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April 17, 2004

The Discretium

String Theory, it has been pointed out, does not have any adjustable coupling constants. In Field Theory (say, the Standard Model), one must do a certain number of experiments, merely to pin down the values of the couplings in the theory, before one can start extracting predictions. These coupling constants are inputs, and not predictable within the context of QFT.

String Theory does not have any adjustable couplings, so one might hope to do better. It does, however, have a huge number of vacua. With enough supersymmetry, these vacua come in continuous families. But with less (or no) supersymmetry, they are typically discrete.

I say “typically”, because in the approximations in which we can compute reliably, there invariably components of the moduli space in which some or all of the flat directions are not lifted. In some cases, that’s simply a result of our inability to reliably compute the effects which lift the degeneracy. In others, there may be stringy reasons for the flat direction.

But let us, for the sake of discussion, assume what we expect “generically” is, in fact true, and the vacua of “interest” are discrete. We’ve gone from having a continuous infinity of such vacua, to having a finite number. But this number is, at first blush, frighteningly large, 10 N10^N where NN is some moderately large number itself. (Proponents of the landscape try to outdo each other in making larger and larger estimates for NN, just as a previous generation of string theorist tried to outdo each other in counting the number of Calabi-Yau manifolds.)

I say “frighteningly,” because these large numbers lead to two distinct, but often conflated anxieties about what it means to do physics in such a situation.

The first anxiety is what I would like to call the “Empirical” question. Which vacuum describes our world? I don’t like to think of myself as being a small fluctuation about the vacuum, but many questions — essentially all of particle physics — can be addressed by studying the physics of small fluctuations about the vacuum.

Rather than doing experiments to determine the values of the coupling constants, we need to do experiments to determine which vacuum to expand about. With a huge number of vacua at our disposal, you might worry that there will still be a large number which are compatible with current observations.

If there are only a small number of such vacua, we might even have predictions for currently-measured quantities. The 19 or so parameters of the Standard Model might not be independently adjustable. Once you pin down some of them, the rest would be determined.

But, if there are enough vacua compatible with current observations, you might worry that we could fit all current observations, and yet have differing predictions for stuff we haven’t measured yet — the mass of the Higgs, the spectrum of superpartners, …

It would be rather depressing if the LHC, and future generations of particle accelerators were simply devoted to pinning down more closely which vacuum we live in, rather than testing predictions.

Fortunately, I don’t think there is a case for the existence of a large number of vacua fitting current observations. Let me pick just two criteria: proton decay and flavour-changing neutral currents. The generic vacuum with approximate N=1N=1 supersymmetry (eg, flux vacua in Type IIB orientifold models — the favourite among proponents of the Landscape) have dimension-4 baryon number-violating operators. Indeed, any theory in which extra coloured junk survives below the GUT scale will generically have baryon number-violating interactions whose magnitudes are too large to be compatible with the observed proton lifetime (>10 32\gt 10^{32} years).

With low-energy supersymmetry, the only way to save the day is to find a discrete symmetry (R-parity, or something similar) and impose it on the theory. Most Calabi-Yau moduli spaces do not have such a discrete symmetry, and those which do only have it on some very high codimension subspace of the moduli space. Pick such a Calabi-Yau. Most of the fluxes you might turn on do not respect the discrete symmetry, so you have to set them to zero (thus cutting down hugely the exponent “NN” in the above estimate) and — even if we restrict ourselves to symmetric fluxes, we need to further restrict ourselves to minima of the resulting superpotential which also respect the symmetry (remember that, just because the scalar potential has a symmetry, its set of minima need not).

If you think avoiding too-fast proton decay is easy (“Hey, we have zillions of vacua to work with!”), then you don’t remember the history of attempts to do String Phenomenology in the late '80s. Back then, people worked in the approximation of vanishing superpotential for the moduli, and simply wished to find a locus on the moduli space of some Calabi-Yau which would yield the correct physics. They allowed themselves to fantasize that nonperturbative effects would later lift the degeneracy and land them precisely where they wanted to be. But, even working with a continuous infinity of vacua (rather than the discretium), finding acceptable solution proved too hard.

I could repeat similar words about FCNC’s, but you get the idea. Finding vacua which fit our current observations is extremely hard. The worry is not that we have too many, but rather that we have too few (i.e. none).

