Greetings from Norway, where we’re about to embark on what is surely the most logistically elaborate conference I’ve ever attended. Setting Time Aright starts here in Norway, where we hop on a boat and cross the North Sea to Copenhagen. The get-together is sponsored by the Foundational Questions Institute, although it came together in an unusual way; I was part of a group that was organizing a conference, and we applied to FQXi for funding, at which point they mentioned they were planning almost exactly the same conference at the same time. So we joined forces, and here we are. Unity ’11!

The topic, if you haven’t guessed, is time. That’s a big subject, one that can hardly be done justice by sprawling books with hundreds of (admittedly quite charming) footnotes. You can see why the conference has to spread over two countries. We’re trying an experiment in interdisciplinarity: while the conference is a serious event meant for researchers, we have a wide variety of specialties represented, including biologists, computer scientists, philosophers, and neuroscientists, as well as the inevitable physicists and cosmologists. (There is also a public event, for those of you who find yourselves in Copenhagen next week.) I can’t wait to hear some of these talks, it should be a blast.

My job is to open the conference with an introductory talk that hits on some of the big questions. Here are the slides, at least as they are right now; last-minute editing is always a possibility. I think I put enough in there to provoke almost everyone at the conference one way or another.

In my last post I described the workshops and conferences – research travel – that I’ve been on during the first part of the summer. But when I returned from Brazil there was one other science trip I went on before taking a few weeks off. In mid June, Sean, Jennifer, Risa, Janna Levin and I were invited speakers at the first of a new initiative – the Discovery Retreats.

These are the brainchild of John Hendricks, the founder of the Discovery Channel and a host of science and education related programming. Taking place at Gateway Canyons Resort (yes, I know, sometimes we’re spoiled), these are several day events at which people come for vacation time in a stunning environment, mixed with lectures, panel discussions, star-gazing, and open discussion events. This inaugural retreat was titled “Secrets of the Universe”, at which Sean (who was organizing the scientific part of the event) gave the introductory overview of cosmology, Janna spoke about black holes, Risa discussed dark matter in the universe, I talked about dark matter and cosmic acceleration, and Jennifer gave a fascinating and fun talk on science and hollywood.

For me, by far the most enjoyable science part of the event was the panel discussion. Janna had left at this point, but we were joined by Nick Sagan, who provided his perspective as a science fiction author. Jennifer moderated this, and had a well thought out sequence of questions that guided us through a set of popular topics. However, it is always interesting to see what topics the audience is most fascinated by, even though they are often the ones you would have suspected. We were led through the nature of the big bang singularity, the ideas of inflation, string theory, the question of whether the universe has an edge, and a bunch of other big issues that frequently arise when one gets into chats about cosmology. We certainly had a great time – I hope the audience did.

One of the more fun non-science events was a tour of John’s extensive car museum at the resort. Here are Janna and I sitting in front of one of the many beautiful exhibits

In many ways this first retreat was a bit of a dry run, in which we were feeling out the right format and exploring the mix of scheduled and free time. There are going to be more of these events, not just focused on cosmology but on the frontiers of other scientific areas. Hopefully our first attempt wasn’t just fun, but also gave enough feedback that these future attempts work as well as possible.

Like many physicists, I spend a reasonable portion of the summer months traveling, delivering talks at conferences and workshops, and taking the opportunity to meet with colleagues and gain first-hand experience of the range of research being done in my field. For me, this began a couple of hours after my classes ended for the semester (congratulations to my General Relativity class, all of whom did very well at the end of the day), when I headed off to California to hang out with Sean for a few days and to give the Caltech physics colloquium.

I always enjoy visiting Caltech, and I find colloquia particularly fun talks to deliver, since they provide the opportunity to explain what’s going on at the frontiers of the field to physicists who spend most of their time working in their own, different areas. But this talk was particularly exciting to give, because of the location. I hadn’t realized, but the Caltech physics colloquia take place in a rather old lecture hall (201 E. Bridge) in which I was told Richard Feynman delivered his renowned lectures on physics. This part of Caltech is about to undergo a round of renovations, which meant that this was probably my last chance to speak in the same place that Feynman did – a wonderful experience. With most academic travel, the main payback from a trip like this is the chance to develop some new ideas with one’s collaborators. This time was no exception, and Sean, a student of his and I started discussions about a new dark matter idea that I’ll attempt to blog about here should it come to anything.

After a week back in Philadelphia, I was on a plane once more, this time for a short hop to my old stomping grounds in Cleveland, to take part in a workshop on gravity being held at Case Western Reserve University. The last decade or so have seen a resurgence of efforts to seek a sensible way in which General Relativity (GR) might be modified, either in ways that might yield new physics of the early universe, or in a manner that might explain phenomena at late times. The main original impetus for this work has been the possibility that the phenomenon of cosmic acceleration might be signaling a modification of gravity on the largest scales. However, among many researchers the current thrust is to gain a comprehensive understanding of the ways in which gravity may differ from GR, and at what scales one might expect any allowed modifications to appear.

