A Bit of Physics History: Ed Witten Introduces M-Theory

The Second Superstring Revolution was, like most revolutions, a somewhat messy affair, with a number of pivotal steps along the way: understanding the role of membranes in 11-dimensional supergravity, the discovery of dualities in supersymmetric gauge theories, Polchinski’s appreciation of D-branes as dynamical extended objects in string theory, and of course Maldacena’s formulation of the AdS/CFT correspondence. But perhaps the high point was Ed Witten’s formulation of M-Theory in 1995. And I just noticed that Witten sharing it with the world was captured on video.

Here is Witten’s paper:

String Theory Dynamics In Various Dimensions
Edward Witten

The strong coupling dynamics of string theories in dimension d≥4 are studied. It is argued, among other things, that eleven-dimensional supergravity arises as a low energy limit of the ten-dimensional Type IIA superstring, and that a recently conjectured duality between the heterotic string and Type IIA superstrings controls the strong coupling dynamics of the heterotic string in five, six, and seven dimensions and implies S duality for both heterotic and Type II strings.

Before this result, we knew about five different kinds of string theory, each living in ten dimensions: Type I, two different Type II’s, and two different “heterotic” theories. Then there was the most symmetric form of supergravity, living in 11 dimensions, which some people thought was interesting but others thought was a curiosity that had been superseded by string theory. To everyone’s amazement, Witten showed that all of these theories are simply different limiting cases of a single underlying structure. Nobody knows what that underlying theory really is (although there are a few different formulations that work in some contexts), but we know what to call it: M-theory.


Now Amanda Gefter, author of the new book Trespassing on Einstein’s Lawn (and a recent guest-blogger at Cocktail Party Physics), takes to Twitter to point out something I wasn’t aware of: a video record of Witten’s famous 1995 talk at USC. (I’m pretty sure this is the celebrated talk, but my confidence isn’t 100%.) [Update: folks who should know are actually saying it might be a seminar soon thereafter at Stony Brook. Witten himself admits that he's not sure.] It’s clearly a recording by someone in the audience, but I don’t know who.

Most physics seminars are, shall we say, not all that historically exciting. But this one was recognized right away as something special. I was a postdoc at MIT at the time, and not in the audience myself, but I remember distinctly how the people who were there were buzzing about it when they returned home.

Nature giveth, and Nature taketh away. The 1995 discovery of M-theory made string theory seem more promising than ever, to the extent that just a single theory, rather than five or six. Then the 1998 discovery that the universe is accelerating made people take more seriously the idea that there might be more than one way to compactify those extra dimensions down to the four we observe — and once you have more than one, you sadly end up with a preposterously high number (the string theory landscape). So even if there is only one unifying theory of everything, there seem to be a bajillion phases it can be in, which creates an enormous difficulty in trying to relate M-theory to reality. But we won’t know unless we try, will we?

Posted in arxiv, Science | 15 Comments

Guest Post: Katherine Freese on Dark Matter Developments

Katherine Freese The hunt for dark matter has been heating up once again, driven (as usual) by tantalizing experimental hints. This time the hints are coming mainly from outer space rather than underground laboratories, which makes them harder to check independently, but there’s a chance something real is going on. We need more data to be sure, as scientists have been saying since the time Eratosthenes measured the circumference of the Earth.

As I mentioned briefly last week, Katherine Freese of the University of Michigan has a new book coming out, The Cosmic Cocktail, that deals precisely with the mysteries of dark matter. Katie was also recently at the UCLA Dark Matter Meeting, and has agreed to share some of her impressions with us. (She also insisted on using the photo on the right, as a way of reminding us that this is supposed to be fun.)

