
I am just trying to solve the SU(N) Matrix model in a more or less full, exact form.
You can prove that this quantum mechanical model contains states with the same scaling laws, up to numerical constants, as the black holes in 11 dimensions. You can show that compactification of this M-theory on circle leads to another matrix model, which is the 1+1 super Yang-Mills on a cylinder, and this matrix string theory exactly agrees with light cone gauge perturbative calculations if you take the perturbative limit.
It’s just bullshit and a lie to say that M-theory is not at all well-defined – or that it is just a collection of dreams. M-theory in 11D is perfectly well-defined, much like string theory is perfectly well-defined in all of its perturbative regimes, and in all these cases, we are already able to actually calculate a huge percentage of the questions. The only missing piece of the definition is a definition of nonperturbative string/M-theory on a *completely general* background – something that may be needed to understand the vacuum selection questions reliably.
Moreover, the very explicit Matrix model above can be shown to be related by dualities to other descriptions of string/M-theory, and you can go to AdS/CFT and others. Be sure that Weinberg knows all these basics of string theory, and the only reason why *you* want to disagree is that so far, you are lazy and you prefer to pray that string theory will die, instead of trying to reduce your 15-year-long delay in particle physics and learning string theory. It is much easier to pray that string theory will die instead of learning something.
Instead of expecting a smart person like Weinberg to distribute wrong, stupid, narrow-minded lies about “string theory being a wrong idea and a collection of dreams and nothing more”, you should try to learn this amazing set of constrained, rigid, and interconnected ideas called string theory, and the matrix model could be a good starting point for you to learn it. Matrix theory was the framework that brought me to active work a couple of years ago, and it is exactly because it is so well-defined. I apologize, but only an idiot could say about this particular model that it is not well-defined.
Try to prove to yourself, to me, as well as others that you are not such a dumbass as D.R. Lunsford and many others. I think that you are much more intelligent, and be sure that there is nothing wrong if you are able to learn something new and change wrong opinions from the past, as opposed to insisting on your wrong position that string theory is a fad.
All the best
Lubos
Hi JC,
as Weinberg emphasized – in his talk that I liked – it is guaranteed that a relativistic quantum theory at low energies will always look as a quantum field theory. Using the tools of QFT, we can restrict some properties of higher spin fields.
Higher spin fields could be excited strings; extra components of gravity from deconstruction; many other choices. We would have to see exactly how they’re produced, how they decay, and what are their quantum numbers. In general, higher-spin fields would look very stringy, and usual quantum field theorists – particle phenomenologists – rarely work with theories with higher spin fields. However, the details may be incompatible with any stringy models. I just can’t give you a universal answer.
Massless higher spin particles should not exist because they require gauge invariance to decouple the negative-norm time-like modes, but such gauge invariance is incompatible with any interactions. Moreover, I don’t know how would the LHC help you to produce new *massless* particles. The LHC is useful to produce new massive particles with the mass above hundreds of GeV.
All the best
Lubos
Dear Matt, your comments about the Higgs sectory in conformal technicolor are interesting. Won’t you post them on sci.physics.strings? This is the kind of discussion that I would love to see there. See
http://schwinger.harvard.edu/~sps
to see different ways how to get to the newsgroup.
The problem with string theory is not that there’s no guiding principles, but that there’s no theory. M-theory is not a theory, but a hope that a theory exists. No one knows what the fundamental variables are or what equations relate them. By any definition of a scientific theory, it’s not one. String theorists would like to believe that not only does the theory really exist, but it’s beautiful and based on some simple principles. At the moment, this is just wishful thinking.
I’m sure that Weinberg is right that there is no “guiding principle” behind string theory. But that’s no big deal — none of our best theories has such a principle. Think of General Relativity, which just says that spacetime is curved and that this curvature is the phenomenon called gravity. Where is the guiding principle? It’s just a clever and beautiful idea that happens to be right. All those “principles” of relativity, equivalence, Mach etc are all just historical junk of no physical importance. So if string theory has no awesome but ultimately trivial “principle” behind it, then it is in good company.
