Hard Evidence for the Multiverse Found, but String Theory Limits the Space Brain Threat

Posted on May 22, 2013 by woit

In recent years there have been many claims made for “evidence” of a multiverse, supposedly found in the CMB data (see for example here). Such claims often came with the remark that the Planck CMB data would convincingly decide the matter. When the Planck data was released two months ago, I looked through the press coverage and through the Planck papers for any sign of news about what the new data said about these multiverse evidence claims. There was very little there; possibly the Planck scientists found these claims to be so outlandish that it wasn’t worth the time to look into what the new data had to say about them. One exception was this paper, where Planck looked for evidence of “dark flow”. They found nothing, and a New Scientist article summarized the situation:

“The Planck team’s paper appears to rule out the claims of Kashlinsky and collaborators,” says David Spergel of Princeton University, who was not involved in the work. If there is no dark flow, there is no need for exotic explanations for it, such as other universes, says Planck team member Elena Pierpaoli at the University of Southern California, Los Angeles. “You don’t have to think of alternatives.”

One of those promoting the idea that “dark flow” was evidence for a multiverse was Mersini-Houghton, who in a 2008 paper with Holman wrote:

Our contention, then, is that these observations of bulk flow can be construed as evidence for the birth of the universe from the landscape multiverse imprinted on the superhorizon sized nonlocal quantum entanglement between our horizon patch and others that began from the landscape. When we calculate the size of the induced dipole in our theory and convert it into a bulk velocity dispersion, we will see that for the constrained values of our parameters we arrive at a velocity dispersion of order 670 km/sec, remarkably close to the observed value of 700 km/sec.

One might think that the refutation of their prediction by the Planck data would be a problem. Instead though, the Sunday Times reported a few days ago that Scientists believe they have found the first evidence that other universes exist. The story got picked up by other news outlets, and appeared in the Daily Mail as “The first ‘hard evidence’ that other universes exist has been found by scientists”. The source for the story was Mersini-Houghton:

Laura Mersini-Houghton, theoretical physicist at the University of North Carolina at Chapel Hill, and Richard Holman, professor at Carnegie Mellon University, predicted that anomalies in radiation existed and were caused by the pull from other universes in 2005.
Now that she has studied the Planck data, Dr Mersini-Houghton believes her hypothesis has been proven.
Her findings imply there could be an infinite number of universes outside of our own.
She said: ‘These anomalies were caused by other universes pulling on our universe as it formed during the Big Bang.
‘They are the first hard evidence for the existence of other universes that we have seen.’

She will be in Britain soon promoting this at the Hay Festival on May 31 and at Oxford on June 11.

According to a New Scientist story just out, this hard evidence for the multiverse should be welcomed, since it (together with string theory) has just been shown to have the power to save us from “Legions of disembodied brains floating in deep space”. The story, which appeared in print as String Theory Limits Space Brain Threat starts with

LEGIONS of disembodied brains floating in deep space threaten to undermine our understanding of the universe. New mathematical modelling suggests string theory and its multiple universes may just provide our salvation – and that could win the controversial theory a few more backers.

It goes on to explain about Boltzmann brains and a recent paper by Bousso and Zukowski, and ends with news of yet another experimental success for string theory:

“This is potentially an added experimental success for string theory and eternal inflation,” says Daniel Harlow, a physicist at Princeton University. “We need to understand it better – [but] the fact that it potentially explains something is motivation to understand it better.”

Update: More here on how string theory will save us from the space brains.

Update: I’ve appended a response from Laura Mersini-Houghton and Richard Holman about this to a later posting, see here.

Posted in Favorite Old Posts, Multiverse Mania, This Week's Hype | 34 Comments

One Ring to Rule Them All

Posted on May 16, 2013 by woit

This week in Sweden the Nobel Foundation is running a symposium on LHC results. It’s invitation only, but the slides of the talks are available here.

One of the scheduled talks today was about string theory, and I was wondering how that would fit into the “LHC results” framework since string theory has nothing to say on the topic. Now that the slides are available I don’t see anything about the LHC, but there are some remarkable revelations. The first is that string theory is not science but “Magic”, with several slides describing the “Magic of String Theory”. The relationship to mathematics is that in string theory “No concept in Math remains unambiguous”, which I guess is about what you would expect when you’re dealing with magic.

An even bigger revelation comes later in the talk: string theory is Sauron’s Ring of Power! It is described as “Concentrated Power” and it seems that the markings on the ring are a SUSY Lagrangian. Part of the Ring Poem is quoted

One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them.

To put this back in context, recall that this is Sauron’s ring he created in order to control everything from Mordor. Here’s more of the poem, including the original language

Ash tug Shakhbûrz-ûr Ulîma-tab-ishi za,
Uzg-Mordor-ishi amal fauthut burgûli.
Ash nazg durbatulûk, ash nazg gimbatul,
Ash nazg thrakatulûk, agh burzum-ishi krimpatul
Uzg-Mordor-ishi amal fauthut burgûli.

One for the Dark Lord on his dark throne
In the Land of Mordor where the Shadows lie.
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the Land of Mordor where the Shadows lie.

