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.

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.

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.

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.

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!

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.

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.

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.

At a first approximation, cosmological perturbations are gaussian: the fluctuations at every point are drawn from a normal (bell curve) distribution. That’s the basic prediction of inflation, and it’s consistent with what we observe. But a more careful calculation shows that perturbations from inflation can be slightly non-gaussian; the search for such a signal in the data is a primary goal of current cosmological observations.

What the EFT of inflation lets you predict is what form the non-gaussianities can take. One way of characterizing the deviation from gaussianity is using the bispectrum, the correlation between fluctuations (in Fourier space) with three different wave vectors.

Although it looks like F depends on the three wave numbers, scale invariance implies that it really only depends on the ratios k₂/k₁ and k₃/k₁. (The directions don’t matter because of rotational invariance.) Long story short, you can plot the possibilities in terms of a function on a two-dimensional parameter space, like so:

The EFT of inflation lets you predict the shape of this function for any given inflationary model. Keep in mind: we haven’t yet observed any non-gaussianity, although we are trying. But it’s a reminder that there’s potentially a wealth of information about the early universe yet to be extracted from observable features today.

(Outfits aren’t as good, though.)

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.

My inaugural dinner with the institute folk set the tone. We went out to a local seafood restaurant. Walking in one passed a number of tanks, filled with live fish, eels, octopus, and various other unrecognizable ocean dwellers. The table next to ours consisted of three Korean women enjoying octopus sashimi. We promptly ordered some for ourselves. The octopuses were extracted from their tank, hauled into the kitchen for a few minutes, and then presented neatly cubed. Octopuses have a fairly unusual autonomic nervous system, with many neurons present in the tentacles rather than the brain. This is a long-winded way of saying that a plate of fresh octopus is a writhing, tangled affair. You rapidly learn to coat the agitating bits in sesame oil before consuming, otherwise the suction cups stick to the interior of one’s mouth, somewhat compromising the whole experience. Needless to say, it is a strange sensation. But entirely delicious.

We were clearly amateurs. George managed to inveigle himself a personal lesson from one of the Korean women in how to eat octopus sashimi (only afterwards did she learn she was teaching a Nobel laureate). The lesson consisted of the woman taking an entire live octopus, carefully wrapping the tentacles around a wooden chopstick (metal doesn’t work), and then consuming the entire octopus popsicle in one fell swoop. As she indulged, there were tentacles coming out of her mouth and desperately grabbing her face, clearly displeased with the turn of events. It was starkly reminiscent of Aliens (with some amount of role reversal). It is one of the more unsettling things I’ve seen.

I got to tour deep into the bowels of the building, where stacks of journals and scientific reports seem to stretch for ages. The library does a brisk job lending books and articles to other institutions; when you need a technical note from 1923 that tells you how a certain bridge was put together, this is the place to go. There is also an amazing rare-book collection, some of which was being put on display as part of an exhibition entitled “Thinking Outside the Sphere: Views of the Stars from Aristotle to Herschel.” I got to leaf through a first edition of Newton’s Principia, which I have to say was pretty awesome. I didn’t find any mistakes, but my Latin is a bit rusty. Here are the three Laws of Motion, right near the beginning of the text.

The library also adds to the intellectual life of Kansas City by sponsoring public lectures. I followed Sara Seager and Seth Shostak in a series about extraterrestrial life. Not my area of expertise by any means, but they asked me to talk about time travel, which I do know something about. (At least by the standards of other human beings, for which neither “time travel” nor “extraterrestrial life” are subjects of true expertise anywhere.)

Dr. Sean Carroll – The Paradoxes of Time Travel from Linda Hall Library on Vimeo.

Of course I also had some BBQ while in KC. One does not live by the life of the mind alone.