Dark Matter Everywhere (at the biannual UCLA Dark Matter Meeting)

The UCLA Dark Matter Meeting is my favorite meeting, period. It takes place every other year, usually at the Marriott Marina del Rey right near Venice Beach, but this year on UCLA campus. Last week almost two hundred people congregated, both theorists and experimentalists, to discuss our latest attempts to solve the dark matter problem. Most of the mass in galaxies, including our Milky Way, is not comprised of ordinary atomic material, but instead of as yet unidentified dark matter. The goal of dark matter hunters is to resolve this puzzle. Experimentalist Dave Cline of the UCLA Physics Department runs the dark matter meeting, with talks often running from dawn till midnight. Every session goes way over, but somehow the disorganization leads everybody to have lots of discussion, interaction between theorists and experimentalists, and even more cocktails. It is, quite simply, the best meeting. I am usually on the organizing committee, and cannot resist sending in lots of names of people who will give great talks and add to the fun.

Last week at the meeting we were treated to multiple hints of potential dark matter signals. To me the most interesting were the talks by Dan Hooper and Tim Linden on the observations of excess high-energy photons — gamma-rays — coming from the Central Milky Way, possibly produced by annihilating WIMP dark matter particles. (See this arxiv paper.) Weakly Interacting Massive Particles (WIMPs) are to my mind the best dark matter candidates. Since they are their own antiparticles, they annihilate among themselves whenever they encounter one another. The Center of the Milky Way has a large concentration of dark matter, so that a lot of this annihilation could be going on. The end products of the annihilation would include exactly the gamma-rays found by Hooper and his collaborators. They searched the data from the FERMI satellite, the premier gamma-ray mission (funded by NASA and DoE as well as various European agencies), for hints of excess gamma-rays. They found a clear excess extending to about 10 angular degrees from the Galactic Center. This excess could be caused by WIMPs weighing about 30 GeV, or 30 proton masses. Their paper called these results “a compelling case for annihilating dark matter.” After the talk, Dave Cline decided to put out a press release from the meeting, and asked the opinion of us organizers. Most significantly, Elliott Bloom, a leader of the FERMI satellite that obtained the data, had no objection, though the FERMI team itself has as yet issued no statement.

Many putative dark matter signals have come and gone, and we will have to see if this one holds up. Two years ago the 130 GeV line was all the rage — gamma-rays of 130 GeV energy that were tentatively observed in the FERMI data towards the Galactic Center. (Slides from Andrea Albert’s talk.) This line, originally proposed by Stockholm’s Lars Bergstrom, would have been the expectation if two WIMPs annihilated directly to photons. People puzzled over some anomalies of the data, but with improved statistics there isn’t much evidence left for the line. The question is, will the 30 GeV WIMP suffer the same fate? As further data come in from the FERMI satellite we will find out.

What about direct detection of WIMPs? Laboratory experiments deep underground, in abandoned mines or underneath mountains, have been searching for direct signals of astrophysical WIMPs striking nuclei in the detectors. At the meeting the SuperCDMS experiment hammered on light WIMP dark matter with negative results. The possibility of light dark matter, that was so popular recently, remains puzzling. 10 GeV dark matter seemed to be detected in many underground laboratory experiments: DAMA, CoGeNT, CRESST, and in April 2013 even CDMS in their silicon detectors. Yet other experiments, XENON and LUX, saw no events, in drastic tension with the positive signals. (I told Rick Gaitskell, a leader of the LUX experiment, that I was very unhappy with him for these results, but as he pointed out, we can’t argue with nature.) Last week at the conference, SuperCMDS, the most recent incarnation of the CDMS experiment, looked to much lower energies and again saw nothing. (Slides from Lauren Hsu’s talk.) The question remains: are we comparing apples and oranges? These detectors are made of a wide variety of types of nuclei and we don’t know how to relate the results. Wick Haxton’s talk surprised me by discussion of nuclear physics uncertainties I hadn’t been aware of, that in principle could reconcile all the disagreements between experiments, even DAMA and LUX. Most people think that the experimental claims of 10 GeV dark matter are wrong, but I am taking a wait and see attitude.