This interpretation of string theory as Sauron’s ring I suppose could explain a lot…

Posted in Uncategorized | 24 Comments

String Theory and the Scientific Method

Posted on May 14, 2013 by woit

There’s a new philosophy of science book out, Richard Dawid’s String Theory and the Scientific Method (available online here if your institution is paying Cambridge University Press appropriately or if you have a credit card). It comes with endorsements from string theorists David Gross and John Schwarz, with Schwarz writing:

Richard Dawid argues that string theory plays a novel role in the scientific process that has been neglected by philosophers of science. I believe that this book is a valuable contribution to the philosophy of science, which should interest practicing scientists as well as those who are more interested in the methodology of science.

Dawid is a particle theorist turned philosopher, and as you might guess from the endorsement, he approaches string theory from an enthusiast’s point of view. The fundamental question addressed is how one can reconcile string theory’s failure in terms of conventional methodology of science with its continuing hold on at least part of the physics community. He explains how the book came about as follows:

This book has been on my mind ever since I left physics and turned to philosophy in the year 2000. A core motivation for making that step at the time was my feeling that something philosophically interesting was going on in fundamental physics but remained largely unappreciated by the world outside the departments of theoretical physics – and underappreciated even within. Twelve years of grappling with the specification of that general idea have considerably changed my perspective on the issue but left the overall idea intact. This book is the attempt to present it in a coherent form.

Reading the book in an odd way reminded me of my recent experience reading Gordon Kane’s reissued book on supersymmetry and string theory written in 2000, especially Witten’s essentially unchanged introduction. The degree of self-confidence of string theorists at that time was much different than now: AdS/CFT was a new idea, with a solution to QCD on its way, SUSY a sure thing at the LHC if not at the Tevatron or LEP, and at least half of the new jobs in the field going to string theorists. No landscape or multiverse pseudo-science was around to sow dissension in the ranks. No failure of SUSY to show up anywhere. No discouraging numbers like those for the last two years which show, in the US at least, 9% of jobs going to string theorists, with more jobs going to lattice gauge theorists in 2011 than to string theorists. And of course, a uniformly positive press, with no naysayers like Smolin and Woit causing trouble.

In Dawid’s description, string theorists are still partying like it’s 1999:

String theory has attained a pivotal role in fundamental physics and has been treated as a well-established and authoritative theory for quite some time by the community of string theorists and by physicists in related fields. As we have described above, large parts of fundamental physics are influenced by string theoretical analysis. The string community is one of the largest communities in all of theoretical physics and for many years has produced the majority of the field’s top-cited papers. Moreover, many string theorists express a remarkably strong trust in their theory’s viability.

For the actual list of last year’s top-cited papers in HEP, see here, and “remarkably strong trust in their theory’s viability” seems to me more 2000 than 2013. He does go on to mention skeptics, but to him a majority of the field is behind string theory, with the skeptics only coming from outside particle theory:

On one side of the divide stand most of those physicists who work on string physics and in fields like inflationary cosmology or high energy particle physics model building, which are strongly influenced by string physics. That group represents a slight majority of physicists in theoretical high energy physics today. Based on an internal assessment of string theory and the history of its development, they are convinced that string theory constitutes a crucial step towards a better and more genuine understanding of the world we observe. On the other side stand many theoretical physicists of other fields, most experimental physicists and most philosophers of physics. They consider string theory a vastly overrated speculation.

Dawid’s main thesis is that string theory critics fail to recognize that a new paradigm of scientific methodology is now needed:

String theory thus should not be taken to announce an end of science but rather to represent a new phase of scientific progress. In this new phase, progress in fundamental physics is no longer carried by a sequence of limited, internally fully developed theories, but rather by the discovery of new aspects of one overall theoretical scheme whose general
characteristics identify it as a candidate for a final theory, yet whose enormous complexity bars any hope of a full understanding in the foreseeable future.

What is the reason you should accept this final theory that no one can understand? Obviously the lack of any empirical support is a problem, so Dawid turns his attention to a detailed study of the subject of “non-empirical theory assessment”: how do you assess scientific progress absent connection to experiment? This is a real and serious problem, which Dawid studies in detail, although from a point of view which just naively accepts all arguments made by string theorists. He considers three main reasons for studying a theory with no empirical support:

  • The No Alternatives Argument. This is the best argument for string theory: there aren’t a lot of viable unified theories out there. Of course, the way science progresses is that there always are unsuccessful ideas with no good alternatives, until the day someone come up with a better idea.
  • The Unexpected Explanatory Coherence Argument. This is the idea that if a theory holds together better after you start studying it and understand it better, that’s a good thing. Dawid repeats uncritically claims of some string theorists that this is the case for string theory. I think you could make an equally good case for string theory unification becoming a more and more dubious idea as it became better understood (see, the Landscape).
  • The Meta-Inductive Argument. Here the idea is that if a theoretical research program worked before, so will a similar later one. Dawid claims that the string theory research program is just like the research program that led to the Standard Model:

    Given the entirely theoretical motives for its creation, the lack of satisfactory alternatives and the emergence of unexpected explanatory inter-connections, the standard model can be called a direct precursor of string theory.

    Honestly, this I just find bizarre, and have no idea what he’s talking about, with the history of the Standard Model and the history of string theory two radically different subjects.

