Back for the third and final day of the Philosophy and Cosmology conference in honor of George Ellis’s birthday. I’ll have great memories of my time in Oxford, almost all of which was spent inside this lecture hall. See previous reports of Day One, Day Two.

It’s become clear along the way that I am not as accurate when I’m trying to represent philosophers as opposed to physicists; the vocabularies and concerns are just slightly different and less familiar to me. So take things with an appropriate grain of salt.

Tuesday morning: The Case for Multiverses

9:00: Bernard Carr, one of the original champions of the anthropic principle, has been instructed to talk on “How we know multiverses exist.” Not necessarily the title he would have chosen. Of course we don’t observe a multiverse directly; but we might observe it indirectly, or infer it theoretically. We should be careful to define “multiverse,” not to mention “exist.”

There certainly has been a change, even just since 2001, in the attitude of the community toward the multiverse. Quotes Frank Wilczek, who tells a parable about how multiverse advocates have gone from voices in the wilderness to prophets. That doesn’t mean the idea is right, of course.

Carr is less interested in insisting that the multiverse does exist, and more interested in defending the proposition that it might exist, and that taking it seriously is perfectly respectable science. Remember history: August Comte in 1859 scoffed at the idea we would ever know what stars were made of. Observational breakthroughs can be hard to predict. Rutherford: “Don’t let me hear anyone use the word `Universe’ in my department!” Cosmology wasn’t respectable. For what it’s worth, the idea that what we currently see is the whole universe has repeatedly been wrong.

So how do we know a multiverse exists? Maybe we could hop in a wormhole or something, but let’s not be so optimistic. There are reasons to think that multiverses exist: for example, if we find ourselves near some anthropic cutoff for certain parameters. More interesting, there could be semi-direct observational evidence — bubble collisions, or perhaps giant voids. Discovering extra dimensions would be good evidence for the theories on which the multiverse is often based.

The only direct observations that currently exists that might bear directly on multiverses is the prediction of giant voids and dark flows by Laura Mersini-Houghton and collaborators.

Carr believes that the indirect evidence from finely-tuned coupling constants is actually stronger. Existence of planets requires a very specific relationship between strength of gravity and electromagnetism, which happens to exist in the real world. There is a similar gravity/weak tuning needed to make supernovae and heavy elements. Admittedly, many physicists dislike the multiverse and find it just as unpalatable as God. But ultimately, multiverse ideas will become normal science by linking up with observations; we just don’t know how long it will take.

9:45: George Ellis follows Carr’s talk with what we’ve been waiting for a while — a strong skeptical take on the multiverse idea.

There are lots of types of multiverses: many-worlds, separated by space or time, or completely disjoint. Anthropic arguments are what make the idea go. The project is to make the apparently improbable become probable.

The very nature of the scientific enterprise is at stake: multiverse proponents are proposing that we weaken the idea of scientific proof. Science is about two things: testability and explanatory power. Is it worth giving up the former to achieve the latter?

The abstract notion of a multiverse doesn’t get you anything; you need a specific model, with a distribution of probabilities. (Does Harry Potter exist somewhere in your multiverse?) But if there is some process that generates universes, how do you test that process? Domains beyond our particle horizon are unobservable. How far should we expect to be able to extrapolate? Into a region which, in principle, we will never be able to observe.

In the good old days we accepted the Cosmological Principle, and assumed things continued uniformly forever beyond our observable horizon. Completely untestable, of course. If all the steps in the extrapolation are perfectly tenable, extrapolations are fine — but that’s not the case here. In particular, the physics of eternal inflation (gravity plus quantum field theory, Coleman-de Luccia tunneling) has never been tested. It’s unknown physics used to infer an unobservable realm. Inflation itself is not yet a well-defined theory, and not all versions of inflation are eternal. We haven’t even found a scalar field!

