Status of Superstring and M-theory

A write-up by John Schwarz of his Erice lectures from this past summer has now appeared on the arXiv, with the title Status of Superstring and M-theory. In his second lecture, Schwarz provides a good review of the various attempts to do “string phenomenology” by trying to find a “string background” that doesn’t conflict with known particle physics. He devotes particular attention to the newest of these backgrounds, so-called “F-theory local models”, providing a summary of the rather complicated constructions involved. Schwarz doesn’t describe any experimental predictions of such models, just noting:

It will be very interesting to see what predictions can be made before the experimental results pour in and whether they turn out to be correct.

For more discussion of these models and the question of whether they predict anything, see here.

Schwarz begins with an account of his interactions with Sidney Coleman at Aspen and elsewhere:

I recall him once saying that there are three things that he does not like, all of which are becoming popular: supersymmetry, strings, and extra dimensions. Obviously, my views are quite different, but this did not lessen my regard for him, nor did it harm our personal relationship. In fact, I respected his honesty, especially as he did not try to impose his prejudices on the community.

About the anthropic landscape issue, he has this to say:

Perhaps the absurdly large number of flux vacua that typically arise in flux compactifications has discouraged people from trying to construct viable particle physics models. In fact, this large number of vacua has motivated the suggestion that various parameters of Nature (such as the cosmological constant) should be studied statistically on the landscape. I don’t really understand the logic of doing this, since this approach seems to assume implicitly that Nature corresponds to a more or less random vacuum. This in turn is motivated by some vague idea about how Universes are spawned in the Multiverse in a process of eternal inflation. Then the story gets even more entangled when the anthropic principle is brought into the discussion. Some people are enthusiastic about this approach, but I find it fundamentally defeatist. It is not the way I like to think about particle physics.

Meanwhile, public promotion of the Multiverse continues, with the opinion pages of Britain’s The Independent today featuring a piece by Bernard Carr entitled Fifth dimensions, space bubbles and other facets of the multiverse. Carr describes the “growing popularity” of the multiverse proposal, ending with:

But is the “multiverse” a proper scientific proposal or just philosophy? Despite the growing popularity of the proposal, the idea is speculative and currently untestable – and it may always remain so. Astronomers may never be able to observe the other universes with their telescopes and particle physicists may never be able to detect the extra dimensions with their accelerators. So, although some physicists favour the multiverse because it may do away with the need for a creator, others regard the idea as equally metaphysical. What is really at stake is the nature of science itself.

Carr characterizes some multiverse proponents as atheists favoring something that doesn’t seem to fit into the conventional scientific method because it gives an answer to the argument from design for a deity. For more about this all-too-common argument for the multiverse, being promoted by Susskind and others, see here. In answer to such claims about religion being promoted by physicists, New Scientist this week is running a sensible piece by Amanda Gefter entitled Why it’s not as simple as God vs the multiverse. It makes the obvious point about the multiverse-God dichotomy:

Science never boils down to a choice between two alternative explanations. It is always plausible that both are wrong and a third or fourth or fifth will turn out to be correct.

Update: For more multiverse mania, see today’s colloquium at Perimeter here. The intense promotion of this pseudo-science continues, but I don’t think it’s getting any traction.

Update: Yet more media attention to the God vs. Multiverse debate, now from the Guardian.

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10 Responses to Status of Superstring and M-theory

  1. Shantanu says:

    Peter, I watched this nice talk on testing inflationary models
    http://pirsa.org/08110021/ and near 46th minute where the
    speakers talked about testing consistency of string theory, there was a lot of
    discussions. Do you think these tests are really testing string thoery?