In one crucial respect, this book is very different though than Kane’s. Kane is well aware that the idea of an inherently experimentally untestable theory is something he can’t sell to his colleagues and the public, so he devotes his book and its argument for string theory to supposed experimental tests. More savvy string theorists than Kane though are seeing the writing on the wall: no SUSY at 8 TeV means almost surely no SUSY at 13 TeV, and thus no prospects for experimental evidence for SUSY during any of our lifetimes. To prop up the string theory unification program past SUSY null results from the 13 TeV LHC in 2016 is going to require relying on Dawid’s “non-empirical theory assessment” and convincing people that string theory and the multiverse represent a new paradigm for how to pursue fundamental science. This book will be welcomed by those pursuing such a goal.

Update: For a different take on the book you can see Lubos Motl’s review (as you might expect, he’s a big fan). The case of the most prominent string theorist blogger reminds me of one of the funnier things in the Dawid book that I forgot to mention, this footnote:

It should be emphasized that physicists on both sides of the divide are aware of the slightly precarious character of the “non-physical” arguments deployed in the debate. Lee Smolin has applied the concept of groupthink to the community of string physicists (which, incidentally, seems a quite accurate representation of what many critics of string physics do think about string physicists) but is careful not to present it as a core argument. String theorists, when entering a discussion with their critics (see e.g. Polchinski in his reasoning against Smolin), try to keep the debate at an entirely physical level.

Posted in Book Reviews | 47 Comments

Number Theory News

Posted on May 12, 2013 by woit

A special seminar has been scheduled for tomorrow (Monday) at 3pm at Harvard, where Yitang Zhang will present new results on “Bounded gaps between primes”. Evidently he has a proof that there exist infinitely many different pairs of primes p,q with p-q less than 17,000,000 70,000,000.

Whether this proof is valid should become clear soon, but there still seems to be nothing happening in terms of others understanding Mochizuki’s claimed proof of the abc conjecture. For an excellent article describing the situation, see here.

Update: The “bounded gaps” talk is now on the Harvard seminar listing with abstract

The speaker proves that there are infinite number of pairs of primes whose difference is bounded by 70 million.

For more on the significance of this, see this Google+ posting by David Roberts.

I haven’t seen a paper, but rumor is that one exists and two referees at a major journal have found it to be correct.

Update: The most recent version of Mochizuki’s lecture notes for a general talk about his work is here. As mentioned in the Caroline Chen article, Go Yamashita has been talking to Mochizuki. Yamashita has now posted a short document FAQ on “Inter-Universality” and promises “For the details of the theory, please wait for the survey I will write in the near future.” He also notes:

I refuse all of the interviews from the mass media until the situation around the papers will be stabilised.

Update: In a weird coincidence, another major analytic number theory result is out today, a proof by Harald Helfgott of the ternary Goldbach conjecture. This says that every odd integer greater than 5 is the sum of three primes. The result had been known for all integers above e3100, and Helfgott’s proof reduces that bound to 1030 which is small enough so that all smaller values can be checked by computer.

Update: Nature has a story up about the Zhang result, including details of one of the Annals referee reports (I gather the paper will be published there).

Update: For some background to the methods being used by Zhang, see here. For Terry Tao on Zhang, see here, on Helfgott, here.


Update: New Scientist has a story about the Zhang result here, with quotes from Iwaniec, who has reviewed the paper, finding no error.

Update: A report from the talk at Harvard is here.

Update: There’s more about the Zhang proof at Emmanuel Kowalski’s blog, including a link to the Zhang paper.

Update: Nice piece about this in Slate from Jordan Ellenberg.

Posted in Uncategorized | 57 Comments

Miscellaneous Links

Posted on May 6, 2013 by woit
  • There’s an interesting discussion amongst philosophers at Brian Leiter’s blog about the effects of Templeton money (and I contributed my two cents…). In other Templeton news, they’re funding a new “literary science magazine” called Nautilus. Also via Leiter, they have awarded $3 million to two philosophers at Saint Louis University (“one of the largest grants SL has ever received in the areas of the humanities or the sciences”) for them to study the subject of intellectual humility.
  • In the category of rumors I’ve heard from so many reputable sources they must be true and I can’t really be violating confidentiality, W. Hugh Woodin is moving from Berkeley to Harvard, and Simon Donaldson from Imperial College to the Simons Center at Stony Brook.
  • Via Simon Willerton at the n-Category Cafe, Edinburgh now has a gallery with a wonderful collection of portraits of seventy mathematicians, including commentary from Michael and Lily Atiyah, an online version is here.
  • The publisher sent me a copy of Tony Zee’s new GR textbook, Einstein Gravity in a Nutshell, which I very much enjoyed looking through. Zee takes the textbook concept to new levels of informality, so it includes a wealth of interesting and amusing comments, spread throughout the text, footnotes and endnotes, including quite a few about quantum gravity. At over 800 pages, it’s a pretty huge book, including a lot of conceptual material (as in his QFT textbook), but more calculational detail than the QFT book. Undergraduate physics students should find this quite an approachable text (unlike the QFT one, which I think you need graduate level training to really follow).

    This definitely is a text for physicists, not mathematicians, with the geometry taking a back-seat. Differential forms and orthonormal frames don’t appear until nearly the end of the book. Personally I’ve found using the same language of connections on principal bundles to do gauge theory and gravity to make the most sense, but this involves getting familiar with quite a bit more formalism than most physicists are willing to deal with.