There is a claim that a multiverse is implied by the fine-tuning of the universe to allow life. At best a weak consistency test. Can never actually do statistical tests on the purported ensemble. Another claim is that the local universe, if it’s inside a bubble, should have a slight negative curvature — but that’s easily avoided by super-Hubble perturbations, so it’s not a strong prediction. We could, however, falsify eternal inflation by observing that we live in a “small” (topologically compact) universe. But if we don’t, it certainly doesn’t prove that eternal inflation is right. Finally, it’s true that we might someday see signatures of bubble collisions in the microwave background. But if we don’t, then what? Again, not a firm prediction.

Ultimately: explanation and testability are both important, but one shouldn’t overwhelm the other. “The multiverse theory can’t make any prediction because it can explain anything at all.” Beware! If we redefine science to accommodate the multiverse, all sorts of pseudo-science might sneak inside the tent.

There are also political/sociological issues. Orthodoxy is based on the beliefs held by elites. Consider the story of Peter Coles, who tried to claim back in the 1990′s that the matter density was only 30% of the critical density. He was threatened by a cosmological bigwig, who told him he’d be regarded as a crank if he kept it up. On a related note, we have to admit that even scientists base beliefs on philosophical agendas and rationalize after the fact. That’s often what’s going on when scientists invoke “beauty” as a criterion.

Multiverse theories invoke “a profligate excess of existential multiplicity” in order to explain a small number of features of the universe we actually see. It’s a possible explanation of fine tuning, but is not uniquely defined, is not scientifically testable, and in the end “simply postpones the ultimate metaphysical question.” Nevertheless — if we accumulated enough consistency tests, he’d be happy to eventually become convinced.

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The previous post on the Philosophy and Cosmology conference in Oxford was growing to unseemly length, so I’ll give each of the three days its separate post.

Monday morning: The Case for Multiverses

9:00: We start today as we ended yesterday: with a talk by Martin Rees, who has done quite a bit to popularize the idea of a multiverse. He wants to argue that thinking about the multiverse doesn’t represent any sort of departure from the usual way we do science.

The Big Bang model, from 1 second to today, is as uncontroversial as anything a geologist does. Easily falsifiable, but it passes all tests. How far does the domain of physical cosmology extend? We only see the universe out to the microwave background, but nothing happens out there — it seems pretty uniform, suggesting that conditions inside extend pretty far outside. Could be very far, but hard to say for sure.

Some people want to talk only about the observable universe. Those folks need aversion therapy. After all, whether a particular distant galaxy eventually becomes observable depends on details of cosmic history. There’s no sharp epistemological distinction between the observable and unobservable parts of the universe. We need to ask whether quantities characterizing our observable part of the universe are truly universal, or merely local.

So: what values of these parameters are consistent with some kind of complexity? (No need to explicitly invoke the “A-word.”) Need gravity, and the weaker the better. Need at least one very large number; in our universe it’s the ratio of gravity to electromagnetic forces between elementary particles. Also need departure from thermodynamic equilibrium. Also: matter/antimatter symmetry, and some kind of non-trivial chemistry. (Tuning between electromagnetic and nuclear forces?) At least one star, arguably a second-generation star so that we have heavy elements. We also need a tuned cosmic expansion rate, to let the universe last long enough without being completely emptied out, and some non-zero fluctuations in density from place to place.

If the amplitude of density perturbations were much smaller, the universe would be anemic: you would have fewer first-generation stars, and perhaps no second-generation stars. If the amplitude were much larger, we would form huge black holes very early, and again we might not get stars. But ten times the observed amplitude would actually be kind of interesting. Given an amplitude of density perturbations, there’s an upper limit on the cosmological constant, so that structure can form. Again, larger perturbations would allow for a significantly larger cosmological constant — why don’t we live in such a universe? Similar arguments can be made about the ratio of dark matter to ordinary matter.

Having said all that, we need a fundamental theory to get anywhere. It should either determine all constants of nature uniquely, in which case anthropic reasoning has no role, or it allows ranges of parameters within the physical universe, in which case anthropics are unavoidable.

