Monday, May 31, 2004
Energy and intelligence
I should tie up some loose ends (read: "potentially misleading intemperate statements") in my post below about the anti-Big-Bang petition.
First, an actual physics point: Does Einstein's general relativity really say that energy is not conserved? You will be unsurprised to hear that the answer depends on what you mean by "energy." and what you mean by "conserved." Before general relativity came along, when spacetime was thought of as a fixed, static background on which all the rest of physics played itself out, the answer was unambiguous; at any moment in time, there was a number we could compute (for any closed system) called the "energy," and that number would be the same as at any other moment in time. (One way to derive this statement is as a consequence of the word "static"; time-translation invariance implies energy conservation.) Most often, we were lucky enough that the energy came in the form of an energy density defined at each point in space, which we could add up over the entire system to get the total energy.
GR changes the rules of the game. Spacetime is a dynamical object whose geometry responds to the presence of matter fields. We can now ask two separate questions: Is energy conserved for the matter fields in a given spacetime background? and Is the total energy of the universe, including matter and gravity (as manifested in spacetime curvature) conserved?
The answer to each question still depends on what you mean. Consider matter evolving in some background spacetime (so we ignore the possible energy of the gravitational field, whatever that may be). There is now no number we can calculate for a closed system that corresponds to "energy" and is conserved. This shouldn't be a surprise, since we have violated time-translation invariance; the background geometry could be expanding or contracting, for example. In cosmology, there is no "total energy of the universe" which is supposed to be conserved -- that's why Lerner's statement was so silly. On the other hand, there is a rule for "covariant" conservation of the local energy-momentum tensor (for experts, it's DaTab=0). This rule can be thought of as telling us exactly how the energy changes in response to changes in the background geometry, and it is what replaces the flat-spacetime notion of energy conservation. So the number of rules is the same in flat or curved spacetime; it's not as if anything goes. But we can't, once again, define a conserved total energy in any reasonable way.
So we should just include the energy of the gravitational field, obviously, right? The problem is there's no good way to do that. If we blindly follow the rules for calculating the energy and apply them to general relativity, we find that they don't give us an energy density at each point in space, but rather a boundary contribution defined solely at infinity. In other words, there is no local definition of energy density in general relativity. In the weak-field limit we can come up with good approximate notions of a gravitational energy density, and these are useful e.g. when calculating the energy lost through gravitational radiation in orbiting bodies. So perhaps we could give up on locality and stick with just a global definition. Given appropriate boundary conditions (typically that spacetime is flat at infinity) this makes sense, and we can define different notions of energy (the ADM energy, the Bondi energy) appropriate to different circumstances. But in cosmology the universe is not flat at infinity, so these circumstances don't apply -- there is generally, once again, no such thing as the conserved total energy. (In a closed universe there is -- and it's always exactly zero, for all the good it does us.)
The situation is thus a little ambiguous; whether energy is conserved in GR depends on the situation you are talking about, and what you would qualify as energy conservation. Don't get me wrong: nobody who understands what's going on has any disagreement about the equations or their solutions, it's just that there are different words we can reasonably apply to them. This is actually a good example of what Thomas Kuhn talked about in The Structure of Scientific Revolutions, where he discusses how words mean different things before and after a paradigm shift. The notion of "energy" is very useful, but its status in GR is different than it is in flat-spacetime physics. The one thing we can all agree on is that background energy density that remains constant as the universe expands, and whose integral over space therefore grows, is perfectly consistent with everything we know about physics.
The other thing I wanted to revisit is my defaming remark that Big-Bang opponents aren't very smart. Peter Woit points out a counterexample: Irving Segal, a well-known mathematical physicist who developed "chronometric cosmology" as an alternative to the Big Bang. Of course there are other counterexamples, notable among whom we should mention Sir Fred Hoyle, who did extremely important work in stellar nucleosynthesis, and later became well-known as a supporter of the Steady State model. (It was Hoyle who actually coined the term "Big Bang," as a derogatory term to belittle the model we now know to be correct.) (Update: Another unfair slander! See the comments.)
I shouldn't have given such a blanket indictment of the intellectual prowess of the anti-Bang folks. For all I know, Eric Lerner is a grandmaster at chess, a gourmet cook, and a crossword-puzzle wizard. What I should have simply said is that the criticisms leveled by these folks at the Big Bang are just not very smart. I can certainly imagine intelligent and reasonable arguments given against almost any scientific position; but the anti-Banging is generally done from a position of deep philosophical conviction, which tends to result in rather weak argumentation. Segal, for example, had a theory in which the apparent velocity of a distant galaxy should be proportional to its distance squared (rather than simply the distance, as in the conventional theory). He would insist that modern statistical techniques reinforced his result; typically, these techniques would involve tossing out the data that manifestly disagreed with his theory. He was extremely bright in some ways, but about this he was blind.
