what is string theory?
string theory is an attempt to describe all the particles and all the forces of nature in a unified theoretical framework. It encompasses quantum mechanics and gravity, and is based on the idea that the fundamental building blocks of matter are not particles, but strings: objects that have a certain length and can vibrate in different ways.
You are reading: Books on string theory for beginners
The first book you’ve chosen is Superstring Theory, Volumes 1 and 2. It’s pretty technical, isn’t it?
As a professional on the subject, I’m attracted to serious accounts. The two volumes by Green, Schwarz, and Witten are an excellent early account of the subject. It was a topic that first came to light in the mid-1980s. The notion of string theory was already around, even in the late 1960s, but it was only in 1984, with the work of Green and Schwarz, that people became aware of it. he realized that string theory could actually be consistent with quantum mechanics, as well as include gravity, and could provide theories that closely resembled the standard model of particles. so there was this tremendous lightbulb moment, where everyone was like, ‘oh my god! this could work.’ And that book, Superstring Theory, captures that era in a very substantive way, as well as being a fairly readable account. In terms of readability, I’d say even non-physicists could get something out of the first chapter, and then later chapters, it’s more for professionals.
yeah, my nephew who did a masters in physics did not recommend it as one to dive into. he said that in a semester-long college course they only got halfway through the first volume…
yes, there is a lot there. what is surprising is that all of this came about in such a hurry: much of the material in that book is the result of two and a half years of activity. there was an incredible boom in creativity at that time and these two books are the record of it.
Isn’t it outdated just because it’s a while ago? it must be a rapidly evolving field.
It’s true. john schwarz once commented to me that the things he most regrets leaving out of that pair of books were the developments that occurred shortly after their publication. but that indicated that he was in fact part of a rapidly evolving field and he captured what was happening in a really convincing way. it really sped up the development of the field for a while. there must be parallels in other fields, where you have a solid contribution that really pushes the field forward in a remarkable way…
You told me you put your books in order, does that mean this is your favorite?
yes. there is something unusual and special about green, schwarz, witten. it was a very current book and yet a classic: the words instant classic come to mind. if we compare it, for example, with Polchinski’s book, another great story – in fact, the one I myself have used the most…
yeah, how about going to polchinski’s book, string theory, vols 1 & 2?
the polchinki book represents a very different achievement. it’s something he spent years writing and refining, which is why this book is superbly edited and refined. he also maintains an impressive website where he has caught all the errors in the equations and corrected them. it’s a work of tremendous care and detail, but it doesn’t have that green, schwarz, witty quality of being an instant classic. it is more considered classic.
Tell me more about why it’s important.
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includes the first key ideas of what is known as the second superstring revolution. people had taken the notions of the green, schwarz, witten textbook (and the many documents on which it was based) as far as they would go, and it seemed that some really new ideas were needed. those ideas had a more geometric flavor. they had to do with dimensions of space and time emerging from the dynamics, and with new objects, as well as strings, entering the story. And that’s where Polchinski made his biggest contribution, which goes by the name of D-Brana. d-branes are a big part of my own book on string theory, and they are string-like objects in the sense that they have a spatial extension; but they are much heavier than ropes. These are some of the ideas that Polchinski brought to the field through his research, and he captured those ideas in his book in an elegant and very systematic way.
On the subject of elegance, what about your next choice, the elegant universe?
This is a very popular account, probably more in line with what you expected. brian greene is a distinguished researcher, and he was the first to go to great lengths to really connect the public with the main ideas of modern string theory. others, like kaku, had made admirable efforts earlier, but greene really got it off the ground, saying we can tell the public what string theory is all about, right now. and he did a great job. the book works on many levels: i gave my mom a copy when she came out, and i also got very positive impressions of the book from norman ramsey, who is a nobel laureate physicist at harvard. so it’s a great achievement, and part of why it’s a great achievement is that it covers not only string theory but also the accepted pillars of twentieth-century theoretical physics, namely quantum mechanics and relativity. he spends half of the book going over these accepted theories in a way that is really accessible to the layman. so two thumbs up.
Can someone without a scientific background read this book and feel like they understand what happened in the 20th century in terms of physics?
I think that’s correct. if you’re looking at the broad spectrum of the fundamental theories of physics, and string theory in particular, the elegant universe is definitely a good choice, though not the only good choice.
Your next book, a first course in string theory, is presumably more for students?
yes. This is a book by my MIT colleague Barton Zwiebach, and it grew out of a year-long course he taught at MIT for undergraduates who wanted to learn string theory. so it reviews much of the material covered in the green, schwarz, witten, and polchinski books at a more detailed level. it does so without pretending to be complete, but you really get the idea. I remember when I learned string theory myself, what was difficult was that it seemed like you had to learn each idea three or four times, because everything had been discussed with different methods, by different groups, competing with each other. and to get the full picture you had to absorb a lot. Zwiebach’s achievement here is that he found a fairly short and direct path through the center of the subject that really makes it more accessible.
so your target audience here is college physics students.
I think that’s fair. or at least students with a strong scientific background and good calculus skills. for example, one of his chapters is “a brief review of lagrangian mechanics”, which is a very challenging topic that has to do with the calculus of multiple variables. so you definitely have to be on your toes to get the full gist of parts of zwiebach’s book. but of all the books I’ve mentioned, this is the most direct path to the heart of the matter.
Finally, we have the string theory lectures by d lust and s theisen.
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, direct prose descriptions of what’s going on. it overlaps the algebra you need to really understand the subject. but at the time I thought it was better than other books to tell you up front, “this is what we’re going to do, and this is 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 is shorter. this is something that I value very much, because nobody really goes through 300 pages of physics. Not even most teachers pick up a 300-page book or article and read it. if you really want to be understood, brevity is a good thing.
