Lectures on String Theory

"This is the book by Dieter Lust and Stefan Theisen, which I included partly for sentimental reasons because it is, in fact, the book from which I learned string theory. But it’s also a great book. Among its advantages is that it has good, straightforward prose descriptions of what is going on. It does layer on the algebra that you need in order to really get the subject. But it seemed to me at the time that it was better than other books at telling you upfront, ‘Here is what we’re going to do – and here’s why.’ So it was a very good pedagogical book on the subject. It shares some of the same positive qualities as Zwiebach’s book, but it’s shorter. This is something that I value greatly, because nobody really gets through 300 pages of physics. Not even most professors really pick up a 300-page book or paper and read it. If you really want to be understood, brevity is a good thing. I would say advanced undergraduates and beginning graduate students. It’s comparable to Zwiebach’s book. Zwiebach’s book is easier to get started on, but hard to finish, because there really is a lot there. In Lust and Theisen, first of all it is string theory of an earlier era, when things were simpler, and there wasn’t quite so much to learn. But they really do get to the heart of the matter of how string theory might be a so-called ‘theory of everything’. A theory that encompasses all forces of nature and includes all the fundamental particles that we see. That doesn’t mean it’s a theory that will allow you to calculate everything. That would be truly a wonderful theory to have, but string theory is not likely to provide that any time soon. Its aim is much more modest – it’s to provide the umbrella under which all fundamental interactions fall. There would still be at least as much scope for condensed matter physics, whose aim is not to discover any fundamental interactions, but instead to describe how objects interact when there aren’t very many of them. You have this broad tapestry, with string theory on one side, trying to get at the most reductive and simple fundamental features, and there are other parts, like condensed matter physics, which are all about what happens when you get more things into your system than you can keep track of one by one. Yes and no. It’s certainly oft-repeated. One quick comeback would be to say quantum field theory is like that too, but nobody complains about it. This is the theory that Richard Feynman won his Nobel Prize for, where you are describing the quantum mechanics of relativistic particles. And if you just start with that as your goal you get a wonderfully broad and inclusive structure, which can deal with all sorts of things – it can deal with electrons, protons, neutrons and so on and so forth. But by itself, it only has so much information and you have to supplement quantum field theory with a lot of specific knowledge of physics before you’re going to get anything out of it. The quick comeback would be to say, it’s always like that – whenever you have a theoretical framework it has always been the case that you have to include facts about the world. It’s true that historically, in the 1980s, people did suggest the idea that string theory might be different. That maybe in string theory, you wouldn’t have to add in facts about the world before you could get something out of the theory; you could just sit down and calculate everything. I never said that. I wasn’t working in string theory at the time. I wouldn’t have expected it, and it didn’t happen, but what else is new? It’s true of all theories that we know – so string theory is no better and no worse in that regard. Support Five Books Five Books interviews are expensive to produce. If you're enjoying this interview, please support us by donating a small amount . Where I really do worry is the extent to which string theory can be connected to modern experiment. It’s one thing to say that you have to put in facts about the world before you can get anything out, but a far greater worry is, once you put in facts about the world, what do you get? So what I’m working on right now is that very question. What can you get out about modern physics, once you are willing to use string theory as a calculational tool rather than saying it’s going to be just a theory which predicts everything from scratch? Instead you say, I’m going to use this set of ideas to understand experiments. In fact there have been a number of calculations in the past five to seven years, where some strikingly successful numerical predictions have come out of string theory. To an extent, the calculations that people have done related to heavy ion physics are in that category. The trouble is that heavy ion collisions are complicated affairs. They’re like an enormous car crash where everything breaks, there’s tremendous confusion, and then you try to sort out afterwards what happened. So any kind of numerical description of it is inevitably going to have some uncertainties, and the degree to which string theory calculations work I would say is a factor of two. String theory might predict that such and such number is one, and the experiment might say well it’s about two, but it could instead be one. That’s the kind of accuracy with which things can typically be done. Now there are some things that are measured, and there is some hope that string theory calculations get them right within 15 to 20 per cent. But I think that’s asking a lot of both the experimental and the theoretical work. There are yet other calculations, say in quantum field theory, where the numbers are known to seven, ten decimal places, both in theory and in experiment – and they work. That’s a bar string theory is not going to clear any time soon. It might get there this century – maybe. I think we’d have to learn a lot more about string theory as a theory, and a lot more about what’s going on at the LHC [Large Hadron Collider in Geneva]. But if 15 years ago you had suggested to me that we’re going to explain some of the aspects of heavy ion collisions that we’ve been working on in the past few years, I would have been very surprised. So no one knows the future. That would be wonderful. I devoted an entire chapter of my book to supersymmetry and the LHC because it’s the great white hope of many theorists that the LHC will discover exactly that. Supersymmetry would be a new way of understanding space and time, and it’s closely tied to string theory – the two ideas grew up together and string theory clearly implies supersymmetry at some level, so to imagine supersymmetry without string theory would be unnatural. And supersymmetry may be experimentally within reach of the LHC. So that would be a big, big deal. Of course it might happen and it might not. One of the reasons I like to talk about some of the recent successes applying string theory to other kinds of collisions – not the LHC collisions, but the heavy ion collisions – is because there I can point out calculations in string theory where they already have been done, and we can already approximately agree that they work. So, come what may, we can point to this stuff and say, ‘String theory did this.’ There is something here to build upon. I wish I could tell you. Positive evidence would be far more compelling than negative evidence. If they discover a whole bunch of new particles and they approximately fit the pattern that supersymmetry predicts, then a lot of people will say, ‘It’s time for the champagne.’ And it would be time for the champagne, even though it would take a lot more than new particles to be sure supersymmetry is right. If they don’t see such particles, then, unfortunately, we would still be left in doubt, because what could happen is that supersymmetry can be part of some theories, but it can predict particles that are a little too heavy for the LHC to produce in any quantity. If that’s the case, it’s bad luck that the LHC is almost powerful enough, but not quite powerful enough, to see supersymmetry. Get the weekly Five Books newsletter I myself would prefer the outcome that the LHC discovers something totally unexpected. And then we would all race around and try to figure out what is actually going on, and no doubt use string theory, among other tools, to try and understand it. If we knew what would happen, there wouldn’t be any point spending the billions of dollars to build the LHC. Supersymmetry is the most likely outcome of the LHC, but only in the sense that it’s more likely than any other comparably specific outcome you can name. I’m not saying it’s particularly likely – I don’t know. There are other possible outcomes: for example, the LHC might produce microscopic black holes. I really don’t think that’s likely. It would take a series of coincidences that seems to be off the wall. Possible, but off the wall."