The other anxiety has to do with what I call the “Historical” question. Given that there exists some appropriate vacuum, how did we end up here, as opposed to one of the zillions of other, inappropriate ones? One possibility is just initial conditions: they were just such that we ended up where we ended up. Given that there’s only one (observable) universe, there’s no sense in which we could reasonably ask whether this was “likely” or “unlikely.” We can’t do statistics with a sample of 1.

On the other hand, in scenarios like Chaotic Inflation, different part of the Universe may sample different initial conditions. Some will inflate, and produce an observable Universe that looks like our own. Others may look very different. Even if “most” such universes don’t look anything like ours, one might try to use anthropic arguments to say that it doesn’t matter what the “typical” universe looks like. It only matters what the typical universe capable of sustaining life looks like. In other words, we should study contingent probabilities.

Weinberg argued, for instance, that the cosmological constant could be explained by such reasoning. If it were too large, galaxies would never form, there would be no supernovæ to produce heavy elements, and we wouldn’t be here. The “expected” value of the cosmological constant turns out to be right in the ballpark of what’s observed.

Unfortunately, Banks, Dine & Gorbatov have extended this sort of analysis to other quantities. Consider again, proton decay. The anthropic bound on the proton lifetime is something like 10 1210^{12} years (otherwise, you and I would glow), twenty orders of magnitude smaller than observations. There’s no anthropic bound on FCNC’s, no anthropic bound on the electron-muon mass ratio, etc.

So, merely demanding the existence of life does not explain the universe that we see.

It’s possible that, once you put in the anthropic bound — say, on proton decay — there might not be enough vacua left over to do Bayesian statistics. We might just have ended up a vacuum which exceeds the anthropic bound on the proton lifetime by 20 orders of magnitude for the silly reason that there is a dearth of vacua which satisfy the bound at all. Alternatively, if there are enough vacua, we might try to get further mileage by including other facts that we know about about our universe (aside from the fact that it supports life) in our conditional probabilities. Demanding, say, the absence of FCNC’s might yield “generalized anthropic” predictions about other quantities. At some point, the game breaks down — not enough vacua to do statistics — but it’s perfectly possible that we might be able to say something beyond Weinberg’s statement about the cosmological constant.

Overall, I’m agnostic about the “Historical” question. If some “anthropic” argument bears fruit, that’s great. If not, well that’s too bad, but I don’t think it impacts our ability to do physics. There are many things in physics which turn out to have no deeper explanation than that they are the result of initial conditions. The choice of vacuum in String Theory might be one of them.

Posted by distler at April 17, 2004 12:44 PM

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14 Comments & 0 Trackbacks

Re: The Discretium

Didn’t Lubos make some comment on sci.physics.strings that said, essentially, that the flux vacua (or metastable states) are the popular vacua right now, but that they probably weren’t very representative of the totality of solutions including completely nonperturbative ones which no-one can calculate at the moment?

So, we find that in the vacua that we can calculate that a stable proton is very rare. But who’s to say whether this statistical property carries over to those where we can’t, and where the SM states are light for a reason that we can’t now fathom?

Unless you want to make a strong claim that we already know about all or most of the types of solution which can possibly lead to something like the SM.

Posted by: Thomas Dent on April 19, 2004 8:57 AM | Permalink | Reply to this

Re: The Discretium

The statement I made is really independent of the details of the superpotential that lifts the degeneracy and leads to isolated vacua.

The computation of the flux vacua is really only valid in the region of moduli space where the size of the Calabi-Yau is large, and the string coupling is small.

More typically, the “Calabi-Yau” is in some non-geometrical phase, and the string coupling is O(1). Which just means that we can’t do the KKLT calculations reliably.

This doesn’t matter for the argument that I gave. The statement that the generic vacuum has baryon-number violation up the wazoo, and “our” vacuum must lie on a very high codimension subspace of the moduli space (if it is to be found at all) is robust.

If you want to count vacua which could not possibly be ours, you will certainly find a very large number. I’m not sure why anyone would find that either surprising or interesting.