10:00: Next up, Philip Candelas to talk about probabilities in the landscape. The title he actually puts on the screen is: “Calabi-Yau Manifolds with Small Hodge Numbers, or A Des Res in the Landscape.”

A Calabi-Yau is the kind of manifold you need in string theory to compactly ten dimensions down to four, picked out among all possible manifolds by the requirement that we preserve supersymmetry. There are many examples, and you can characterize them by topological invariants as well as by continuous parameters. But there is a special corner in the space of Calabi-Yau’s where certain topological invariants (Hodge numbers) are relatively small; these seem like promising places to think about phenomenology — e.g. there are three generations of elementary particles.

Different embeddings lead to different gauge groups in four dimensions: E₆, SO(10), or SU(5). Various models with three generations can be found. Putting flux on the Calabi-Yau can break the gauge group down to the Standard Model, sometimes with additional U(1)’s.

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Greetings from Oxford, a charming little town across the Atlantic with its very own university. It’s in the United Kingdom, a small island nation recognized for its steak and kidney pie and other contributions to world cuisine. What you may not know is that the UK has also produced quite a few influential philosophers and cosmologists, making it an ideal venue for a small conference that aims to bring these two groups together.

The proximate reason for this particular conference is George Ellis’s 70th birthday party. Ellis is of course a well-known general relativist, cosmologist, and author. Although the idea of a birthday conference for respected scientists is quite an established one, Ellis had the idea of a focused and interdisciplinary meeting that might actually be useful, rather than just bringing together all of his friends and collaborators for a big party. It’s to his credit that they invited as many multiverse-boosters as multiverse-skeptics. (I would go for the party, myself.)

George is currently very interested and concerned by the popularity of the multiverse idea in modern cosmology. He’s worried, as many others are (not me, especially), that the idea of a multiverse is intrinsically untestable, and represents a break with the standard idea of what constitutes “science.” So he and the organizing committee have asked a collection of scientists and philosophers with very different perspectives on the idea to come together and hash things out.

It appears as if there is working wireless here in the conference room, so I’ll make some attempt to blog very briefly about what the different speakers are saying. If all goes well, I’ll be updating this post over the next three days. I won’t always agree with everyone, of course, but I’ll try to fairly represent what they are saying.

Saturday night:

Like any good British undertaking, we begin in the pub. I introduce some of the philosophers to Andrei Linde, who entertains us by giving an argument for solipsism based on the Wheeler-deWitt equation. The man can command a room, that’s all I’m saying.

(If you must know the argument: the ordinary Schrodinger equation tells us that the rate of change of the wave function is given by the energy. But for a closed universe in general relativity, the energy is exactly zero — so there is no time evolution, nothing happens. But you can divide the universe into “you” and “the rest.” Your own energy is not zero, so the energy of the rest of the universe is not zero, and therefore it obeys the standard Schrodinger equation with ordinary time evolution. So the only way to make the universe real is to consider yourself separate from it.)

Sunday morning: Cosmology

9:00: Ellis gives the opening remarks. Cosmology is in a fantastic data-rich era, but it is also coming up against the limits of measurement. In the quest for ever deeper explanation, increasingly speculative proposals are being made, which are sometimes untestable even in principle. The multiverse is the most obvious example.

Question: are these proposals science? Or do they attempt to change the definition of what “science” is? Does the search for explanatory power trump testability?

The questions aren’t only relevant to the multiverse. We need to understand the dividing line between science and non-science to properly classify standard cosmology, inflation, natural selection, Intelligent Design, astrology, parapsychology. Which are science?

9:30: Joe Silk gives an introduction to the state of cosmology today. Just to remind us of where we really are, he concentrates on the data-driven parts of the field: dark matter, primordial nucleosynthesis, background radiation, large-scale structure, dark energy, etc.

Silk’s expertise is in galaxy formation, so he naturally spends a good amount of time on that. Theory and numerical simulations are gradually making progress on this tough problem. One outstanding puzzle: why are spiral galaxies so thin? Probably improved simulations will crack this before too long.

