Michael Douglas gave a colloquium at City College this afternoon, with the title “Are there testable predictions of string theory?” I went up there to the talk, figuring that I knew more or less what he would say, but he really surprised me. Douglas has given many talks over the last year or so about his program for trying to get predictions out of string theory by doing statistical analyses of string vacuum states. He has concentrated on what looks like the most promising case, trying to see whether vacua with low-energy supersymmetry breaking are favored over ones where supersymmetry is broken at much higher scales (e.g. the GUT or Planck scales). If he could make a prediction that the LHC will see supersymmetry, that would count as the first real prediction of string theory, and in 2008 or so we would see if it was right. I was expecting Douglas today to explain this whole program, report on what he had achieved so far, and offer hope that he and his collaborators would have a yes or no answer about supersymmetry sometime soon.
Instead he very much downplayed hopes for this kind of prediction, answering a question about it by Nair at the end of the talk by explaining some of the difficulties. Presumably he now agrees with recent claims by Dine that it is too difficult, even in principle, to decide whether or not the landscape predicts supersymmetry. Given this, in the conclusion of his talk, I was expecting him to answer the “Are there testable predictions” question in the negative. Instead, he did something very strange. He announced that string theory does make predictions, lots of them, adopting the Lubos Motl definition of a “prediction” of string theory as being anything consistent with string theory. Examples he gave included Polchinski’s cosmic string networks, where one could tell from the behavior of the network whether the strings were fundamental or not, and short distance modifications to GR. Of course these are not in any sense real predictions; all sorts of different modifications of GR at short distances are compatible with string theory, as are either no visible fundamental cosmic strings, or visible ones with a huge variety of possible different properties.
The weirdest part of his talk was when he explained what he considered the best prediction of string theory. This involved the negative prediction that the fine structure constant can’t have varied with time in the early universe, since effective field theory arguments would imply a corresponding variation in the vacuum energy, something inconsistent with observation. So his best prediction from string theory isn’t really a prediction of string theory at all, but actually a prediction of effective field theory. Furthermore this “prediction” is the purely negative one that something that hardly anyone expects to be true actually isn’t true.
In the question section, some obnoxious guy who has a weblog asked him whether it was really true that the best prediction string theory could come up with was the no variation of the fine structure constant one that was really an effective field theory prediction, and didn’t that mean there was no hope of string theory ever really predicting anything. For some reason this made him rather defensive, and he began by saying it depended on the meaning of the word “prediction”. After having it explained to him what most physicists consider a prediction to be, he launched into a sequence of analogies designed to explain why you can’t get real predictions out of string theory. They all were of the same genre: imagine some situation where you can only observe phenomena that are related in a very complicated and hard to calculate way to the underlying fundamental theory, and somebody tells you what the fundamental theory is. Shouldn’t you work on it and believe in it?
This argument makes it clear where the whole subject is going to end up. The standard scientific method of deciding whether a theory is true or not by figuring out its implications and comparing them to observations is no longer operative. In the case of string theory there’s a new method. You just believe because authorities tell you to, and from now on the activity of professional theorists will consist solely in the construction of elaborate scenarios designed to explain why you can’t ever predict anything. Feynman’s line that: “string theorists don’t make predictions, they make excuses” has been changed from a criticism into a new motto about how to do science.
Concerning the Feynman quote, accidentally I stumbled upon the following (from
http://infoproc.blogspot.com/2005/01/string-theory-quotes.html)
“…I do feel strongly that this is nonsense! …I think all this superstring stuff is crazy and is in the wrong direction. … I don’t like it that they’re not calculating anything. …why are the masses of the various particles such as quarks what they are? All these numbers … have no explanations in these string theories – absolutely none! … ”
–Richard Feynman: in Davies and Brown, Superstrings, Cambridge 1988, pp. 194-195.
Chris –
I was trying to point out that Minkowski space also has features that are not realized in naive experience, but we seem very happy with it and find it “useful”.