We also heard about the hints of detection of a completely different dark matter candidate: sterile neutrinos. (Slides from George Fuller’s talk.) In addition to the three known neutrinos of the Standard Model of Particle Physics, there could be another one that doesn’t interact with the standard model. Yet its decay could lead to x-ray lines. Two separate groups found indications of lines in data from the Chandra and XMM-Newton space satellites that would be consistent with a 7 keV neutrino (7 millionths of a proton mass). Could it be that there is more than one type of dark matter particle? Sure, why not?

On the last evening of the meeting, a number of us went to the Baja Cantina, our favorite spot for margaritas. Rick Gaitskell was smart: he talked us into the $60.00 pitchers, high enough quality that the 6AM alarm clocks the next day (that got many of us out of bed and headed to flights leaving from LAX) didn’t kill us completely. We have such a fun community of dark matter enthusiasts. May we find the stuff soon!

Posted in Guest Post, Science | 23 Comments

Effective Field Theory and Large-Scale Structure

Been falling behind on my favorite thing to do on the blog: post summaries of my own research papers. Back in October I submitted a paper with two Caltech colleagues, postdoc Stefan Leichenauer and grad student Jason Pollack, on the intriguing intersection of effective field theory (EFT) and cosmological large-scale structure (LSS). Now’s a good time to bring it up, as there’s a great popular-level discussion of the idea by Natalie Wolchover in Quanta.

So what is the connection between EFT and LSS? An effective field theory, as loyal readers know, an “effective field theory” is a way to describe what happens at low energies (or, equivalently, long wavelengths) without having a complete picture of what’s going on at higher energies. In particle physics, we can calculate processes in the Standard Model perfectly well without having a complete picture of grand unification or quantum gravity. It’s not that higher energies are unimportant, it’s just that all of their effects on low-energy physics can be summed up in their contributions to just a handful of measurable parameters.

In cosmology, we consider the evolution of LSS from tiny perturbations at early times to the splendor of galaxies and clusters that we see today. It’s really a story of particles — photons, atoms, dark matter particles — more than a field theory (although of course there’s an even deeper description in which everything is a field theory, but that’s far removed from cosmology). So the right tool is the Boltzmann equation — not the entropy formula that appears on his tombstone, but the equation that tells us how a distribution of particles evolves in phase space. However, the number of particles in the universe is very large indeed, so it’s the most obvious thing in the world to make an approximation by “smoothing” the particle distribution into an effective fluid. That fluid has a density and a velocity, but also has parameters like an effective speed of sound and viscosity. As Leonardo Senatore, one of the pioneers of this approach, says in Quanta, the viscosity of the universe is approximately equal to that of chocolate syrup.

So the goal of the EFT of LSS program (which is still in its infancy, although there is an important prehistory) is to derive the correct theory of the effective cosmological fluid. That is, to determine how all of the complicated churning dynamics at the scales of galaxies and clusters feeds back onto what happens at larger distances where things are relatively smooth and well-behaved. It turns out that this is more than a fun thing for theorists to spend their time with; getting the EFT right lets us describe what happens even at some length scales that are formally “nonlinear,” and therefore would conventionally be thought of as inaccessible to anything but numerical simulations. I really think it’s the way forward for comparing theoretical predictions to the wave of precision data we are blessed with in cosmology.

Here is the abstract for the paper I wrote with Stefan and Jason:

A Consistent Effective Theory of Long-Wavelength Cosmological Perturbations
Sean M. Carroll, Stefan Leichenauer, Jason Pollack

Effective field theory provides a perturbative framework to study the evolution of cosmological large-scale structure. We investigate the underpinnings of this approach, and suggest new ways to compute correlation functions of cosmological observables. We find that, in contrast with quantum field theory, the appropriate effective theory of classical cosmological perturbations involves interactions that are nonlocal in time. We describe an alternative to the usual approach of smoothing the perturbations, based on a path-integral formulation of the renormalization group equations. This technique allows for improved handling of short-distance modes that are perturbatively generated by long-distance interactions.