Posted by: Jacques Distler on April 19, 2004 9:16 AM | Permalink | PGP Sig | Reply to this

Re: The Discretium

Let’s see if I correctly understand your point of view on the Landscape:

1. The idea of making statistical predictions using the anthropic principle is doomed because it seems to be unavoidable that these will say that the universe is likely to have features that are anthropically allowed but experimentally false (too fast proton decay, too much FCNC)

Is that right? If so, we agree (although we might have different interpretations of why prominent people in particle theory are running around promoting an idea which clearly can’t work).

2. One doesn’t need to worry that there will be so many KKLT-type vacua consistent with all standard model predictions, since it is much more likely that there will be none.

Here I don’t agree or disagree since all indications are that the KKLT picture will turn out to be inherently vacuous or to be wrong and it is not clear why one should spend any time worrying which one it is.

3. It is plausible that there will be some small number of vacua consistent with known properties of the standard model in which case one might be able to make predictions.

This seems to me sheer wishful thinking. I can’t imagine why anyone would want to spend their time engaging in wishful thinking that the universe is extremely ugly and complex, so much so that one can’t do get any other explanation of basic features of the standard model other than “initial conditions”.

Posted by: Peter Woit on April 20, 2004 12:11 PM | Permalink | Reply to this

Damned if they do, damned if they don’t

1. The idea of making statistical predictions using the anthropic principle is doomed because it seems to be unavoidable that these will say that the universe is likely to have features that are anthropically allowed but experimentally false (too fast proton decay, too much FCNC)

There are two questions here that you are conflating.

  1. Are there enough vacua hospitable to life (small cosmological constant, proton lifetime greater than 10 1210^{12} years, etc.) so that one can even apply the anthropic principle?
  2. If there are enough such vacua, does the anthropic principle work?

I don’t really know the answer to the first question. But, assuming the answer is yes, I’m hard-pressed to believe that the “most probable” proton lifetime is actually 20 orders of magnitude longer than the hard limit.

But, as I said, the anthropic principle as elaborated in the Weinberg paper cited above, is but one of a general class of explanations based on contingent probabilities: given that “X” is true, what is the probability distribution for “Y”?

So we might put in more fact about our universe (say, that the proton lifetime is actually >10 32\gt 10^{32} years), and ask for statistical predictions about the remaining quantities.

Here I don’t agree or disagree since all indications are that the KKLT picture will turn out to be inherently vacuous or to be wrong and it is not clear why one should spend any time worrying which one it is.

Well, since you seem to be convinced that, no matter what happens, it’s a disaster for string theory, this hardly comes as a surprise.

You would make a more convincing “critic” of string theory if you allowed — purely for the sake of argument — that there were some possibility for it to turn out to be correct.

But, even so, I am puzzled by what you might mean by the contention that the “KKLT picture will turn out to be inherently vacuous”. Could you perhaps explain your definition of “inherently vacuous”? KKLT could be right or wrong, but “vacuous”?

3. It is plausible that there will be some small number of vacua consistent with known properties of the standard model in which case one might be able to make predictions.

This seems to me sheer wishful thinking. I can’t imagine why anyone would want to spend their time engaging in wishful thinking that the universe is extremely ugly and complex, so much so that one can’t do get any other explanation of basic features of the standard model other than “initial conditions”.

Umh … because having a small number, or even a unique vacuum compatible with current observations would be incredibly predictive?

Seriously, you trash the contention (by KKLT) that there are a large number of vacua compatible with known physics as a disaster for string theory, and then you turn around and trash the contention that there is (at most) a small number of vacua compatible with known physics.

Make up your mind!

Your visceral distaste for string theory appears to hinder your ability to present a coherent argument for longer than a paragraph.

Put the question in the context of QFT, or of “Theory X”, or whatever it takes to give yourself some emotional distance. Then try your argument again.

Posted by: Jacques Distler on April 20, 2004 7:50 PM | Permalink | PGP Sig | Reply to this

Re: Damned if they do, damned if they don’t

“But, even so, I am puzzled by what you might mean by the contention that the “KKLT picture will turn out to be inherently vacuous”. Could you perhaps explain your definition of “inherently vacuous”? KKLT could be right or wrong, but “vacuous”?”

If KKLT or similar constructions give you a large enough number of consistent vacua and the theory has no way of choosing amongst them, the whole set-up is vacuous, in the sense that it can’t predict anything because it is consistent with everything. Right now it is vacuous as a theory of the cosmological constant. If there are enough vacua of the right kinds to play the same game with all the other unexplained parameters of the standard model, the whole thing will be completely vacuous. If this turns out to be true, I don’t see how you avoid acknowledging this is a new disaster for string theory.