Lubos,
How would string theory deal with a hypothetical LHC scenario where a set of “new” massive “gauge particles” are found, which have a spin of:
a – spin-2
b – spin-3, spin-4, or higher
Also how would string theory change if a massless spin-3 or spin-4 gauge particle is discovered, which is found to be significantly weaker than any of the other four known forces?
I suppose it’s true that one generally does have a new scalar field in these models, but these have a very different origin from the SM Higgs. In Randall-Sundrum type Higgsless models one has the radion to worry about, but it might be stabilized in some way such that the LHC does not see it. (Of course there are also RS models with a Higgs.) In the “conformal technicolor” approach, they predict a scalar resonance below the TeV scale, which would look approximately like a heavy Higgs with couplings that aren’t quite the Standard Model values.
My inclination at this point is to say that it’s an open question whether one generically expects a Higgs-like scalar to show up at the LHC even in “technicolor-like” approaches. The phenomenological difficulties in building a model with no such scalar are large, but it’s not entirely clear yet that this is impossible.
As for motivation, I think these models are well-motivated in the sense that they’re the closest we can come to a solution to the hierarchy problem that does not involve supersymmetry. (Well, that is not entirely true; there are also Little Higgs models, which lately it appears can be made consistent with experimental results so far.) Should the LHC see signs of a Higgs-like scalar but no SUSY, one must either admit to fine-tuning, or look for other new physics that explains the hierarchy. In this case it would be good to have examples like “conformal technicolor.” But there is always the chance the LHC will see no Higgs-like scalar at all, in which case we would like to have at least some idea of what any model with unitary WW scattering but no Higgs-like scalar looks like. We’re getting there, although as you say the examples are convoluted. One would ideally like to have such a model solve the hierarchy problem with no fine-tuning, but it’s not clear we can do this yet.
Then, one could always take the position that everyone should just wait for the experimental results before going too far on some of these lines, as one experimentalist expressed to me, but I think we stand to learn interesting lessons in the meantime from trying to understand the space of possibilities.
OK, Matt (Reece), let me soften the statement a bit. The discovery of strongly interacting physics behind electroweak symmetry breaking would mean the expansion of some (as of today) very new models with approximate conformal symmetries in some energy regime, their AdS duals, and consistent incorporations of technicolor. I’ve heard a couple of talks about it. My feeling is that these models always require you to have some scalar field at the beginning anyway, and the physics of this scalar field just becomes complicated because of the strong interactions. My opinion is that these models are not terribly well motivated and unnecessarily convoluted – especially because the Standard Model with the fundamental Higgs works fine. But of course, I am slightly open-minded about that.
New generations, preons, new gauge groups.
A – If new generation(s) are found, it’s not a big deal. The neutrino from this new generation can’t really be light because the number of neutrinos lighter than the Z bosons is measured from the decay rate of Z, and the experimental result is 3+-0.01 or so. One can imagine a very heavy new generation with a heavy neutrino, but it just sounds weird – especially because the fermions, if they get mass through the Higgs, have Yukawa couplings to the Higgs proportional to their mass, and a fermion much heavier than the electroweak scale seems to imply a very large Yukawa coupling (greater than one), which seems to lead to Landau poles (divergence of the coupling at some slightly higher energy scale) and inconsistencies in the field theoretical description.
An explicitly seen new generation that is really similar to the known 3 generations would be weird, and it would probably be interpreted as an evidence for many Higgs doublets – and the new heavy generation would mostly couple to this much heavier Higgs doublet.
I don’t know – it’s a weird phenomenological gedanken experiment that would have no impact whatsoever on 99.9% of the string theoretical research.
B – On the other hand, preons would mean a significant shock for string theorists because – as of today – we have not seen anything like that in the stringy models. Especially the idea of a composite gauge bosons – made of spin 1/2 subparticles – seem very weird from the string theoretical point of view. If the experiments supported the thesis of some people that only spin 1/2 particles can be fundamental, string theory would be in trouble. String theory as we know it today pretty clearly implies that all light particles with spins 0, 1/2, 1, 3/2, 2 can be equally elementary.
