Greene on String Theory

"Greene's point here is that while we have some idea of what string theory predicts experimentally, there is also a lot that is unknown. So if our first attempts at experimental validation go wrong, that might mean simply that we need to refine the theory, rather than discard it altogether. He is not saying we should simply ignore apparently contrary data just to preserve the theory, and he is not saying the string theory will forever be immune to experimental refutation.

That is starkly different from the situation with belief in God. Such belief is not a theory that gets modified in the light of new data. People are not conducting experiments meant to refine our conception of God. With God belief the situation is simply that scientists have failed to turn."

Not to mention the fact that Greene is still talking about holding string theory ultimately to accountability through observation and experimental testing before full acceptance, a standard of evidence to which supernatural entities are, by definition, both inadequate and exempt.

~David D.G.

Other issues aside (though expert critics such as Smolin can hardly be easily dismissed), string theory definitely has an image problem partly of its own making. I find Greene, in particular, often adopts a worryingly millenarian tone.

It has the capacity to complete the Einsteinian revolution and could very well be the concluding chapter in our species' age-old quest to understand the deepest workings of the cosmos. (emphasis added)

A little less hubris might be more effective in the string club's PR campaign (and PR after all is what op-ed pieces are about).

He can study strings all he wants. The hype by Greene Kaku and others has raised unreasonable hopes to something which makes no testable predictions. Smolin's complaint is that it has sucked up ffunding and attention from other ideas.

About the supposed mathematical consistency of string theory:

You should notice that string theorists regularly say things like "we don't yet know what string theory is". What one can actually write down is a series of terms that is supposed to be the perturbation series for some unknown true version of string theory. This series is only asymptotic, thus divergent (and yes, mathematically inconsistent) at any finite value of the string coupling.

There have been many proposals for what non-perturbative string theory is supposed to be. So far they all have been either in violent contradiction with what we observe, or, well, mathematically inconsistent. So, it is important to keep in mind that the situation is not that we have a well-defined theory, and as we study it more do not find inconsistencies. It's rather that there is a dream that a certain theory exists with certain properties. No one knows what this is, and it has eluded decades of efforts to find it. It is true that no one can prove that the existence of this conjectural theory is mathematically inconsistent. But this may just be because the conjecture is too vague to ever be pinned down, not the sort of thing one can ever show to be inconsistent.

Peter-

Thanks for the comment. Your book has been sitting on my shelf for a while, but I haven't had time to read it yet. Guess I should!

One point that impressed me in Greene's essay is this:

While accessibility demands that I describe these developments using familiar words, beneath them lies a bedrock of rigorous analysis. We now have more than 20 years of painstaking research, filling tens of thousands of published pages of calculations, which attest to string theory's deep mathematical coherence. These calculations have given the theory countless opportunities to suffer the fate of previous proposals, but the fact is that every calculation that has ever been completed within string theory is free from mathematical contradictions.

Moreover, these works have also shown that many of the prized breakthroughs in fundamental physics, discovered over the past two centuries through arduous research using a wide range of approaches, can be found within string theory. It's as if one composer, working in isolation, produced the greatest hits of Beethoven, Count Basie and the Beatles. When you also consider that string theory has opened new areas of mathematical research, you can easily understand why it's captured the attention of so many leading scientists and mathematicians.

So it sounds to me like string theorists have been making steady progress over the last two decades, and that it is providing fertile ground for further research. You might be right that string theory will forever be too vague to be pinned down one way or the other, but it seems a bit hasty to reach that conclusion at this time. Do you agree with what Greene says in the paragraphs above? Or do you think it's largely propaganda?

One argument that I've seen raised in string theory's defense is that it's really the only game in town. Lee Smolin argues that there are other promising approaches to these problems that are being ignored because of the dominance of string theory. Do you agree with Smolin? In other words, do you think studying string theory is a complete waste of time or merely that it should be investigated as one interesting idea among many?