The Aspen Center runs a public lecture series in conjunction with each conference. So last Wednesday Andrea Ghez gave a lecture on the black hole at the center of our galaxy. It’s our closest big black hole, roughly 25,000 light years (2×10¹⁷ kilometers) away, and four million times the mass of our Sun. Andrea has been leading a team studying the motion of stars orbiting around this black hole. These orbits are one of the best ways (short of the detection of gravitational waves from black hole mergers) of confirming that black holes exist. The orbits tell us the mass of the central object. And the innermost passage of the closest orbit gives us an upper limit on the size of the central object. Combining these numbers gives us a lower limit to the density of the “dark object” at the center of our galaxy. At this point, a black hole is the only viable model for what we see. There is no way to make sense of the orbits using a cluster of (dark) stars at the center, or a massive gas cloud, or anything else we can think of. Gravity tells us that any normal stuff we put there (including “conventional” dark matter) will evaporate or collapse to a black hole. We are not yet probing the horizon of the black hole (in some sense, its surface), but we are getting closer and closer with each passing year.

But, more importantly, Andrea is responsible for one of the coolest movies in all of science:

This shows the orbits of stars around our galactic center. This isn’t an artist’s conception. This isn’t some abstraction of other data. This is a real movie of stars circling the black hole over the last 15 years. In particular, watch S-02. It loops around the black hole, and closes its orbit; we have watched it over one full S-02 “year”. It is an incredible feat of observational astronomy to make these movies. It requires adaptive optics on the largest telescopes in the world (the Keck telescopes on Mauna Kea). We used to think of the heavens as eternal and unchanging. Now we watch movies of stars orbiting black holes.

Shuttle launches witnessed: 1
Shuttle launches since 1981: 129
Shuttle launches remaining: 5
“Shuttle Experience” rides experienced: 1

Cost of Space Shuttle Atlantis [dollars]: 1.7 billion
Total cost of the International Space Station [dollars]: 157 billion
Science publications resulting from research by the International Space Station: ~200
Total cost of the Hubble Space Telescope [dollars]: ~4-6 billion
Science publications resulting from Hubble Space Telescope data: >8500

Years between first trans-Atlantic air passenger and first man walking on Moon: 42
Years since last human walked on moon: 37
Moons of earth where water was found: 1

Cities visited, where snow was visible: 2
Cities visited, where it has never snowed: 2
Cities visited with a “Disney Land/World”: 2
Mickeys seen: 0
Alligators seen: 2
Geckos seen: 1
Astronauts met: 1
Space geeks met: ~ 40

Tweets sent at first “tweetup”: 24
Tweets sent in lifetime: 24
Number of distinct words heard starting with an extraneous “tw”: >15
Days after my first tweet that Palin decided to resume tweeting: 4
Books released by Sarah Palin: 1
Stewardesses I saw that were the spitting image of Sarah Palin: 1

Oceans swum in: 1
Oceans I was close enough to swim in: 2
Places visited that are the Holiest site of a religion: 1
People met that are writing a book about escaping that religion: 1
Points bowled: 67
Team place out of nine teams of bowling scientists: 1st

Flights taken: 7
Amount of carbon emitted by those flights [lbs]: 2240
Net amount of energy generated by my solar panels [kW/hrs]: ~100
Equivalent amount of carbon not emitted [lbs]: 100
Cost of offsetting that 2240 lbs of carbon [dollars]: 12.63

Talks given on completely different topics: 3
Talks listened to: 41
Talks listened to without my laptop open: 15
Non-astrophysics talks I heard that mentioned dark matter: 10

NSF proposals submitted (as Co-PI): 2
HST Multi-Cycle Treasury proposals submitted (as Co-I): 2
Total number of HST MCT proposals submitted by the community: 39
Total number of HST orbits requested by those 39 proposals: 26801
Interviews given: 3
Days with at least 3 nearly identical deadlines: 2

Emails received @ work address: 768
Emails sent: 253
Emails still in my inbox: 361

Average number of hours slept per night: 5
Brain cells lost by multi-tasking: Uncountable.

It’s been a mixed bag so far; while I’ve had great fun interacting with people here in Australia, I’ve also been struggling with a nasty cold I picked up on the flight over. Spent yesterday mostly in bed, too fogged up to even work on my talk for Friday. But when I’ve had the strength to be up and about, it’s been a treat. Here’s an iPhone snap of the University of Sydney; that clocktower in the middle houses, appropriately enough, the Centre for Time.