Preons would be very hard to reconcile with string theory, I think – even though, obviously, a few people would try. This holds in all cases.
C – on the other hand, new gauge symmetries, new heavy Z’ bosons, and so forth, would be compatible with strings, and they could tell us a lot about the origin of the gauge groups (geometry of the branes etc.). String theory can generate the Standard Model and bigger groups, but it can also produce some pretty different gauge groups, and in some 4D realistic SM-like models, the amount of new U(1) (broken) symmetries can be rather large. C would certainly be an interesting arena for string theorists and many people would become string builders, having a new material to explain.
If SUSY is found but Higgs is not, I don’t have any immediate solution, but no doubt, all people in particle physics and string theory would try to explain why the thing that seems like the electroweak symmetry is broken. I think that string theorists and particle physicists would be comparably surprised why one of the more solid and older predictions is not confirmed while the newer one (SUSY) is. I think that the normal particle physicists would be surprised more than string theorists because they view SUSY pragmatically as a natural mechanism to protect the mass of Higgs, and if there is no explicit Higgs, their main reason for SUSY disappears. For string theorists, SUSY is first of all a natural and nice friend of string theory that makes it work better, and Higgses are independent.
At any rate, if SUSY is found, it will be a boost for the type of thinking led by string theorists because it will show that we are on the right track. The electroweak symmetry would have to be broken otherwise, or it will have to be shown as an approximate phenomenon.
Lubos wrote:
If (2) is realized and the Higgs is replaced by some more complicated physics breaking the electroweak symmetry, phenomenology will have very new, moderately interesting topics to study, and they will have very little to do with the teachings of string theory. String theory will be, in this case, disappearing from experiment-oriented physics departments, moving again to math departments – maybe even more rapidly than in the case (1).
I’m not at all convinced of this. If electroweak symmetry is broken in some way corresponding to new strongly interacting physics (a lot of the models being considered here involve new conformal physics, as suggested by Vafa, Strassler, and others), I think we might have to turn to stringy dualities to better understand these strong interactions. This is certainly true at large N (in the Randall-Sundrum models of this type that we have now, we find flavor-related difficulties in the third generation, just as in traditional technicolor, but it’s not yet clear that these difficulties are insurmountable). Markus Luty and Takemichi Okui recently argued for the small N conformal case in a paper called “Conformal Technicolor,” and there we have even fewer tools for understanding possible CFTs at small N. In general I think we have a poor understanding of the possible behavior of new gauge theories, and supersymmetric dualities have shed the most light on these issues so far. So, if confronted by data indicating new strong interactions, I think we’ll have a difficult time figuring out what strongly interacting theory is right, and I think SUSY and string theory might give us the tools that allow us to do this. I also think such a scenario would be more than “moderately interesting,” and I would greatly prefer it to clear evidence of SUSY (it would leave us with more fun things to explore).
oops sorry…..that was suppose to be back ground dependancy
Lubos,
On a slightly different twist, how would the string theory picture change in these hypothetical LHC scenarios (or a future particle accelerator):
A – a 4th vertical family of quarks/leptons is found
B – quarks and/or leptons are found to have an additional substructure, made out of “preons”
C – a “new” and different set of gauge bosons are found, which don’t fit into any of the known theories like the standard model, grand unified theories, nor any of their supersymmetric extensions
Lubos,
Linear extrapolation can only be done on back dependancy?
sol
Lubos,
Just to be complete, how about an LHC scenario where no standard model Higgs is found, but SUSY particles are found?
Hi JC,
if the standard Higgs is found at the LHC, but no SUSY (and probably no other stringy things), then the activity, funding, and the number of new students going to string theory will decrease visibly.
People will have to live with the fact that the world is fine-tuned a bit, even as far as the mass of the Higgs goes, and they will create new models where various things are “unnaturally” fine-tuned. The influence of the anthropic principle will increase.
