Bert Schellekens has posted on the arXiv an extended 87 page argument for the anthropic string theory landscape, entitled The Emperor’s Last Clothes? While most string theorists find the existence of the landscape and the corresponding inability to get any predictions out of the theory about particle physics rather discouraging, Schellekens instead sees this as an argument in its favor:
Initially, when string theory was touted as the “theory of everything” around 1984, there were hopes it would lead to exactly the opposite: a unique derivation of all the laws of physics. Evidence that quite the opposite was true started emerging almost immediately after 1984, but most people chose to ignore it. In 2003, after important additional evidence had been found, Leonard Susskind published a paper  entitled “The Anthropic Landscape of String Theory”, which finally started a debate that should have started fifteen years earlier. What is at stake in this debate is not only the uniqueness of our universe, but also the fate of string theory as a fundamental theory of all interactions.
In my opinion string theory gives the right answer, and the fact that it does adds to the evidence in its favour. I can say this without being accused of trying to put a positive spin on the recent developments, because I actually wrote in 1998  that I hoped string theory would ultimately lead to a huge number of possible choices for the laws of physics, a point of view I have been advocating since the late eighties. I reached that conclusion after having been involved in one of the first papers  pointing out that the number of possibilities was humongous…
We all hope to live during a time when big things are happening in our field, and I have never doubted that this is one of those things. I have spent the last twenty years trying to convey my sense of excitement to my colleagues, but with little success. But in the last few years I have been delighted to see more evidence coming in supporting this point of view, so that the mood has started to change. I hope this is the right time to make one more attempt.
Schellekens describes in great detail the anthropic argument and the arguments for the string theory landscape. He addresses some of the counter-arguments, especially in three appendices. He doesn’t explictly deal with the main counter-argument that I’ve made repeatedly here: the anthropic landscape is not science (since it is not testable), rather it is just an elaborate excuse for the failure of the speculative idea of getting the SM out of a 10/11d string/M-theory.
Schellekens has the following comments about “string phenomenology”, noting that he worked in the area around 1987 and recently, finding not much has changed:
I have been active in this are around 1987 (which led me to the conclusions presented here) and again in the last few years, and to me the similarities are more striking than the differences. There has certainly been progress: we can obtain string solutions that are more similar to the Standard Model than twenty years ago, and we have more methods to construct them. There has been major progress in moduli stabilization and supersymmetry breaking. There is more interest in “landscape statistics”. But very little seems to have changed in the way many people view the problem we are facing. Although many of my string phenomenology colleagues claim that it was clear to them a long time ago that there are many solutions, I cannot help noticing that they still talk about their most recent “model” as if it would actually have a chance to be the Standard Model. And even nowadays one still hears the occasional expression of hope for the unknown and elusive dynamical principle that will select the vacuum. The most common way of dealing with the large vacuum degeneracy is to say “I do not care about the other 10500vacua, I only care about the one that describes our universe”. That may sound reasonable, and fact it may sound like the very definition of phenomenology, but it is actually an escape from reality.
First of all, if indeed there are 10500 vacua, it is highly unlikely that anyone will find “the Standard Model” in string theory. One should expect to find a huge number that satisfy all current experimental constraints. In addition, although we now have many techniques at our disposal to construct string theories in four dimensions, it is quite clear that we are just scratching the surface. Statistically speaking, our chances of finding even one of the expected huge number of Standard Model realizations is essentially zero. Furthermore, even if we do find one, we can only make predictions about novel phenomena if we know all the other solutions and their predictions for the same phenomena. This is a crucial change in comparison to the state of the art about ten years ago: with 1020 solutions (the largest number anyone may have expected), if one is found that agrees with all current data, the probability that there is a second one is extremely small. With 10500, the same probability is astronomical. So we should forget about the idea of finding the Standard Model and then making predictions based on it.
As for LHC predictions, Schellekens argues against the idea that it will see supersymmetry:
One could say that supersymmetry is a non-solution to a non-problem: the large weak scale hierarchy is already understood anthropically, and supersymmetry by itself does not even explain it…
With the start of the LHC just months away (at least, I hope so), this is more or less the last moment to make a prediction. Will low energy supersymmetry be found or not? I am convinced that without the strange coincidence of the gauge coupling convergence, many people (including myself) would bet against it. It just seems to have been hiding itself too well, and it creates the need for new fine-tunings that are not even anthropic (and hence more serious than the one supersymmetry is supposed to solve). But even if evidence for low energy supersymmetry emerges at the LHC, in the context of a landscape it will not be the explanation for the smallness of the weak scale. The explanation will in any case be anthropic. The landscape will undoubtedly allow a distribution of values for the weak scale, including values outside the anthropic window.