Jason,

I do think those two paragraphs of Brian's are kind of over-the-top and misleading, mixing some valid points with some exaggeration. It's certainly true that work on string theory has led to interesting mathematics. Some of this is directly related to string theory, but much of it comes from the fact that studying string theory raises all sorts of interesting questions and problems about things that are just pieces of the string theory story, e.g. 2d conformally invariant quantum field theories, and 6 d manifolds. Unfortunately the parts of string theory that are mathematically interesting are pretty much disjoint from the parts that people hope to use to connect to particle physics and the real world.

I'm not sure what Brian is referring to when he claims that "prized breakthroughs" from the past can be found in string theory, there are several possible things he could have in mind. It's true that string theory does reproduce important parts of theoretical physics, but often this is because the theory has been constructed explicitly to do so, or because any consistent framework has to reproduce certain things.

Saying that every string theory calculation is free of mathematical contradictions is kind of misleading. As I wrote earlier, people believe that calculating all the terms in the string perturbation series will give you a divergence and thus a mathematical contradiction. There is now no well-defined mathematical framework that one can point to and say that it is "string theory". It does seem quite possible that there may be such a mathematical framework, but from what is known about these things, they appear to have very unphysical features (e.g. exact supersymmetry). The calculations Brian is referring to in generally have very unphysical features.

Neither I nor Smolin claim that studying string theory is a waste of time. Both of us argue that it is an interesting idea, but that the way it has been pursued has been such as to make pursuing other very different interesting ideas quite difficult. Smolin and others are working on a set of ideas about quantum gravity quite different than string theory which show some promise, and I think he's right that the dominance of string theory has made it hard for people to get jobs if they work on those non-string theory ideas. Personally I'm much more interested in mathematical approaches to particle theory, not quantum gravity. In this area, again it is very difficult for anyone to get a job in a physics department doing this. There are a wide range of things we don't understand about quantum field theory that few if any people work on, partly because the problems are very difficult, partly because the way string theory dominates mathematically sophisticated particle theory research makes getting a job doing this very difficult. One thing to keep in mind is that the job situation in this field is terrible for everybody. As a result, there's a lot of pressure to be working on the most popular thing, in order to optimize one's chances of getting one of the few permanent jobs.
(By the way, I'm in a math, not physics, department, and have a permanent job I'm very happy with. These comments about job difficulties have nothing to do with my situation.)

Peter-

I read some of your book last night, specifically the preface, first two chapters and chapters 16-18. You seem even harder on string theory than Lee Smolin is. For example, the subtitle of your book is “The Failure of String Theory...” as opposed to, say, “The Unfulfilled Promise of String Theory...” At the end of your preface you write: “Without unexpected experimental data, new theoretical advances are likely to come about only if theorists turn their attention away from this failed program and toward the difficult task of better understanding the symmetries of the natural world.” That's pretty blunt, but it also seems a bit premature to me. Brian Greene makes a similar point in his essay.

Another point I found interesting in the portion of your book that I have read is that you seem unimpressed by the landscape idea promoted by, for example, Leonard Susskind. Perhaps because I spend a lot of time arguing with creationists, who frequently invoke the “fine-tuning” of the fundamental constants of the universe as evidence for God, I found Susskind's idea very appealing. In describing the landscape idea as an admission of failure, you seem to be taking for granted that a proper physical theory should uniquely predict the values of the constants of our universe. If I understand you correctly, that seems to be the majority view among physicists as well. But I'm not sure why that should be the default position, since it seems to me that such a theory would leave a big unsolved mystery: namely, why is there only one way of building a universe, and why is that one way conducive to the development of life.

The idea of the landscape solves this problem, and I'm not sure why the anthropic principle, as Susskind describes it, is so anathema to physicists. At any rate, it's not an admission of failure if it turns out to be true.

I think that you and Smolin and other string theory critics are persuasive when you argue that string theory has been vastly overhyped, and that it's difficult to imagine how we could ever test the theory. You also raise a lot of troubling concerns about the hegemony of string theory among physicists. I was surprised to read, for example, that you could not find a university press to publish your book. It seems to me that books like yours are precisely what university presses should be publishing: challenging, well-informed, provocative, and probably too technical for a popular-level book.