If KKLT is just wrong (no vacua consistent with the standard model), then string theorists are in the same situation they’ve been in for twenty years, unable to predict anything and engaging in wishful thinking that they will ever be able to do so. In this case KKLT is not a new disaster, just the same old ongoing one.

About the possibility of a small number of KKLT consistent with some anthropic or standard model constraints, thus allowing non-trivial predictions: this is not logically inconsistent, but to me looks like pure wishful thinking.

Posted by: Peter Woit on April 22, 2004 9:56 AM | Permalink | Reply to this

Vacuous

If KKLT or similar constructions give you a large enough number of consistent vacua and the theory has no way of choosing amongst them, the whole set-up is vacuous, in the sense that it can’t predict anything because it is consistent with everything.

Again, I think you are conflating the “Empirical” and the “Historical” questions above. “Predictivity” has to do with the “Empirical” question. Whether the theory “has [a] way of choosing amongst them [its possible vacua]” addresses the “Historical” question.

I argued above that there is little reason for pessimism about the “Empirical” question.

I am less optimistic that KKLT, the Anthropic Principle, or similar sorts of reasoning can give a satisfactory answer to the “Historical” question.

But that has zero impact on the predictivity of the theory. These are separate issues.

Right now it is vacuous as a theory of the cosmological constant.

I think you need to go read the Weinberg paper cited above.

If KKLT is just wrong (no vacua consistent with the standard model), then string theorists are in the same situation they’ve been in for twenty years…

No, if there are no vacua of String Theory consistent with the Standard Model, then String Theory is ruled out.

That’s called falsifiability — something most people consider a desirable feature of scientific theories.

About the possibility of a small number of KKLT consistent with some anthropic or standard model constraints, thus allowing non-trivial predictions: this is not logically inconsistent, but to me looks like pure wishful thinking.

Yet again, your basic premise is that String Theory is useless or wrong from the 'git-go, so there’s no point even trying.

If the number of vacua consistent with the Standard Model is large, then the theory is unpredictive. (Again, I think it’s unlikely to be large.)

Conversely, believing that the number is small is, according to you, “wishful thinking”. And hence, presumably, trying to find such vacua is a waste of time.

We can conclude that you believe one of two thing: either that the number is large, or that the number is zero.

Which is it?

To sharpen the question, replace the phrase “String Theory” with “Theory X”. I don’t see how any of the words would change. Are you, therefore, of the opinion that any work (whether using String Theory, or any other conceivable approach) in this particular branch of theoretical physics is a waste of time?

Posted by: Jacques Distler on April 22, 2004 11:36 AM | Permalink | PGP Sig | Reply to this

Re: Vacuous

“I think you need to go read the Weinberg paper cited above.”

Weinberg’s anthropic argument that the cosmological constant can’t be too big just tells you that when you throw up your hands and say “I have no clue at all what is going on with the cosmological constant”, the fact of your existence still allows you to say something. The “prediction” that the discretium proponents are so proud of is exactly the same one I can make when I confess to having no idea what determines the cosmological constant. It isn’t a prediction at all, it’s an elaborate and contrived excuse for why they can’t make a real prediction and is scientifically vacuous. I’ve heard Feynman quoted as having said “String theorists don’t make predictions, they make excuses” and this is more of the same.

Posted by: Peter Woit on April 25, 2004 12:31 PM | Permalink | Reply to this

Re: Vacuous

“No, if there are no vacua of String Theory consistent with the Standard Model, then String Theory is ruled out.”

That’s true, but I was explicitly referring to KKLT vacua. I don’t believe that if these are ruled out string theorists will give up. Instead they’ll keep hoping that some other vacua (or non-perturbative or cosmological effects) will do the trick.

“Are you, therefore, of the opinion that any work (whether using String Theory, or any other conceivable approach) in this particular branch of theoretical physics is a waste of time?”