Those who survive in string theory will forget about phenomenology, and they will incline to mathematical aspects of string theory and the application of string theory to more formal questions related to mathematics and nice, but not realistic quantum field theories. Those that will survive in string theory, trying to match the experiments, will appreciate those who predicted that string theory naturally predicts SUSY breaking at very high energies (like Mike Douglas), but many people will be discouraged by this game.
Well, honestly, the activity in particle phenomenology, including all models with extra dimensions (which, to some extent, work *for* the LHC), will decrease even more in the case (1) – only the fathers of the Standard Model who have already made it will have a new confirmation that their theory is really far-reaching, and Higgs, Goldstone and some others will become hot candidates for a Nobel prize (after the Higgs experimental Nobel prize). The progress in particle physics will slow down even more than today, and the ideas about the new expensive colliders will probably be killed.
Unfortunately this is a very possible scenario although I’ve made a $1,000 bet against it (for SUSY) haha. Well, so if (1) occurs, I will also lose 1,000 dollars with Michal Fabinger but there will be more serious things haha.
Concerning your second choice. If no Higgs is found, there must be *something else* that corrects the unitarity in the WW goes to WW scattering. The pure interactions of the gauge theory, from the vertices that we know, just do not give you a consistent result. There simply must be some deviations from the no-Higgs standard model – simply because the no-Higgs standard model is not consistent; it is not unitary which means that it does not preserve unitarity (the total sum of probabilities of alternatives is not 100 percent).
If no SUSY, no extra dimensions or strings of course, but also no Higgs is found, the electroweak symmetry must be broken by other means. Let me declare technicolor to be the primary example – the Higgs may be composite and complicated enough so that it is not seen explicitly at the LHC. What happens with string theory if such scenarios get an experimental support? I think that it will suffer as much as in the case (1) because the composite Higgs models and stuff like that do *not* naturally follow from string theory (even though some people will try to make stringy models of technicolor); on the other hand, string theory has no problems with fundamental scalar particles and fields.
If (2) is realized and the Higgs is replaced by some more complicated physics breaking the electroweak symmetry, phenomenology will have very new, moderately interesting topics to study, and they will have very little to do with the teachings of string theory. String theory will be, in this case, disappearing from experiment-oriented physics departments, moving again to math departments – maybe even more rapidly than in the case (1).
As you can see, there is no permanent safety, and if you believe that string theory is fundamentally on the wrong track, you should also think that there are only 3-4 years left before string theory will be punished for that. Of course, such a punishment is never definitive – it is a crazy idea to think that a failure on one collider will remove *all* proponents of a model. One can always move the arguments to higher energies and make them less convincing, but still plausible. Maybe the next collider after the LHC could prove string theory anyway – it is a game of probabilities and arguments, and they are rarely 100% clear until the experiments are done.
Lubos,
Lubos’s “less than” problem has been fixed.
Lubos,
How would the string perspective change in the following hypothetical LHC scenarios:
1 – the standard model Higgs is found, but no SUSY particles are found
2 – no Higgs is found, and no SUSY particles are found
It ate my paragraph because after “d” I used the sign “is smaller”, which is not compatible with HTML too well. Could not you, Peter, fix it, so that the sign “smaller than” would be translated to the appropriate HTML sentence, so that it’s not interpreted as a HTML command?
I said that the models in d less than four dimensions are disconnected with reality, and their only virtue is that they are solvable. But Nature does not care whether we can solve something exactly. The qualitative conclusions from 2D gravity cannot be reliably extrapolated to 4D and other realistic spacetimes.
The anthropic principle cannot fuel a revolution or an explosion; the anthropic principle is, on the contrary and because of its very nature, a symptom of a *missing* revolution. If there will be many more years without really new and sharp results in theoretical particle physics, the anthropic principle will grow stronger, together with philosophical, non-quantitative hand-waving, and with satisfaction with not-too-impressive matches.
