When it comes to quantum gravity and fundamental physics more generally, there is a lot we don’t know, and many different approaches to making progress. A top-down kind of approach attempts to figure out what the ultimate laws are, and then see what they might have to say about reality; string theory is the obvious example. But you could also take a bottom-up or phenomenological approach, in which you try to figure out what kinds of physical effects might arise due to quantum gravity and then go look for them, even if you haven’t derived them from a more complete theory. Tests of Lorentz invariance provide a good example, and the subject of my recent paper. This is the paper I mentioned writing with my (former) student Eugene; he now has a follow-up paper extending our work.

Lorentz invariance is simply the idea that there is no preferred direction in spacetime. Not only do the laws of physics not care about the direction in which you are looking (invariance under spatial rotations), it also doesn’t care about the speed at which you are moving relative to other stuff in the universe (invariance under “boosts,” as physicists would say). The idea is a cornerstone of relativity, and got a big boost (pun unintended, but accepted) when the Michelson-Morley experiment showed that the speed of light seemed to be the same in all reference frames — there was no evidence for a background “aether” with respect to which you could measure your velocity. But its roots actually go back to Galileo, who first proposed that all velocities were relative; to people on the sidewalk, the road is stationary and the cars are moving, but to people in the cars, they are stationary and the road is moving, and each perspective is perfectly valid.

Actually, this violating-Lorentz-invariance stuff is not new to me. Using cosmology to test Lorentz-violating theories was the subject of my first published paper. It was a collaboration with George Field (my Ph.D. advisor) and Roman Jackiw, both very accomplished theorists in astrophysics and field theory, respectively. My role as the meek young graduate student was largely to type in the data and make plots, but that’s how you get started in this business.

The theory we considered had a fixed timelike vector field without any independent dynamics, but it was coupled to electromagnetism in a specific way that violated parity as well as Lorentz symmetry. We showed that the coupling would cause a “twist” in the polarization of light coming from distant galaxies, and George knew that for certain radio galaxies you could determine what the polarization should be without any Lorentz-violating effect, allowing us to put a very tight limit. The data I typed in came from different sources, and consisted of the polarization information plus the distance (actually, the redshift) of the different galaxies. Years later, much to our surprise, this same data appeared on the front page of the New York Times. Two researchers had used our dataset but analyzed it in a different way, trying to constrain a vector pointing in a spacelike direction rather than a timelike one. The big difference is that they claimed to actually find a nonzero effect, which they announced in a press release before they made the paper available to other experts. Unfortunately they were just mistaken, as I recount in great detail here. But it’s still worth thinking about; indeed, some candidates for dark energy in the universe could lead to a very similar effect, so it’s worth improving the present data to put much tighter constraints (or to discover dark energy!).

What Eugene and I have done is a little different. We imagine there is a vector field through spacetime that violates Lorentz invariance (since you could, in principle, measure your speed with respect to it in an absolute sense), but we worry about its gravitational effects rather than its interactions with ordinary matter and radiation. Interestingly, we find that the vector field has no effect if there is no matter lying around, but it works to alter the strength of the gravitational field caused by matter. In other words, it changes the effective value of G, Newton’s constant of gravity. This would be an unobservable effect if it just changed Newton’s constant once and for all, since we have no experimental knowledge of what the constant was before the vector field messed with it. Fortunately, it changes it in different ways in different circumstances: making the effective value of Newton’s constant larger in the Solar System, but smaller when we consider the expansion of the entire universe.

Thus, we have an observational constraint: measure the value of G in the Solar System, use that to predict something about cosmology, and compare with the data. The most straightforward example is actually the primordial abundance of light elements such as Helium and Lithium. These were created at an early time after the Big Bang (between one second and a couple of minutes) as the universe expanded and cooled, and the precise amount of different elements you get depends sensitively on the expansion rate of the universe, and thus on Newton’s constant. We find that our vector field must be less than ten percent of the Planck scale, the fundamental unit in physics where gravity and quantum mechanics come together. The Planck scale is pretty big, so it’s not a great limit, but still an interesting one.

