In conjunction with his receipt of the Newton Medal, Edward Witten gave a public talk in London (now available on-line here), and an interview (available as part of a pod-cast here).
Witten’s talk was a rather polemical argument for string theory, in which he laid out his reasons for still feeling that string theory is on the right track. The video is in two parts, with the first part not especially interesting since it is pretty much word-for-word the standard arguments for string theory that he and others have been making since the mid-eighties. The second part has some more interesting content, including Witten’s comments on the evolution of his own personal relationship to the subject. This started in the early eighties, before the 1984 “First Superstring Revolution”, when he began studying string theory, feeling that it was an approach to unification that deserved more attention than it was getting.
A recurring theme in his talk is that “string theory” has gone through unexpected changes in perspective over the 30 years he has been working on it, with the unspoken argument being that some new change in perspective may yet make the current deadly problems of string unification go away. He takes an ambiguous attitude towards attempts in recent years to argue for a change in perspective to the pseudo-scientific “landscape”, explaining the arguments of proponents while not signing on to them. In the podcast he says “I don’t know if this is the right picture of the universe”, in the talk it’s “to my thinking we still need more clues to have a better picture whether that is the right interpretation.”
It’s interesting to compare Witten’s pro-string theory arguments to the somewhat similar ones of his much less mild-mannered thesis advisor David Gross (see here for a posting about a recent talk by Gross). Unlike Witten, Gross is clear where he stands on the anthropic landscape, denouncing it as pseudo-science. One other crucial difference has to do with their discussion of upcoming LHC results. Here, Gross argues strongly that the LHC will see supersymmetry, and is willing to put money on the table to back this up. Witten’s talk barely refers to the LHC, and while he argues that a point in string theory’s favor is its relation to supersymmetry, all he’s willing to say about supersymmetry or extra dimensions at the LHC is “it might happen if we’re fortunate enough.” The next interesting part of the string theory story may very well be what happens in 2013-4 when it becomes clear that supersymmetry and extra dimensions are not going to be seen at LHC energies. After paying off his debts, I wouldn’t be surprised to see Gross take this as an opportunity to back off from the idea of string theory unification. On the other hand, in his talk Witten seems to be positioning himself for carrying on as before, by now making arguments for string theory that don’t at all involve low-energy supersymmetry.
Witten ends his talk with the argument that we still know very little about what string theory is, and this implies that it remains an excellent subject for young physicists to start to work on. I suspect he sees all too well that among physicists the tide has changed, with students turning to other subjects as jobs in string theory dry up and prospects for progress on string theory unification look increasingly dismal. He’s trying to counter this by restating the arguments that have continued to keep him interested in the subject.
Update: Clifford Johnson has some very interesting and accurate comments on Witten and on how others react to him here.
Thanks for posting this Peter! I agree, the second half is more interesting than the first.
I second those thanks.
For me, the overwhelming impression is one of ongoing stagnation, combined with a melancholy nostalgia for the excitement that once surrounded the subject.
I really wonder if a truly good and insightful idea in this field would be recognized as such by its current practitioners.
In a recent video interview, David Gross said explicitly that he would still believe in supersymmetry even if the LHC did not find it. So it seems that your opinion about Gross changing his mind if experiments contradict theory is somewhat optimistic.
In addition, it is a pity how little string theorists are following the advice of Murray Gell-Mann, who suggested to clarify its basic principles, in the same way that Einstein clarified the basic principles of general relativity before starting calculations. A search on arxiv essentially leads to zero results.
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Makes me wish I could have a look at the future and see what people will think about string theory in 30 years from now. Will you still be blogging then and keep us updated? 😉
“The next interesting part of the string theory story may very well be what happens in 2013-4 when it becomes clear that supersymmetry and extra dimensions are not going to be seen at LHC energies.”
You don’t know that. I agree that there’s no reason to believe in SUSY or extra dimensions of any kind at the TeV scale (unless the latter is related to EWSB), but you make that statement with an (apparent) certainty that far exceeds what you know for sure.
On a related note, 2014 is WAY too early to be ruling out anything that Tevatron won’t have already excluded.