-drl
Danny – I’m not following you. Mass is not dilatation invariant & therefore a universe with massive particles is not dilatation invariant & therefore not conformally invariant. Why is it more complicated than that?
Following on from Peter’s point, the twistor theorists I knew at Oxford: Lane Hughston, Paul Tod and William Shaw (a graduate student at the time) were always completely honest about what the framework could and could not do (“Don’t ask about massive particles, don’t ask about interacting particles” is what I seem to remember William saying to me once). Although they were called the “Mathematical Physics” group they were well aware of the fact that what they were doing was much more mathematics than physics. One thing, though where physicists could learn from them is the elegance with which they deal with the SL(2,C) covering group of the Lorentz group (which of course is one of the steps on the road to Twistors).
I think that the key question is “At what level the conformal invariance is realized?”.
Assume the representatibility of physically realizable space-times as 4-surfaces of space M^4xS, S some compact space. Assume that 3-D lightlike boundaries of space-time surface act as “causal determinants”. Causal determinants could correspond also to light-like surfaces representing “shock waves”.
From these assumptions you end up with the realization that these light-like 3-surfaces allow generalized conformal invariance by their metric 2-dimensionality. Hence conformal invariance and 4-dimensionality of space-time are very tightly related. This conformal invariance has nothing to do with the rather trivial conformal invariance (as compared to 2-D conformal invariance) of M^4.
Best,
Matti Pitkanen
Chris,
It’s somewhat more subtle than that – as an affine space Minkowski space allows dilations, and we should experience these along with boosts and rotations. Since we don’t, we “can’t” be in Minkowski space. So the argument goes both ways.
I think conformalism will eventually play a crucial role, but the existing approaches based on SO(4,2) will not.
-drl
In a word, no.
The idea of using twistors to do fundamental physics has mainly been pursued by people working with Penrose’s group at Oxford. It never became very popular outside of people associated with this group. The new work on twistor string theory is a big change, causing a lot more people to learn about twistors.
There’s been a steady interest in twistor techniques among mathematicians for quite a while. Some of this goes back to the late seventies, when Atiyah and others got interested in the fact that you could use holomorphic techniques on twistor space to get solutions to the YM self-duality equations. There’s been a lot of work by Ward and others on using twistors to solve various kinds of equations.
Claude leBrun has gotten a lot out of twistor techniques in his work on 4-manifolds, and there are lots of other examples of the idea being useful in mathematics.
Peter, Chris
Were twistors ever hyped up in the past in a similar manner to string theory, except maybe at a smaller scale?
I vaguely remember a popular press book by Peat which discussed string theory and twistors in the 1980’s. Other than that, I don’t recall twistors being really excessively hyped up.
Hi Thomas,
There weren’t that many string theorists in the audience. Of those who were there, some of them have told me in the past they’re not at all happy with the present state of string theory. I had to leave fairly quickly after the talk, didn’t get a chance to talk to too many people afterwards. I can say there definitely were some people in the audience sympathetic to my complaints.
By the way, Nair isn’t really a string theorist, although he has done some work on string theory. He has worked on a lot of different things over the years. This includes YM amplitudes in twistor space, where 15 years ago, he was one of the first (if not the first, I don’t really know the history) to calculate them this way. Before the colloquium we talked about what is going on in twistor string theory, which he’s working on (and about “Not Even Wrong”, of which he is a reader).
I can very easily think of so many very serious topics in science that are not getting far as fast as they should mainly because not enough people are working on them. One example that comes to mind is twistors.
No way. Twistors are dead in the water. They rely on conformal invariance. Particles have mass & therefore the world is not conformally invariant. Twistors are therefore a blind alley.
Feynman evidently used similar reasoning to rule out superstrings. Superstrings require ten dimensions. The world is not ten-dimensional. Ergo, superstrings are wrong.
Peter,
I’m curious about how the string theorists in the audience (like Nair) reacted to Douglas’ talk and your question. Were they upset about your irreverence? Do they think that the Landscape makes predictions? Do they care at all whether string theory disagrees with observation?