Everyone was (justifiably) jealous sometime back when I revealed that I had received a WMAP beach ball in the mail. WMAP, the Wilkinson Microwave Anisotropy Probe, is a NASA satellite that has taken the most precise images yet of the leftover thermal radiation from the early universe. Who wouldn’t want to have a beach ball with a snapshot of the whole universe on it?

Well, I can’t help you with the beach ball, but here’s something almost as good: a CafePress store selling all sorts of WMAP merchandise, including the boxer shorts shown at right. (Found linked from Licia Verde‘s home page.) There’s something for everyone — T-shirts, teddy bears, thongs, more than you would ever want, really.

Someday I will follow through on the idea of setting up a store for Preposterous Universe merchandise. (In the meantime, just go buy my book!) Maybe I’ll stick with coffee cups, though.

Continuing the story from yesterday, let’s say you’re an enthusiastic amateur about to go out in the field (led by one of the world’s experts) and dig up some dinosaur fossils. Your first question would of course be: what do I wear? You’ll want good hiking boots, sturdy but not too heavy. It’s beastly hot, so you’ll be tempted to wear shorts, but don’t; you’ll be tromping through cactus and sagebrush, and then spending hours kneeling on rocks and dirt, so jeans are definitely called for. Hat and sunglasses are mandatory. Some of us wore lightweight long-sleeved shirts, although I did fine with T-shirts and heavy doses of sunscreen; despite the relentless Wyoming sun, I managed to keep my healthy pale complexion largely intact.

Besides clothes, the only necessary items we were personally responsible for were our water canteens. Bug spray is a good idea, and cameras or binoculars are useful, but not required. The serious equipment was provided for us by the Project Exploration folks: GPS units, walkie-talkies, brushes, awls, hammers, gloves, shovels, pickaxe, hardener, glue, tinfoil, burlap, plaster, measuring tape. Nothing very high-tech, other than the GPS. The physicist in me was sure that there must be some X-ray-like technique to probe into the soil to distinguish fossils from the surrounding rock; but nobody knows of any good way to do it, and I didn’t have any useful ideas. (I’m a cosmologist, okay?)

Suitably equipped, we head out to the site. Let me just mention that none of the skills one develops by spending one’s days doing theoretical physics and one’s evenings at jazz clubs really come in handy out in the field. The work involves serious physical labor and tremendous patience; the good news is that, although it requires years of training and practice to be really good at it, essentially anyone can be productive after a short tutorial. It helps that Paul seems to have an endless (or at least substantial) supply of patience and confidence in his motley crew of city folks; in his shoes, I would be scared to death of what these klutzes were likely to do to my fossils, and would simply ask them to watch from a respectful distance while I did the work myself.

Our group was about fifteen people, of whom two (Paul and Bob Masek, a fossil preparator at the University of Chicago) really knew what they were doing. Of the rest of us, about half had been along the year before, and the newcomers would occasionally (in their naivete) look to us for guidance. We took two vans from the ranch where we were staying out to the site, or at least as close to the site as we could get in the vans. From there we have to lug the aforementioned equipment to the actual fossils, at which point the excitement starts.

The main fossil Paul and his students had found was a vertebra from the tail of what is likely to be a Camarasaurus, a large sauropod common in the Jurassic. (Yes I know the link says that Camarasaurus was “much smaller” than other sauropods, but when you’re 60 feet long and weigh 20 tons, you’re large in my book.) Sauropods are the hulking big four-legged herbivorous dinosaurs with long necks, like Brachiosaurus and Diplodocus; other major categories are the mostly-carnivorous theropods such as raptors and T. Rex, and ornithischians or bird-hipped dinosaurs, including most of the funky armor-plated species like Triceratops and Ankylosaurus. Just to add an element of confusion, actual birds evolved from the lizard-hipped theropods, not from the bird-hipped ornithischians.

What you typically find, of course, is a single bone sticking out of the rock. In this case, Paul had found a vertebra from the tail. The question then is, if you follow the trail into the rock, do you just find the tip of the tail, or most of the dinosaur? In other words, in which direction is the tail pointing? You just have to dig and find out.