10:30: Andrei Linde talks about inflation and the multiverse. The story is laden with irony: inflation was invented to help explain why the universe looks uniform, but taking it seriously leads you to eternal inflation, in which space on extremely large (unobservable) scales is highly non-uniform — the multiverse. The mechanism underlying eternal inflation is just the same quantum fluctuations that give rise to the density fluctuations observed in large-scale structure and the microwave background. The fluctuations we see are small, but at earlier times (and therefore on larger scales) they could easily have been very large — large enough to give rise to different “pocket universes” with different local laws of physics.

Linde represents the strong pro-multiverse view: “An enormously large number of possible types of compactification which exist e.g. in the theory of superstrings should be considered a virtue.” He said that in 1986, and continues to believe it. String theorists were only forced to take all these compactifications seriously by the intervention of a surprising experimental result: the acceleration of the universe, which implied that there was no magic formula that set the vacuum energy exactly to zero. Combining the string theory landscape with eternal inflation gives life to the multiverse, which among other things offers an anthropic solution to the cosmological constant problem.

Still, there are issues, especially the measure problem: how do you compare different quantities when they’re all infinitely big? (E.g. number of different kinds of observers in the multiverse.) Linde doesn’t think any of the currently proposed measures are completely satisfactory, including the ones he’s invented. A big problem with Boltzmann brains.

Another problem is what we mean by “us,” when we’re trying to predict “what observers like us are likely to see.” Are we talking about carbon-based life, or information-processing computers? Help, philosophers!

Linde thinks that the multiverse shows tendencies, although not cut-or-dried predictions. It prefers a cosmological constant to quintessence, and increases the probability that axions rather than WIMPs are the dark matter. Findings to the contrary would be blows to the multiverse idea. Most strongly, without extreme fine-tuning, the multiverse would not be able to simultaneously explain large tensor modes in the CMB and low-energy supersymmetry.

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Travel is broadening, and in particle physics we get to do a lot of it. In July, having temporarily settled my father into a nursing home after being hospitalized (the subject of my last post, Part 1), I was able to meet my commitment to travel to Krakow, Poland, to give a plenary talk on the search for the Higgs boson at the annual Europhysics conferenceheld at the Jagiellonian University there (where Copernicus studied for four years, 1491-1495).

Central Krakow emerged from World War II, which began nearly exactly 70 years ago, nearly unscathed. The central square is one of the more beautiful in Europe, similar in a way to that of Prague. But it was hard to avoid waling there without imagining what it must have looked like during the war, occupied by German soldiers who had made Krakow the center of their regional government during the war.

From the square one can take tours in little golf-cart-like jitneys, and see some of the interesting historical sites, including the Jewish Quarter (Kazimierz) and Schindler’s famous enamelware factory. Some of the apartment buildings in Kazimierz are still in the state they were at the end of the war, a rather grim reminder of the central role Krakow played in the Holocaust.

From Krakow one can take day trips to a number of interesting places, and we visited the spectacular salt mines of Wielicka, a UNESCO World Heritage site, which have amazing, huge rooms carved out of the rock.

But there was another interesting place to tour that we were hesitant about – Auschwitz. Others who took the tour came back saying that it was well worth the journey, over an hour by bus each way, but tended not to say much more about it…hmmm.

So on our last free day we took the plunge, signed up for the tour, and went. The bus traveled through quite rural countryside on two-lane roads, past farms and villages, roughly following the Vistula river, until reaching the town of Oswiecim, which the Germans called Auschwitz.

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I’m spending most of this week at the 2009 meeting of the Division of Particle and Fields of the American Physical Society (DPF2009), which is being held at Wayne State University in Detroit. I’m here giving a talk, and convening, with Corbin Covault from Case Western, a couple of sessions on particle astrophysics and cosmology.