As useful as the EFT of LSS approach is, our own contribution is mostly on the formalism side of things. (You will search in vain for any nice plots comparing predictions to data in our paper — but do check out the references.) We try to be especially careful in establishing the foundations of the approach, and along the way we show that it’s not really a “field” theory in the conventional sense, as there are interactions that are nonlocal in time (a result also found by Carrasco, Foreman, Green, and Senatore). This is a formal worry, but doesn’t necessarily mean that the theory is badly behaved; one just has to work a bit to understand the time-dependence of the effective coupling constants.

Here is a video from a physics colloquium I gave at NYU on our paper. A colloquium is intermediate in level between a public talk and a technical seminar, so there are some heavy equations at the end but the beginning is pretty motivational. Enjoy!

Posted in arxiv, Science | 8 Comments


Almost forgot again — the leap-year thing always gets me. But I’ve now officially been blogging for ten years. Over 2,000 posts, generating over 57,000 comments. I don’t have accurate stats because I’ve moved around a bit, but on the order of ten million visits. Thanks for coming!

Nostalgia buffs are free to check out the archives (by category or month) via buttons on the sidebar, or see the greatest hits page. Here are some of my personal favorites from each of the past ten years:

Here’s to the next decade!

Posted in Blog, Personal | 12 Comments

God/Cosmology Debate Videos

Here is the video from my debate with William Lane Craig at the 2014 Greer-Heard Forum. Enough talking from me, now folks can enjoy for themselves. First is the main debate and Q&A:

It took a while for the Saturday talks by Maudlin, Collins, Rosenberg, and Sinclair to appear on line, but I’ve posted them here.

Posted in Philosophy, Religion | 134 Comments

Particle Physicists and Cosmologists on Twitter

Katie Freese, a well-known particle cosmologist who has a new book coming out, was asking if I had an tips about publicity. Short answer: not really, no. I haven’t really figured that one out. But one of the most obvious things to do, in terms of possible benefit per unit effort, is to join Twitter and start talking about science with the other denizens there.

I thought I would suggest to her a dozen or so good scientists to follow — after all, there aren’t that many working physicists in this field who are active on Twitter. I went to send her a few recommendations, at which point I realized there are actually quite a few! Some of whom deserve a lot more recognition.

So here is a list I compiled, consisting of people who are (1) active researchers in particle physics or cosmology; (2) on Twitter; (3) known to me. (More specifically, “recalled or noticed by me while making this list.”) Obviously one could compile a much longer list if we expanded it to include science communicators of all stripes, or even active scientists (or even just physicists) of all stripes. But I’m only one guy here. If I’m missing anyone who certainly qualifies, leave a comment; I’ll be happy to add them if I feel like it. I’m sure this isn’t more than half of the people who might be included in such a list. Obvious systematic error in favor of English-speakers, sorry. Entries listed in no particular order.

See also Lucretius’s lists of physicists, astronomers, and philosophers on Twitter, and CERN’s list of physicists.

Continue reading

Posted in Internet, Science and the Media | 23 Comments

Paco de Lucía

Sad news for guitar fans: the brilliant Spanish musician Paco de Lucía just passed away yesterday. While I’m not a major flamenco fan myself, I am a jazz fan — and like many others, I fell in love with Friday Night in San Francisco, an astonishing collaboration between de Lucía and fellow guitar masters Al Di Meola and John McLaughlin. Here are the three gents showing the rest of the world how its done.

After composing this post, I just noticed I used the same tune above for the very first post in this newly-constituted blog.