This is where the patience and determination come in. You basically poke gingerly at the area around the fossil with an awl, then remove the dirt and stones with a brush. (Try not to use your fingers, or let the stones fall onto the fossil; a brush is gentlest.) The awl and brush are your most common tools. From the variations in texture and color, you should be able to tell the bone from the surrounding rock if you are careful, although sometimes it’s tricky even for the experts. (The experts, by the way, refer to the rock in which the fossil is embedded as “matrix,” thus adding to the science-fictiony feel of the whole enterprise.) At first you have to move extremely carefully and tentatively, as you don’t know where the rest of the bones are. Every time you uncover a little bit of bone, you pour hardener over it to help protect it from being scratched or shattered. As often as not, the bones are not “articulated” — arrayed in a nice dinosaur shape — but rather are jumbled together. But after you make a little bit of progress, you can begin to get a feeling for the way in which the skeleton is arrayed in the rock. At that point, you might decide that the three feet of rock above your fossil can be removed more rapidly than awls and brushes allow; that’s where the pickaxe and shovels come in, or even jackhammers or heavy machinery in extreme cases.

Here’s a picture of Paul hugging the part of the tail we have uncovered. He’s feeling protective because we had very good news: the fossil seems to be pretty much articulated, and even better the vertebrae are growing as we move into the rock! Which means there is an excellent chance of finding a substantial portion of Camarasaurus skeleton lying in there, undisturbed for the last 150 million years.

Our rate of progress wasn’t nearly enough to imagine actually excavating the thing; the picture here basically shows the end result of a day and a half of work. Further trips will be required before the entire fossil can be shipped to Chicago. To protect what we have uncovered, we first cover the bones with tinfoil, then with strips of burlap dipped in plaster. The plaster will not only protect the bones once we cover them with dirt again, it also will make it much easier to eventually bring the fossil home. In fact you don’t nearly dig away all the rock from the bone; you intentionally leave an inch or two of matrix surrounding the bottom half of bones, dig out from the bottom, and then plaster around the entire collection, which gets shipped back to the lab. (A ton or two of shipped materials is common.) There a real expert, working in decent conditions (presumably involving air-conditioning), can remove the rest of the matrix. Then you take appropriate pictures and measurements, and possibly think about mounting the specimen for display. If (as Paul often does) you went to Niger or Mongolia or Argentina to collect the fossils in the first place, the original country will typically want it back; you get the science out of it, and it ultimately returns home. For our Wyoming fossils, we hope to build up a collection at the University; the previous collection was short-sightedly given away.

This is the second year I’ve gone on one of these trips with PE. The first year was great fun but I was exhausted by the end; this year I wanted to stay out there and keep digging. It’s an exhilarating experience, and utterly different from being a theoretical physicist. On the other hand, dinosaurs and cosmology are two topics that readily engage the public imagination, and the folks at PE hope to extend their reach into other areas of science, so I hope I can be some help. At the end of a long day in the field I gave an informal lecture on black holes — amazingly, despite myriad other distractions and every reason to be tired (the lecture began at 9:30 p.m.), everyone on the expedition attended and asked great questions about the curvature of spacetime. Just one more reminder, as if any were needed, that most people are intrinsically fascinated by science, and it’s our duty to do a better job of sharing the excitement that professional scientists get to feel all the time.

Found over at Talking Points Memo, this amazing story from Associated Press:

Some Democrats seeking Cheney tickets had to sign oath

By RICHARD BENKE

Associated Press Writer

RIO RANCHO, N.M. (AP) — Some Democrats who signed up to hear Vice President Dick Cheney speak here Saturday were refused tickets unless they signed a pledge to endorse President Bush.

The measure was a security step designed to avoid a disruption, which Bush campaign spokesman Dan Foley alleged Democrats were planning. Democratic Party officials denied it.

Several Democrats, at least, encountered the screening measures Thursday after calling from a line that self-identified as ACT, America Coming Together, an activist group that supports Kerry, Foley said. Others attempted to give false names and were denied tickets, he said.

Two men who had sought tickets reported they were required to give name, address, phone number, e-mail address and driver’s license number, then were presented the pledge of endorsement when they arrived to pick up the tickets Thursday.

