Not Even Wrong

Stephen Hawking and Thomas Hertog have a new paper out, called Populating the Landscape: A Top Down Approach. It contains his version of the anthropic landscape idea, based on his “no-boundary” idea of quantum cosmology (sometimes also referred to as the “Hartle-Hawking wavefunction”), and he refers to it as “top-down cosmology”.

Here’s part of the summary:

In a top down approach one computes amplitudes for alternative histories of the universe with final boundary conditions only. The boundary conditions act as late time constraints on the alternatives and select the subclass of histories that contribute to the amplitude of interest. This enables one to test the proposal, by searching among the conditional probabilities for predictions of future observations with probabilities near one. In top down cosmology the histories of the universe thus depend on the precise question asked, i.e. on the set of constraints that one imposes…

The top down approach we have described leads to a profoundly different view of cosmology, and the relation between cause and effect. Top down cosmology is a framework in which one essentially traces the histories backwards, from a spacelike surface at the present time. The no boundary histories of the universe thus depend on what is being observed, contrary to the usual idea that the universe has a unique, observer independent history. In some sense no boundary initial conditions represent a sum over all possible initial states. This is in sharp contrast with the bottom-up approach, where one assumes there is a single history with a well defined starting point and evolution.

and

We have also discussed the anthropic principle. This can be implemented in top down cosmology, through the specification of final boundary conditions that select histories where life emerges. Anthropic reasoning within the top down approach is reasonably well-defined, and useful to the extent that it provides a qualitative understanding for the origin of certain late time conditions that one finds are needed in top down cosmology.

I haven’t completely understood this yet, especially the author’s claims that they can use these ideas to say something about the shape of primordial fluctuation spectra, but the whole thing doesn’t sound obviously any more promising than other approaches to the anthropic landscape. Here, as usual, I agree with Lubos’s comments on this. But despite this, he seems to have removed me from his blogroll. My feelings are hurt.

Various things from the past couple days related to Susskind, the Landscape, and his book The Cosmic Landscape:

The paper Computational Complexity of the Landscape I by Frederik Denef and Michael Douglas is out. They show that even in simplified models of string theory vacua the problem of finding a model with CC in the anthropically determined range is NP-hard. This strongly indicates that in practice you can’t ever do what landscapeologists have optimistically hoped might be possible: pick out those vacua with anthropically acceptable values of the CC, and somehow use them to make predictions. Denef and Douglas end by bringing up a peculiar possibility: what if direct evidence for string theory is found (they’re kind of vague on how this going to happen…), but the problem of actually identifying our vacuum state remains intractable?

This raises the possibility that we might someday convince ourselves that string theory contains candidate vacua which could describe our universe, but that we will never be able to explicitly characterize them.

Lubos has a posting about this, including an exchange of comments with Frederik. On the whole I tend to see eye to eye with Lubos about the Landscape, although not here, where he’s frantically trying to dismiss the Denef-Douglas results, which look pretty solid to me.

The Philadelphia Enquirer has an editorial entitled A scientific leap, but without the faith, by Amanda Gefter, who did the recent interview with Susskind in New Scientist. Gefter tries to argue that string theory, unlike intelligent design, is science despite not being falsifiable. I don’t have the time or energy here to do justice to her argument or the problems I have with it (for one thing, she thinks string theory is breathtakingly beautiful). Science and Theology News has an article about the Gefter editorial called Intelligent design versus string theory which kind of misses the point, claiming that string theory can be falsified.

This month’s American Scientist has a review of Susskind’s book by cosmologist John Peacock entitled A Universe Tuned for Life . The review is pretty much uncritical, and mainly happy that Susskind is anti-religion:

These obligatory small criticisms should in no way detract from Susskind’s tremendous achievement. This book is a fine piece of popular science writing, but it is particularly significant for the timeliness of its message. Susskind emphasizes that the whole structure of the universe requires an active Creator no more than does the human eye or the temperature of the Earth. At a time when more and more people seem happy with a creation that took place 6,009 years ago, this lesson needs repeating.

Peacock actually feels that Susskind doesn’t go far enough in trashing the 20th century idea that there is some simple, compelling physical theory that explains the way the world works:

But if life on Earth is a random accident in a universe where only chance yielded laws of physics suitable for life, why stop there? Perhaps string theory itself is nothing special and only part of a wider spectrum of possible prescriptions for reality. If the search for a unique and inevitable explanation of Nature has proved illusory at every step, is it really plausible that suddenly string theory can make everything right at the last? Reading Susskind’s book should make you doubt that possibility, in which case we may have reached the end of the search for underlying simplicity that has driven physics since the beginning.

