Lawrence Krauss has just put up a long interview with Lenny Susskind. I was listening to it while doing something else, found myself shocked when the discussion got to the current state of string theory to find that I mostly agreed with what Susskind has to say. Here’s some of it (around 1:23):
I can tell you with absolute certainty String theory is not the theory of the real world, I can tell you that 100%…. My strong feelings are exactly that String theory is definitely not the theory of the real world.
Here he’s referring to “String theory” (with a capital S) as the superstring theory which has a known definition, at least perturbatively. He goes on to explain that he thinks it possible that some very different generalized or “string-inspired” theory might have something to do with the real world but that:
we don’t know and we don’t know if String theory will help us find those things…. We are still uncertain about whether whatever it is: “generalized”,
“boundaries pushed”, “string-inspired theories”. We don’t know and I think that’s the bottom line now.
As for the idea that we just need to understand how to break the superstring supersymmetry to get the real world:
People will say oh all you have to do is spontaneously break supersymmetry, blah blah well it’s been 25 30 40 years by now that nobody’s figured out how to do that.
On the “failure” description, he thinks String theory has not been a failure in the sense of showing gravitation and QM can coexist, but, as for particle theory:
Whether it’s been a failure in producing a theory of Elementary particles I would guess remains to be seen but String theory (with a capital S) is not the right theory.
On the wormhole publicity stunt (1:38):
Not a very good experiment… the experimental implementation of it left more than a little bit to be desired. That got much too much hype, yeah.
Krauss brings up the the string theory hype problem, with
we owe it to the public I think to be careful in what we say. I understand we get excited and it’s fine for physicists to get excited with each other but we have to be careful of what we say we can do because if we don’t it’ll come down and bite us in the butt.
Susskind’s response is:
I completely agree with that, but on the other hand there is a tension between that and the importance of keeping excited and letting the public know why we’re excited.
I’d have liked to hear Susskind’s thoughts on what should be done when a bunch of theorists write popular books hyping ideas that don’t work. Whose job is it to explain to the public that they were misled by overenthusiastic scientists? If he’s not going to do that, would he at least be publicly supportive of those who do this job?
Another question I’d have liked to ask him would be: given that he agrees that the idea of using string theory to understand particle physics hasn’t worked and we don’t know that “string-inspired” is the right direction to go, what next? If “string-inspired” is not the way to go, should we just give up on going beyond the SM? If not, how would he encourage young people to work on non-“string-inspired” ideas about unification that might take us beyond the SM?
Susskind pushed really hard for landscape theory, which is a consequence of string theory, in his pop book The Cosmic Landscape. And as I recall he said on The Edge website that if you reject landscape theory you’re probably a creationist. So now he’s telling us that he takes it all back?
This is awesome, he is the first high profile string theorist that I know that is publicly recognizing the things are they really are, it may start something… or not..
John,
I was thinking of writing more in this blog post, about the last part of the conversation with Krauss, where he talks about the landscape. There he’s much more careful in his argument, no nonsense about rejecting the landscape anthropic argument as being akin to creationism, or rejecting the theory of evolution.
He’s much more careful there than in his popular book. His argument now is that if you had a unified theory and then knew “The equations” governing the world, you could look at the solutions to these equations. He acknowledges that maybe if we find “The equations”, they will have only one, or a few solutions. Then any multiverse created by bubbling baby universe would have the same laws everywhere, no role for anthropics possible.
But, he argues, if you have “The equations” and they have a vast number of solutions, then the multiverse picture with our universe a randomly chosen anthropically allowed solution is a consistent possibility. He tries to argue that no one has any other viable theory for the size of the CC, so this deserves some sort of status as best known explanation.
The problem of course is that we don’t know “The equations”. My problem with what he was doing in the past was always that he and others were trying to argue that this was somehow evidence for string theory, or at least explained why string theory can’t predict anything. What struck me about his current argument is that he no longer is doing this. Instead of arguing that the equations of the String theory-based KKLT construction were “The equations”, now he’s saying that String theory doesn’t work as a unified theory, that we don’t at all know what “The equations” are.
So, now his anthropic landscape argument is just an abstract argument about an unknown theory with conjectured properties, no longer an argument based on string theory.
To pin him down, Krauss should have asked him specifically what he now thinks about KKLT as an explanation of the string vacuum and a unified theory. I’m guessing he might now take the correct position: you can’t tell whether KKLT vacua make sense since you don’t know “The equations” that they are supposed to solve.
Z Y,
I think what he’s saying here is what most prominent string theorists have been thinking for a long time. In private, I’ve found they often agree that known string theory is useless to do particle theory, say only reason to work on it is as a model of quantum gravity.
The question then is why the hype about string theory and unification is still getting heavily promoted by many people, with no push-back from those who know it is nonsense. It’s one thing to justify hype from scientists overly enthusiastic about a new idea they are working on, but I don’t think there’s any way to justify hype from scientists trying to evade acknowledgement that their ideas from 20-30-40 years ago haven’t worked out.
amazing, thanks for digging up
Of course String theory is not a theory of the real world, it is not a theory of anything. The real STRING THEORY – M-Theory cannot be built upon shaky handwaving grounds.