The point really is that the anti-Big-Bang crowd are not visionary mavericks being unfairly undermined by a narrow-minded scientific establishment; they are just crackpots. The difference can be quite subtle and even subjective, but in this case it's pretty clear.
Sunday, May 30, 2004
I think of myself as realistic and even cynical, but reality keeps surpassing my lowest expectations. The Poor Man points out something I didn't believe until I checked for myself: the front page of the official George W. Bush campaign website features (on May 30, anyway) not a single image of George W. Bush. It does have four pictures of John Kerry, though. Lead with your strengths, I suppose.
Saturday, May 29, 2004
Doubt and dissent are not tolerated
PZ Myers of Pharyngula fame has pointed me to an online petition that was apparently first published in New Scientist. No, it's not complaining about the Bush administration making a travesty of science (although David Appell points to one of those, too); it's about the terrible dominance of the Big Bang model.
The complaints are not new. The Big Bang just rubs some people the wrong way, and they won't believe in it no matter how many successes it accumulates. Some of the disbelief stems from religious conviction, but in other cases it seems to be a particular kind of philosophical outlook. Most of the skeptics, of course, have their own favorite alternatives. The most popular is undoubtedly the Steady-State model (or one of its increasingly twisted modern incarnations), but there is also something called the "plasma cosmology", championed by the late Nobel Laureate Hannes Alfven. (His Nobel was for plasma physics, not cosmology; and the fact that he was Swedish didn't hurt.) If you want to know in detail why the various alternatives are wrong, Ned Wright tells you.
Here is the kind of thing the petition says:
What is more, the big bang theory can boast of no quantitative predictions that have subsequently been validated by observation. The successes claimed by the theory's supporters consist of its ability to retrospectively fit observations with a steadily increasing array of adjustable parameters, just as the old Earth-centered cosmology of Ptolemy needed layer upon layer of epicycles.Really? How about acoustic peaks in the power spectrum of temperature fluctuations in the cosmic microwave background? And the polarization signal, and its spectrum? And the baryon density as deduced from light-element abundances agreeing with that deduced from the CMB? And baryon fluctuations in the power spectrum of large-scale structure? And the transition from acceleration to deceleration in the Hubble diagram of high-redshift supernovae? And the relativistic time delay in supernova light curves? These are just the very quantitative predictions that have come true in the last few years; the Big Bang has had a long history of many observational successes. (This is a very incomplete list; usually one doesn't pay much attention to straightforward tests of the Big Bang framework, since they are taken for granted.)
But here is the important issue, again from the petition:
Whereas Richard Feynman could say that "science is the culture of doubt", in cosmology today doubt and dissent are not tolerated, and young scientists learn to remain silent if they have something negative to say about the standard big bang model. Those who doubt the big bang fear that saying so will cost them their funding.Something actually interesting is being raised here: at what point does a scientific theory become so well-established that it's no longer worth listening to alternatives?
There's no easy answer. Scientific theories are never "proven" correct; they simply gather increasing evidence in their favor, until consideration of alternatives becomes a waste of time. Even then, they are typically only considered correct in some domain. Einstein's general relativity, for example, works very well in a certain regime, but that doesn't stop us from considering alternatives that may be relevant outside that regime.
So, shouldn't we devote a certain fraction of our scientific resources, or our high-school and secondary curricula, to considering alternatives to the Big Bang, or for that matter Darwinian evolution? No. Simply because resources are finite, and we have to use them the best we can. It is conceivable in principle that the basics of the Big Bang model (an expanding universe that was much hotter and denser in the past) are somehow wrong, but the chances are so infinitesimally small that it's just not worth the bother. If individual researchers would like to pursue a non-Big-Bang line, they are welcome to do so; that's what tenure is for, to allow people to work out ideas that others think are a waste of time. But the community is under no obligation to spend its money supporting them. And yes, young people who disbelieve in the Big Bang are unlikely to get invited to speak at major conferences, or get permanent jobs at research universities. Likewise astrophysicists who believe in astrology, or medical doctors who use leeches to fight cancer. Just because scientific claims are never proven with metaphysical certainty doesn't mean we can't ever reach a conclusion and move on.