I’m enjoying the conference, and have managed to fit in a number of very illuminating discussions with colleagues from other institutions, which is really the most useful part of conferences anyway, as everyone knows. Plus, I finally got to meet the only one of my co-bloggers that I’d never met before. Furthermore, the talks have generally been good – well thought through, and eloquently presented – and we had a lovely reception last evening at the beautiful Detroit public library, where I snapped the mural, which seemed appropriate (except for the pecs). Nevertheless, the overwhelming vibe that I’m getting here is one of extreme impatience and anticipation. This, of course, is all about the Large Hadron Collider (LHC).

There have been talks presenting rather recent and significant results, of course – for example Angela Olinto‘s talk about high energy cosmic rays and gamma rays was a lovely survey of the combined data from Fermi, Auger, PAMELA, ATIC, and other experiments; Josh Frieman‘s talk on cosmology, and particularly cosmic acceleration, provided a clear picture of the vibrancy of the field and the great progress that has been made over the last decade; and there are numerous other interesting talks coming up on QCD, heavy ion physics, neutrinos, etc.. But in high-energy particle physics I think we’re mostly seeing talks, albeit good talks, summarizing things we’ve seen again and again for a long time. The details of the LHC detectors (ATLAS, CMS, LHCb and ALICE); how one hopes to tease out evidence for the Higgs from the data; ditto for supersymmetric particles, and those arising from large extra dimensions; and a talk by Lyn Evans summarizing the progress towards getting the LHC back online after last year’s calamity.

Particle physics is screaming out for a new result pointing the way to the physics that we know must lie beyond the unreasonably successful standard model. We know this physics should be there because of purely particle physics problems, such as the hierarchy problem – why is the weak scale so much lower than the Planck scale, and stable against quantum corrections – but also because cosmological observations such as the matter antimatter asymmetry of the universe and dark matter tell us that new particles and interactions must be out there, perhaps at the energy scale of the LHC.

People aren’t sitting around twiddling their thumbs and just waiting for the machine to turn back on, of course. And none of what I’ve written above is in any way intended as a criticism. Ongoing work at existing experiments (such as those at the Tevatron, for example) is placing new limits (for example the recent claimed exclusion of a Higgs mass between 160 and 170 GeV), and experimentalists are busy refining their techniques for extracting the maximum amount of information from the upcoming LHC data. This is all extremely important work, and certainly very interesting. But it doesn’t change the fact that people really want the LHC.

And it isn’t just pure particle physicists who feel this way. Those of us in particle cosmology have been getting a wealth of data from cosmology for a while now. But this has left us in a position where dark matter and cosmic acceleration are on such a firm footing that more than ever we desperately need to understand how these phenomena fit into our understanding of fundamental physics. The LHC is the next essential tool in this quest. New physics discovered there may have direct implications for cosmology. And if it doesn’t, then proposed theoretical explanations will be constrained by, and may well open up new vistas for, cosmology.

So we’re all, particle physicists and cosmologists, keeping our fingers tightly crossed for the planned turn on later this year.

For a long time, the government has been responsible for space travel in the United States. That’s about to change.

Government is the appropriate agent for certain forms of collective action: roads, public schools, national defense. It’s also good for big-picture things without immediate financial payoff, like support for the arts or basic scientific research. It makes perfect sense for the government to shoulder the burden for developing the technologies to get us into space, and it will continue to make sense for them to play an active role in astronomical research in space. But for commercial purposes, like launching satellites, it ultimately makes a lot more sense for space travel to be a private-sector enterprise. We’re on the brink of seeing it happen.

SpaceX is a private company founded by Elon Musk, who previously co-founded PayPal and the electric car company Tesla Motors. For a while now, SpaceX has been developing reusable launch vehicles and space capsules. They’ve been awarded a contract from NASA to take over re-supplying the International Space Station after the Shuttle fleet is mothballed next year. And they’ve had one launch that reached orbit, but also a few failures; until yesterday, they hadn’t succeeded in putting a satellite into orbit.

But now they’ve done it. I was watching on live webcam last night as the Falcon 1 rocket launched a Malaysian satellite into orbit.