But I also think you cross a line in going from the assertion that string theory might forever remain untestable, to concluding that it is probably wrong.

One final point. Your description of the cultural pressures that force young physicists into trendy areas is interesting and important. I wonder, though, how many young graduate students really have to be dragged kicking and screaming into string theory, at the expense of their own novel ideas that are being suppressed as a result. For example, when I was a graduate student in mathematics my research area was governed more by a desire to work with a particular faculty member than it was to pursue a particular problem. I suspect many physics graduate students find themselves in the same situation.

Anyway, thanks again for your comments. You certainly provide a lot of food for thought.

Jason,

A few comments:

My claim that "string theory has failed" is elaborated in great detail in the book, and I try and make it clear that I am referring to something very specific: the idea of getting particle physics out of a 10 or 11d superstring/M-theory. After more than 20 years of an incredible amount of work by very smart people on this, it has been an utter failure. This program has explained nothing about particle theory, makes no predictions, and now there is not even a plausible idea about how it might be used to make predictions.

I think you're making a terrible mistake in trying to answer IDers by defending Susskind et. al. If a theory is inherently untestable, it's not science, whether it's string theory or ID. It is very important that scientists stick to that principle, and not give ID an opening by supporting a small group of people who have decided to try and prop up a failed theory by abandoning the standard criterion of what is science. A much better answer for IDers would be to point out that the issue of whether the Landscape is science is a topic of heated debate, with virtually all physicists agreeing that if it's not testable, it's not science.

I'm not claiming that grad students are being forced to abandon their own ideas and go into string theory. Frankly, I think the problems we face are very hard, and solving them will require someone very smart getting very good training and working very hard for quite a few years. The question is how to get good people the right kind of training and the opportunity to try to do something very ambitious. The problem is that almost the only viable career path in mathematically oriented particle physics is through string theory, and if you spend the years necessary to master that subject, you can't simultaneously do other things.

String theory continues to offer profound breadth and enormous potential...

I can't remember who said it, but the context was baseball: if you're a rookie, "potential" is a compliment. If you've been around a few years already, "potential" is no longer a compliment.

Peter-

Just to clarify the point about the Landscape: I agree completely that if it is not testable it is not science. I am not saying that the landscape is a done deal or has the authority of science behind it. The creationists, however, want to claim that fine-tuning is, all by itself, evidence for the existence of an intelligent designer to the universe. In that context I think it's perfectly reasonable to point out that there are other explanations for fine-tuning, such as the landscape, that are seriously discussed by physicists.

The bigger issue I was raising is that you seem to think the landscape should be dismissed out of hand as some sort of betrayal of science. Skepticism I can understand, but the hostility seems misplaced. Surely it's a real possibility that the string theorists are right, and the fact that there are an enormous number of possible vacuum states is telling us something important about the universe. And as Susskind argued in The Cosmic Landscape, if we reject the landscape altogether then fine-tuning remains a serious mystery.

Jason,

I'm not dismissing the Landscape out of hand, I'm claiming it is not science because it makes no predictions. And I've spent a lot of time looking at what the proponents of this idea have to say in terms of proposed scientific tests. The only "prediction" Susskind mentions in his book and his lectures is one about the sign of the curvature of the universe (not the magnitude...), and a paper last night on the arXiv argues that you can just as well get the opposite sign.

Personally I don't know what to make of the "fine-tuning" problem, and I'm not convinced it's a real problem. What I do know though is that I'm not very interested in proposed solutions to it that are not scientifically testable, whether they invoke a deity or a multiverse.

WOW!

Great Great discussion. I'm an old math major and failed physics student but I've kept up on cosmology and the evolution of the universe. One idea that I thought string theory hatched (an untestable one, granted) was that universes (and space) existed as membranes and that gravity can be thought to "leak" off the branes and connect from one to the other...and that the big bang was caused by branes intersecting. (best as I can remember it)

I forget the arguments but they were REALLY COOL IDEAS. Most likely that has something to do with the popularity of the subject.

Jason, great blog a fire man....have fun in your new house...

kd