While quote about pathological science rings true to my ears regarding string theory. There was a time when it appeared perilously close to being revolutionarily right. It is still in fashion in high circles and so will correspondingly incur opprobrium when it does fall.
Of the six Langmuir attributes of pathological science, 5. is the most recognizable re: string theory, namely,
5. Criticisms are met by ad hoc excuses.
The other points would need rewording for string theory, e.g.,
3. It makes claims of great accuracy &
4. It puts forth fantastic theories contrary to experience
Regarding Feynman’s remark, I would like to make a remark on the sociology of science: The amount of effort that’s invested over so many years, by so many remarkable people, to keep string theory afloat is incredible.
I can very easily think of so many very serious topics in science that are not getting far as fast as they should mainly because not enough people are working on them. One example that comes to mind is twistors.
If it were not for Witten’s involvement, strings would probably have remained in 10D with E8xE8 to this day. (Happily, Witten has revived interest in twistors recently.)
Hi Dave,
I answered that it one of the previous comments. It comes from Lawrence Krauss.
If you look at what Feynman did say about string theory in print, one thing he complains about is that the most straight-forward prediction was for d=10 and SO(32) or E8xE8 symmetry groups. He refers to the activity of string theorists invoking Calabi-Yau compactifications, etc. to get around these naive predictions as making excuses for a theory that gives the wrong answer.
And to the previous commenter with the quote on “pathological science”. Do you want to defend string theory as “pathological science?” Or do you have some other off-topic pathological science you want to promote, in which case I should delete the comment.
Peter, where did you get that Feynman quote? Or is it a paraphrase? I’ve seen quite a few things by Feynman about string theory (and who knows, his personal “war” with Gell-Mann may as much to do with his opinion as anything else) but I’d never seen it worded quite this harshly.
“Nothing is to be gained by castigating those who followed false paths in good faith and with the honest determination to add to human knowledge. ‘Pathological science’ is an epithet applied to potentially revolutionary discoveries that did not pan out. The passionate disdain implied by the phrase is not justified by the actions of those who have been so criticized. Rather, it may be an instance of odium scholasticum: the criticism is so furious not because the thing is so far removed from the acceptable, but because it comes so infuriatingly close to being remarkably right. ”
{ From the conclusion of
http://www.hyle.org/journal/issues/8-1/bauer.htm }
About the Feynman quote: kind of funny, given the other discussion here, but the source is Lawrence Krauss. Two or three years ago the Museum of Natural History here in New York organized a public debate about string theory. On the pro side were Brian Greene and Jim Gates, on the negative side Krauss and Glashow, with Lisa Randall also on the panel. It was during this debate that Krauss quoted Feynman. You’d have to ask Krauss for his source, possibly he heard Feynman say this.
Peter,
“String theorists do not make predictions. They make excuses”.
Do you recall where you read that?
Hey,
This string ‘theory’ thing is turning pretty much as the ‘intelligent-design’ (ID) type of argument. I hope these guys adding ID to its curriculum didnt hear about what physics is ending up being (nor about Lubos please! 🙂 )
It will be very difficult to argue with them if, as probably lumo would love to do, people go around saying strings should be taught in highschools as the theory of everything…..”The last one turn off the light please” 🙂
Dont get me wrong, I find both fascinating human construction (and a beatiful one in the case of strings indeed), nothing to do with nature so far though :). We should be careful where do we put each of them, it might sound silly but don’t forget who is supporting science and what’s the main purpose of it.
By the way, Michael Dine is a great guy, I asked him recently about the landscape, SUSY and string theory, I asked “let’s assume we dont find SUSY at the LHC, does that mean string ‘theory’ would be possibly wrong?’ He answered that so far the evidence is not conclusive and he wouldnt be able to conclude that either case. That kind of honesty I find remarkable. I asked Ed Witten some time ago how it was possible string theory, originally developed in a fixed background, could describe fluctuations of the geometry. His answer was: “I’m afraid we still dont fully understand that”, as one would expect of a real genius. By the way, did anybody see the word ‘landscape’ written in his papers? 🙂
I love math as much as l love nature, and I agree there is a deep and meaningful conection between the two we still ‘dont fully understand’, perhaps, as I have been recently discussing with some friends of mine, Math is not invented but discovered, and there is always a ‘deep reason’ for any one of its many branches.