  2. Peter Woit says:

    Shantanu,

    Because of the low audio quality, I found it hard to hear exactly what was going on in that discussion. But, as far as I can tell, what’s going on is that the speaker and others are talking about their “dream” to “constrain string theory”. So, in their most optimistic scenario, all they will ever be able to do is to, amongst the infinite complexity of the landscape, say that some regions of it are consistent with various string inflation scenarios, and some aren’t. This can never actually produce any evidence for or against string theory itself, it just takes physics where some string theorists want it to go. String theory is accepted as the “best guess” as to how the universe works, taught to high schools students, and fundamental physics becomes reduced to just parametrizing observations with string vacua, vacua that are capable of parametrizing anything that is ever likely to be observed. Doesn’t really look like science to me….

  3. Samuel Prime says:

    You wonder if the enormous advancements in physics (and science) are prompting researchers to delve into, speculate, and think about superquestions and more adventerous ideas that, by nature, go beyond the currently accepted methods of science. It’s hard to imagine science not evolving in its methods. (Causality was an integral part of classical physics 100 years ago but now no longer—or rather is still part of relativity but not quantum theory.)

    Perhaps new and/or additional methods will need to be added in order for us to advance (or define) our ‘knowledge’ of such huge unknowns (and which are so intriging to us humans). (Perhaps string theory is a symptom of this trend to ‘push’ the methods of science.) Of course, as Socrates would say, I know nothing, but I do think it is safe to say that science in 200, or even 500, years from now will not be the same as it is today. Our perspective will change. Perhaps even our methods and manner of perception.

    We are hitting the boundaries of experiment. (More specifically, experiment that can be humanly done.) These many superquestions and supercuriosities show that we are more curious by questions that now seem untestable by our current state of abilities, at least to a high degree of certainty. Nevertheless, the big curiosities could play a role how the methods of science develop.

    I have wondered if the development science will become similar to that of mathematics. (Research in high energy physics is already beginning to look like that!) For example, even though the Axiom of Choice and the Continuum Hypothesis are unprovable from the other Zermelo-Frankael axioms of set theory, mathematicians generally feel free to assume them in their work (even though they are ‘untestable’). So if in science we hit boundaries of experiment and the unknowable, we may have to find ways to penetrate such boundaries.

  4. Chris W. says:

    Samuel Prime: (Causality was an integral part of classical physics 100 years ago but now no longer—or rather is still part of relativity but not quantum theory.)

    It sounds like you’re confusing causality with strict determinism. Causal structure is very much a part of quantum theory. (By the way, see Scott Aaronson’s latest post.)

    More to the point: When delving into superquestions and more adventurous ideas leads a scientist—or someone who claims to be a scientist—to start regarding testability against empirical observation as fundamentally irrelevant, it becomes utterly self-defeating. Some theorizing about the multiverse flirts with that attitude. Metaphysical speculation is (in my opinion) an inescapable part of formulating scientific questions that seek deep understanding, but the ultimate goal is a challenging confrontation with observation, hopefully with a successful outcome for the theory in question.

  5. Esornep says:

    It’s rather amusing that Schwarz can dismiss the nucleation of bubble universes ["This in turn is motivated by some vague idea about how Universes are spawned in the Multiverse in a process of eternal inflation"] just after his reverential remarks about Coleman, in view of the fact that Coleman, with De Luccia, was the author of the massively influential paper on which all current multiverse theorizing is based. Yes, Coleman, who was opposed to higher dimensions, etc etc etc, started the whole multiverse ball rolling. Coleman was quite explicit about the possibility that our own Universe is a bubble, and of course there is no reason for such a bubble to be unique.

  6. Samuel Prime says:

    Chris W., could you please tell me where I mentioned determinism so that it would be confused (by yourself) with causality? Any educated physicists who read my comment would know what I meant by causality in the contrasting contexts of relativity and QM that I stated.

    As for the unhappy comments toward the multiverse idea, I would hardly consider Steven Weinberg as among those ‘self-defeating’ physicists. In his essay Living in the Multiverse, Weinberg notes:

    There is also a less creditable reason for hostility to the idea of a multiverse, based on the fact that we will never be able to observe any subuniverses except our own. Livio and Rees and Tegmark have given thorough discussions of various other ingredients of accepted theories that we will never be able to observe, without our being led to reject these theories. The test of a physical theory is not that everything in it should be observable and every prediction it makes should be testable, but rather that enough is observable and enough predictions are testable to give us confidence that the theory is right.