The purported explanation is that a speech by the Vice-President is a “reward” for loyal supporters, and consequently need not be open to ordinary non-supporting Americans.

Outrage fatigue. That’s the only way they can get away with this stuff.

Having returned from dinosaur hunting basically intact, I’d like to explain a little of the process that is actually involved. Also very interesting, of course, would be to talk about the dinosaurs themselves, and what we hope to learn by studying them. But that’s another huge subject, which I am fairly unqualified to talk about, so you’d do better to hunt about on the web (or visit a museum). What I can try to do is give you an idea of what it is like to actually go out there and dig up some bones.

In particular, there are two questions I’ve always had about the nuts and bolts of paleontology: how do you actually find the fossils in the first place, and how to you dig them out once found? Answer in both cases: a little bit of know-how, and a huge amount of effort.

This is the second year I’ve gone to Wyoming with Project Exploration; last year I did manage to find a fossil myself, but this year we focused more on digging than on prospecting for new specimens. The prospecting was done ahead of time by Paul Sereno and some of his students. (Paul is the paleontologist half of the husband-and-wife team who founded PE; Gabrielle Lyon is the educator half.) The relevant know-how, as best I can make out, involves a combination of geological background and word of mouth. In fact, it seems from my outsiders perspective that there isn’t much of a bright line separating the disciplines of geology and paleontology; each relies heavily on knowledge from the other field, and experts in one are typically well-versed in the other. For dinosaur-hunting purposes, the geology comes in once you realize that your prospecting efficiency is greatly enhanced if you spend your time peering at rocks that actually date from the Mesozoic (the dinosaur era, between 248 and 65 million years ago), rather than before or after. Not only that, but you would especially like to have a layer that has been uplifted by geological activity, so that exposed rock faces (out of which might be sticking dinosaur fossils) are plentiful. Formations of this sort are common in certain areas of the Rocky Mountains, which is one reason why this region has produced so many significant dinosaur finds. We were working in the Morrison Formation, a limestone bed dating from the Jurrasic. (The Mesozoic is divided into three periods: the Triassic [248-108 Mya], the Jurassic [208-146Mya], and the Cretaceous [146-65 Mya].) The Morrison has long been a prolific source of dinosaur fossils. Here is a picture of the layered topography through which we were poking around; click for a bigger view.

Word of mouth comes in for the simple reason that, once you find a dinosaur somewhere, the surrounding regions are (sensibly) thought to be more likely to contain fossils than other randomly-chosen regions of the countryside. Not only that, but much of the land in this region of Wyoming is owned by ranchers who know the terrain like the back of their hand, and have become adept at spotting fossils. So sometimes it’s as simple as being told by a local rancher that there are some interesting-looking fossils at a certain site, or at least that a certain region seems promising.

In this particular case, Paul and some of his students had visited some formations around Shell, Wyoming (pop. approx. 50) a few weeks before we went for our expedition. They found some interesting fossils the old-fashioned way: waking up early in the morning and spending their days walking for miles, paying close attention to every rock that was part of the relevant geological layer. Fossils, of course, are bones (or other parts of an organism) that have been preserved in rock, and the process of fossilization typically involves much of the original organic material actually being replaced by rock. Which is to say, the fossils look awfully rock-like. So you have to know what you are doing, and long practice is at least as important as being told ahead of time what to look for. (Most common sentence heard at these digs, spoken by we amateurs to one of the people who knew what they were doing: “Is this a bone?”) Once you’ve seen some examples, you begin to recognize the striations characteristic of fossilized bone marrow, in contrast with the relative smoothness or unstructured graininess of rock. Still, wandering through long stretches of hillside, you need to have an eagle eye to be effective at prospecting.

In addition to a few dinosaur fossils, they found the trunk of a tree from the Jurassic that was over five feet wide; if it continues into the rock in which it is embedded, it may very well be over forty feet long. Paul calculated that two semi trucks should be able to haul the thing back to Chicago, where it could be used as part of an exhibition for PE of dinosaurs in their native habitat. At the same time, by looking at the interior we could learn something about the climatological conditions at the time the tree was alive (over perhaps a century or two). Personally, I’m happy to spend my time doing the delicate work of digging out dinosaur bones, and not having to be responsible for a several-ton fossilized tree.