We first need to understand what is a full non-rational 2-dimensional 𝜎-model conformal field theory, then what is full non-perturbative YM and finally then we can have the true rigorous non-perturbative M-Theory.
Yes this might be a 10 000 year problem, but every problem needs to start somewhere and there is no other game in town other than String theory which serves as a good starting point.
“Krauss brings up the the string theory hype problem”
Dear Prof Woit, do you think that the laughable Quantum Wormhole hype is inadvertently crippling the String Theory bandwagon, by exposing the hype machine?
It would be ironic if the downfall of hype-driven String Theory is catalysed by hype itself, in this case, by non-ST (wormhole) hype.
Jim Eadon,
The wormhole hype was so bad that even the string theorists recognized it and some got annoyed. Susskind though here seems to me to be taking the position that
1. This was “much too much hype”.
2. But it is understandable that people do this kind of thing, it’s not something worth doing anything to stop.
For some context, Susskind himself has a lot of responsibility for what happened. He was one of very few people the hypesters chose to let in on the planned hype campaign before it started. As part of this, he and a collaborator wrote the article in Nature accompanying the hype announcement. When I told the people at Quanta that they had been had, I was told that I was wrong, that they had consulted four experts while writing their article and producing their video. One of the four was Susskind.
Seems Susskind still hasn’t grappled with Sabine’s point that it is not possible to specify a probability distribution over possible universes even if we had ‘the equations.’ Good for him for divorcing the anthropic nonsense from “String theory” landscape, but still would be better if he grappled with the deeper point.
Adam Treat,
I can imagine versions of “The equations” which would give dynamics of a big bang and baby universe creation which would imply a probability distribution. I can also image versions that wouldn’t. The basic fact is that we know nothing at all about this, are ignorant not just of “The equations” but of what variables they are in.
One can entertain oneself by arguing about the properties of completely unknown equations, but this isn’t science in any sense. You might as well argue about the Gods and what they are up to when they create universes.
I studied physics. It was a long time ago, and I never really practiced it, but the debate about string theory has reached the point that I don’t even understand anymore what the expression “the equations” is supposed to mean in this context.
Does anyone have an idea what these equations should look like, other than being able to describe 10^500 (or whatever) universes, of which our universe might be one?
Peter,
What Susskind is saying is:
1. For String theory, with a capital S, meaning the theory we know the equations for, there is no possible connection to the real world.
2. For some unknown better theory that does describe the real world, “string-inspired” or not, there will be “The Equations” which define it. We have no idea what these are. He wants to go on and make an argument for the anthropic explanation of the CC, based on assuming certain properties of the these unknown equations.
There are many papers on supersymmetry breaking in string theory, as well as on non-supersymmetric string theories, so I don’t know what Susskind is talking about. Does anyone know what he’s talking about?
Mitchell Porter,
Anyone who has actually been looking at those thousands of papers on supersymmetry breaking knows what Susskind is talking about: they don’t solve the problem. As Susskind explains, people have been trying for 40 years without success.
You can claim supersymmetry as a good idea because it has new symmetry generators so makes new predictions (equal mass superpartners with specific properties) if unbroken. The predictions are wrong, so you have to break SUSY (explicitly or dynamically). The problem is that any known way of doing this complicates the theory while either making new wrong predictions, or just allowing you to move the new predictions somewhere unobservable, predicting nothing at all observable. This has been the problem with SUSY, in QFT or string theory, since the beginning.
Peter,
Perhaps I can explain myself by analogy with KKLT. The 2000s debate over KKLT was on matters like, is this really how reality works, is this still science, and so on. But the 2020s debate over KKLT is on matters like, is this how *string theory* works, or do nonperturbative effects actually collapse the whole construction.
What I can’t tell, is if Susskind is calling stringy models of SUSY breaking into doubt *as rigorous string theory* (analogous to KKLT 2020s debate), or if he’s just saying that they are all empirically unsatisfactory.
Mitchell Porter,
The debate over whether non-perturbative effects stabilize moduli goes way back, KKLT itself was just part of that ongoing debate. In the 2000s leading figures in the field were debating this, the debate over whether this turned the subject into pseudo-science was a follow-on. Yes there are still people debating whether KKLT works now, but no one of any significance is paying attention any more.
It became clear early on in this debate that when you don’t have a non-perturbative theory, arguing over the technicalities of non-perturbative effects doesn’t really make sense. Susskind now I think is just following the mainstream opinion that, as currently understood, String theory with a capital S can’t answer this kind of question.
Whether he’d put some specific string theory model of SUSY breaking in the capital S category or not I don’t know, you’d have to ask him. But I think it’s clear he now realizes this is irrelevant: the ones that you could argue for don’t say anything about the real world anyway. From what he says, for a connection to the real world he’s not looking for some minor improvement on what is known, but for some different theory (his interest in ER=EPR indicates something very different than capital S string theory).