Finally, last night’s new papers on the arXiv included the surprising inclusion of one on Emergent Gravity by Jack Sarfatti. Famously, supposedly Sarfatti had been banned from publishing on the arXiv, but now I guess the arXiv has changed it standards. I still haven’t heard from them about why they’re banning trackbacks to this site, but perhaps there’s an explanation as to why they have finally found a paper by Sarfatti acceptable. It includes exactly two references, one to a 1976 paper about defects in condensed matter physics, the second to, you guessed it, Susskind’s The Cosmic Landscape.

Update: Chris W. correctly points out that I mistakenly had Gefter referring to string theory as “strikingly beautiful”, when it was general relativity she was referring to in this context.

Update: As a commenter pointed out, there’s a long, uninformed discussion of string theory and intelligent design over at The Panda’s Thumb. Many of the people commenting over there seem to believe that string theory is testable, and even invoke the latest SLAC story. Please don’t bring that particular discussion over here now. If you’re in the mood, contribute over there. Unfortunately I don’t have the time or energy at the moment…

There’s a beautiful survey paper about elliptic cohomology that Jacob Lurie, an AIM 5-year fellow in the math department at Harvard, has recently put on his home page. This paper has been discussed a bit already by David Corfield and by Urs Schrieber.

I don’t have time right now to try and write up something comprehensible about those parts of the elliptic cohomology story that I kind of understand, and in any case I want to spend more time reading Lurie’s paper. It brings into the elliptic cohomology story several of my favorite pieces of mathematics (Atiyah-Segal completion, Freed-Hopkins-Teleman), in a way that I don’t yet understand. But in any case there’s a lot of very beautiful and very new mathematics in this paper, mathematics that has tantalizing relations to quantum field theory.

The SLAC web-site today has a feature article entitled SLAC Physicists Develop Test For String Theory*. The “*” refers to a footnote to the title saying “Under Certain Conditions”. This is about the 10⁵⁰⁰th news story making this kind of announcement that has appeared over the past twenty years (like this recent one), and the title is just as incorrect and misleading as all the others.

The story starts with

String theory solves many of the questions wracking the minds of physicists, but until recently it had one major flaw — it could not be tested. SLAC scientists have found a way to test this revolutionary theory, which posits that there are 10 or 11 dimensions in our universe.

and is about a paper by JoAnne Hewett, Ben Lillie and Thomas Rizzo entitled Black holes in many dimensions at the LHC: testing critical string theory. This paper is perfectly reasonable, discussing a proposal for getting information about the number of extra dimensions, assuming Tev-scale gravity (a huge assumption most people think unlikely) and thus production of black holes at the LHC. If the number of extra dimensions is bigger than 6, then 10d superstring theory is ruled out (one can make similar comments about 11d M-theory, whatever that is).

Like all news articles of this kind, this one is misleading in the extreme, since “SLAC Physicists Develop Test For String Theory” is likely to make the unwary think that string theory is now testable. In addition, it’s flat out wrong, since the writer made the critical decision to replace “critical string theory” by “string theory”. Granting the unlikely assumption that the LHC sees extra dimensions and measures their number. if this turns out to be more than 6 or 7, string theorists will likely just point out that it is only “critical” string theory which lives in 10 dimensions. In recent years there has been much talk about string theories outside the critical dimension. For some discussion of this, see the comment thread of a recent Cosmic Variance posting, where string theorists Eva Silverstein and Clifford Johnson maintain that they see reasons to believe in the existence of string theories in dimensions other than 10. For some flavor of the discussion, here’s what Clifford has to say:

…the “person on the street” all too often hears (or implicitly gathers from posts like this) the phrase “string theory requires D=10/11″, and it is simply not true and in some years we may well have to be spending a lot of time undoing yet another uncautious claim when/if after doing phenomenology better we find that we don’t need to start in higher D and then “compactify”. We’ll have to go around telling everyone (on the tv shows and radio shows and magazines) “oh…that thing we said about extra dimensions? We were just kidding”…. Just like we’re doing now with the whole “unique vacuum” and “theory of everything” phrases…

Clifford seems fond of the idea of sub-critical strings, perhaps even strings in four dimensions (another enthusiast of this idea is Warren Siegel), while Stanford string theorist Silverstein advocates the study of super-critical strings, exactly the ones that would get around the “test” promoted today by her colleagues at SLAC.