And to be sure, the alternatives to the Big Bang are just silly. Usually I try to keep my intellectual disagreements on the level of reasoned debate, rather than labeling people I disagree with as "dumb" (that I reserve for the President); but in this case I have to make an exception. They just aren't, for the most part, very smart. Consider this quote by Eric Lerner, petition signatory and author of The Big Bang Never Happened:
No Conservation of EnergyActually, there is a field of physics in which energy is not conserved: it's called general relativity. In an expanding universe, as we have known for many decades, the total energy is not conserved. Nothing fancy to do with dark energy -- the same thing is true for ordinary radiation. Every photon loses energy by redshifting as the universe expands, while the total number of photons remains conserved, so the total energy decreases. An effect which has, of course, been observed.
Just because a person doesn't understand general relativity doesn't mean they are dumb, by any means. But if your professional activity consists of combating a cosmological model that is based on GR, you shouldn't open your mouth without understanding at least the basics. So if I get to decide whether to allocate money or jobs to one of the bright graduate students working on some of the many fruitful issues raised by the Big Bang cosmology, or divert it to a crackpot who claims that the Big Bang has no empirical successes, it's an easy choice. Not censorship, just sensible allocation of resources in a finite world.
Thursday, May 27, 2004
Vital questions addressed
Will Baude at Crescat Sententia asks two profound questions that we at Preposterous are happy to answer.
First: What is the appropriate honorific for a professor at the University of Chicago? There's a story one sometimes hears to the effect that everyone (students, faculty, presumably researchers) at the UofC refers to each other by Mr/Ms, in sort of a charming reverse-snobbery. (As Brian at That's News to Me points out, the story is promulgated through the UofC student guide.) We're all supposed to be a community of scholars or some such thing. But is it really true?
The existence of this story makes things more awkward than they should be, if anything; the transition from Dr/Professor to first names as students get to know professors better is ambiguous and difficult enough, and throwing the possibility of Mr/Ms in there muddles things beyond hope. But we can look at the data. A quick perusal of emails from students in my current undergrad class reveals about a 50/50 split between "Professor Carroll" and a complete absence of name (just "Hi" or some such thing). No "Mr. Carroll"'s in evidence. But perhaps email is slightly more formal than face-to-face? I recall at least one student last quarter using "Mr." Not that I care; students are welcome to call me by Sean, or Professor, or Dr. I would think that the rules should be close to what they are outside the academic environment; if you are meeting someone for the first time, the relevant title seems appropriate, and once you get to know them better you can use the first name. Note to students: not every professor feels this way, and some quite like being called "Professor." And there's no easy way to tell.
To be honest, I'm not always clear on what I should call other professors. In particular, if I am sending email to someone in my field, whose work I am familiar with but whom I've never met in person nor corresponded with previously, should I call them Dr/Professor or just use their first name, as would be common if we were introduced at a conference? For no especially good reason, I tend to jump right in with the first name if the person is actually in my field, but use an honorific for someone in another discipline. Presumably I feel as if we physicists are a band of brothers and sisters, all in this together and somehow all friends even if we haven't actually been introduced. Whereas the more abstract ties of academia aren't quite enough to allow for such assumed intimacy. I think this compromise is not unusual, actually, although I don't have any real data.
(As I was writing this I noticed an update. Seems like the Professors are taking over.)
Will's second question: Should an omnipotent God be omnicontracting (able to make any promise, but not to ever break those promises) or omnibreaching (able to do anything at any time, even break past promises)? That one is much easier. The concept of an omnipotent God is incoherent. There is no sensible way to define what is meant by "omnipotent." That's okay, because there doesn't exist anything resembling an omnipotent God, so the logical impossibility of the concept shouldn't bother anyone.
Never let it be said that we don't tackle the important issues.
Wednesday, May 26, 2004
It all makes sense now
In case you had any doubt, the human rights situation worldwide is now the worst it's been in 50 years. Ruben Bolling offers one possible explanation (click for more):
Don't laugh, I've heard crazier ideas.
Tuesday, May 25, 2004
Was Friedmann wrong?
Yesterday we wondered out loud whether cosmological evidence for dark matter might actually be pointing to something more profound: a deviation of the behavior of gravity from that predicted by Einstein's general relativity (GR). Now let's ask the same question in the context of dark energy and the acceleration of the universe.
We have (at least) two problems to face. First, if you do a back-of-the-envelope estimate of what the vacuum energy (the energy density inherent in empty space, or equivalently Einstein's cosmological constant) should be, you get an answer that exceeds the observationally allowed value by the ridiculous factor of 120 orders of magnitude (10120). That's bad, as far as agreement between theory and experiment is concerned. But nevertheless the universe is accelerating, which could be explained by a tiny amount of vacuum energy -- about 10-8 ergs per cubic centimeter, if you care. Or perhaps by something else.