It is, in fact, an extremely tricky proposition to modify GR, with almost any idea one might think of running into trouble either with established tests of the theory within the solar system, or with serious theoretical inconsistencies such as the appearance of particles with negative energies, known as ghosts. Many of the more interesting ideas involve models arising from extra dimensions, which have led not only to interesting modified gravity models, but also to new ideas about field theories in four dimensions, that I will discuss in another post soon. The gravity workshop focused on many of these new ideas, and, as often happens at small intense meetings, I left with lots of new ideas about my own work.

In June, I left for a lightning trip to Brazil, to speak at the very first meeting of the whole of the Brazilian Physical Society. This conference was held in the beautiful location of Iguassu Falls. Although I was, unfortunately, too ill from a flu I had caught to be able to travel to the falls themselves, I was lucky enough to see them from the air a couple of times. I will clearly have to go back! The meeting had several thousand people, and it was clear that Brazilian physics is undergoing a period of rapid expansion, something it is heartening to see given the pressures science is facing in many other parts of the world. One of the highlights was an event launching the new South American branch of the International Center for Theoretical Physics (ICTP). The ICTP, in Trieste, Italy, was founded in 1964 by Abdus Salam, with the goal of providing educating scientists from developing countries. Their new branch, in Sao Paulo, will be directed by Nathan Berkovitz and should extend the great work of the original. It’ll be interested to see how this endeavor develops – I wish them all the best.

I’d intended three days in Brazil, but ended up there for an extra twenty-four hours because the airport at Iguassa Falls was closed for a day by particulates from the Chilean volcano. I get delayed many times every year and find myself cursing airlines (I’d missed an important meeting in Cambridge a few weeks earlier thanks to USAir), but it’s hard to be furious at a volcano. The people at the Brazilian Physical society were wonderfully helpful and I’d like to thank them as publically as I can for taking such good care of us, dealing with our hotel rooms, and getting us rebooked on new flights.

Now I’m back to work, taking a few weeks without travel and trying to get new projects up and running, while finishing writing up a few papers before the new semester creeps up on me.

Not much blogging this week, as I’m at the World Conference of Science Journalists in Doha, Qatar. I’m informed that the technical term describing my role here is that of trailing spouse. But I did give a little talk on upcoming discoveries we should be looking forward to in particle physics and cosmology.

I’ll try to put up a more full report later. Right now I’ve just been guilted into blogging because I’m listening to a sexy and exciting panel about science blogs, staring Mo Costandi, Maryn McKenna, Jennifer Ouellette, Ed Yong, and Mohammed Yahia. And just to prove I’m here, I can show you what a sign inside Starbucks looks like in an Arab country.

Afterwards I’ll be headed to the souk to shop for scimitars.

Yesterday’s talks were devoted to the idea of dark matter, which as you know is the hottest topic in cosmology these days, both theoretically and experimentally.

Eric Armengaud and Lars Bergstrom gave updates on the state of direct searches and indirect searches for dark matter, respectively. John March-Russell gave a theory talk about possible connections between dark matter and the baryon asymmetry. The density of dark matter and ordinary matter in the universe is the same, to within an order of magnitude, even though we usually think of them as arising from completely different mechanisms. That’s a coincidence that bugs some people, and the last couple of years have seen a boomlet of papers proposing models in which the two phenomena are actually connected. Tracy Slatyer gave an update on proposals for a new dark force coupled to dark matter, which could give rise to interesting signatures in both direct and indirect detection experiments.

This is science at its most intense. A big, looming mystery, a bounty of clever theoretical ideas, not nearly enough data to pinpoint the correct answer, but more than enough data to exclude or tightly constrain most of the ideas you might have. It wouldn’t be at all surprising if we finally discover the dark matter in the next few years; unfortunately, it wouldn’t really be surprising if it eluded detection for a very long time. If we knew the answers ahead of time, it wouldn’t be science (or nearly as much fun).

Today is our last day in Avignon, devoted to cosmic acceleration. My own talk later today is on “White and Dark Smokes in Cosmology.” (The title wasn’t my idea, but I couldn’t have done better, given the context.) It’s the last talk of the conference, so I’ll try to take a big-picture perspective and not sweat the technical details, but (following tradition) I will admit that it’s an excuse to talk about my own recent papers and ideas I think are interesting but haven’t written papers about. At least it should be short, which I understand is the primary criterion for a successful talk of this type.