What level of knowledge does this book target?
I would say advanced college students and beginning grad students. It is comparable to Zwiebach’s book. Zwiebach’s book is easier to start, but harder to finish, because there really is so much in there. in lust and theisen, first of all, it’s string theory from an earlier age, when things were simpler and there wasn’t much to learn. but they really get to the heart of how string theory could be a so-called “theory of everything”.
what is it?
a theory that encompasses all the forces of nature and includes all the fundamental particles that we see. that does not mean that it is a theory that will allow you to calculate everything. that would be a wonderful theory indeed, but string theory isn’t likely to deliver it any time soon. its goal is much more modest: it is to provide the umbrella under which all fundamental interactions fall.
There would still be at least as much scope for condensed matter physics, whose goal is not to discover fundamental interactions, but to describe how objects interact when there aren’t many of them. you have this broad tapestry, with string theory on one side trying to get to the most reductive and simple fundamental features, and there are other parts, like condensed matter physics, dealing with what happens when you stuff more things into your system. than you can keep track of one by one.
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One problem with string theory that I’ve heard is that there isn’t just one string theory, there’s a number that coexists, which nullifies the predictive power of string theory, its ability to explain physical phenomena. is that a valid criticism?
yes and no. It certainly repeats itself frequently. a quick answer would be to say that quantum field theory is like that too, but no one complains about that. this is the theory for which richard feynman won his nobel prize, where you describe 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 handle all sorts of things: it can handle electrons, protons, neutrons, etc, etc. but by itself, it only has so much information, and you have to supplement quantum field theory with a lot of physics-specific knowledge before you can take advantage of it. the quick answer would be to say, it’s always like this: whenever you have a theoretical framework, it’s always been the case that you have to include facts about the world. it’s true that historically, in the 1980s, people suggested the idea that string theory might be different. that maybe in string theory, you wouldn’t have to add facts about the world before you could get something out of the theory; you could just sit back and calculate everything. I never said that. I wasn’t working on string theory at the time. I wouldn’t have expected it, and it didn’t happen, but what else is new? it’s true for all theories we know of, so string theory is neither better nor worse in that regard.
what really worries me is to what extent string theory can be connected with modern experiments. It’s one thing to say you have to put facts about the world before you can get anything, but a much bigger concern is that once you put 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 of modern physics, once you’re willing to use string theory as a computational tool instead of saying it will just be a theory that predicts everything from scratch? Instead you say, I’m going to use this set of ideas to understand the experiments. in fact, there have been a number of calculations in the last five to seven years, where some surprisingly successful numerical predictions have come out of string theory.
yes, I was going to ask, will string theory lead to testable predictions this century?
To some extent, the calculations people have done related to heavy ion physics fall into that category. the problem is that heavy ion collisions are messy business. they’re like a huge car accident where everything breaks, there’s tremendous confusion, and then you try to figure out what happened. so any kind of numerical description is inevitably going to have some uncertainties, and the extent to which string theory calculations work I’d say it’s a factor of two. string theory might predict that such and such a number is one, and experiment might say it’s approximately two, but it might be one instead. that’s the kind of precision you can usually do things with.
Now there are some things that are measured, and there is some hope that string theory calculations will get them within 15 to 20 percent. but I think that is asking too much of both experimental and theoretical work. there are still other calculations, let’s say in quantum field theory, where the numbers are known to seven, ten decimal places, both in theory and in experiments, and they work. that’s a bar string theory that won’t be cleared up any time soon. could get there this century, maybe. i think we should 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 were going to explain some of the aspects of heavy ion collisions that we have been working on in recent years, I would have been very surprised. so no one knows the future.
what is your opinion about the lhc? do you think the first evidence of supersymmetry will come from the next experiments?
that would be wonderful. I devoted an entire chapter of my book to supersymmetry and the lhc because it is 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 is closely tied to string theory – the two ideas grew up together and string theory clearly implies supersymmetry at some level, so imagining supersymmetry without string theory of strings would be unnatural. and supersymmetry may be experimentally within the reach of lhc.
so that would be a big, big problem. of course it can happen and it can not. one of the reasons i like to talk about some of the recent successes applying string theory to other kinds of collisions, not lhc collisions, but heavy ion collisions, is because there i can point to calculations in string theory strings where they have already been made. done, and we can already roughly agree that they work. so whatever happens, we can point to these things and say, ‘string theory did this.’ here’s something to build on.
so even if things don’t work out in gin… when will we know, by the way?
I wish I could tell you. positive evidence would be much more convincing than negative evidence. if they discover a bunch of new particles and they roughly fit the pattern that supersymmetry predicts, a lot of people will say, “it’s champagne time” and it would be champagne time, but it would take a while. much more than new particles to be sure that the supersymmetry is correct. if they don’t see such particles, then unfortunately we would still be left wondering, because what could happen is that supersymmetry may be part of some theories, but it can predict particles that are a bit too heavy for the lhc to detect. produce in any quantity. if that’s the case, it’s just bad luck that the lhc is almost powerful enough, but not powerful enough, to see supersymmetry.
I myself would prefer the result of the lhc discovering something totally unexpected. and then we all ran around and tried to figure out what was really going on, and we certainly used string theory, among other tools, to try to understand it.
It is impossible to predict what will happen.
if we knew what was going to happen, it wouldn’t make sense to spend billions of dollars to build the lhc. supersymmetry is the most likely result of the lhc, but only in the sense that it is more likely than any other comparable specific result you can name. I’m not saying it’s particularly likely, I don’t know. There are other possible outcomes: for example, the LHC could produce microscopic black holes. I really don’t think that’s likely. it would take a series of matches which seems to be out of place. possible, but off the wall.
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