Space-time, Quantum Mechanics and the Large Hardon Collider

Posted on February 28, 2011 by woit

The title of the posting is that of Nima Arkani-Hamed’s public lecture last week at the IAS, with the spelling that of the title at the beginning of the video (available here, lower resolution version here).

The bulk of the talk is devoted to expounding the idea that the central problems of fundamental physics are two hierarchy problems, that of the CC (why isn’t it at the Planck scale?) and that of the Higgs mass (why isn’t it also at the Planck scale?). Given that we don’t understand quantum gravity, and don’t know that the Higgs phenomenon is due to an elementary scalar, it’s not clear to me that these are yet real problems. In any case, Arkani-Hamed gives the anthropic multiverse argument for the CC problem, and claims that if the LHC doesn’t see supersymmetry or large extra dimensions, then we’re stuck with the anthropic multiverse argument also for the electroweak scale.

The LHC only puts in an appearance in the last fifteen minutes of an hour and a half talk. Back in 2005 (see his talk at Strings 2005) Arkani-Hamed claimed that we would know whether supersymmetry solves the hierarchy problem within a year or so of first collisions at the LHC (then scheduled for summer 2007). Now that initial results from the LHC are in, showing no evidence of supersymmetry, his estimate is:

We’re going to have answers one way or another to this question on the time scale of 2020.

One of his slides estimates production of 1 squark/minute given 1 billion collisions/sec, which would mean about 50 squarks already produced in each detector. While it’s true that the LHC won’t be running at full energy until 2014, no explanation is given for why we need to wait until 2020 to find out about supersymmetry. Back in 2005, before the machine was turned on, enthusiastic predictions of quick results were being made. Now that the data is coming in, the story seems to have changed.

Update: Nature News has a new article up by Geoff Brumfiel: Beautiful theory collides with smashing particle data (also available here). While Arkani-Hamed is arguing that one will have to wait until 2020 (the sLHC perhaps?) before knowing whether supersymmetry is at LHC energies, John Ellis appears willing to give up much earlier, maybe the end of next year:

“I’m wouldn’t say I’m concerned,” says John Ellis, a theorist at CERN, Europe’s particle-physics lab near Geneva, who has worked on supersymmetry for decades. He says that he will wait until the end of 2012–once more runs at high energy have been completed–before abandoning SUSY. Falkowski, a long-time critic of the theory, thinks that the lack of detections already suggest that SUSY is dead.

“Privately, a lot of people think that the situation is not good for SUSY,” says Alessandro Strumia, a theorist at the University of Pisa in Italy, who recently produced a paper about the impact of the LHC’s latest results on the fine-tuning problem. “This is a big political issue in our field,” he adds. “For some great physicists, it is the difference between getting a Nobel prize and admitting they spent their lives on the wrong track.” Ellis agrees: “I’ve been working on it for almost 30 years now, and I can imagine that some people might get a little bit nervous.”

The article ends with a very sensible quote from experimentalist Chris Lester, who evidently doesn’t share Arkani-Hamed’s view that it’s SUSY or the Multiverse:

“Plenty of things will change if we fail to discover SUSY,” says Lester. Theoretical physicists will have to go back to the drawing board and find an alternative way to solve the problems with the standard model. That’s not necessarily a bad thing, he adds: “For particle physics as a whole it will be really exciting.”

Update: It seems that the video files have been temporarily removed, presumably for editing. I fear that some poor tech person is having a bad morning…

Update: New video files with typo fixed are now available.

Posted in Favorite Old Posts, Uncategorized | 69 Comments

Implications of Initial LHC Searches for Supersymmetry

Posted on February 22, 2011 by woit

There’s a new paper out this evening from a large collaboration entitled Implications of Initial LHC Searches for Supersymmetry. Instead of just adding it to the bottom of my recent posting, I thought it would be a good idea to start a new one, and add a bit more explanation of what is going on.

For a good news story from today by Kate McAlpine, see this at Physics World. For excellent more technical explanations, see the latest blog postings at Tommaso Dorigo’s blog (today) and at Resonaances (yesterday). Physics World, Tommaso and the new arXiv preprint discuss published results from CMS and ATLAS, while Resonaances discusses even more stringent preliminary limits on SUSY from ATLAS made public last week at Aspen.