Posted in Music | 5 Comments

Post-Debate Reflections

We’ve returned from the lovely city of New Orleans, where within a short period of time I was able to sample shrimp and grits, bread pudding soufflé, turtle soup, chicken gumbo, soft-shelled crab with crawfish étouffée, and of course beignets. Oh yes, also participated in the Greer-Heard Forum, where I debated William Lane Craig, and then continued the discussion the next day along with Alex Rosenberg, Tim Maudlin, James Sinclair, and Robin Collins. The whole event was recorded, and will be released on the internet soon — hopefully within a couple of days.

[Update: Here is the video.]

In the meantime I thought I’d provide some quick post-debate reflections. Overall I think it went pretty well, although I certainly could have done better. Then again I’m biased, both by being hard on myself in terms of the debate performance, but understandably of the opinion that my actual ideas are correct. I think I mostly reached my primary goal of explaining why many of us think theism is undermined by modern science, and in particular why there is no support to be found for it in modern cosmology. For other perspectives see Rational Skepticism or the Reasonable Faith forums.

Clockwise from top left: William Lane Craig, Alex Rosenberg, Sean Carroll, James Sinclair, Robert Stewart (Greer-Heard organizer), Tim Maudlin, and Robin Collins.

Clockwise from top left: William Lane Craig, Alex Rosenberg, Sean Carroll, James Sinclair, Robert Stewart (Greer-Heard organizer), Tim Maudlin, and Robin Collins. Screenshot by Maryanne Spikes.

Short version: I think it went well, although I can easily think of several ways I could have done better. On the substance, my major points were that the demand for “causes” and “explanations” is completely inappropriate for modern fundamental physics/cosmology, and that theism is not taken seriously in professional cosmological circles because it is hopelessly ill-defined (no matter what happens in the universe, you can argue that God would have wanted it that way). He defended two of his favorite arguments, the “cosmological argument” and the fine-tuning argument; no real surprises there. In terms of style, from my perspective things got a bit frustrating, because the following pattern repeated multiple times: Craig would make an argument, I would reply, and Craig would just repeat the original argument. For example, he said that Boltzmann Brains were a problem for the multiverse; I said that they were a problem for certain multiverse models but not others, which is actually good because they help us to distinguish viable from non-viable models; and his response was the multiverse was not a viable theory because of the Boltzmann Brain problem. Or, he said that if the universe began to exist there must be a transcendent cause; I said that everyday notions of causation don’t apply to the beginning of the universe and explained why they might apply approximately inside the universe but not to it; and his response was that if the universe could just pop into existence, why not bicycles? I was honestly a bit surprised at the lack of real-time interaction, since one of Craig’s supporters’ biggest complaints is that his opponents don’t ever directly respond to his points, and I tried hard to do exactly that. To be fair, I bypassed some of his arguments (see below) because I thought they were irrelevant, and wanted to focus on the important issues; he might feel differently. I’m sure that others will have their own opinions, but soon enough the videos will allow all to judge for themselves. Overall I was moderately satisfied that I made the responses I had hoped to make, clarified some points, and gave folks something to think about.

Longer version (much longer, sorry): the format was 20-minute opening talks by each speaker (Craig going first), followed by 12-minute rebuttals, and then 8-minute closing statements. Among the pre-debate advice I was given was “make it a discussion, not a debate” and “don’t let WLC speak first,” both of which I intentionally ignored. I wanted all along to play by his rules, in front of his crowd, and do the best job I could do without any excuses. Continue reading

Posted in Philosophy, Religion | 149 Comments

God and Cosmology Debate with W.L. Craig

Tomorrow (Friday) is the big day: the debate with William Lane Craig at the Greer-Heard Forum, as I previously mentioned. And of course the event continues Saturday, with contributions from Tim Maudlin, Alex Rosenberg, Robin Collins, and James Sinclair.

I know what you’re asking: will it be live-streamed? Yes indeed!

[Update: Here is the video.]