I don’t know how you’re defining “this particular branch of theoretical physics”. If you mean particle theory beyond the standard model,then, no, I don’t think it’s a waste of time, but I do think people need to acknowledge when ideas don’t work and move on. I do think any work on the KKLT vacua is a waste of time. They are contrived and hideous, don’t explain anything about the world and never will. For the moment they are just being used to make excuses, not predictions. I have no idea how many there are that are consistent with the standard model, but the idea that is is a small number that will all of a sudden allow predictions seems to me almost infinitely unlikely. This work wouldn’t be a waste of time if people understood string theory well enough to see that all of its realistic vacua had to be of the KKLT type, so by studying them they potentially could make progress by falsifying the theory.

Personally I think that at this point the idea of unifying everything by a 10/11 dimensional supersymmetric string/M-theory of extended objects has failed so miserably that further work on it is a waste of time. But if someone believes otherwise, their time is likely to be better spent trying to figure out what string/M-theory really is, and thus maybe really understand its vacuum states or find some use for the thing where it really works (e.g. strong-coupling gauge theory).

Posted by: Peter Woit on April 25, 2004 12:43 PM | Permalink | Reply to this

Not vacuous

Weinberg’s anthropic argument that the cosmological constant can’t be too big just tells you that when you throw up your hands and say “I have no clue at all what is going on with the cosmological constant”, the fact of your existence still allows you to say something.

No, it says something much more precise and quantitative than that. Again, I suggest you read it.

“No, if there are no vacua of String Theory consistent with the Standard Model, then String Theory is ruled out.”

That’s true, but I was explicitly referring to KKLT vacua. I don’t believe that if these are ruled out string theorists will give up.

By “KKLT vacua” do you mean “Type IIB orientifold compactifications with fluxes, stabilized at large radius and weak string coupling,” or do you mean something else?

If you mean something else, please tell us what you mean, so that we can have an intelligent discussion.

If you do mean the above definition, why do you think such vacua are the ones string theorist should concentrate on (i.e. that string theorist should give up on the subject if the Standard Model turns out not to be in that class)?

I do think any work on the KKLT vacua is a waste of time. They are contrived and hideous, don’t explain anything about the world and never will.

KKLT vacua are interesting because the effects which lift the degeneracy and give you isolated vacua are under good quantitative control (using current string theory technology). That’s the only reason to study them in preference to some other class of string vacua.

I have no idea how many there are that are consistent with the standard model, but the idea that is is a small number that will all of a sudden allow predictions seems to me almost infinitely unlikely.

Since you haven’t disputed my contention that the number is not large, this must mean you think the number is zero. Why do you think it’s zero (rather than small, but nonzero)? On the basis of what evidence or intuition did you arrive at this conclusion?

Personally I think that at this point the idea of unifying everything by a 10/11 dimensional supersymmetric string/M-theory of extended objects has failed so miserably that further work on it is a waste of time.

When did you not think that?

My understanding is that your opinion on the matter has not changed since you first heard about string theory in the mid '80s.

But if someone believes otherwise, their time is likely to be better spent trying to figure out what string/M-theory really is, and thus maybe really understand its vacuum states or find some use for the thing where it really works (e.g. strong-coupling gauge theory).

A startling admission on your part. That is exactly what the rest of us are doing.

String Theory has served as a rich source of insights into the behaviour of strongly-coupled gauge theories. But there are many fundamental aspects of the theory that are still not understood. It is therefore, in my opinion, premature to attempt a detailed comparison with phenomenology.

Posted by: Jacques Distler on April 25, 2004 1:24 PM | Permalink | PGP Sig | Reply to this

Re: Not vacuous

I’m really not the person to argue the details of the KKLT story with since I think it’s a waste of time. Maybe you’re right that demanding certain features of the standard model reduces the number of vacua to something manageable, I have no idea. Until I see some positive evidence though, I’ll continue to think it is extremely unlikely that this will ever lead to a prediction about the real world that works. I’m agnostic about whether this will be because you’ll get predictions that don’t work or find predictions impossible.

“When did you not think that?”

In the mid-eighties I wouldn’t have described string theory as a miserable failure. When I first heard about it in late 84 and learned more about it in 85, it seemed to me a not obviously compelling idea, but one that someone should investigate (and there was no lack of people doing this). Throughout the early eighties there were many different ideas that came up, attracted a lot of attention for a year or two, then fell by the way-side as it became clear they didn’t really work out as well as initially hoped. I thought string theory would be like these: either people would get somewhere with it within a couple years, or interest would die off.