HI Peter,
I haven’t looked at the talk, as am travelling. However did Witten discuss
any high energy prospects for string theory (such as proton decay,
neutrino oscillations). 5 years ago, I attended this where Witten
talked about proton decay predictions in some string theory model.
Also I am guessing he said nothing about loop quantum gravity?
after seeing the talk I am more sympathetic to string theory, since
1. unifies gauge, diffeomorphism and super symmetries
2. interactionns are global only, so e.g. space could be emergent 3. it is not some theory that gets quantized, but starts with both parameters a’ and h (just imposing a commutator is not principled)
these are important to the foundations of quantum mechanics
“Dismissing” the landscape is itself not a scientific act. It is a question of fact as to whether string theory inherently predicts chaotic inflation; it is a question of fact as to whether we live in such a universe; and as things stand, an answer of yes to both questions is consistent with all the evidence.
I can understand people being skeptical about anthropic reasoning – though Weinberg’s argument is a good one – but you don’t get to reject string theory just because it suggests that particle physics is highly contingent. Practical field theory is about model-building meant to explain observations; in the era of the landscape, so is string phenomenology. In other words, it’s actually a return to business as usual, except that now you have a class of models that contain gravity.
Theorist,
Sure, that’s my prediction, confidence level 95%…. For people who do expect supersymmetry to show up at the LHC, I’m quite curious to know what amount of data they will need to see before they admit it’s not going to happen. In particular, what exactly are the terms of David Gross’s bet? I was using the date 2014 based on the expectation that by then there should be about 10 inverse-fb of analyzed data at design or near design energy (6.5 or 7 TeV/beam). If no signs of supersymmetry are seen in that data set, I’d think that prospects for them showing up in 100 or even 1000 inverse fb would be slim.
The LHC is going to be around for a long time, a very long time if you count various proposed upgrades. If those saying they are willing to bet on supersymmetry at the LHC are including the full life span of the machine with upgrades, betting with them would not be a good idea for someone middle-aged like me since I might not be around to collect.
Shantanu,
In the talk you refer to, Witten’s conclusion is negative:
“There is no hope of getting a real answer right now, because even if string theory is correct and even if nature is based on one of its GUT-lik realizations, there are far too many possibilities.”
He described string theory as the “only real idea about quantum gravity”, which I guess was a comment on LQG (I told you the talk was kind of a polemical one…).
Pmer,
In Witten’s talk he described the speculative hopes like the ones you mention that he still holds for the theory. I characterize his talk as polemical since he makes repeated overly-strong claims, not even mentioning the reasons for skepticism about them. I thought for a minute about examining these claims and explaining the problems with them, then realized “wait a minute, this is a complicated topic that I wrote a whole book about…”
Mitchell Porter,
Witten gave an exhaustive list of the reasons he thinks string theory is still worth pursuing. “String phenomenology” was not mentioned, I think for good reason.
Hi Mitchell Porter,
I had been under the impression that within the context of the KKLT or similar landscapes, it is not possible or practical to point to a specific model and say, “This specific model definitely has a cosmological constant small enough to be consistent with observations,” the problem being that large positive and negative contributions, of very different origin, have to cancel to a precision of about 1 part in 10^{120}, and neither type of contribution is controlled to anywhere near the necessary degree of precision.
If this is incorrect, or no longer the case, what would be a good reference to look for examples in?
Just a perhaps obvious remark about the previous comment by C. Austin
:
I believe it is correct, but it is not a specific limitation of string theory.
In no current framework for theoretical physics do we have the ability
to compute contributions to vacuum energy to sufficient accuracy to claim that one would definitely reproduce today’s vacuum energy to high precision.
It is important not to confuse specific problems of string theory (of which there may be many, or may be none), with general difficulties that face all theoretical models, and are not specific to one framework.
I believe the small value of the vacuum energy is in the latter category,
if one wants a normal predictive theory that explains the value, and does not want to resort to Weinberg arguments and vacuum multiplicity.
Chris, I think you’re right, but matching the observed cosmological constant is likely to be one of the last considerations in building a phenomenological model from string theory, precisely because it is supposed to arise from a complicated near-cancellation of many positive and negative contributions. Just reproducing the qualitative structure of the Standard Model in string theory is quite difficult and this offers guidance to the model builder that is far more direct. There are only a handful of ways to do it (that are known) and there are plenty of other issues to preoccupy the people working on each such possibility. At the moment I’m interested in orbifold models and there are a few papers on getting the cosmological constant there, e.g. hep-th/0603088. But it’s an issue for the final stage of string phenomenology, along with particle masses and moduli stabilization.