Well, to be honest, the revolution is not too likely to start with some lower-dimensional models in d less than 4 either because they are disconnected from the real world by their very nature, and the qualitative conclusions of these vacua can never be reliably extrapolated to real physics. Their only virtue is that many things are calculable exactly, but Nature does not care whether we can calculate something exactly.
There are potential sources of breakthoughs – discrete-like structures at the Planck scale (see Vafa et al. papers). Even if these things are relevant, they will be very different in details from all discrete approaches to QG that have been tried so far. An example – the “quantum of entropy” is often naively said to be log(2) or log(3) because the system is thought of as a system of discrete blocks with 2 or 3 states. I believe that it will be shown that the entropy of some special objects, such as extremal rotating black holes, is always a multiple of 2.pi (or at least a rational multiple of pi).
OK, let me say something about what I really believe. Below, there are two different scenarios, theory-dominated, and experiment-led. Let’s start with the theoretical one.
I think that the next revolution – and not sure whether next year or in 20 years – will start with revealing some underlying “string theory” behind the theories which we do not consider to be stringy today – namely the CFT on the worldsheet. Well, N=2 strings whose target space behaves as a worldsheet of strings is the pre-example. This will allow us to interpret all effects on the worldsheet – handles, boundaries, crosscaps – as results of underlying “2D quantum gravity” that will have a more complicated nonlocal extension.
A new non-geometrical generalization of the principles of CFT will be found, and it will allow to extend the success of S-matrices etc. to the non-perturbative realm. A geometry-like original of dualities – such as E_k in supergravity – will be clarified. Non-perturbative physics on general backgrounds will become calculable, and supersymmetry breaking will be shown to be very different in details than previously anticipated. Realistic N=1 4D vacua with SUSY breaking will be connected and the potential will pick up a rather small number of priviliged points – close to the “heterotic strings on Calabi-Yau three-folds” and/or “M-theory on G2 manifolds” and/or “intersecting brane models with some warping”. Two years after the beginning of the revolution, the people will calculate the masses of the heaviest quarks, the (small) QCD theta-angle, and other things, and they will predict the first new physics beyond the SM, which will be only confirmed several years later experimentally.
Meanwhile, the structure of M-theory in 11D will be solved exactly, and the position of poles of the scattering amplitudes in 11D will be known more or less exactly.
Alternatively, theorists won’t be that fast, and the revolution will start experimentally in 2007, most likely with the LHC. A rather simple pattern of masses of superpartners will be found, together with supersymmetry, and it will match one of the popular SUSY scenarios within string theory. Alternatively, small black holes or excited strings are gonna be seen, and their precise patterns will be used to reversely engineer the shape of branes and hidden dimensions.
Most string theorists will jump on stringy phenomenology, and they will refine the models. It will be soon realized that these models have a very natural explanation why they’re special from the string theory viewpoint, and the structure of the “landscape” will be viewed very differently than today. It will have a very hierarchic, organized structure, as opposed to the stupid chaotic democratic structure with 10^300 equal members, and the people will start to realize how the early cosmology “creates” the right compactification/branes naturally.
Lubos,
What do you think will be the main ideas which will propel the “third” superstring revolution starting next year? Hopefully it will be something which does not involve the anthropic principle.
Hi JC,
that’s a very nice gedanken experiment.
First of all, if they abandoned string theory, there would probably be some immediate reason for their decision, and be sure that I would study this reason in detail because this would become one of the most important questions. If the reason were right (or at least very convincing), I would join them.
Second, if I found this reason to be unconvincing or plain wrong, I would just declare that these guys got mad. I would probably also write a couple of humiliating articles about them (so be careful, Gentlemen!). ๐ In some sense, it would be even more encouraging to do research, and I am sure that some young people would be attracted to the field *because* of the feeling of being highly original and dissenting, but it is likely that we would have no funding of it without these guys, at least for some time. ๐ Who knows.
This is not a truly realistic scenario. The reality is that the people will probably stay in the field, but they are reducing their optimism and excitement about more specific aspects of string theory and its subfields. Although all of them still certainly view string theory as a unified structure, a single theory, the opinion which directions of research and questions are “big” but also “doable” start to differ. It is completely obvious that the more thrilling period in the scientific field we live in, the more focused the research becomes. If less impressive results are created, the interests become fragmented, and it is the case of today.