Update: SLAC has replaced this article on their website with a new, much more accurate version, entitled “SLAC Physicists Develop Framework-Dependant Test For Critical String Theory”. The original version got wide distribution, even appearing on Slashdot.

The CERN Council Strategy Group is putting together a document proposing a European stategy for particle physics, in a process to be completed this July. As part of this process, earlier this week the group held an Open Symposium at Orsay, and the presentations are now available on-line.

I’ve often written here about possible future plans for particle physics in the US, but these presentations give an excellent overview of what is going on in Europe, where the situation is quite a bit better than here. Several of the presentations discuss possible upgrades to the LHC: the SLHC (increased luminosity), the DLHC (doubled beam energy using more powerful magnets), and the LHeC (colliding electrons or positrons with protons, like HERA at DESY, but with 1.4 TeV center of mass energy). Pretty much all the presentations are worth taking a look at, several of them involve an impressive amount of work in putting together a lot of information into a very professional PowerPoint format.

The one presentation about particle theory is by Nigel Glover and compares the performance of European and American theorists by looking at citation counts. There’s a lot of interesting data, much of it showing American dominance, but keep in mind that there is a strong “Witten effect” in the data, since he is by far the most influential theorist around, especially in terms of number of citations.

Back here in the U.S., on Monday the Bush administration is releasing its FY2007 budget proposals. An outline of the DOE budget lists an 8% increase in HEP spending to $775.1 million, as well as full funding for RHIC. The NSF should also see a sizable increase as part of the so-called American Competitiveness Initiative. The folks over at Cosmic Variance are experiencing some cognitive dissonance.

David Goss wrote to tell me that Physics Web has an article about physics weblogs. The theorists quoted are Sean Carroll, Paul Cook and Dave Bacon.

There’s an article by Michael Green in the latest Nature Physics reporting on the recent 23rd Solvay conference (mentioned here and here). Green notes that “Much of the discussion focused on string theory” but that “the structure of string theory is still so badly understood that it does not yet deserve to be described as a ‘theory’ at all — it is more a ‘work in progress’.” He describes the discussion at Solvay about anthropic explanations of the CC as “lively”, but that Polchinski et. al. have yet to carry the day: “there is a strong body of opinion that holds that it may be premature to decide which parameters are environmental within string theory, as the structure of the theory is still poorly understood and will surely hold further surprises.”

The same issue of Nature has an essay by Lawrence Krauss entitled Anthropic Fever where he starts by explaining the standard Landscape story that fundamental physics is really an “environmental science”, but then goes on to write:

But I have been wondering whether things might actually be much worse. It could be that many different combinations of laws could allow life to form, and that it is a pure accident, not favoured by any particular probabilistic explanation, that the constants of nature result in the combinations we experience in our Universe. Or, it could be that the mathematical formalism is so complex that the ground states of the theory might not be mathematically determinable, even in principle.

Whether or not nature is ultimately ‘undecidable’ in this strong sense, these ideas point to a possible future for particle physics that is very different from the past. Fundamental physics might not be restricted by any underlying grand mathematical structure that would ‘explain’ why the Universe is the way it is. It’s a possibility that I hope will be wrong, but it’s a possibility nonetheless.

This kind of worry seems to me completely misplaced. There’s not a shred of evidence for it, and the only reason people have been engaging in this kind of speculation is that string theory led them down this sorry path. Finding a trackless swampland at the end of the path doesn’t mean that physics is destined to spend the rest of eternity mucking about in a swamp, it is far more likely that a wrong turn was taken quite a ways back.

Last night a write-up by Nati Seiberg of his rapporteur talk at Solvay appeared at the arXiv, entitled Emergent Spacetime. It does a very good job of laying out the reasons that people often say that string theory suggests that our standard concept of “space” needs to be revised, that perhaps space is an “emergent” concept. Personally I don’t think the different reasons that Seiberg lays out add up to a very convincing case. For one thing, string theorists have been trying to come up with a new “stringy” version of space for twenty years now, without much success at all. They are still far from anything like a consistent proposal of what this new idea about space will be, and the various evidence given by Seiberg is rather incoherent, leading to different kinds of generalizations of space, not pointing to any one of them in particular.