One of the perks of civilization that hasn’t quite caught on in these parts is affordable internet access in hotel rooms, so don’t expect a lot of blogging over the next week or two. Instead, I can point you to a couple of recent videos. One is an extended interview for Edge, entitled Why Does the Universe Look the Way it Does? It is an interview (presented in text and video), not a carefully pre-planned document, so not all thoughts are arranged as elegantly as one might like. Here is some of the flavor:

We are in a very unusual situation in the history of science where physics has become slightly a victim of its own success. We have theories that fit the data, which is a terrible thing to have when you are a theoretical physicist. You want to be the one who invents those theories, but you don’t want to live in a world where those theories have already been invented because then it becomes harder to improve upon them when they just fit the data. What you want are anomalies given to us by the data that we don’t know how to explain.

The other one is a panel discussion on Time Since Einstein, from the World Science Festival. As the description there says, it features Roger Penrose, David Albert, and some other people it would be too exhausting to list individually. Here’s part 1 of 5:

World Science Festival 2009: Time Since Einstein, Part 1 of 5 from World Science Festival on Vimeo.

Now if only my immune system would finish off the little viral buggers inside me, I could get out and see a bit of this interesting country.

So we rearranged our schedules, met yesterday in Denver, woke up at 6 am this morning, and are now at Kennedy Space Center with 100 space twitterers. They’ve got a full program here with astronauts and a tour today, and the launch of mission STS-129 to the space station at 2:29 pm tomorrow. The event just started… So stay tuned, we’ll keep you posted. We will be blogging as well as loosing our tweeting virginity @cosmicvariance. You can follow the rest of the gang by looking for #nasatweetup.

But the reason I’m blogging about it is because I’ll be giving some public talks, and it would be great if any local CV readers dropped by to say hi. I’ll be hitting three different cities:

• Monday 16 November: Sydney
• Friday 20 November: Melbourne
• Monday 23 November: Adelaide

With all these public talks in a row, you would almost think I’m touring in support of some sort of book. That was part of the original idea, but now the book won’t be officially released until January 7. So instead I’ll just be talking in support of … Science! And trying to stay clear of dangerous creatures.

p.s. Wow, I almost did an incredibly boneheaded thing by showing up at the airport without a visa. Why in the world do you need a visa to go from the USA to Australia? I thought it was like a southern version of Canada. Fortunately, when you check in online you get “reminded” that a visa is required; even more fortunately, there is an online instant-visa service that seems to work. This is why I’m a theoretical physicist and not put in charge of anything important.

Now that I’m basically settled at Penn, I’ve been focused primarily on working on some exciting new projects, while also trying to bring to completion a lot of different research projects that have been languishing somewhat as I got myself settled. I think most of these are back on track now (although some of my collaborators, whose Skype calls I’ve moved several times, may have a different opinion), which is a nice feeling to have after a few months of concerted effort. I even managed to get a conference proceedings finished. I’ll probably post about some of these projects when they’re done.

But over the last few weeks, I’ve also been traveling a little, starting with a colloquium at the University of Wisconsin, Milwaukee, where I talked about modifying gravity to a very knowledgeable audience, and where I was treated to a wonderful (and very late) Friday night out, courtesy of Luis Anchordoqui and Patrick Brady, to whom I owe many thanks. That trip was followed by a colloquium at NASA Goddard Space Flight Center, which seems to employ an inordinate number of people I went to graduate school with. I certainly enjoyed the talk, but particularly enjoyed the meetings they had set up for me during the day. I learned about the long-term possibility of using observations of gravitational wave sources by the LISA experiment, accompanied by optical follow-ups, as a new way to construct a Hubble diagram to trace the cosmic expansion history. While not feasible right now, I found this a fascinating possibility for the future, and I’m hoping that our resident expert may tell us more about it some time.

My final two trips of the recent period were both to New York, and both to NYU. The first was to deliver a seminar at the Center for Particle Physics and Cosmology. The second was for the first of a set of one day meetings that the Center for Particle Cosmology has begun with the NYU Center. That took place a week ago, and featured talks by my colleagues Justin Khoury and Daniel Wesley, and NYU speakers Neal Weiner and Roman Scoccimarro. This was a very fun intellectual exchange, with talks on modified gravity, cyclic cosmologies, and interacting dark matter. Certainly the NYU people set the hospitality bar pretty high for us for when they visit us at Penn next semester.