The third superstring revolution should start roughly next year – by linear extrapolations of the 1st and 2nd revolutions, and also because 2005 is the World Year of Physics. ๐ So stay tuned.
All the best
Lubos
Lubos,
Imagine a hypothetical scenario where guys like Gross, Witten, Schwarz, Greene, Polchinski, Susskind, etc … all abdicated from string theory and stopped believing in it. Would you personally continue doing string theory research if this particular scenario happened?
BTW, Peter, Weinberg, although he loves to attack nonsense – like philosophers ๐ – is not fighting against string theory, and the reason is simply that string theory is not quite nonsense ;-), and he knows about it.
Weinberg wrote the foreward to the book “String theory” by Joe Polchinski:
“From the beginning it was clear that, despite its successes, the Standard Model of elementary particles would have to be embedded in a broader theory that would incorporate gravitation as well as the strong and electroweak interactions. There is at present only one plausible candidate for such a theory: it is the theory of strings, which started in the 1960s as a not-very-successful model of hadrons, and only later emerged as a possible theory of all forces. … There is no one better equipped to introduce… than … Polchinski … D-branes … Polchinski has a rare talent for seeing what is of physical significance in a complicated mathematical formalism, and explaining it to others. In looking at his book, I was reminded … Texas… where I had benefits from his patient, clear explanation of points that puzzled me in string theory. I recommend this book to any physicist who wants to master this exciting subject.”
By the way, each string theorist is also using the name of ‘t Hooft roughly 3 times a day in average. We don’t view him as a string theorist, but he would certainly joined the top ten if he described himself as a string theorist. ๐ ‘t Hooft is teaching a course on string theory and has extensive lecture notes, see
http://www.phys.uu.nl/~thooft/lectures/string.html
The Dutch are very good in our field, I would say, and of course support of their leaders is helpful.
Sorry, Peter, but these people probably know more about string theory than you do. It is great to criticize nonsense, but it is also good to learn in advance whether something is nonsense or not.
Well, Edward Witten is (still, I hope) on our anti-anthropic side, although he is not as radical as Gross. ๐
Today, Weinberg is not a string theorist, and therefore he can afford a completely neutral – and to some extent detached – point of view. He knows that the progress in particle physics is significantly slowed down because of missing new experiments, and he also knows that string theory is the only really big idea/framework around, but it is not understood well enough to beat the absence of the new experiments.
Peter, I am pretty sure that if Weinberg thought that the reason why string theory is not connected to new experiments is different – namely that it is not the right idea – he would definitely say it. Instead, he says that he does not know, but he guesses that the prediction that the current string theory research will be viewed (in 100 years) as a heroic path towards the theory of everything is more likely (PBS, The Elegant Universe).
We would be happier if Weinberg remained in the business of quantum field theory because even though it is not the most intriguing era right now, it still looks much more interesting to many of us than the calculations of acoustic waves from early cosmology… ๐
SUSY breaking is a big issue. My feeling is that many people sort of think that they already understand at least some mechanisms of it quantitatively, but it’s not quite my feeling, and I can still imagine that the correct final understanding of SUSY breaking will imply a small cosmological constant, for example.
Peter
Weinberg is certainly one of the smartest people in the business, and he in general is a very clear and elegant expositor and thinker. Normally he’s very skeptical of nonsense (see his public comments on religion), so I’ve always been surprised that he is not more critical of string theory. However, if anyone is a leading member of the particle theory establishment it is him, and I think he’s not willing to countenance the idea that he and many of his colleagues have gone off on a 20 year wild goose chase.
In this same vein, I wonder why you have been so critical of string theory, and have remained silent on the issue of gravitational waves?
The relationship is obvious in terms of what is needed for proof, yet we have modelled a deeper reality in our methodology for proof?
I ask this in a nice way, to point out the discrepancy I see as a outsider, looking in.