Schellekens ends up making the currently fashionable argument that it doesn’t matter that string theory doesn’t predict anything testable about particle theory, that the important thing is that it is a theory of quantum gravity:
During the last two decades there was some reason to hope that we might be able to do that [get experimental confirmation of string theory] by means of some prediction of a Standard Model feature. That hope is fading now. I am not saying that this will never happen, but I have seen too much wishful thinking to make an optimistic statement about this. Essentially, we came to that conclusion already in 1986 . We are dealing with a theory of gravity. Getting information about it through the back door of particle physics is a luxury that we once had good reasons to hope for, but that may not exist. Rejecting a theory of gravity that makes no particle physics prediction may be like rejecting the theory of continental drift because it does not predict the shape of Mount Everest
He then goes on to acknowledge that we’re not going to get any experimental tests out of the quantum gravity aspect of string theory either:
One cannot count on any direct experimental check of a theory of quantum gravity, since any observable consequences it might have are extremely small, unless we are extremely lucky.
In the end, he seems to argue that the only evidence for string theory we may ever get is its consistency, something which is a very long ways from being shown. He does argue that string theory is in principle falsifiable, but the example he gives (that string theory would be wrong if coupling constants varied observably on astronomical scales) is not an uncontroversial one since other string theorists have argued that varying coupling constants would be evidence for string theory. There’s also the usual “who knows?” argument used against anyone who points out that evidence against an idea is overwhelming:
On longer timescales, it is clearly ridiculous to pretend that what we currently know will be the state of the art forever. When Darwin formulated his theory of evolution he was unaware of Mendel’s results on inheritance, and could not even have imagined DNA.
The truly peculiar thing about this is to see a scientist almost gleeful at the idea that a theory they have worked on their entire professional lives doesn’t predict anything:
To me, what is emerging looks very appealing. It fulfills and even exceeds the hopes I expressed in 1998. It is has been amazing to see this theory leading us in the right direction, sometimes even against the initial expectations of most of the people working on it. We should continue to follow its lead, and do everything in our power to strengthen its theoretical underpinnings. The emergence of a huge landscape” makes this more worthwhile then ever before.
Unfortunately, Schellekens is far from alone in this. At the FQXI web-site there’s an article about the string theory/cosmology couple Andrei Linde and Renata Kallosh entitled A Perfect Match (“How do you tie down the physics of the multiverse? With string.”) In it, Kallosh explains how “string cosmology” is now the hot topic:
These days, in fact, collaboration be-tween string people and cosmology people is all the rage.
“To give you a funny example, I had an invitation to give a talk at the Strings 2008 conference at CERN,” Kallosh says. “The way the invitation was writ-ten was, ‘Of course you are welcome to speak about any topic . . . but we would be very happy if you would give us a mini-review on string cosmology!’”
Suddenly, everyone is interested in their kind of union.
“I’m also working on other very formal, very stringy topics, which were always part of my skills,” Kallosh says. “But, at this moment, people want to know about string cosmology. I’m happily working on it . . . with Andrei’s help.”
Cosmology is now ascendant, with Kallosh arguing that it will be needed to explain what is seen at the LHC:
“Soon the LHC will start giving new information on particle physics,” she says. “But we know it will be difficult to interpret this data unless you also can digest all the data from the sky—all the observations from astrophysics and cosmology.”
The article ends with a large picture of one of the LHC detectors, captioned “Cradle of Collaboration: Will the LHC provide evidence for string theory?”
FQXI is funded by the Templeton Foundation, the goal of which is to bring science and religion together. Cormac O’Raifertaigh is at another Templeton funded event, a conference in Cambridge on From the Big Bang to the Brain: Current Issues in Science and Religion. This Wednesday will be devoted to cosmology, featuring talks on the anthropic principle, fine-tuning, God and time, and God and the Big Bang.
For another take on cosmology, this October the ENS in Paris will host a conference on Evolution and Development of the Universe. For more about this parallel universe of cosmologists who also study anthropics and the multiverse , see EvoDevoUniverse.
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