For example, maybe general relativity works for ordinary bound systems like stars and galaxies, but breaks down for cosmology, in particular for the expansion rate of the universe. In GR the expansion rate is described by the Friedmann equation, which sets the expansion rate proportional to the square root of the energy density. Ordinarily the density drops as the universe expands, and the expansion rate follows suit; if the density is constant (as with vacuum energy), the expansion rate can stay constant. (We would then interpret that as "accelerating", oddly enough. A distant galaxy has a recession velocity v=Hd, where d is the distance and the Hubble constant H tells us the expansion rate. If H is constant and d is increasing, we would measure v to be increasing.)
So maybe Friedmann was somehow wrong. For example, maybe we can solve the problem of the mismatch between theory and experiment by saying that the vacuum energy somehow doesn't make the universe accelerate like ordinary energy does. There are different ways to make this happen, none of which strike you as perfectly compelling. Arkani-Hamed, Dimopoulos, Dvali, and Gabadadze have proposed a "filter" by which smoothly-distributed energy doesn't affect spacetime in the same way as localized energy; it's somewhat ad hoc, but definitely interesting. With Laura Mersini I suggested that the pressure of a substance, as well as the energy density, contributes to the expansion rate, in just such a way as to make vacuum energy (which comes with a negative pressure) cancel exactly and leave spacetime unaffected. We were inspired by models with extra dimensions of space, but the particular models in question don't quite work, so I've since spent a lot of time looking for models that are strictly four-dimensional. No luck yet.
Solving the cosmological constant problem is hard, and a popular strategy is to ignore it and try to account for dark energy separately. That is, imagine that the vacuum energy is set to zero by some mysterious mechanism, and something else is responsible for the acceleration of the universe. There are different approaches to this possibility as well. One is to be largely phenomenological, and just write down alternatives to the Friedmann equation to see what might work. This approach (inspired again by extra dimensions, but basically phenomenological) has been pursued by Dvali and Turner, and by Freese and Lewis. One issue here is that ultimately it wouldn't be possible to distinguish experimentally between a modified Friedmann equation and some model of dark energy; both would only show up in the behavior of the expanding universe.
So the complementary approach is to come up with a whole new theory of gravity that can make the universe accelerate, and see if what that theory has to say about other tests of gravity. I've been thinking about this recently, having spent the weekend talking to Mark Trodden. He and I have written a paper with Duvvuri and Turner that explores an especially simple approach to doing this. We just suggest that the curvature of spacetime somehow resists going to zero, and can bounce back to infinity when it becomes small. (There are actually a lot of equations involved, but that's the basic philosophy.) Then we can compare our model to experiments. Sadly, it fails. The simplest way to see this was pointed out by Chiba, who rewrote our theory in a way that resembles other models, and showed that ordinary tests of GR in the solar system are incompatible with our suggestion. Basically, the orbit of Mars would look different. But that's okay; it just goes to show that GR is actually quite robust, and even an attempt to change it exclusively in cosmology ends up affecting all sorts of other things.
A more elaborate approach has been suggested by Dvali, Gabadadze, and Porrati (see a recent review by Dvali for more details). They again use an extra dimension of space, imagining that our world is a three-dimensional "brane" embedded in a four-dimensional space (plus one time dimension, as usual). They have invented a scheme whereby gravity can behave differently on and off the brane, become much weaker in the four-dimensional "bulk." But at very large scales the bulk begins to affect our universe. They claim that, if we choose parameters appropriately (there is always a great deal of unnatural fine-tuning involved in these scenarios), we can straightforwardly explain the accelerated expansion of the universe. Even better, they are not yet (as far as we know) ruled bout by solar-system tests, but improved measurements might be able to detect new affects from the extra dimensions in the orbit of the moon (see also this paper by Lue and Starkman). This theory is not very well understood as yet, and deserves a lot more work to be fully explicated; but it's interesting and promising, so we'll have to see what happens as people think about it further. (There is a nice popular-level exposition by Dvali in Scientific American, although you have to pay for the full article online.)
So we don't have any extremely compelling alternatives to Einstein's theory, but there are a lot of possibilities and we have our work cut out for us. The good news is that observed phenomena (the dynamics of galaxies and clusters, the cosmic microwave background, the accelerating universe) are pushing us to think of profound new scenarios for gravity and cosmology.