Tommaso also refers to a 2008 guest blog posting by Ben Allanach explaining how statistical predictions for SUSY masses were being made, adopting various simplifying assumptions (CMSSM) and assuming supersymmetry solves the problems it is advertised as solving (muon g-2 anomaly, dark matter, etc.). Allanach discusses the 2008 version of this kind of calculation by the same group that has just put out a new, 2/22/2011 version this evening.

The usual model for how science is done is that theorists make predictions before an experiment is done, then when the experimental results come in, they get compared to the predictions. That’s not quite what is going on here, where as far as I can tell, the new paper doesn’t directly compare the 2008 predictions to the new experimental results. Instead, the new experimental results are used to make new predictions. Since a large part of the parameter space favored in the 2008 predictions has now been ruled out, the new ones move the favored part of parameter space up to higher particle masses. The authors do make clear what is going on, showing on their plots a “snowflake” where the 2008 best-fit value was, and “stars” for where the new best-fit values are based on data from the two experiments. Note that the paper does not include the latest, stronger results from ATLAS announced last week, which presumably would move the “stars” up to even higher mass.

While the question this paper addresses about where supersymmetry might be given that it hasn’t been seen yet is interesting, it leaves unaddressed the more conventional question: do the LHC experimental results show that the theoretical predictions about supersymmetry made in 2008 before the machine was turned on were wrong? This is a statistical question, so should have a statistical answer. Assuming that the LHC continues to not see supersymmetry as it collects more data, I’m interested in the question of how the experimental data will falsify the theory. Will its proponents just keep calculating statistical predictions of higher and higher masses as lower ones get ruled out? Most will undoubtedly at some point throw in the towel, although there will be some who will never, never, never, never give up (see here):

SUSY may still be there even if it remains invisible to the LHC, indeed. And yes, I don’t hide that I will be convinced that SUSY is there even if the LHC doesn’t find it. The LHC will only confirm or exclude effects at particular regimes – usually low energy but it’s not quite accurate a description of the regime that may be excluded.

What I have been scared for several years is the pseudoscientific propaganda of your kind trying to claim – without any justification – that not seeing SUSY at the LHC should imply that physicists shouldn’t be allowed to work on SUSY or believe that it is a key feature of our Universe. There are many reasons to think it’s the case and theorists whom I consider any good will continue to treat SUSY as an essential feature whether or not it shows up at the LHC.

Update: See figure 1 of this evening’s What if the LHC does not find supersymmetry in the sqrt(s)=7 TeV run? to see how how much of the predicted region of superpartner masses was ruled out by initial LHC results, and how much of the rest is likely to be ruled out during by the 2011-2 7 TeV run.

Update: There’s a very new up-to-the-minute survey of LHC results concentrating on supersymmetry by John Ellis here. Unfortunately no figures that superimpose CMS/ATLAS exclusion regions on the statistically favored regions for supersymmetry that are discussed (based on assuming supersymmetry explains dark matter and the muon g-2 anomaly). It does look like this year’s data should be able to convincingly rule out the idea that supersymmetry explains both of these phenomena.

Update: The ATLAS results providing the strongest limits so far on SUSY are now out, see the paper here.

Posted in Experimental HEP News | 64 Comments

This Week’s Hype

Posted on February 21, 2011 by woit

A session on results from the LHC at last week’s AAAS meeting has generated some news reports about results from the heavy ion run, see here and here. Under the heading “String theory supported”, MSNBC reports:

Previous experiments conducted at another particle accelerator, the Relativistic Heavy-Ion Collider in New York, showed that quark-gluon plasma took on the form of a liquid. Some scientists expected the plasma to go to a gaseous state at the higher temperatures achieved by ALICE, but it didn’t. Instead, it was a “perfect liquid, which flows without resistance and is completely opaque,” Schutz said.

That in itself was a big surprise. But Schutz told me that the results were consistent with what had been predicted by a particular variant of string theory known as AdS/CFT correspondence, which also addresses such mysteries as quantum gravity and extra dimensions. “I’m surprised that they can make a prediction and that it matches what we measured,” Schutz said.