We ended up asking the following questions: Could one predict the existence of mathematics? Is logic an emergent phenomena?
Do have these questions any meaning, or even answer? Are they even wrong? 🙂
Oh well, I never said I was doing science :p
Some of us are necessary I admit (I hope 🙂 ) but taking control of the community hasnt been, and it isnt, healthy in particular for young people like us. I still believe philosophy is what motivate us in the first place, the ‘big questions’, we shouldnt forget though to look for a REAL answer, or at least something we can confront with Nature somehow. It is a tough call in any case, specially ‘standing up and taking a look around’……
best regards.
OK, sorry to get sidetracked by referring to Krauss and the “fate question” (only as an example, should have left it nameless). I did not realize he is a hot item here, not everyone pays such close attention to who said what to whom.
Needless to say he is a good physicist and a great populizer etc etc.
The point I was making (playing a bit of the devil advocate) is that the main paradigm of particle physics research- finding a simple model and comparing the properties of its ground state to reality is a theoretical prejudice. In the context of cosmology the question of the true minimum is unphysical, and simplicity is there only to help with the analysis. Maybe the true model of the world is complicated, and it’s properties that are relevant to reality are not that of the ground state. Think about protein folding- if you were presented with the relevant potential (especially if you were told it came from string theory) you would give up immediately trying to find any structure. Maybe quantifying the kind of structure which exists there and using it is doable and interesting, even if it is motivated by string theory. It is a matter of judgement if you think it is plausible, but it is certainly not outrageous to give it a try.
Sociologically, what I find funny is that a large majority of string theorists would agree with you completely on landscape issues. You will have to take issue mainly with some cosmologists and particle phenomenologists (no names
this time).
If you really didn’t know that Krauss recently was quoted in the New York Times describing string theory as a “colossal failure”, and is one of a very small number of people in the field who has been willing to publicly criticize string theory in recent years, then apologies for my tone.
But frankly I find this hard to believe. If you want to attack someone for being obssesed with something whose physical effects can’t be measured, it’s kind of strange you chose the one string theory skeptic around instead of the huge number of string theorists one could make the same criticism of. Maybe you know Krauss personally so know about his obsessions. But looking at his papers from the last few years, including the one you mention, I see no evidence of this obsession.
For the first paragraph, agreed, so far it looks like there is no structure, though the methods for determining such structure are underdeveloped (and possibly interesting), and it is hard to find structure without looking for one. The set of questions to do with such structures is independent of string theory, and in fact finds much more resistance among string theorists than among say cosmologists.
As for the second paragraph, look for example at astro-ph/9902189. Krauss and Turner actually proved there is no finite set of measurments that will determine the “fate question”, which would make that question uninteresting, I think you would agree. I was not aware that Krauss challenges string theorists, but I’m sure you know more about that than myself.
The tone has turned unpleasant, some would say “abusive”, so I guess I am no longer invited. Have fun talking to your regulars.
The problem with assuming physics is determined by a metastable state of some complicated potential function is that the idea is completely vacuous unless you have some idea what this potential function is and its metastable states have some non-random properties. Current ideas about string theory produce such a wide variety of extremely complicated possible potential functions that there is zero hope of ever getting any predictions out of the idea (something that should have been obvious to Douglas, Dine and others from the beginning). In the case of the vacuum energy, the complete randomness of this property of the potential function is actually considered a virtue. You may think this idea is an “intellectual challenge”, but it’s one that has nothing to do with doing physics. If it weren’t for the iron grip of the string ideology, no one serious would be paying much attention to this idea that obviously goes nowhere.