By the late eighties I was starting to find the whole thing a bit strange, but there was a lot of formally interesting stuff being learned, especially about conformal field theory. This was creating a really healthy interaction between math and physics. On the days Witten wasn’t thinking about string theory, he was revolutionizing mathematics with a fantastic array of new ideas about QFT, many related to CFT.

By the mid-nineties I would have said that it was pretty clear that string theory had failed as a unification idea. I admit to being intimidated for a while by the “Second String Theory Revolution” hype. It took me a while to understand the whole M-theory circle of ideas well enough to be sure that they did nothing to address the problems with unification and didn’t even go very far towards getting a real non-perturbative theory that had any hope of addressing the question of what the vacuum states of the theory really were.

By the late 90s I certainly did think that string theory was a miserable failure, one that was being heavily promoted as a success and increasingly dominating particle theory research in an extremely unhealthy way. I first wrote something for publication about this in 2000, partly with the idea that maybe I was wrong, that there were good arguments for string theory I didn’t know about. The reaction to what I wrote has convinced me that I was more right than I knew at the time. The whole anthropic nonsense of the last couple years seems to me strong evidence that not only is string theory a miserable failure, but that many of its practitioners are so unwilling to ever admit that it is wrong that they will abandon the most basic principles of what it is to do science in order to keep promoting string theory. The level of public hype about string theory has increased in recent years as its failure has become glaringly obvious. I don’t see how anyone who cares about theoretical physics cannot find this disturbing.


Posted by: Peter Woit on April 26, 2004 11:21 AM | Permalink | Reply to this

Re: Not vacuous

I’m really not the person to argue the details of the KKLT story with since I think it’s a waste of time. Maybe you’re right that demanding certain features of the standard model reduces the number of vacua to something manageable, I have no idea.

But you have vociferously argued that KKLT “proves” the failure of string theory, both here, on your own blog, and in other venues.

Now you concede that you “have no idea” whether the central pillar of your argument has any merit.

The whole anthropic nonsense of the last couple years seems to me strong evidence that not only is string theory a miserable failure, but that many of its practitioners are so unwilling to ever admit that it is wrong that they will abandon the most basic principles of what it is to do science in order to keep promoting string theory.

There is nothing “unscientific” about anthropic reasoning. It is even falsifiable, as I have argued above.

In the mid-eighties I wouldn’t have described string theory as a miserable failure. … either people would get somewhere with it within a couple years, or interest would die off.

It was very charitable of you to have been willing to give string theory “a couple years” to develop before dismissing it entirely.

But, really, it is hard to imagine an alternative universe in which progress in string theory would have come at a pace sufficient to satisfy you.

To the contrary, you are dismissive both of work on fundamental aspects of the subject (because they don’t directly address the “problems of unification”) and of work on its phenomenological aspects (because they don’t bring us closer to “a real nonperturbative theory”).

And applications of string-theoretic ideas to answer hard questions in quantum field theory, like — to pick one example out of many — Dijkgraaf & Vafa’s work on the vacuum structure of supersymmetric gauge theories, doesn’t even merit any mention at all.

As to applications to Mathematics, there is much more going on now than there ever was in the late ’80s. Take (again choosing one example, more or less at random) the work of Gopakumar/Vafa/Bryan/Panharipande. Or, to go a few years back, the revolutionary impact of Seiberg&Witten on the theory of the topology of 4-manifolds.

Even in phenomenology (you do look at hep-ph from time to time, don’t you?), it is fair to say that the two most significant new ideas in the past decade: deconstruction and Randall-Sundrum have both come from string theory.

Your anti-string theory screeds have engendered a strong reaction in some quarters, not because string theorists are defensive, rather because your diatribes are utterly unrelated to the actual intellectual state of affairs in the field of High Energy Theory.

Personally, I find them simply boring. They are an uninspired rehashing of the same set of complaints voiced a decade and a half earlier, minus the wit and vigour of the original.

It is generally a mistake to complain that scientists are, as a group, collectively barking up the wrong tree. Scientists (especially high energy theorists) are a restless and ambitious lot. If one avenue of research proves less than promising, they will generally shift to another.

You remind me of nothing so much as a certain class of “cultural critic”, decrying the vulgar tastes of a general public, brainwashed by corporate advertising into desiring things that are not really good for them.