    It seems that two standards are being applied. One standard to theories we don’t like, where we raise the standards to too rigorous a level (and requiring each idea to be testable), and another standard to theories we like and where we do not pick on them for not being testable in every respect. One could have picked on the Big Bang theory for the idea of an infinitely small point or particle when it exploded, even though that idea by itself alone is untestable.

    As for myself, I can’t say I believe in the multiverse idea. However, I am open to the concept. It can’t just be an idea that stands by itself, but along with a broader mathematical framework, coupled with other hypotheses, so that the whole theory (not just one single idea) can either be tested or related to other known or accepted facts or scenarios that may be worthy of examination and further study.

    I don’t know of any grand and successful physical theory in which all of its ideas and concepts are each testable. So let’s not push the testability thing too far or it will haunt our efforts.

  7. anon. says:

    ‘The test of a physical theory is not that everything in it should be observable and every prediction it makes should be testable, but rather that enough is observable and enough predictions are testable to give us confidence that the theory is right.’ – Weinberg, quoted above.

    That’s wishy-washy brane-washing, because there is no unequivocal definition of ‘enough’ and ‘us’. It’s easy for string theorists to find even today ‘enough’ ad hoc prediction of gravity, etc., in string theory to give them confidence (to the point of arrogance) that their pet theory or religion is the right one. The fact that they are confident that they are right isn’t worth much, however. Try applying Weinberg’s statement to flat-earth theory, Piltdown man, cold fusion, Phlogiston, Caloric, etc.

  8. Jake the Snake says:

    The God v. Multiverse issue is an amusing one. Intelligent Design types love the dichotomy because it “forces” atheists to accept their God or postulate a weird hypothesis about multiple universes as an alternative explanation for the apparent “design” or “fine-tuning.” Likewise, some atheists might not like it because they do not understand the real nature of the God-Multiverse dichotomy and think they may be compelled to postulate a multiverse, while those who do believe in a multiverse think they have an answer to the arguments of intelligent design. The problem is that any hypothesis which appeals to either a supernatural entity or multiple cosmoses fails the test of Occam’s razor. Fortunately, the dichotomy between God and Multiverse is simply not true.

    There is no compelling to declare either of the two scenarios true, or, on the contrary one can combine the two. Bradley Monton considered the ramifications of doing this to resolve the so-called problem of evil.

    http://bradleymonton.wordpress.com/2008/12/03/the-many-universe-solution-to-the-problem-of-evil/

    Why would Monton, who claims to be an atheist, attempt the above argument? It seems to be a thought experiment. As Monton reveals, he did not come up with the proposed solution, but he analyzes it briefly and considers the multiverse proposal to be an unsatisfactory answer to the problem of evil.

  9. Samuel Prime says:

    ” … there is no unequivocal definition of ‘enough’ and ‘us’. “

    Yes, there is a practical one: the general consensus of the physics community. I’ll grant you, though, that string theory has much more work to do before it proves its mettle (if it is the right approach). (I think they know that themselves, even if some are arrogant, just as arrogance can be found on any side of an issue.)

    I had an arrogant and mean math teacher in school, but for some reason I separated his arrogance from my fascination for math. After him there was a much kinder teacher, and that made it all the more sweeter.

  10. TCO says:

    Skepticism about string theory has penetrated the popular culture. Its days of supremacy in funding and tenure positions are numbered. You should not feel defeated or defensive. You’ve practically won already.

    Lubos is smart and young so he can find something useful to do, still. Though if he doesn’t have tenure yet, I would be rather worried. Harvard is bad enough but with his field falling out of favor…anyhow, it just means he’ll latch on at a less prestigious school after Harvard dings him in a couple years.

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