Also, few people have strong feelings about non-gaussianities or neutrinos, but many people have strong feelings about reductionism. Quelle surprise!

Every academic who attends conferences knows that the best parts are not the formal presentations, but the informal interactions in between. Roughly speaking, the perfect conference would consist of about 10% talks and 90% coffee breaks; an explanation for why the ratio is reversed for almost every real conference is left as an exercise for the reader.

Yesterday’s talks here in Avignon constituted a great overview of issues in cosmological structure formation. But my favorite part was the conversation at our table at the conference banquet, fueled by a pretty darn good Côtes du Rhône. After a long day of hardcore data-driven science, our attention wandered to deep issues about fundamental physics: is the entire history of the universe determined by the exact physical state at any one moment in time?

The answer, by the way, is “yes.” At least I think so. This certainly would be the case is classical Newtonian physics, and it’s also the case in the many-worlds interpretation of quantum mechanics, which is how we got onto the topic. In MWI, the entirety of dynamics is encapsulated in the Schrodinger equation, a first-order differential equation that uniquely determines the quantum state in the past and future from the state at the present time. If you believe that wave functions really collapse, determinism is obviously lost; prediction is necessarily probabilistic, and retrodiction is effectively impossible.

But there was a contingent of physicists at our table who were willing to believe in MWI, but nevertheless didn’t believe that the laws of microscopic quantum mechanics were sufficient to describe the evolution of the universe. They were taking an anti-reductionist line: complex systems like people and proteins and planets couldn’t be described simply by the Standard Model of particle physics applied to a large number of particles, but instead called for some sort of autonomous description appropriate at macroscopic scales.

No one denies that in practice we can never describe human beings as collections of electrons, protons, and neutrons obeying the Schrodinger equation. But many of us think that this is clearly an issue of practice vs. principle; the ability of our finite minds to collect the relevant data and solve the relevant equations shouldn’t be taken as evidence that the universe isn’t fully capable of doing so.

Yet, that is what they were arguing — that there was no useful sense in which something as complicated as a person could, even in principle, be described as a collection of elementary particles obeying the laws of microscopic physics. This is an extremely dramatic ontological claim, and I have almost no doubt whatsoever that it’s incorrect — but I have to admit that I can’t put my objections into a compact and persuasive form. I’m trying to rise above responding with a blank stare and “you can’t be serious.”

So, that’s a shortcoming on my part, and I need to clean up my act. Why shouldn’t we expect truly new laws of behavior at different scales? (Note: not just that we can’t derive the higher-level laws from the lower-level ones, but that the higher-level laws aren’t even necessarily consistent with the lower-level ones.) My best argument is simply that: (1) that’s an incredibly complicated and inelegant way to run a universe, and (2) there’s absolutely no evidence for it. (Either argument separately wouldn’t be that persuasive, but together they carry some weight.) Of course it’s difficult to describe people using Schrodinger’s equation, but that’s not evidence that our behavior is actually incompatible with a reductionist description. To believe otherwise you have to believe that somewhere along the progression from particles to atoms to molecules to proteins to cells to organisms, physical systems begin to violate the microscopic laws of physics. At what point is that supposed to happen? And what evidence is there supposed to be?

But I don’t think my incredulity will suffice to sway the opinion of anyone who is otherwise inclined, so I have to polish up the justification for my side of the argument. My banquet table was full of particle physicists and cosmologists — pretty much the most sympathetic audience for reductionism one can possibly imagine. If I can’t convince them, there’s not much hope for the rest of the world.

By this point in my life, when I attend a large-ish conference like this one the chances are good that I’m older than the average participant. Certainly true here. It’s a great chance to hear energetic young people tackling the hard problems, and I certainly have the feeling that the field is in very good hands. It’s also a good reminder that we old people need to resist the temptation to fall into a rut, churning out tiny variations on the research we’ve been doing for years now. It’s easy to get left behind!

Still, it’s also nice to hear a talk on a perennial topic, especially when you hear something you didn’t know. Yvonne Wong gave a very nice talk on “hot relics” — particles that were moving close to the speed of light in the early universe. (They may have slowed down by now, or maybe not.) Neutrinos, of course, are the classic example here; they are known to exist, and were certainly relativistic at early times. If the neutrinos have masses of order 10 electron volts, they would contribute enough density to be the dark matter. But that doesn’t quite work in the real world; “hot dark matter” tends to wipe out structure on small scales, in a way that is dramatically incompatible with the world we actually observe. Also, ground-based measurements point to neutrino masses less than 0.1 electron volt — not for sure, since what we directly measure are the differences in mass between different kinds of neutrinos, rather than the masses themselves, but that seems to be the most comfortable possibility.