Fun starts at 8pm Eastern, 5pm Pacific. (Corrected from earlier goof.) The format is an opening 20-minute speech by WLC and me (in that order), followed by 12-minute rebuttals, and then 8-minute closing statements, and concluding with 40 minutes of audience questions. Official Twitter hashtag is #GreerHeard14, which I believe you can use to submit questions for the Q&A. I wouldn’t lie to you: I think this will be worth watching.

You can find some of WLC’s thoughts on the upcoming event at his Reasonable Faith website. One important correction I would make to what you will read there: Craig and his interlocutor Kevin Harris interpret my statement that “my goal here is not to win the debate” as a strategy to avoid dealing with WLC’s arguments, or as “a way to lower expectations.” Neither is remotely true. I want to make the case for naturalism, and to do that it’s obviously necessary to counter any objections that get raised. Moreover, I think that expectations (for me) should be set ridiculously high. The case I hope to make for naturalism will be so impressively, mind-bogglingly, breathtakingly strong that it should be nearly impossible for any reasonable person to hear it and not be immediately convinced. Honestly, I’ll be disappointed if there are any theists left in the audience once the whole thing is over.

Feel free to organize viewing parties, celebrations, discussion groups, what have you. There should definitely be a drinking game involved (it’ll be happy hour on the West Coast, you lightweights), but I’ll leave the details to you. Suggested starting points: drink every time WLC uses a syllogism, or every time I show an equation. But be sure to have something to eat, first.

If it seems worthwhile, I will follow-up with thoughts after the debate, and try to answer questions. Let’s have some fun.

Posted in Philosophy, Religion | 167 Comments

Dept. of Energy Support for Particle Theory: A “Calamity”

One of the nice things that governments do is support basic scientific research — work that might help us better understand how the world works, but doesn’t have any direct technological or economic application. Particle physics and cosmology are great examples. In the U.S., much of the funding for these fields comes from the Office of High Energy Physics within the Office of Science at the Department of Energy (DOE).

Now that support is crumbling — drastically. In the last couple of years, the DOE has radically changed how it carries out reviews of different university theory groups, to decide how much grant support each will get. All for ostensibly good reasons — leveling the playing field and all that. But, without much fanfare, the actual result has been a significant drop in funding for almost every major theory group in the country.

Laurence Yaffe of the University of Washington, a respected particle and nuclear theorist, just released an analysis he informally carried out after serving a temporary assignment at the DOE. Here is his abstract (emphasis mine):

Impacts of Recent Comparative Review Cycles on DOE-funded High Energy Theory
L.G. Yaffe, University of Washington
February 19, 2014

A summary is presented of data obtained from a grass-roots effort to understand the effects of the FY13 and FY14 comparative review cycles on the DOE-funded portion of the US high energy theory community and, in particular, on graduate students and postdoctoral researchers who are beginning their careers. For a sample comprised of nearly all of the larger groups undergoing comparative review, total funding declined by an average of 23%, with numerous major groups receiving reductions in the 30–55% range. Funding available for postdoc or graduate student support declined over 30%, with many reductions in the 40–65% range. The total number of postdoc positions in this large sample of theory groups is declining by over 40%. The impacts on young researchers raise grave concerns regarding continued U.S. leadership in high energy theory.

A 20% cut in funding in one year is kind of a big deal. A picture is worth a thousand words, so here are two of them; overall funding changes for all the different groups:


and changes specifically in support for graduate students and postdocs:


Obviously this is unsustainable, unless as a society we make the decision that particle physics just isn’t worth doing. But hopefully things can be rectified at least a bit, to restore some of that money. Everyone I know is bemoaning the cuts, complaining that they have been turning away prospective grad students and postdocs more than ever before. I’m not necessarily against decreasing the number of postdocs (as opposed to grad students); the pipeline has to narrow somewhere, and there’s a sensible argument to be made to do it at that point. But we should do it deliberately and after thinking and talking about it, not as the haphazard result of some new bureaucratic procedures. It would be a shame to destroy our future prospects in this centrally important area of science.

Posted in Science, Science and Politics | 31 Comments