It’s incredibly exciting, but just the beginning. The idea behind the Shuttle was to make trips to orbit cheap, reliable, and routine; it failed spectacularly on all counts, and NASA’s capabilities and plans for space flight have become somewhat disjointed (while its science missions continue to have amazing success). Hopefully we’re moving past the point where we have to rely on the government to get us to space.

The internets have spoken, and it’s a good thing I listened. A few months ago I had the idea to organize a session at the upcoming meeting of the American Association for the Advancement of Science, in San Diego next February. It’s a giant cross-scientific-disciplinary meeting, offering a great chance for journalists and scientists in diverse fields to catch up on what’s happening in other areas.

But I couldn’t decide between two possible topics, both of which are close to my heart: “The Origin of the Universe” or “The Arrow of Time.” (My original book subtitle was “The Origin of the Universe and the Arrow of Time,” before that was squelched by the marketing department and replaced with “The Quest for the Ultimate Theory of Time.” Quests are big these days, apparently.) So I did the natural thing: I Tweeted the question. And the internet spoke with a fairly unambiguous voice: “Arrow of Time” sounded more interesting. So that’s what I proposed.

And now we’ve just been accepted, so it’s on for San Diego 2010. We have a fantastic line-up of speakers (and also me), spanning quite a range of topics:

• Me, setting the stage about the arrow of time and the Second Law, and drawing some connections to cosmology.
• Huw Price, director of the Centre for Time, Department of Philosophy, University of Sydney, and author of Time’s Arrow and Archimedes’ Point. Huw will be talking about the arrow of time in philosophy.
• Anthony Leggett, winner of the Nobel Prize in 2003 for his work on superfluidity. Tony will be talking about the arrow of time in quantum mechanics and condensed matter physics.
• Michael Lässig, University of Cologne. Michael specializes in statistical physics and quantitative biology, and will be speaking on the arrow of time in biology, in particular in evolution. He recently gave a talk on this topic at the KITP.
• Daniel Schacter, Harvard, author of The Seven Sins of Memory. Dan was the leader of the studies showing that the brain predicts the future in the same way that it remembers the past. He’ll be talking about the arrow of time in psychology.

That’s the fun part about this topic; it ranges naturally from the birth of the universe to the operation of your brain. Should be a good symposium.

Update: Unfortunately, Daniel Schacter won’t be able to make the symposium. Instead, we are very fortunate to have Kathleen McDermott of Washington University in St. Louis. Her research involves how we remember the past and forecast the future.

I’m back from the World Science Festival, which was a rousing success, leaving thousands of smiling attendees chattering excitedly about the mysteries of the universe as they dispersed through the streets of Manhattan. So naturally I want to talk about how it could be improved. Writing about one’s travels can be one of the least compelling arrows in the blogger’s quiver, but it would be great if the science-festival idea caught on more widely, so perhaps there is something to be learned from the experience.

A science festival, one presumes, aims to bring science to a wide audience through a series of events concentrated in space and time. But there are a lot of different approaches we could imagine taking to achieve that goal. Kirsten Sanford insightfully compares the WSF to the San Diego Science Festival — two similar-sounding events that end up having a very different look and feel. The WSF appeals to the cultural and cool, while the SDSF aspires to be a noisy bring-the-family affair. Neither is right or wrong, and in these cases each is appropriate to the venue; but the choices of how to proceed should be made consciously.

Public events for science can be placed in a two-dimensional parameter space, where one axis ranges from “observational” to “participatory,” and the other ranges from “inspiring” to “informative.” Again, none of these reflects a normative judgment; inspiring and informing are both laudable goals, and sometimes the best way to achieve those goals is to have the audience observe a performance, while other times it’s better to have them participate more directly. The point is not to say what’s better or worse, it’s to figure out what is appropriate for the circumstances.

The parts of the WSF I experienced directly — the opening gala, the two events in which I participated, and two events where I sat in the audience — were roughly speaking more observational than participatory, and more inspiring than informative. For the three events I watched, I think that was exactly right, but for the two events I participated in, I think they could have been even better had the balance been shifted. (Which obviously raises the possibility of some sort of bias on my part, left for you to decide.) In other words, I think a slightly more diversified portfolio of approaches could be beneficial to future science festivals.