  • I’d been curious to hear more about recent work of Jacob Lurie and Dennis Gaitsgory on Tamagawa numbers, and had been waiting to see a paper from them. Turns out there’s something much better: Lurie has been teaching a course about this at Harvard this semester, with notes appearing here. For some indication of why you might take an interest in this if your interest is gauge theory, see here.
  • While about the only bipartisan agreement in Washington these days is that something must be done to deal with the terrible problem of the shortage of STEM graduates in the US, someone has noticed that there actually is no shortage, see here.
  • This past weekend there was a conference in honor of Bruno Zumino’s 90th birthday at Berkeley, and one can hope that some version of the talks might become available online here.
  • A hot topic in HEP remains that of when the failure of the SUSY picture that has been heavily over-sold for several decades will finally be acknowledged. From the list of titles at the Zumino conference, Maiani’s was “Supersymmetry: not time to give it up, yet”. Cormac O’Raifertaigh reports here that Nati Seiberg is saying “only certain aspects of minimal models had been ruled out so far. As for the future, who knows?”. (now corrected, see Cormac’s blog). Physics World has a story here, with Ben Allanach claiming that “data taken at the LHC have excluded roughly half of supersymmetry’s parameter space” and that now one has to wait until 2015 when, if SUSY is right, it will be found nearly immediately:

    “My hopes are pinned on the next run,” he says. “The energy jump now is going to make the big difference. And if supersymmetry is the correct theory of nature, I would be expecting to see a big signal within the first month. If it doesn’t crop up, I’ll then be getting pretty depressed.”

    Bill Murray of ATLAS makes the excellent point that

    “Proving [supersymmetry] wrong would be as important as proving it right,” he says. “Null results are hard to sell to newspapers, but they are really important to scientific progress.”

    Killing SUSY will be one of the great achievements of the LHC, and complaining about this might be kind of like being upset that Michelson-Morley didn’t find the ether.

Update: I’ve been pointed to an impressive photo of Robbert Dijkgraaf that unfortunately did not make the Atiyah Gallery.

Update: The New York Times has a story today about the new Templeton-funded science magazine Nautilus.

Update: The news from Britain is that Stephen Hawking has joined the academic boycott of Israel, cancelling plans to attend a conference there this month. Please discuss your views on the Israeli/Palestinian conflict elsewhere. There’s no way I’m going to moderate such a discussion, and there are now surely dozens of other sites carrying this story where comments are encouraged.

Posted in Uncategorized | 37 Comments

Supersymmetry and Beyond

Posted on May 1, 2013 by woit

Back in the year 2000, Gordon Kane published Supersymmetry: Unveiling the Ultimate Laws of Nature, a popular book promoting supersymmetry and string theory. The thrust of the book was that there was already indirect evidence for SUSY, with confirmation by discovery of superpartners due to come soon from LEP (which was running at energies near 100 GeV/beam) and the Tevatron (where Run II at high luminosity and nearly 1 TeV/beam was to start in 2001). The LHC was also discussed, mainly as the place that would confirm and extend the LEP/Tevatron superpartner discoveries.

Thirteen years later, with no hint of SUSY showing up as promised, not only at LEP/Tevatron energies, but also at the much higher energies and luminosities of the 8 TeV LHC, Kane has a new popular book promoting supersymmetry and string theory, entitled Supersymmetry and Beyond. It includes his claim to have predicted the Higgs mass using string theory (see Matt Strassler’s take on this here, mine here). Much of the book though consists of exactly the same text as the 2000 version.

How does Kane handle the detailed failed predictions of the 2000 edition in the new 2013 version? Basically by editing them out, with no indication to the reader that this has been done.  What’s the right word to describe the result of an Orwellian exercise like this? You can make up your mind about that yourself, since I’ve gathered together here some examples of the book text, showing the edits that were done to create the new version.

Pages xvii/xviii

Supersymmetry is still an idea as this book is being written (mid-1999) in late 2012. There is considerable indirect evidence that it is a property of the laws of nature, but the confirming direct evidence is not yet in place. That is not an argument against nature being supersymmetric; rather, the accelerator collider facilitiesy that could confirm it (the LHC) are  is just beginning to cover the region where the signals could appear (Chapter 5)[about LEP and Fermilab].

Pages 2-3/3

If we understand supersymmetry and its implications correctly, direct experimental evidence for supersymmetry will be found in the next few years – possibly soon after this book is published (or, with great luck, even before).

Pages 13/16

Only now are colliders and detectors at laboratories are now achieving the energies and luminosities (amounts of data) and sensitivities needed to explicitly detect the superpartners explicitly, at least if our thinking about their properties is more or less right.

Pages 70-71/77-79

The manner in which supersymmetry explains the Higgs physics is elegant and has important consequences for how we expect to test supersymmetry experimentally. It is rather technical. A more detailed description is given in Appendix B; here I will give a short version. There are three parts…

Therefore, the supersymmetric Standard Model explanation of the Higgs mechanism would not make sense unless the some superpartner masses were not much larger than the Standard Model masses they explain. That gives us an estimate of the masses we should expect the superpartners to have as we search for them, and it tells us at what stage we should question the validity of the theory if the superpartners have not been detected. Such estimates are only approximate, but luckily the expected masses are small enough that they imply the superpartners should be detected soon.