And if it weren’t for the nasty way string ideologues deal with anyone who challenges them, you wouldn’t be making stupid attacks here on Lawrence Krauss.
The problem is not specific to string theory- suppose you have a theory complicated enough to have many meta-stable states. For example a few scalar fields with scalar potential function that has a few valleys and peaks. In a cosmological context the classic problem of finding the true minimum of that function becomes irrelevant- no doubt you will find yourself there eventually, but we are not interested in that- we may well be in process of getting there now, stuck in a false vacuum for a while. What happens eventually (at asymptotically large times) is an unmeasurable and uninteresting question (though some cosmologists, such as Lawrence Krauss, are obsessed with it).
Thinking how to do physics in this context is a new intellectual challenge, there is no reason really a theory should be “simple” in a superficial sense (just a few degrees of freedom) and it is not clear there is nothing at all that can be said in more “complicated” theories. I think Douglas’ work is a good starting point, though not very satisfying as of now.
On Aaron’s comment, I must say that I have never understood how “more probable” amounts to “a prediction”. Even if Douglas’ program would show that low energy susy is more probable, and that doesn’t turn out to be true, that wouldn’t rule out string theory.
I think the only (kind) thing to say about string theory right now is that it has had it’s many ups and downs in the past, and that right now it’s in one of its down phases.
What about the running of alpha with energy though? I don’t want to get off topic and will be very brief. Quantoken, at around 100Gev, alpha becomes alpha ~ 1/128 and not 1/137. This has been measured. Please respond or comment at your own blog though and not here.
The talk of the so called “running” physical constants, like an alpha that varies against time, is NONSENSE.
Tell me what “TIME” you are talking about when your alpha is “time”-dependent, since Einstein’s special relativity tells us time is all different from one reference frame to the next one, and from one location to the next. You would have to set up a Newtonian absolute spacetime, against the relativity principle, to be able to talk about “time-dependency” of alpha.
The fine structure constant alpha, being a dimentionless parameter, is a TRUELY Lorentzian invariant quantity. You could measure spectrum lines of star lights in a fast moving spaceship. Although each spectrum line is red-shifted or blue-shifted, when you calculate alpha the end result is the same as measured on earth.
A “running” alpha would also be against the strong equivalence principle, which says that physics laws do not depend on position, time, or reference frame. If alpha and other physics constants are “running”, we could well measure them and use it as a basis to establish a Newtonian absolute spacetime reference frame, against the insight Einstein’s relativity, and hence turn clock back and retro-progress the science back a couple hundred years!
Why would people even start to talk about “running” costants?
Quantoken
As everyone I know is sick of hearing, I don’t think Douglas’s program has much hope of being predictive. Counting of vacua has no experimental consequences — as long a vacuum is consistent with current experimental results, it matters little how generic it is amongst other consistent vacua. We still could be in it.
So, while it is an interesting question to understand what vacua are out there (if only because exclusion is predictive), it’s still taxonomy and not prediction.
Douglas explicitly said that his argument was an effective field theory argument, not really a string theory argument. Sure you can write down effective field theories where changing alpha doesn’t change the vacuum energy, but if this is an effective field theory for some unknown short distance dynamics that determines alpha and the vacuum energy, you would generically expect changing alpha to change the vacuum energy. This doesn’t have anything to do with string theory specifically.
Dear Peter,
you’re wrong about all the points that are not completely fuzzy.
Concerning varying alpha: you can find hundreds of people who argue that it is perfectly OK to have a time-dependent alpha that changes independently of the cosmological constant and other parameters. In effective field theory, there is nothing wrong if you make a parameter time-dependent. In string theory, it’s not possible to vary these parameters separately because string theory does not allow any non-dynamical parameters. These things are powerful and nontrivial predictions.
Best
Lubos
Actually, I don’t know who posted it, except that their IP is in Russia.
But maybe that could be a motto string theorists could put over their office doors.
Wow, I think such a comment as below
deserves to be booed!
Worse, we all know who posted it!
If you want a prediction, go see an astrologer or palm reader.
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