The opening gala, a science-and-art extravaganza that both set the stage for the festival and celebrated E.O. Wilson’s 80th birthday, was a great example of an event for which the inspiring/observational paradigm worked perfectly. It was a big production, at Lincoln Center, with a rapid-fire series of performances bridging the gap between art and science; it would have been crazy to try to invite audience participation. And inspiration is just what you need to kick off a big festival. Brian Greene, who along with Tracy Day (“the first couple of New York science“) founded the WSF, did a tag-team presentation with violinist Joshua Bell. Brian would talk a bit about string theory or various wonders of the cosmos, while videos from The Elegant Universe played in the background, and then Bell would play some music appropriate to the mood. Very little educational was going on — nobody came out of the performance considerably more knowledgeable about the secrets of string theory than they went in. But it was an artistic success, putting people in the frame of mind to excitedly tackle meatier fare over the next few days.

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I spent most of last week back in Cleveland, at Case Western Reserve University, where I spent three delightful years working with Tanmay Vachaspati, Glenn Starkmann, and Lawrence Krauss (who recently left). The occasion was a workshop on Tests of Gravity and Gravitational Physics, organized by the Center for Education and Research in Cosmology and Astrophysics (CERCA).

An interesting mix of theoretical cosmologists, relativists, particle physicists, observers and experimentalists participated, and the aim was to pull and tug at the loose threads in various ideas of modifying gravity, while talking about how we might perform sensible tests of the theory in new regimes in the near future. Most of the program consisted of talks, both theoretical and experimental, which you can download from the main site. The theory talks ranged from Nima Arkani-Hamed‘s talk on why general relativity (GR) is so remarkably robust, with the take-home message “Don’t modify gravity – understand it”, to Bill Unruh‘s description of his “dumb Hole” analog models for black holes, and the discussion of the associated analogous phenomenon to Hawking radiation. On the experimental side, there were great talks covering tabletop tests of sub-millimeter gravity – like the one by Blayne Heckel – all the way up to tests of gravity on the scale of galaxies and above – like Stacy McGaugh‘s talk on tests of MOND.

As well as taking part in the general discussions and chairing a session, my role in all this was to sit on a panel for a discussion of “Modified Gravity: What does it buy? and at What Price?”

Nima moderated the discussion, and the other panel members were Eanna Flanagan, Nemanja Kaloper, Glenn Starkman and Bob Wald. While everyone had their own particular take on this, a common theme was that the theoretical and observational constraints are such that we currently do not have any modifications to GR that are compelling, despite our attempts to find a way to address the cosmological constant problem, explain cosmic acceleration, and search for alternatives to the dark matter paradigm. This doesn’t mean it isn’t worth trying, of course, although Nima made a spirited argument that it is hard to maintain the important holographic (and hence nonlocal) features of general relativity in any modified theory that isn’t of the simple scalar-tensor type, and that this is an argument that we shouldn’t expect nontrivial modifications. Rather, he argued, we should search for a dual S-matrix description that applies in flat space (unlike AdS/CFT) and that may allow us a better understanding of, for example, the cosmological constant problem.

This was all heavy stuff, but it didn’t consume our entire time in Cleveland. Tuesday evening saw the workshop dinner and it turned out that our hosts had arranged some after dinner entertainment, in the form of an improvisational comedy team. These guys were big on audience participation, and I was one of the first people chosen to mildly (I hope) embarrass themselves. At first this seemed like a heavy price to pay for dinner. However, trust me, subsequently seeing Tanmay Vachaspati, Bill Unruh and George Pickett up there made me realize it was well worth every blush and squirm.

Update: Dmitry Podolsky has a couple of very nice posts over at NEQNET with quite a few details about the talks, including Nima’s.

Springtime in the Imperial Palace gardens, Tokyo:

Nikon D200.