Appendix B was deleted entirely, it contained the text (page 156)

Therefore, the superpartner masses cannot be very much larger than the Z boson mass if this whole approach is valid. This is the only place where we can use the theory to relate the unknown superpartner masses to known masses, so on the one hand, it is a major test of the correctness of the supersymmetry explanation of the Higgs physics, and on the other, it is the most significant reason whey we expect the masses of the superpartners to have values that allow them to be produced at Fermilab or even LEP. This connection also suggests that if the superpartner masses are much larger than the Z boson mass, then the apparent success of the supersymmetry theory in explaining the origin of the Higgs physics of the Standard Model could be an accident.

Pages 88-89/89

Several arguments imply that some sparticles are within the reach of Fermilab the LHC. The strongest One of the most appealing is based on the explanation supersymmetry gives for the Higgs mechanism of the Standard Model, as described in the last chapter Chapter 7. Basically the qualitative argument is that because since supersymmetry provides the Higgs mechanism that accounts for the masses of W and Z, the some sparticle masses cannot be much heavier than the W and Z masses themselves. Fermilab has already produced and detected thousands of W’s and Z’s. When this argument is framed put in a technical form, it implies that gluinos and probably charginos and neutralinos and stops should be in the Fermilab within the LHC reach. If they are not, the impressive successes of supersymmetry listed at the beginning of Chapter 4 may be meaningless coincidences. There are some arguments, both theoretical and phenomenological, suggesting that squarks and sleptons will be too massive to produce at the LHC.

Chapter 8, on SUSY implications for matter/anti-matter asymmetry, proton decay, rare decays like mu to e-gamma, and CP violation has been deleted. Appendix D, on large extra dimensions, has also been completely deleted.

Witten’s preface has been edited:

Experimental clues suggest that the energy required to produce the new particles is not much higher than that of present accelerators. If supersymmetry plays the role in physics that we suspect it does, then it is very likely to be discovered by the next generation of particle accelerators, either at Fermilab in Batavia, Illinois, or Large Hadron Collider (LHC) or its upgrades, at CERN in Geneva, Switzerland.

Posted in Book Reviews | 51 Comments

Arkani-Hamed Colloquium

Posted on April 30, 2013 by woit

Nima Arkani-Hamed was here at Columbia yesterday to give the physics colloquium, which clocked in at a bit over 1 hour and 45 minutes. He did reveal the secret of why his talks are this long: when invited to give a 1 hour colloquium, he plans on talking for at least 1 hour 30 minutes. The content of the talk was similar to many others he has given recently that are available on the web, see for instance this one at the IAS, this recent one at BNL, or for a written version, see here.

As a performer, he’s a powerful speaker: smart, vigorous, and supremely self-confident. His arguments lead to “inevitable” conclusions, not just implying results but “nailing” them. It’s clear why he’s the most influential person in the field these days. With most theorists made worried and unsure by 40 years of failure to get anywhere in their efforts to improve on the Standard Model, he knows exactly what he thinks and will tell you forcefully what you should think. The fact that none of the ideas about BSM physics he is famous for (large extra dimensions, split SUSY, Little Higgs, etc…) have ever worked out doesn’t seem to slow him down, and he has a professorship at the IAS and a $3 million prize from Yuri Milner to back him up.

Despite his long-time advocacy of SUSY, according to Arkani-Hamed, the negative results from the LHC are “not making many of us worried about SUSY”. He (accurately) points out that he’s not one of those like Gordon Kane who for decades has been predicting the discovery of superpartners to be six months away. It has long been clear that the simplest versions of SUSY should have shown up at LEP and the Tevatron, and pre-LHC the lack of any indirect evidence for SUSY indicated to him that it was unlikely to show up at the 8 TeV LHC. So, by his lights, there’s no reason that LHC results so far should cause any new worries about SUSY, beyond those he already had pre-LHC. On the more limited question of whether a “natural” version of SUSY will work out, one where the superpartner masses just barely avoid large amounts of fine-tuning, a year ago (see here) he was saying we were at the “eleventh and a halfth hour” for this possibility. Now that the 8 TeV results are here (and negative), he argues that it is only with the 2015 data that the results will be decisive. The current wisdom about “natural SUSY” I guess is summarized in slide 8 here: Keep Calm and Wait for 14 TeV.

The main point of the talk was one that Arkani-Hamed has been consistently making for nearly a decade, that the role of the LHC is to decide between two possible futures for fundamental physics:

  • The small value of the Higgs mass (in Planck units) has a “natural” explanation, most likely using SUSY, in which case we spend the rest of our lives unraveling the complexities of a SUSY-extended Standard Model.
  • The small value of the Higgs mass (in Planck units) indicates “fine-tuning” that can only have an anthropic explanation, just like the one for the CC. In that case, we live in a multiverse, with physics determined by something like the string theory landscape. About this whole conceptual framework, he says the “ideas are so poorly defined, not clear if they make any kind of mathematical sense”, and it’s “not clear progress will happen anytime soon” but, no need to worry or get discouraged, since this is an “attractive problem”.

Based on the LHC results so far, it looks like all evidence is that we’re headed to the second alternative.

