Question: What is string theory?
Peter Woit: Well the first thing I can say is, when people sit and talk about String Theory, they're actually talking about a very complex set of ideas that lots of people, a very large amount of people have worked on and have done a lot of different things with. Probably what it's best known for and what got people all excited about it in the physicist community is the conjecture that, at the most fundamental level, you can understand matter and the universe in terms not of point particles, which is the way our best theory is, currently, you can understand things, but in terms of, if you like, vibrating in loops of some elementary objects here, your elementary object instead of being a point-like thing is something you should think of more as a one dimensional loop, or a string which is kind of moving around.
So, it has a lot more - it can do a lot more complicated things than a point, this kind of loop of elementary matter, whatever it is. And so, it gives you a very different class of theories than the ones that have been so successful before. So, during the '60's, this idea was initially developed and initially people tried to do one thing with these passive theories something which didn't work out that well. And then starting in the late '70's and '80's, people then came up with a conjecture that maybe you really could unify all of physics and solve some of the open problems in physics by replacing our standard theories, what we call the standard model with some kind of string theory. So, since this idea because very popular in 1984, and so it's been now 25 years people have been working very hard on that. And I just think the initial thing that got people excited was I would claim it really hasn't worked out and it really can't work out. And that's kind of been, I think, a lot of the reason the controversy has been an argument over this issue of whether this very speculative idea about whether you could use these strings, and you don't have to get a unified theory, about whether that has - is that an idea that's failed or is there still some hope for it, is what I think is really what's the controversial part of it.
Question: What is the “grand unifying theory” that physicists are trying to formulate?
Peter Woit: Well, the main thing to understand about the current state of physics is that we have - are in some sense, a kind of victim of our own success. We have an incredibly successful theory called the Standard Model. And it really explains everything that we can observe about and in terms of a very small number of elementary particles and some basic forces between them. And it's a quite beautiful theory and it really is just absurdly successful. Every experiment anybody knows how to do that in principle can be - that this theory has something to say about, it works out perfectly to whatever experimental and in whatever detail you can do an experiment to whatever precision, it come out to exactly as predicted by the model.
So, we have a semi-unified theory. This quite nice, beautiful structure which explains everything we can see, but it still leaves open several questions. Some of the questions are just kind of why we have all these different particles and they all have different masses. Why do they all have different masses? We don't understand why the electron has a certain mass, quarks have other masses. So, there's just kind of things which the theory doesn't address. It just doesn't answer these questions and then questions which as physicists we think there should be answers to.
But then there is also one remaining force which isn't part of the standard model, which is the gravitational force. The gravitational force is much, much weaker than these other forces and it has a somewhat different nature, so the problem with it is we don't - these other forces we have something which is a quantum theory. It's quantum mechanics. It really works down to the microscopic level and in terms of this very fundamental idea about reality called quantum mechanics.
The theory of gravity that we still use is Einstein’s Theory of General Relativity and it's what we call a classical theory. It's not a quantum mechanical theory. The problem is, you can make an estimate within this theory of how big quantum mechanical effects would be and then that estimate tells you that they are just so absurdly small you can never hope to see them. So, there's kind of this problem of principle. We have this theory which is not a proper quantum mechanical theory and we know that there's things which it can't quite properly explain, but they're far too small for us to study them and to try to see them.
So anyway, we are not completely happy with this setup. We have a force which doesn't quite fit with the others and which for logical reasons we would like to unify it with these others and understand it as a quantum mechanical theory, and we've never been quite successful in being able to do that. And this is what String Theory was, kind of a promise of a way of how to do that. And that's why people got so excited.
Question: What do you mean when you say that string theory is “not even wrong”?