The areas in which one finds these specimens are what most of us would describe as the middle of nowhere. Once you’ve found an interesting site, you mark its location in the handheld GPS unit that you remembered to bring with you while prospecting, so that you can actually find the thing when you come back. (Don’t ask me what they did before GPS.) There is also the worry that someone else comes along and digs up the fossil that you have found (expeditions like ours take place largely on government land, maintained by the Bureau of Land Management, so in principle anyone can come and dig them up, so long as they get a government permit to dig). So after finding a fossil and perhaps doing some preliminary digging to verify that it’s worth a follow-up visit, you then take a shovel and cover it back up with dirt so that it’s not obvious to future visitors. It’s not even obvious to you when you come back some time later, so a certain amount of time is spent re-finding the sites that were found on a previous visit.

Tomorrow: we go out and actually dig up some bones.

In the Denver airport on the way back from Wyoming, I used one of the public terminals to check email etc. (I haven’t completely mastered the wireless setup on my new laptop.) Of course I stopped by Preposterous to check on how things were going. Or at least I tried; the site was blocked for containing inappropriate content. Still trying to think what might have set that off. Questioning Einstein, maybe?

I am back from the wilds of Wyoming, having happily scampered through the Jurassic for a few days. A full report will be forthcoming. But first I wanted to thank Gretchen for filling in while I was gone; hopefully she will post again in the future.

A lot can happen in one short week, so I just wanted to hit some high points of the events you had to endure without me.

• The big news, my man Barack Obama hit a home run at the convention. I didn’t get a chance to see the speech, which I hear was not even broadcast by the networks. Also we were staying at a ranch owned by folks who were quite hospitable, but whose politics didn’t really align with mine. The first clue was the large painting of Jesus kneeling in front of the Liberty Bell. I must have missed that chapter in the Gospels. So Democratic-convention-watching wasn’t one of the scheduled activities.
• But actually, is the lack of coverage by the networks worth all of this hand-wringing? Don’t most people have cable TV by now, and don’t the cable networks cover the thing to death? I never understood why there was supposed to be a moral imperative for all three networks to provide essentially the exact same pictures. They could just rotate, like with the Olympics (without the dizzying rights fees).
• Despite the Dems having apparently stage-managed the convention quite skillfully, the free speech zones are a travesty. (Images found linked at Majikthise.) Their existence is a travesty on basic philosophical grounds, but their appearance is a disaster purely on craven political grounds. I mean, barbed wire?
• In one last bit of Hawkingiana, it’s worth pointing to this statement by John Preskill about the bet he won with Hawking (found linked at Michael Nielsen’s blog). Poor Preskill, who is a world-class theoretical physicist in his own right, but can only get in the newspapers by winning bets with Hawking.
• Francis Crick passed away. Most of what I know about Crick comes from reading The Double Helix, which I’m sure isn’t the most reliable source. More discussion at the Panda’s Thumb.
• Atrios unmasked! With his permission. (That’s the mysterious Atrios of Eschaton, for you scientists out there.)
  
• Finally, let’s give some props to Allen Iverson of my beloved Philadelphia 76ers, one of only two NBA stars (along with Tim Duncan) to fulfill his initial obligation to go to Greece as part of the Olympic team. Iverson has his issues, but he has always been treated far worse than he deserved, just because of his hairstyle and tattoos. Now he’s the co-captain of the Olympic team, which has to feel good.

I’m sure other interesting things “happened,” but if I wasn’t paying attention to them, how real can they be?

I wanted to mention why I will be in Wyoming next week: I’ll be hunting dinosaurs. (Fortunately they will have been dead for millions of years, otherwise it would not really be a fair fight.)