Mitchell Porter
How close are string theorists to “reproducing the qualitative structure of the Standard Model” along with such string phenomenology, along with particle masses and moduli stabilization and cc?
I can’t say. I’m a neophyte and remote from the centers of research. But from the literature I can say that what people try to do is to find string vacua which produce the “minimal supersymmetric standard model” (MSSM) or a supersymmetric GUT which reduces to the standard model. This is the qualitative part and has been done several times in several ways. Usually such models must contain extra particles that haven’t been seen, and so you have to find vacua where those extra particles are heavy or otherwise undetectable at present energies.
So far as I can see, people are not yet presenting further refinements of these models in which *all* the standard model parameters are derived at once, such as the masses of the *observed* particles, but instead this final step is being tackled piece by piece. There are a lot of papers on how to get a heavy top quark, there are a few papers on how to give the up and down quarks small nonzero masses, and so on. It’s all very incremental, and also framework-dependent (a mechanism for giving quarks mass in one class of model may not be transposable to another class of model), and even dependent on basic theoretical progress (e.g. arXiV:0707.1871).
In the history of all of science, has a hunch such as string theory ever received so much attention and funding?
Truly, string theory and its handwaving ethos shped physics over the past twenty-thirty years. Even LQG adopted some of its tenor and tone, often stating in its own defense, “hey if string theory can be not even wrong then we have every right to be not even wrong too!”
One must wonder about all the lost opportunities. What physicists were shut out from the academy and funding? How many bright young minds were lead down a seemingly dead-end street? How many gained tenure not by science, but by politics?
As Witten was an undergraduate history/politics major, it would have been interesting to hear his take on how string theory politicized and polemicized science.
Best,
Bruno Galileo
As Thomas Kuhn stated in his book about Scientific Revolutions, in many cases new ideas/theories do not just replace the old ones by being more successful, but they simply outlive them: they only get established when the older generation of supporters for the one theory gradually die out and the younger generation prefers the new theory. I think this is the most likely scenario how String theory will fade out, if it does at all, and this can take a very long time(Too long for middle-aged people to witness?). However, if the theory continues to win the souls of the younger physics students, whether by its rosy prospects(?), virility(?) or career pressure from its established, influential practitioners, then it may still be able to survive many more generations. In that case, even during the entire lifetime of a younger person such as me fundamental theoretical physics may continue in its current shape, monopolized by one seemingly promising theory which does not live up to its huge expectations.
In many cases, like… what?
hello all,
i believe the quote you were looking for comes from max planck:
A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up with it. –Max Planck
A great collection of quotes. This single post should serve as a resource for anyone on the fence and looking for string theory opinions from scientists who haven’t lost their sanity.
Getting back to the Witten topic: Everyone talks about how much of a genius he is but yet he is a string theorist. What contributions has he made that will stand the test of time when string theory eventually falls by the wayside? I’m not being a wiseguy and I’m not taking shots at Dr. Witten – I was just wondering if anyone could clear that up?
Mark Decker,
I wrote extensively about Witten and his non-string theory achievements in my book.
Witten made clear in his talk that one cannot start with a classical theory and then quantize it to get M-theory. Rather, you have to start with alpha’ and hbar together.
Can anybody expand on this? What is he talking about?
Pmer,
I think this is just one aspect of the fact that we don’t actually have any definition of M-theory or really know what it is. We really just have a conjecture that a theory exists with certain properties.
For first-quantized string theory, you can proceed in the usual way, “quantizing” a classical theory of strings. But this can only give you interactions through a series expansion in the string coupling (expansion in the genus of the string world-sheet) one that doesn’t converge. M-theory, among other things, is supposed to give a theory valid at non-zero string coupling. One doesn’t know how to get M-theory by “quantizing” a classical theory and Witten suggests will be impossible. But until you understand what the theory is a lot better, I don’t see how you can be sure that it might not have some formulation that could be interpreted as a “quantization” of something.