String theory is a long-debated conception of the subatomic world that envisions matter as being composed of incredibly tiny strings or membranes that vibrate in an 11-dimensional universe. Skeptics have criticized the concept as being untestable and unfalsifiable, but if findings from the LHC can confirm some hypotheses and falsify others, that could increase string theory’s acceptance.

The campaign to deal with the failure of string theory unification by confusing it with AdS/CFT as an approximate calculational method continues. No matter how successful or unsuccessful AdS/CFT is at describing heavy-ion collisions, this has nothing to do with string theory as a unified theory of gravity and the Standard Model. I am curious though about the question of how well AdS/CFT does work as an approximation for describing heavy-ion physics. Can anyone point me to distinctive AdS/CFT predictions about what the LHC should see that are now being tested? The news reports just seem to refer to evidence that at LHC energies the quark-gluon plasma seems to continue to exhibit the perfect liquid behavior seen at RHIC.

Update: See the comment section for an extensive discussion by someone expert in the field (Hans Juergen Pirner) relevant to the question I was raising.

Posted in This Week's Hype | 23 Comments

First LHC Winter Conference Results

Posted on February 17, 2011 by woit

This week the Aspen Center for Physics is hosting one of the first of this year’s “Winter Conferences” where results from last year’s LHC run are being reported. Appropriately, the title of the conference is New Data from the Energy Frontier. The most dramatic result has to do with what is not being seen: any evidence of supersymmetry, with new limits reported today by ATLAS. The new ATLAS results rule out gluino masses up to 7-800 GeV, improving on the first limits of this kind from CMS which were about 600 GeV.

For more detailed discussion courtesy of the blogosphere, see Resonaances and Cosmic Variance. For some indication of what this means for string theory, Michael Dine’s lecture notes for his talks on “What LHC might tell us about String Theory” at last summer’s TASI summer school are now out, with the title Supersymmetry from the top down. These lecture notes start off with a section very unusual for this kind of thing, entitled “Reasons for Skepticism”. and he notes:

Our enthusiasm for supersymmetry, however, should be tempered by the realization that from existing
data – including early LHC data – there are, as we will discuss, reasons for skepticism.

For some historical perspective about what pre-LHC expectations were, I happened to run across today a copy of Witten’s lecture notes from a string theory conference at Hangzhou in 2002, where he gives the muon magnetic moment discrepancy as one piece of evidence for supersymmetry, and says:

Assuming this discrepancy holds up, we would expect to interpret it in terms of new particles, but these are highly constrained; one explanation that does work is supersymmetry, with masses of new particles of order 200 – 300 GeV.

Of course, even the minimal supersymmetric extension of the Standard Model is ferociously complicated, with over a hundred unknown parameters, so all quoted limits make various simplifying assumptions. Relating LHC data to limits on supersymmetry will be a subject keeping many physicists busy for the next few years, for more about this, see this talk at Aspen by Jay Wacker. He doesn’t expect this year’s run to as dramatically increase limits on gluinos as last year’s run did, describing early results as “full coverage up to 300 GeV, reach up to 600 GeV”, increasing to “full coverage up to 375 GeV, reach up to 800 GeV” after an inverse femtobarn of data is analyzed (that’s the official LHC goal for 2011, although it’s hoped they can double or triple that).

The last sentence of his last slide refers to something that I’ve always worried about, but am not expert enough to know whether such a worry is serious. He describes the web-site http://LHCNewPhysics.org where simplified models based on supersymmetry and other BSM ideas are given, and notes:

ATLAS studying 10 Simplified Models from 0 in August. Changing their triggers.

The worry I’m not so sure about is to what extent the LHC detector triggers are being optimized to look for supersymmetry, potentially missing un-expected non-Standard Model physics. Since there were always reasons to be skeptical of LHC-scale supersymmetry, and these have now become so compelling that even Michael Dine is writing about them, one hopes that the trigger designers will keep that in mind.

Meanwhile, back at the LHC, powering tests are finished, the ring is closed and will be put through full tests of its operational cycle the next couple of days. Official start of beams for this year is planned for Monday.

Update: More details about the latest on this at Resonannces.