Arkani-Hamed’s talk was structured so as to present a long chain of argument (needing at least 1h 30 min to explain) leading to these two alternatives. One of the alternatives (SUSY naturalness) is essentially already dead, with the die-hards intent on hanging on a couple more years. The other is essentially what David Gross has called “giving up”: you just announce that the problems you haven’t been able to solve can never be solved. In this vision, the 20th century with its huge success at finding a highly predictive, mathematically beautiful fundamental theory was an aberration caused by only being able to see physics at energies way below the Planck scale. In this new 21st century physics, you just postulate that at higher energies things are much more complicated, in ways we can’t hope to ever know, and theorists devote their lives to making excuses, not predictions. Witten may end up being right that “string theory is 21st century physics that fell into the 20th century”, in a much more negative way than he intended.

If a long, complicated argument leads you to the conclusion that the only viable alternative is to give up, then it seems to me you have two choices: give up, or examine more carefully your argument. A much more interesting and more useful talk than Arkani-Hamed’s would be one less devoted to forcefully insisting on the conventional chain of argument based on the technical problem of sensitivity of the Higgs potential to the cut-off, instead looking carefully for weaknesses in the argument (one possibility is discussed here). Arkani-Hamed is a brilliant physicist, but this may be a time when what is needed is not self-confidence in the power of one’s arguments, but instead a suspicion that one has been making a mistake somewhere for quite a while now.

Posted in Favorite Old Posts, Uncategorized | 45 Comments

Time Reborn

Posted on April 23, 2013 by woit

Lee Smolin’s new book, Time Reborn, is out today. For more about the ideas in the book, see video of a talk here, and an interview here.

While I mostly vehemently agreed with what Smolin had to say in his last book, The Trouble With Physics, I find myself equally vehemently in disagreement with this one. On some of the topics covered, I’m indifferent to his arguments mostly as a matter of taste. While my views on human society are likely similar to Smolin’s, I’ve never found the scientific insights of fundamental mathematics or physics to have anything significant to tell me about this part of life. Similarly, while I’ve spent some time studying philosophy, I’ve mostly found this of little help in gaining deeper understanding of math or physics. Others though have a very different experience than me, and I’m not about to argue against people looking for enlightenment wherever they happen to find it.

On some of the scientific issues dealt with in the book, again I’m mostly just indifferent. Smolin accurately explains how the lack of predictivity makes typical multiverse models empty, but I’m not convinced that his favored alternative (“cosmological natural selection”) does much better. While I understand well the human appeal of wondering about what came before the big bang, I’ve yet to see any specific models of this that carry enough explanatory power about anything to make them particularly attractive or interesting.

Many of the ideas Smolin is arguing for are clearly labeled as what they are: speculative challenges from a very much minority point of view to some of the received wisdom of this kind of science. Unfortunately, parts of his argument that are most problematic are ones which are in danger of becoming the new received wisdom of the subject. The refusal to admit the failure of the idea of string/M-theory unification has left many of our most prominent theorists pushing the idea that fundamental physics is based on some new and very different degrees of freedom, with dynamics that just happens to be too complicated to allow them to find vindication by seeing how the Standard Model emerges at low energies. For his own reasons, Smolin signs on to a version of this point of view, writing:

I’m inclined to believe that just about everything we now think is fundamental will also eventually be understood as approximate and emergent: gravity and the laws of Newton and Einstein that govern it, the laws of quantum mechanics, even space itself…

A large part of the elegance of general relativity and the Standard Model is explained by understanding them as effective theories. The beauty is a consequence of their being effective and approximate. Simplicity and beauty, then, are the signs not of truth, but of a well-constructed approximate model of a limited domain of phenomena.

The notion of an effective theory represents a maturing of the profession of elementary-particle theory. Our young, romantic selves dreamed we had the fundamental laws of nature in our hands. After working with the Standard Model for several decades, we are now simultaneously more confident that it’s correct within the limited domain in which it has been tested and less confident of its extendability outside that domain.

This notion that the SM is “just an effective theory”, with its fascinating and deep mathematical structures nothing but an artifact of low-energy approximation has become the reigning ideology of the last few decades. One impetus for this has been string/M-theory, with its conjectured very different physics at short distances. This has been put together with our modern understanding of renormalization, according to which non-renormalizable theories make perfect sense as effective theories. The argument is then made that this is all there is to the SM, neglecting to note that to a large degree the SM couplings are asymptotically free, meaning that (most of) the quantized geometric degrees of freedom make perfectly good sense at all energy scales.

Smolin’s view that the recent history of particle physics makes us “less confident of its [the SM’s] extendability outside that domain [where it has been tested]” is one I strongly disagree with. Despite endless “naturalness” and “fine-tuning” predictions based on the “nothing but an effective theory” argument, the SM has not only been vindicated at the LHC over a large new energy range, but the discovery of the Higgs has shown it to have just the right characteristics to make perfectly good sense up to extremely high energies, far beyond anything we can test.

I’ve been teaching a course this past year on quantum mechanics for mathematicians, emphasizing the role of Lie groups, unitary representations and symmetries in providing not only useful calculational methods, but governing the underlying structure of the theory. Smolin argues instead that, based on Leibniz’s “identity of the indiscernibles”, symmetries cannot be fundamental (although a footnote says this doesn’t apply to gauge symmetries):

Symmetries are common in all the physical theories we know. Several of the most useful tools in the physicist’s toolbox exploit the presence of symmetries. Yet if Leibniz’s principles are right, they must not be fundamental.