Of course, we know about three kinds of neutrinos (associated with electrons, muons, and taus), but there could be more. So it’s fun to use cosmology to see if we can constrain that possibility. An extra neutrino species, even if it were very light, would slightly affect the expansion rate of the early universe, which works to damp structure on small scales. This is something you can look for in the cosmic microwave background, and the WMAP team has diligently been doing so. Interestingly — the best fit is for four neutrinos, not for three! Here’s a plot from Komatsu et al.’s analysis of the WMAP seven-year data, showing the likelihood as a function of the effective number of neutrino species. (“Effective” because a massive neutrino counts a little less than a massless one.)

Now, maybe this isn’t worth getting too excited about. There’s a nice discussion of this possibility in a recent paper by Zhen Hou, Ryan Keisler, Lloyd Knox, Marius Millea, and Christian Reichardt. I’m not sure how a new neutrino could affect the CMB in this way without being ruled out by primordial nucleosynthesis, but I haven’t looked at it carefully. Regardless, it’s best not to just trust any one measurement, but do every measurement we can think of and make sure they are consistent. Certainly something worth keeping an eye on as CMB measurements improve.

Greetings from Avignon, where I’m attending a conference on “Progress on Old and New Themes” in cosmology. (Name chosen to create a clever acronym.) We’re gathering every day at the Popes’ Palace, or at least what was the Pope’s palace back in the days of the Babylonian Captivity.

This is one of those dawn-to-dusk conferences with no time off, so there won’t be much blogging. But if possible I’ll write in to report briefly on just one interesting idea that was discussed each day.

On the first day (yesterday, by now), my favorite talk was by Leonardo Senatore on the effective field theory of inflation. This idea goes back a couple of years to a paper by Clifford Cheung, Paolo Creminelli, Liam Fitzpatrick, Jared Kaplan, and Senatore; there’s a nice technical-level post by Jacques Distler that explains some of the basic ideas. An effective field theory is a way of using symmetries to sum up the effects of many unknown high-energy effects in a relatively simple low-energy description. The classic example is chiral perturbation theory, which replaces the quarks and gluons of quantum chromodynamics with the pions and nucleons of the low-energy world.

In the effective field theory of inflation, you try to characterize the behavior of inflationary perturbations in as general a way as possible. It’s tricky, because you are in a time-dependent background with a preferred (non-Lorentz-invariant) frame provided by the expanding universe. But it can be done, and Leonardo did a great job of explaining the virtues of the approach. In particular, it provides a very nice way of calculating non-gaussianities. (more…)

Electrical outlets in O’Hare are like gasoline in The Road Warrior.

(Outfits aren’t as good, though.)

Last week I found myself on a tram in Hiroshima, heading to the stop “A-bomb dome”. I was surrounded by Japanese passengers, and for the first time in Japan I felt self-conscious and uncomfortable. I am an American working at Los Alamos, the literal and figurative birthplace of the atomic bomb. Memories of my visit to Trinity Site are still fresh. The weight of history is unavoidable. As in a classic Bruegel painting, however, nobody seems to pay particular notice. Everyone moves forward with their lives. A few days after the bomb, they restored streetcar service to parts of the city. There is no evidence of that terrible instant. None, that is, until you get off the tram stop and confront the dome. You’ve seen images of it countless times. But standing in front of it, surrounded by the bustling city of Hiroshima, is an altogether different experience.

There is a museum near the dome, with the impossible task of presenting the bomb to the residents of Hiroshima, the inhabitants of Japan, and the rest of the world. The museum is split into two parts. The first focuses on the history of Hiroshima, and the build-up to war. It dwells on the extended decision-making process through which Hiroshima was selected as the first target. The city had strategic significance. The city hadn’t been (conventionally) bombed, which meant that the full effect of the new device could be estimated. It didn’t have significance for the post-war reconstruction plans (in the way that Kyoto did [and the US Secretary of War apparently honeymooned in Kyoto, and had a sentimental attachment]). It didn’t contain American prisoners-of-war. Hiroshima ended up at the top of the list. One thing I found surprising: the museum implies that the timetable for the bombings was heavily influenced by the Russians. The US wanted to pre-empt Russian participation in the Pacific, and were hoping to elicit a Japanese surrender before the Russians could formally enter the war. The other half of the museum focuses on the immediate aftermath of the bomb. It contains artifacts from the day, including stopped watches and bits of clothing and hair. And countless stories, almost entirely of children returning home to their parents in horrific condition, and dying in the subsequent hours or days. There is a focus, both in the museum and in the memorial peace park which surrounds it, on the youngest casualties.

Sixty-five years ago the first atomic bombs were used in war. There is something depressing that humanity finds it necessary to develop such terrible weapons. But perhaps there is something hopeful in that, in the ensuing half century, we’ve had enough sense not to use them again.