Weinberg is certainly one of the smartest people in the business, and he in general is a very clear and elegant expositor and thinker. Normally he’s very skeptical of nonsense (see his public comments on religion), so I’ve always been surprised that he is not more critical of string theory. However, if anyone is a leading member of the particle theory establishment it is him, and I think he’s not willing to countenance the idea that he and many of his colleagues have gone off on a 20 year wild goose chase.
In my experience, good mathematicians and physicists often keep doing very original and impressive work into their sixties, but almost all slow down a lot when they hit 70 or so. Weinberg is 71.
While I find a lot of Weinberg’s accomplishments impressive, I do find that I have a very different set of prejudices about particle theory. I came into the field just after the advent of the standard model, when people were just realizing how beautiful and geometric the model was. So my prejudice is kind of like Dirac and Einstein’s: one should be looking for new beautiful, simple and powerful geometric principles as the way to make progress. My prejudice is also that QFT is not just an effective theory, but is the most fundamental theory we know of. The example of asymptotically free theories shows that QFTs can make perfect sense to arbitrarily short distances.
Weinberg began his career in the era of S-matrix theory, when the prejudice was that QFT could not be a fundamental theory, that it was just an effective theory at low energies. He never swallowed the full S-matrix nonsense, but has always thought of QFTs and geometry as being just low energy approximations to something unknown. His prejudice is that QFT is not fundamental, but a structure one is lead to by imposing consistency of locality, special relativity and quantum theory. This prejudice shows up in his comments against geometry in his GR book and in the way he does gauge theory in his QFT books. I think it is still showing up in his talk yesterday when he hypothesizes that string theory may also be some non-fundamental framework not based on a simple geometric principle, but whose necessity is imposed by consistency.
“Nonsense” does not quite get there.
“We think that SU(3)xSU(2)xU(1) is pretty good although there are a number of unresolved problems; we invented something called ‘Supersymmetry’ which is an interesting but thus far inapplicable idea; we can’t seem to quantize gravity, what shall we do? I know, let’s start again. Let’s suppose that instead of point particles we have one-dimensional strings. Unfortunately it does not work in less than 26 dimensions. Is the theory wrong? No, the world must be 26-dimensional. Hey, if it’s supersymmetric it could be only 10-dimensional! Great! Only six unobserved dimensions to explain away (plus the fact that the world is not supersymmetric, but never mind that). Oh dear, there seem to be five ways of doing this, what shall we do? How about adding another (unobserved) dimension? Great! Now what? Um …”
I remember seeing a Feynman quote apropos string theory. He said something like: ” when I was a young man, I heard all these old physicists saying all this newfangled stuff is nonsense. Now I’m an old physicist, and I must say I think all this newfangled stuff [string theory] is nonsense.”
It sure sounds like a lot of the older particle and/or string guys have fallen into the “old fogey” pattern of cynicism, after many years of hype which didn’t pan out and the eventual letdowns. I wonder if this is an “age” thing in general, independent of the topic of discussion?
Anyone can give “state of the union” type speeches about their subject in general terms; the real test is when they talk technical. The vast majority of technical seminars I went to when I was in theoretical physics were incomprehensible to anyone who had not worked in the specific area related to the seminar. Not so Weinberg: he spoke with clarity and precision, and I was able to follow everything despite not being a specialist in all the topics that he covered. Being able to communicate effectively seems to me to be a trait of a good scientist. I seriously doubt that people can really have understood something if they are unable to communicate it in a comprehensible way.
He’s a curious fellow. In his gravity book, he goes to great lengths to explain how the geometric approach is wrong-headed, that Lorentz invariance and the Principle of Equivlance force gravity to be a spin-2 gauge field, then he gives the most perfectly geometric exposition possible. One can never really tell what he “believes” rather than “knows”.
For the record, I talked to Weinberg about a few things last weekend when he was here at the Perimeter Institute and he didn’t seem old, tired or discouraged. Indeed, I remember thinking that he had remarkable energy for someone who has been in the game so long.