This applies to the very structure of quantum mechanics:

Quantum mechanics, too, is likely an approximation to a more fundamental theory.

since it is linear, and he bets thus just a linear approximation to some fundamentally non-linear theory. Again, mathematical simplicity is seen as an artifact of approximation, not indication of something fundamental.

Smolin ends with a vision that is pretty much the exact opposite of mine, one with a vastly diminished role for mathematics in understanding the nature of reality:

The most radical suggestion arising from this direction of thought is the insistence on the reality of the present moment and, beyond that, the principle that all that is real is so in the present moment. To the extent that this is a fruitful idea, physics can no longer be understood as the search for a precisely identical mathematical double of the universe. That dream must be seen now as a metaphysical fantasy that may have inspired generations of theorists but is now blocking the path to further progress. Mathematics will continue to be a handmaiden to science, but she can no longer be the Queen.

Unfortunately it seems possible that Smolin’s arguments about mathematics will resonate well with the current backlash against sophisticated mathematics that one sees at many physics departments in the wake of the failure of string theory. In a footnote he explicitly argues that the problem with string theory was too much symmetry:

Indeed, we see from the example of string theory that the more symmetry a theory has, the less its explanatory power.

I don’t understand this argument at all. The problems with string theory are something I’ve written about endlessly here, but too much symmetry is not one of these problems.

Smolin has been quite right to point out in recent years that fundamental physical theory is in a state of crisis, but I think his diagnosis in this book is the wrong one. Abandoning the search for a more powerful mathematical understanding of the world because the huge success of this in the past has made further progress more difficult is the wrong lesson to draw from recent failures (the nature of which he lucidly described in his previous book).

My own interpretation of the history of the Standard Model is that progress came not from finding more, larger symmetries, but from a deeper appreciation of the various ways in which gauge symmetry could be realized (spontaneous symmetry breaking, confinement, asymptotic freedom). The arrival of string theory pushed the study of gauge symmetry into the background, and these days one often hears arguments against its fundamental nature, such as this one from Arkani-Hamed

What’s as a misnomer called gauge symmetry, whose beauty is extolled at length in all the textbooks on the subject, is completely garbage. It’s completely content free, there’s nothing to it.

Smolin’s arguments against the fundamental nature of symmetries, even if gauge symmetry is let off the hook in a footnote, just reinforce some of the attitudes at the root of our present-day crisis. The problems that remain in fundamental theory are difficult, but denigrating now the powerful ideas that have led to success in the past won’t help find a way forward.

Update: For more about this, there’s a review in the NYRB, and a piece at edge.org (with responses).

Posted in Book Reviews | 83 Comments

Anderson on Anderson-Higgs

Posted on April 16, 2013 by woit

Philip Anderson was here at Columbia yesterday, and gave a very interesting talk, mostly discussing what was going on in the late 50s and early 60s at the intersection of condensed matter and particle physics. This has attracted a lot of interest around the question of who first came up with what is now called the “Higgs mechanism” and who first predicted a “Higgs particle” (I’ve written a long blog posting about this here).

After the discovery of the BCS model of superconductivity, Anderson did important work on understanding the “gauge problem” of how gauge symmetry acts in such a theory, publishing a series of papers on this in 1958. He joked that he was “pretty naive about field theory” at the time, so much so that the spelling he was using was “guage”. He had the advantage of regularly talking with Bardeen and with Nambu, and he described some of Nambu’s work on the so-called “Nambu-Jona-Lasinio” model. His 1958 work explained how one avoids getting massless Goldstone bosons in superconductors due to the singular long range nature of the Coulomb force. His talk included an anecdote about escaping from handlers in the Soviet Union to get a chance to explain this to Shirkov during a visit there (he wasn’t allowed to meet Bogoliubov).

He was at Bell Labs in the summer of 1962, and talked to J.G. Taylor, who told him that the problem of massless Goldstones was something those in particle theory were actively worrying about. Taylor also gave him a copy of Schwinger’s Gauge invariance and mass paper, which had been published that January. This led to Anderson’s Plasmons, gauge invariance, and mass paper, finished in November, and published in April 1963. This paper clearly explains the nature of what is now generally referred to as the “Higgs mechanism”, in the Yang-Mills case, not just the Abelian case, ending with the very modern point of view

We conclude, then, that the Goldstone zero-mass difficulty is not a serious one, because we can probably cancel it off against an equal Yang-Mills zero-mass problem.

In 1964 the papers by Brout-Englert-Higgs-Guralnick-Hagen-Kibble appeared that have drawn the most attention as earliest instances of the Higgs mechanism, but Anderson had the correct idea a couple years earlier. He described the situation as one where he and the 1964 authors all had the right explanation for why the W and Z have mass, although none of them (including him) had the actual physical Higgs particle, which he claimed first appears in a 1966 paper of Higgs.

The main point of his talk was the fruitful nature of research at the intersection between problems in condensed matter and particle theory, with the 50s-60s a happy period of such work. He ended with some comments on “supersolids”, see his recent paper about this here. Anderson will be 90 years old later this year (he’s almost exactly the same age as Freeman Dyson) and it was great to see him still going strong.