I’ve blogged previously about the great work that Gabe Lyon and Paul Sereno do through Project Exploration, a non-profit organization devoted to bringing the excitement of science to city kids. Doing good work requires money, so PE has to devote a lot of energy to fund-raising, and has been blessed with a devoted and enthusiastic set of donors. One thing they like to do each year is to take a trip with some of the donors to do some honest fossil excavation at a site in the U.S — sifting for microfossils, prospecting for larger dinosaur bones, and gently digging up and preserving the major fossils that have been found. (One goal is to build a teaching collection for the University.) So, while the expedition is fun and certainly educational, it’s also quite serious; those are the bones of an actual Tyrannosaur that you are digging up and perhaps breaking in pieces if you’re not too careful.

All of this seems perfectly sensible; less clear is how they got the idea that it would be amusing to bring along a cosmologist. But far be it from me to ask questions. (I did check that they wouldn’t expect me to be able to identify constellations in the night sky, but they said that was okay.) I went last year, had a great time, even found a serious fossil myself. (A hadrosaur, if you must know — the “cattle of the Cretaceous,” a major contributor to T-Rex’s diet.) And I’m going again this year. I’ll report on any major discoveries when I get back.

Update: Here is the post-expedition report, part one and part two.

Enough talking about Hawking, I would think. There are a bunch more newspaper stories out there (including these from Newsday and the New York Times that were nice enough to quote me). Also, Juan Maldacena and I appeared together today on Odyssey, the program hosted by the very same Gretchen Helfrich who will soon be our official guest-blogger. No quid-pro-quo was involved, I promise. You are welcome to listen to the program if you have RealAudio.

Just to show you that scientists don’t always agree, two representative quotes. First, Leonard Susskind in the Times:

Until Stephen’s recent reversal, he was about the only person still getting it wrong.

Susskind is a string theorist who thinks that it’s already been figured out, nothing to see here, time to move on. Next, from my colleague Bob Wald:

Hawking is completely revising his prior belief that what goes into a black hole is washed out. Now he believes that anything emitted from a black hole can be identifiable back to its source. He’s running away from what we still believe.

Wald is a general-relativist, quite skeptical about the claimed mechanisms for getting the information out. This is the problem with thought-experiments; they’re not nearly so conclusive as actual experiment-experiments.

As anticipated, Stephen Hawking gave his talk on black hole information loss at the GR17 conference in Dublin today; newspaper stories are already popping up, although they don’t tell us much we didn’t already know. I’ll try to have some incisive commentary soon; in the meantime, why not just get right to the heart of the matter? Here are the press release and transcript for Hawking’s talk.

• Press release from GR17 conference on Stephen Hawking’s talk
• Transcript of Hawking’s talk on black hole information loss

Judge for yourselves! At least, if you’re up on the background reading about Euclidean quantum gravity and the AdS/CFT correspondence. Thanks to Dennis Overbye of the New York Times for forwarding the material.

Update: Peter Woit has a parsing of what Hawking is trying to say. I think the most direct paragraph is probably this one:

So in the end, everyone was right, in a way. Information is lost in topologically non trivial metrics, like the eternal black hole. On the other hand, information is preserved in topologically trivial metrics. The confusion and paradox arose because people thought classically, in terms of a single topology for spacetime. It was either R4, or a black hole. But the Feynman sum over histories, allows it to be both at once. One can not tell which topology contributed the observation, any more than one can tell which slit the electron went through, in the two slits experiment. All that observation at infinity can determine, is that there is a unitary mapping from initial states, to final, and that information is not lost.

The idea seems to be that, so far as information measured at infinity is concerned, when we integrate over all possible geometries the relevant ones are those that don’t have black holes at all, merely apparent horizons. Some evidence for this point of view is adduced from AdS/CFT (the connection, first discovered by Juan Maldacena, between certain configurations of quantum gravity and certain field theories in one less spacetime dimension).

When we ultimately agree on the resolution of the information paradox, this idea may very well be part of the story. For the moment, it doesn’t seem like a very practical suggestion, to say the least; it amounts to a promise that, if only we could actually do the path integral for Euclidean quantum gravity, all the information would be preserved and end up in the outgoing Hawking radiation. It remains the case that most people are quite skeptical that we will ever make sense of the Euclidean path integral, especially in the semi-classical regime Hawking makes use of. So one way or another, there’s still a lot of work to be done!