Update: More from Tommaso Dorigo (LHC Excludes SUSY Theories, Theorists Clinch Hands), and a Physics World article by Kate McAlpine here. Tommaso links to a 2008 posting by Ben Allanach that discusses predictions for SUSY masses made (using various assumptions one can read about there) around that time. One of these, by a large group including John Ellis, predicted that 50 inverse picobarns at 10 TeV would be enough to explore most of the region they expected SUSY masses to be in, at 68% confidence level. The latest data, which is about that luminosity but at 7 TeV, does rule out much of that region, with the most likely SUSY mass right around the boundary of the region ruled out by ATLAS (although the tan(beta) values are different). According to the Physics World article:

John Ellis of CERN and King’s College London disagrees that the LHC results cause any new problems for supersymmetry. Because the LHC collides strongly interacting quarks and gluons inside the protons, it can most easily produce their strongly interacting counterparts, the squarks and gluinos. However, in many models the supersymmetric partners of the electrons, muons and photons are lighter, and their masses could still be near the electroweak scale, he says.

Posted in Experimental HEP News | 23 Comments

Budget News

Posted on February 14, 2011 by woit

Almost five months into FY 2011, the US still has no budget for the year, operating on a continuing resolution that funds the government at FY 2010 levels until March 4. The House Republicans have come up with a proposal for huge budget cuts, which would arrive late in the fiscal year, probably requiring national labs like Fermilab to essentially shutdown for the remainder of the fiscal year. John Conway has more about this here. No one seems to seriously believe this proposal will pass into law.

This morning, the White House announced its budget proposal for FY 2012. The DOE Office of Science would get a healthy increase, to $5.416 billion from $4.964 billion in 2010. Similarly, the NSF would go from $6.873 billion to $7.768. However, the Administration’s FY 2011 request ended up being pretty much irrelevant, and it’s not clear that this one will fare any better. No information yet on how HEP and mathematics fare specifically in these requests.

So, bottom line is that no one really knows what this year or next year’s budget numbers will be, with the House proposal a lower bound for this year, and I suspect the president’s proposal will be an upper bound for next year.

Midday tomorrow, Fermilab director Oddone will give an all-hands talk at Fermilab to discuss the implications of all this for the lab.

Update: At the DOE, the FY2012 request for High Energy Physics is $797 million, versus $791 million in FY 2010 (last year at this time, the FY 2011 request was for $829 million). More details here.

Details of the NSF budget request are here. Mathematics research goes from $241 million in FY 2010 to $260 million in FY 2012. Physics from $290 million to $300 million. The NSF has pulled the plug on the DUSEL lab, freeing up $36 million/year, which is repurposed towards what they describe as their three priority areas: physics of the universe, quantum information science and the physics-biology interface.

Posted in Uncategorized | 6 Comments

Celebrity News

Posted on February 8, 2011 by woit

A selection of celebrity math/physics news:

  • Jane Fonda’s blog has a report on My Meeting With Stephen Hawking. Hawking told her “You were my heart throb”, admitting that Barbarella was what he had in mind.
  • MIT has put online a video of a lecture given in December by Jim Simons. Simons tells some of the story of his remarkable career and gives some advice about doing math. The financial news this morning reports that Simons’s Renaissance Technologies is now down in Brazil doing high-frequency trading. The story is illustrated with a photo of Gisele Bundchen super-imposed on what appears to be Simons lecturing on differential K-theory.
  • Someone has come up with an Einstein Index to rank theoretical physicists using a new citation analysis. According to this index, Juan Maldacena outranks Steve Weinberg and Ed Witten by a sizable amount. Further down the list, Sean Carroll and Murray Gell-Mann get the same ranking.
  • Brian Greene’s new book on the Multiverse is getting a lot of attention, with mostly laudatory reviews. For an alternate take, see the review at Bookforum by Charles Seife.
    • Posted in Uncategorized | 22 Comments

    IAS 80th Anniversary Talks

    Posted on February 3, 2011 by woit

    This may be old news, but I just recently noticed that talks given at the IAS in Princeton last fall to celebrate its 80th anniversary are now available on-line here. They include talks by Voevodsky on the foundations of mathematics, Zaldarriaga on cosmology, Wilczek on supersymmetry and quantum computing, and Arkani-Hamed on Fundamental Physics in the 21st Century.