The past two days there was a conference at Harvard on Black holes, topological strings, and invariants of holomorphic submanifolds, which also included some lectures in memory of Raoul Bott by Sir Michael Atiyah, S. T. Yau, and Dan Freed. Lubos has reports on two talks there, one by Robbert Dijkgraaf about a hoped for “Universal Wave Function”, a Hartle-Hawking kind of wave-function that would give the amplitude for the universe to be in various parts of the Landscape. The second was by Frederik Denef who spoke on “D-brane ground states, multicentered black holes, DT/GW correspondence, and the OSV conjecture [or: why OSV is probably right].” Lubos reports on a conversation with Frederik about his two forthcoming papers with Michael Douglas on computational complexity and the Landscape. It seems that what Denef-Douglas show is that, even if everything one would like to calculate is in principle calculable, the problem of identifying a specific string theory background realizing anthropically small values of the CC is NP-hard. This means that in practice you’ll never be able to do what landscapeologists would like to do: use the observed values of the CC and maybe some other standard model parameters to identify a tolerably small number of backgrounds, then use the properties of these backgrounds to make predictions. I believe it is this possibility that Krauss is alluding to in his quote above about how the “ground states of the theory might not be mathematically determinable, even in principle.”

Update: The last paragraph has been modified to better reflect reality. In its initial version I had assumed from Lubos’s blog entry that these computational complexity issues had been what Frederik’s talk was about.

Update: Polyakov also has a new preprint based on remarks at the Solvay conference, entitled Beyond Space-time. It’s a mixed bag, mostly about various ideas related to AdS/CFT, as well as the cosmological constant. He begins with critical but not completely dismissive comments about anthropic arguments:

Another danger is to get distracted by non-dynamical anthropic arguments, which recently acquired some popularity. I find the anthropic principle irrelevant. It is unlikely to uncover fundamental ideas and equations governing the universe. But, in spite of these misanthropic remarks, I believe that in special cases anthropic arguments may be appropriate.

At the end of the paper, he characterizes the various topics he has discussed as follows:

As it is clear from the list of the references below, these ideas (except for the gauge/strings correspondence) did not attract any attention. Perhaps they don’t deserve it. My best hope, however, is that some of them may serve as small building blocks of the future theory, the vague contours of which we can
discern at the horizon.

One of Fermilab’s recent colloquia was by James Rozensweig of UCLA on the topic of Reinventing the Accelerator for the High-Energy Frontier. Video and Powerpoint slides of the talk are now available.

Current accelerator technologies are up against very fundamental physical limits. In the case of circular proton colliders like the LHC, the fundamental limiting factor is the strength of the magnets and the size of the ring. The LHC uses a 27km ring and 8.36 Tesla superconducting magnets, and the energy scales linearly with the magnet strength and ring size. So one could get beams an order of magnitude more energetic than those in the LHC by using a 270km ring, but the cost of such a thing is likely to be prohibitive. One could also try and design higher field magnets, but the current record for this kind of magnet is only about 16 Tesla.

For circular electron colliders, the limiting factor is the energy loss to synchrotron radiation and these energy losses scale as the fourth power of the energy. LEP was probably the highest energy collider of this kind anyone is ever likely to build, since it already was using an amount of power a sizable fraction of that of the city of Geneva. One could try and use muons, which are much heavier so synchrotron radiation is not a problem, but they decay quickly so there are lots of problems with storing them in a collider.

These considerations mean that there is only one viable route to much higher energies, a linear collider, and this is the path that the ILC project is pursuing. What limits the energy in a linear collider like the ILC is the combination of the energy gradient one can achieve and the length of the machine. The superconducting RF cavities being studied for use in the ILC are inherently limited to gradients of less than 40-50 MV/m, with something like 35 MV/m a likely realistic number. With this gradient, to get up to a TeV in energy would require a length of about 33 km, about at the outer limits of what is possibly affordable. Realistically, to get to higher energies than this, one needs to find some way to get much higher energy gradients.

Rozensweig’s talk covers this material, but goes on to discuss various exotic new technologies that in principle can provide these much higher gradients. He describes progress on a long list of these, the most advanced of which is the CLIC project at CERN which uses the wake-field from a drive beam (a second accelerator). Much more exotic are various proposals involving lasers and plasma waves, some of which have been used to achieve gradients of 40 GV/m over short distances in the laboratory.

So, now all one has to do is to achieve a stable, high luminosity beam and make this work over kilometers not centimeters…. Not going to happen any time soon, but the distant future of high energy physics may depend on this kind of technology.

Update: I should also have mentioned here an article on this topic in the current (February) Scientific American entitled Plasma Accelerators.

Thanks to Wolfgang Beirl for the news that there’s an exciting new string theory blog, called The Official String Blog.