Peter Woit: Well, so that's a famous phrase among physicists, it goes back to a well-known theorist called Wolfgang Pauli in the '50's and the story is that toward the end of his life someone - I guess he was well-known for being very hyper-critical of things that were going on and would get up in the middle of a seminar and start saying it's wrong, it's completely wrong. And then late in his life someone asked him about some work of some speculative idea that someone - and shook his head and said, "Well that one's not even wrong." And so, it's a well-known phrase among physicists. It kind of carries I think two meanings. One of them is more somewhat of a term of abuse, "well that's so bad it's not even wrong." But there's a more interesting it is, it's a more technical meaning that very often you have a speculative idea and if it's not a very good idea, or it turns out that you end up not being able to do very much with it, you end up not being able to predict with it, or to - it's just not useful, so there's also a notion of not being "not even wrong," in the sense that it's not an idea which can be fully developed or can be turned into something which is powerful enough to actually predict something and actually be wrong.
So, one problem of the String Theory is that it's kind of a theory which can explain what the problems are, but the problems are such that you can't even pin it down and say this is exactly what it predicts, so lets go out and test it. So, it's not even capable of being wrong, or being falsified, or being showed to be wrong. So, that's the more relevant meaning here really.
Question: Will string theory ever be verifiable or unverifiable?
Peter Woit: Yeah, well as I said, String Theory is actually a very complicated story. If you start out with this hypothesis that maybe your ephemeral objects are not points, but are these strings, there's a lot of different things you can try and do that you have a whole different class of theories you can play with. So, I think a lot of - if you look at what most people, who are still going String Theory are doing, they're actually not directly trying to develop this unified theory anymore. They're off doing other things with String Theory. People these days are trying to apply it to problems in nuclear physics; they're applying it to problems in Solid State Physics, understanding super conductors. So, the people who are still interested in it are often kind of - even if they may or may not explicitly admit that they've given up on the unified theory idea, but they're often doing other things. So, there's a very active pursuit of String theory with other applications that don't have anything to do with unification.
It's also turned out to be very interesting in mathematics. There's a very, one of the things that I'm most interested in is the intersection between mathematics and physics and the way the two fields affect each other and ideas from physics lead to very interesting things about mathematics, ideas in mathematics get used in **** in physics. And String Theory has been very, very fruitful in terms of raising questions which have led to very interesting mathematics. So, there's a very active field of research kind of in between math and physics in String Theory. But it just doesn't seem to be relevant to this question of unification.
Question: What model would you propose as an alternative to string theory?
Peter Woit: Well, I certainly shouldn't - my criticism of String Theory is not that, well, if you guys have just realized that you should be doing such and such, that would solve a lot of problems. I don't actually - I don't think that I - I have some of my own ideas which I am very excited by it which I'm pursuing which are also new ideas about how to use mathematics to do things in physics which don't have anything to do with String Theory and which kind of, at least lead me to see there's a lot of areas, a lot of things we don't understand which are closer to the standard model which we do know works. There's a lot of mathematical structure behind the standard model which is still kind of mysterious and which is not well understood and I think pursuing that is more likely to get us somewhere than the String Theory Unification idea. Just because you're starting from something which you know - a theory which you know is right and trying to further develop your understand of that theory is maybe a more fruitful thing to do than trying to just throw all that out and start afresh with something more speculative.
I guess one thing to talk about is this - what's happening in Geneva this month, so a lot of the problem with the field, as I said, is this business of being a victim of our own success, not having any experimental results that disagree with the theory, which makes it very, very hard to figure out how to improve a theory if you've got no clues as to what might possibly be wrong about it and needs to be changed. And so, there's this accelerator called the LHC, it's **** in Geneva and it's been in development for quite a few years now and it's been a long process getting it working, but just over the past month, they are finally going to subject beams into the accelerator and start colliding these beams and start doing some new physics. Over the next year they'll start raising the energy of this accelerator to the point where it will be in a new energy regime which allows us to get experimental data about what's happening at a high energy, which corresponds to what's happening at shorter and shorter distances.
To see what's happening in shorter and shorter distances, in some sense you need to use shorter and shorter wave length, or higher and higher energy probes, and this new accelerator, it finally promises some data in this new energy range beyond what we've been able to see so far. And there are reasons to believe that this new energy range is one where we can start getting some answers to some of these questions especially about mass. About why do different particles have different masses?
Recorded on December 16, 2009
Interviewed by Austin Allen