    According to Arkani-Hamed, the 21st century will be all AdS/CFT, all the time, using it to justify:

    the slogan is that string theory/quantum gravity is particle physics…

    There is a less interesting but amusing sociological fact associated with this, that since this realization there aren’t really different camps in theoretical physics any more. There aren’t string theorists and particle theorists, there’s one big structure “Good Ideas in Theoretical Physics”, OK. That structure has many different facets and you can work on different parts of it, but it’s all connected. You can really see it in the way the field developed since the late 90s, we’re much, much more one big happy family than was the case in the 1980s. I should say of course there are still people that do bad theoretical physics, but they’re not at the Institute. So all good ideas in theoretical physics are combined in one very big structure that no longer is there such a big difference between strings, QFT…

    This puts into practice Nati Seiberg’s 2005 prediction:

    “Most string theorists are very arrogant,” says Seiberg with a smile. “If there is something [beyond string theory], we will call it string theory.”

    Both Wilczek and Arkani-Hamed advertise supersymmetry, with Arkani-Hamed making the peculiar claim that physicists need to throw space-time out the window, but for some reason, before doing so it is important that they add supersymmetry to it. Wilczek states definitively that if superpartners don’t show up at the LHC, as far as he is concerned the idea will be gone.

    The minimal supersymmetric standard model is an important pillar of the “Good Ideas in Theoretical Physics” that those at the Institute cling to, despite efforts to use them to unify physics failing miserably over the last 25 years. If the LHC doesn’t see supersymmetry and Wilczek and others give up on the idea, it will be interesting to see if the IAS faculty revise their point of view and start to develop more of an interest in what they now claim is “bad theoretical physics”. I suppose though that when they do, they’ll call whatever they change over to “string theory”.

    Update: By the way, for the latest on what initial results from the LHC are saying about extra dimensions and supersymmetry, see this talk given today at CERN by Alessandro Strumia.

    Posted in Uncategorized | 34 Comments

    The 4% Universe

    Posted on January 31, 2011 by woit

    I’ve written a review of Richard Panek’s quite good new book The 4% Universe, which has appeared at the Wall Street Journal. The main topic of the book is the supernova searches that led to what seems to be a non-zero value of the cosmological constant. It also discusses the astronomical evidence for dark matter, as well as on-going searches for a dark matter particle.

    One of the most interesting themes of the book is that of the encounter between the two different cultures of particle physics and astronomy. Astronomers have begun to worry not only about a new culture of large collaborations, but about the danger of an over-emphasis on certain specific measurements of fundamental significance. For more about this, see the article by Simon White from a few years ago Fundamentalist Physics: why Dark Energy is bad for Astronomy. Now that cosmologists have their own highly successful Standard Model, they’re starting to take a look at what happened after the arrival of the Standard Model in particle physics, and worry that they too may someday become victims of their own success.

    Posted in Book Reviews | 12 Comments

    Is the Multiverse Immoral?

    Posted on January 29, 2011 by woit

    [Warning, somewhat of a rant follows, and it’s not very original. You might want to skip this one…]

    In the last week or so, I’ve run into two critiques of the currently fashionable multiverse mania that take an unusual angle on the subject, raising the question of the “morality” of the subject. The first of these was from Lee Smolin, who was here in New York last week talking at the Rubin Museum. I probably won’t get this quite right, but from what I remember he said that discussions of a multiverse containing infinite numbers of copies of ourselves behaving slightly differently made him uneasy for moral reasons. The worry is that one might be led to stop caring that much about the implications of one’s actions. After all, whatever mistake you make, in some other infinite number of universes, you didn’t do it.

    Over at Scientific American, yesterday they had John Horgan’s Is speculation in multiverses as immoral as speculation in subprime mortgages?. There’s more about this in a Bloggingheads conversation today with George Johnson, where Horgan describes his current reaction to multiverse mania as “I can’t stand this shit.”