However, perhaps the most time consuming activity of the last few weeks, in comparison with the rest of the year, has been writing and editing letters of recommendation. This is something I think everyone realizes professors do, but usually doesn’t realize the amount of time it takes.

I don’t know what it is like for everyone, but the first time I was asked to write a serious letter of recommendation was when I was a postdoc, and one of the graduate students I’d been working with asked me if I’d be one of his letter writers as he applied for his first postdoc. This first letter keeps you up at night. One wants to be enthusiastic about the candidate, while realizing that your letter is supposed to provide a service for your colleagues who will evaluate the application, as well as for the candidate. Thus one gets excessively stressed about painting a balanced picture of the candidate’s strengths and weaknesses, while competing with some of the glowing letters that one knows other people will write for their candidates. Nevertheless, you get over it, and you write the best letter you can and hope that it is helpful.

As a faculty member, one rather rapidly comes to realize that writing letters of recommendation is a crucial and time-consuming part of one’s job. So how does one go about it? Well, suppose that someone has asked you for a letter. They might be an undergraduate, a graduate student or a postdoc, asking for a letter for positions ranging from an REU position to a faculty job. The first thing to decide is whether you are prepared to write for them. For me, I tell a person I will write if I think my letter will leave a better impression than receiving no letter at all. If not, then I turn them down and tell them they would do better asking someone else. I also like to tell people roughly the type of letter they can expect. Obviously I don’t give details, but I don’t want there to be any confusion. They might decide they can get a better letter from someone else, or that they just don’t want me to write, and I like them to have enough information to make that decision.

But assuming you’ve made the decision to write for someone, and that they still want you to write, then you have a lot of work ahead of you. There’s some basic stuff to get out of the way up front – how long have you known the person and in what capacity? This is where you lay out why you have sufficient experience with them to be able to provide a complete and authoritative account of their skills, track record and potential. When one is rather junior and writing, this part is important to demonstrate your qualifications before you discuss the applicant’s. As a physicist becomes more senior and well-known, this part of the letter remains just as important, although now it is more because it reassures the reader that the writer actually does know this person well, as opposed to them just being another of the presumably huge number of people clamoring for letters.

Now one moves on to the meat of the matter – evaluating the technical talents of the applicant. Are they deep, broad, sophisticated, creative, and calculationally skilled? There are many nuances involved in this part. Obviously, one wants to be accurate, while highlighting the skills that have impressed you most about the person. If you have written papers with the candidate, then this is the place you’ll write about some of the details and what the candidate brought to the project. If there are relevant weaknesses, you may want to point them out; but in my case, if I’ve decided to write, then I will typically think that these are outweighed by strengths, and you want to make that clear. Perhaps the most important thing to bear in mind here is that these are just your opinions. Yes, they are informed opinions; and yes, they have been solicited by both the candidate and/or the hiring institution because they feel you are qualified to give them. Nevertheless, there is no way around the fact that there is a significant subjective component to a recommendation letter, and it is important to make sure that you recognize this and consider it carefully before making any strong statements.

This, of course, makes letter writing somewhat terrifying (naturally, having to ask for them is also scary). Of course, one doesn’t resent students, postdocs and colleagues for asking for letters (not least because we have all relied on others to do this for us many times), and you want to see your talented colleagues succeed. It’s just that because of this you owe it to everyone involved to do a good job, and this is what makes for the required time commitment. October and November are the time when most letters are requested, and so if you find yourself writing for many people (ten or more people sometimes, at various levels), then it comprises a significant portion of your work over those weeks. It’s a not-often-mentioned, but important part of an academic’s job.

Yes, that’s a pretty suave picture of me on the image capture. What can I say? I’m just one of those lucky folks with an effortless magic in front of the camera.

If you prefer to get your talks about entropy unadulterated by voice and motion, and don’t mind a more technical presentation, I’ve put the slides from my recent Caltech colloquium online. These are aimed basically at grad students in physics, so there is an equation or two, and the caveats are spelled out more clearly. But the punchline is the same.

Then it’s off to the Francophone sector with me, where I’ll be visiting McGill University in Montreal to give another public talk on Monday night. I don’t know of any recordings there, but the talk won’t be that different from Saturday’s. But if there are any CV readers in Montreal, be sure to say hi!