Update: See here for a write-up by Anderson of a talk a few years ago covering much the same material as yesterday’s Columbia talk (including the story of meeting Shirkov).

Update: A commenter points to this recent talk by Guralnik at Brown and mentions some comments that might be about Frank Close.

I just watched the talk, and he explicitly refers to this exchange in the London Times. I don’t see how when he says

the person involved as far as we can tell has no understanding whatsoever of mass renormalization and how these things work.

this can refer to anyone except Close (about whom it is completely absurd). The point of contention here is a very simple one. Guralnik’s paper (unlike Higgs’s) has no potential term for the scalar field. In his talk he says this is because it was the practice at Harvard not to write such terms down, while knowing they had to appear to renormalize the theory. Close’s point I think is just that Higgs went further than the other authors at the time in terms of exhibiting what would be needed to study the dynamics of the physical mode of the scalar field. From what I can tell, of course everyone writing these papers knew about potential terms for the scalar and how they worked, but the whole issue is a bit irrelevant: none of the people involved at this period seem to have thought seriously about this physical mode that describes the Higgs particle itself. They were thinking about something entirely different, the mass of the gauge field, and in any case these are Abelian models that have nothing to do with the real non-Abelian model that describes the Higgs particle.

The main point of Guralnik’s talk as far as this controversy goes I think is his explicit and repeated claim (which seems to me debatable) that Higgs’s paper was just wrong for technical reasons, in the sense of reaching correct conclusions by an incorrect argument. This appears to be Guralnik’s argument for why he and his collaborators should be preferred to Higgs as candidates for a Nobel, and is made much more strongly here than in other places (such as here, where he doesn’t use the term “wrong”, emphasizes more why Higgs’s argument was “incomplete”).

Leon Cooper was in the audience, and asks Guralnik about the Anderson explanation for why the Goldstone theorem is violated here: the long range nature of the Coulomb potential. Guralnik seems to acknowledge that this is the right physical way to understand what is going on, but says that relativity makes things more complicated. He explicitly acknowledges that he understood not at all Anderson’s arguments, saying

we were woefully ignorant, had barely heard of superconductivity.

About the crucial question of priority, the fact that his competitor’s papers were published earlier, were read by him before submission of his paper, and explicitly referenced in his paper, all he says is

as we published it, we found out about other papers.

which really doesn’t do justice to the situation. In this piece, written after the Higgs discovery, he describes the history as

We finally submitted our paper to PRL with the proof of the general mechanism to avoid the Nambu Goldstone theorem (the only work to have this) and the special example. We were surprised to discover that two very different but related papers, with parts of the example, one by Englert and Brout and the other by Higgs also existed. All three papers appeared in the same volume of PRL in 1964.

which is highly misleading.

Posted in Uncategorized | 54 Comments

To Mars and Beyond

Posted on April 14, 2013 by woit

I’m not sure either of these stories from the past week is particularly important in and of itself, but since I try and keep up on trends in theoretical physics, and two is a trend, here’s some news from two of the greats of the field:

  • There’s an interview here (via John Baez) with Gerard ’t Hooft about his role as “ambassador” for the Mars One project, which plans to send people on a one-way trip to Mars in 2023. This will be financed with an associated reality TV show, and already 40,000 people have signed up for a chance to get to go.
  • Stephen Hawking has even more radical ideas, which he talked about in a visit to Cedars-Sinai in LA last week. He believes humanity is guaranteed to trash this planet, so our best hope is to use M-theory to find a way to move on to another one:

    For him, the answers to the largest and tiniest questions lie in M-theory.

    “To understand the universe at the deepest level, we have to understand why is there something rather than nothing,” Hawking said, speaking through a computer program that converts his eye and cheek movements into spoken speech. “Why do we exist? Why this particular set of laws, and not some other? I believe the answers to all of these things is M-theory.”

    The theory, he said, combines multiple ideas about math and physics. It suggests that there are multiple dimensions or universes, and offers solutions for the behavior of super-massive black holes and the properties of the fabric of space-time. M-theory is a work in progress, but Hawking said he believes that it’s the most promising lead to a unified theory.

    The payoff to solving M-theory, Hawking said, is understanding where we fit in — and, perhaps, how we can thrive.

    “We must continue to go into space for humanity,” Hawking said. “We won’t survive another 1,000 years without escaping our fragile planet.”

Update: On Tuesday Hawking gave a talk to students at Caltech, with a report here that includes smuggled audio of the talk. Evidently Hawking told students that they don’t need God, but they do need M-theory and anthropics:

During his talk, he cited M-Theory — a wide-ranging and as-yet-incomplete explanation of the universe that attempts to unite the factions within String Theory — as the only workable theory going forward that can explain the true nature of the cosmos.

M-Theory suggests that the multi-dimensional “strings” of the universe are bound together by a strange material sometimes called membranes, but also known by other names. It suggests that matter, space, time and every possible history exists simultaneously across dimensional planes that were created out of nothing at the moment of the Big Bang some 13.8 billion years ago. Only in very few of these dimensions can a species like humanity come into being.

Posted in Uncategorized | 45 Comments