    I’m in agreement with Horgan there, but my own moral concerns about the issue are different than the ones he and Smolin describe. The morality of how people choose to live their everyday lives doesn’t seem to me to have much to do with whatever the global structure of the universe might be. The world we are rapidly approaching in which a multiverse is held up as an integral part of the modern scientific world view isn’t one in which many people are likely to behave differently than before, so I don’t share Smolin’s concerns. Horgan’s exasperation with seeing the multiverse heavily promoted by famous physicists appears to have more to do with the idea that this is a retreat by physicists from engagement with the real world, something morally obtuse in an era of growing problems that scientists could help address. For what he would like to see instead, I guess a good model would be John Baez’s recent decision to turn his talents towards real-world problems facing humanity, see his blog Azimuth for more about this. Personally, I’m not uncomfortable with the fact that many mathematicians and physicists find that they don’t feel they are likely to be of much help if they go to work on the technology and science surrounding social problems. Instead, one can reasonably decide that one has some hope of making progress on fundamental issues in mathematics or physics and choose to work on that instead. One can try and justify this by hoping that new breakthroughs will somehow, someday help humanity, although this may be wishful thinking. Or one can argue that working towards a better understanding of the universe is inherently worthwhile, so pursuing this while taking some care to avoid worsening one’s local corner of the world is a morally reasonable stance.

    My own moral concerns about the multiverse have more to do with worry that pseudo-science is being heavily promoted to the public, leading to the danger that it will ultimately take over from science, first in the field of fundamental physics, then perhaps spreading to others. This concern is somewhat like the one that induced Alan Sokal to engage in his famous hoax. He felt that abandonment by prominent academics of the Enlightenment ideals exemplified by the scientific method threatens a move into a new Dark Ages, where power dominates over truth. Unfortunately, I don’t think that revelation of a hoax paper would have much effect in multiverse studies, where some of the literature has already moved beyond the point where parody is possible.

    For a while I was trying to keep track of multiverse-promoting books, and writing denunciatory reviews here. They’ve been appearing regularly for quite a few years now, with increasing frequency. Some typical examples that come to mind are Kaku’s Parallel Worlds (2004), Susskind’s The Cosmic Landscape (2005), and Vilenkin’s Many Worlds in One (2006). Just the past year has seen Sean Carroll’s From Eternity to Here, John Gribbin’s In Search of the Multiverse, Hawking and Mlodinow’s The Grand Design, and Brian Greene’s new The Hidden Reality. In a couple weeks there will be Steven Manly’s Visions of the Multiverse. Accompanying the flood of books is a much larger number of magazine articles and TV programs.

    Several months ago a masochistic publisher sent me a copy of Gribbin’s book hoping that I might give it some attention on the blog, but I didn’t have the heart to write anything. There’s nothing original in such books and thus nothing new to be said about why they are pseudo-science. The increasing number of them is just depressing and discouraging. More depressing still are the often laudatory reviews that these things are getting, often from prominent scientists who should know better. For a recent example, see Weinberg’s new review of Hawking/Mlodinow in the New York Review of Books.

    While most of the physicists and mathematicians I talk to tend towards the Horgan “I can’t stand this shit” point of view on the multiverse, David Gross is about the only prominent theorist I can think of known to publicly take a similar stand. One of the lessons of superstring theory unification is that if a wrong idea is promoted for enough years, it gets into the textbooks and becomes part of the conventional wisdom about how the world works. This process is now well underway with multiverse pseudo-science, as some theorists who should know better choose to heavily promote it, and others abdicate their responsibility to fight pseudo-science as it gains traction in their field.

    Posted in Multiverse Mania | 107 Comments

    News From Chamonix

    Posted on January 28, 2011 by woit

    The people responsible for the LHC are meeting in Chamonix this week to make plans for the upcoming run, slides of many talks are available here. The results of discussions there are:

  • The recommendation will be to run at 3.5 TeV/beam, not increasing to 4 TeV/beam as widely expected.
  • The long shutdown to fix splices and allow going to 7 TeV/beam will be delayed until 2013, with 2012 devoted to a physics run. The 2013 shutdown will likely last more than a year.
  • Officially, the integrated luminosity goal for 2011 remains at 1 inverse femtobarn. However, unofficially, they expect to be able to do at least twice this, ending up with 2-3 inverse femtobarns by the end of the year

    • For a discussion of the possible physics that can be done with these parameters, see here. By the end of 2011 the LHC should be able to do better than the Tevatron on the Higgs search over most of the possible mass range, except for the low end of the range, where higher energy isn’t much help, and the Tevatron’s more than 10 inverse femtobarns and longer experience with the data analysis may give them an edge.

    Posted in Experimental HEP News | 6 Comments