Posted by: Jacques Distler on April 26, 2004 3:39 PM | Permalink | PGP Sig | Reply to this

Re: Not vacuous

“But you have vociferously argued that KKLT ‘proves’ the failure of string theory”

What I’ve argued is something different:
1. KKLT seems to kill the standard wishful thinking about string theory that was pretty universal in the mid-eighties: that there are only a small number of theoretically consistent vacua, our world is in one of them and string theory is thus highly predictive.
2. Susskind and others who accept the idea that KKLT implies that string theory has an astronomically large number of consistent vacua have chosen the course of abandoning doing physics over the more rational course of abandoning string theory. They are now proudly promoting their success at constructing a convoluted excuse for not understanding the cosmological constant, and claiming that further such successes lie ahead.

I actually suspect David Gross might even agree with 2.

About the pace of progress in string theory:

1984-5: 0 predictions, but many people believe some are just around the corner.

2004: 0 predictions, no one seems to believe any are likely soon. Arguments rage about whether any are even possible.

As far as I can tell, the pace of progress in string theory is slow and negative. Maybe before we’re both dead the pace will pick up, and string theory will be well enough understood to prove that it can’t ever work. Maybe not.

I talk to senior people in the math community all the time about what is going on in math and physics. I can assure you that there are a lot of them saying that the number of interesting ideas coming out of physics has declined in recent years and few if any share your belief that there is “much more” going on now than in the late 80s.

Posted by: Peter Woit on April 27, 2004 1:15 PM | Permalink | Reply to this

Re: Not vacuous

What I’ve argued is something different: 1. KKLT seems to kill the standard wishful thinking about string theory that was pretty universal in the mid-eighties: that there are only a small number of theoretically consistent vacua, our world is in one of them and string theory is thus highly predictive.

Utter nonsense.

From the very start it was clear that string theory had a large number of vacua. In perturbative string theory (i.e. as the theory was understood in the '80s), these even came in continuous families, so the number was not even finite, but uncountably infinite.

And it was well known early on (from the work of Dine and Seiberg) that none of the perturbative string vacua known in the mid '80s could possibly be consistent with the Standard Model. If you use the known values of M GUTM_{GUT}, M PlM_{Pl} and α GUT\alpha_{GUT} to fix the fundamental parameters, α\alpha', R compactificationR_{\text{compactification}} and g stg_{st} in conventional perturbative string theory, you find that the string theory is not weakly-coupled (i.e. your approximation was not self-consistent).

It was not until much later that we even had the tools (still woefully inadequate) to discuss vacua which might plausibly give the correct values for M GUTM_{GUT}, M PlM_{Pl} and α GUT\alpha_{GUT} within the realm of validity of the approximations involved.

I have tried to explain to you the significance of KKLT: that the effects which lift the continuous degeneracy of vacua are under good quantitative control. It has nothing to do with an “increase” number of vacua.

2. Susskind and others who accept the idea that KKLT implies that string theory has an astronomically large number of consistent vacua…

I have tried, very patiently, to explain to you why you are wrong. Evidently, nothing I say penetrates.

There are zillions (counting vacua with at least 8 unbroken supercharges, an uncountably infinite number) of perfectly consistent string vacua that look nothing like the real world. This has nothing — a priori — to do with the predictivity or lack of predictivity of String Theory.

I talk to senior people in the math community all the time about what is going on in math and physics.

Ah, yes, more of your “anonymous sources.” I talk to plenty of mathematicians too. And the folks I talk to say different.

The level of collaboration is deeper and more far-ranging now than the shallow, and mostly one-sided state of affairs in the '80s.

Posted by: Jacques Distler on April 27, 2004 6:09 PM | Permalink | PGP Sig | Reply to this

Enough is enough

I sat down this evening and reread this entire comment thread. I have come to the conclusion that our discussion has degenerated quite far enough.

If you want to discuss the physics issues involved, please feel free to respond, and I will be happy to lend whatever further insight I can.

If, however, your interest is simply to post anti-String Theory diatribes, I would ask that you post them on your blog, rather than here.

You can be assured that all of my readers will know where to find you, should they feel the need for another dose of spleen.

Posted by: Jacques Distler on April 27, 2004 10:26 PM | Permalink | PGP Sig | Reply to this

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