By the way (as it were), John Scalzi links here and suggests that GR is bound to give way pretty soon. I hope that's not the impression I'm actually giving; I personally think it's an incredible long shot, but one worth pursuing. Probably the "standard" story of cold dark matter and vacuum energy are exactly right; this is a robust model that makes many more predictions than there are free parameters, and it's well-motivated (although far from completely understood) in fundamental physics. In contrast, when we start modifying gravity, we're just flailing around, hoping some good will come of it. But that's how science works; the flailing always looks a little silly until it smacks into a bit of miraculous insight, which we then clean up and proclaim to be genius. Right now there's a lot of talk of modifying gravity, because it's well worth considering; but if you're going to bet, Einstein gets much better odds even than Smarty Jones.
Monday, May 24, 2004
Was Einstein wrong?
Gravity is the most obvious of the four forces of nature (gravity, electromagnetism, and the strong and weak nuclear forces). It's also the first for which we had a sensible physical theory: Newton's law of universal gravitation. Now we have sensible theories for all four of the forces, and Newton's theory has been superseded by an even better theory, Einstein's general relativity (GR).
GR has passed a series of experimental challenges with flying colors: the precession of Mercury, deflection of light, gravitational redshift and time delay, gravitational radiation from the binary pulsar, and the expansion rate of the early universe during the nucleosynthesis era. But it doesn't quite fit in with the rest of physics, since the other three forces seem to be compatible with quantum mechanics in a way isn't so obvious for gravity. So very few people really believe that general relativity is the final answer; at some point we'll have to invent a better model (string theory being the leading candidate) that is intrinsically quantum-mechanical yet reduces to GR in the appropriate regimes.
Usually in field theory, if a model works well in a certain regime, you might expect it to break down at shorter distances or higher energies, but continue to be successful at long distances and lower energies. Nevertheless, people have begun to ask whether general relativity might be okay in the solar system but break down on much larger scales (galaxy- or universe-sized distances). The primary motivation for such suggestions is the fact we need to hypothesize dark matter and dark energy to make sense of our universe if GR is correct. It is very likely that GR is correct, and dark matter and dark energy are both for real, but since we can't be sure we consider the possibility that our understanding of gravity is to blame.
Of course, it's easy to say "let's modify gravity," much harder to come up with a good model. Indeed, it's not even obvious what issue you'd like your model to address -- the need for dark matter in galaxies, clusters, and large-scale structure; or the perplexingly small value of the cosmological constant; or the acceleration of the universe conventionally attributed to dark energy.
Modifying gravity with the goal of replacing dark matter is a long-standing project that has met with mixed success, most famously pursued by Milgrom and his friends. Milgrom has an idea called "Modified Newtonian Dynamics," or MOND for short. For some introductions see pages by Greg Bothun or Stacy McGaugh, or this review by Sellwood and Kosowsky. The idea is to slightly increase the Newtonian gravitational acceleration when that acceleration is very small, so that slowly-moving particles feel more force than they ordinarily would, mimicking the presence of unseen matter. This idea works extremely well for individual galaxies; indeed, Milgrom made predictions for the behavior of low-surface-brightness galaxies before they were directly observed, and the predictions were later confirmed very nicely.
Unfortunately, there are problems with the MOND paradigm itself. For one thing, it's not really a "theory", it's just a rule for making predictions in a very specific set of circumstances -- slowly-moving particles orbiting around massive bodies. (Just as an observational matter, it doesn't even seem to work very well for clusters of galaxies, although it does quite well for individual galaxies.) Since it's not a full-blown theory, it's hard to make predictions for other tests you might like to do, like deflection of light or cosmology. So people have been trying to invent an actual theory that reduces to MOND in the appropriate circumstances. In a recent proposal, Bekenstein has claimed to succeed; now people are at work putting this idea to the test, to see both if it makes sense and if it agrees with other things we know about cosmology.
In addition to the theoretical difficulties, there is at least one model-independent reason to think that no modification of gravity will ever replace the idea of dark matter: we seem to be accumulating evidence (tentatively at the moment, to be sure) for gravitational forces pointing in directions where there is no ordinary matter. The most basic such clue comes from studies of gravitational lensing of clusters of galaxies, which can be used to reconstruct the distribution of dark matter in the clusters. The upshot is that the dark matter seems to be distributed much more smoothly than the ordinary matter; see this reconstructed cluster image for an example. Less direct evidence is found in the acoustic peak structure of the temperature anisotropies in the cosmic microwave background. (For an intro, see Wayne Hu's tutorial.) Density fluctuations in the plasma of the early universe lead to sound waves, in which regions become more dense and therefore hot, and then bounce back and become less dense, in a repeating cycle; this leads to peaks in the plot of temperature fluctuation as a function of angular scale. But fluctuations in the dark matter don't heat up (they don't interact with light, since they're dark), so they only increase with time. Consequently, odd-numbered peaks have ordinary matter and dark matter in phase, and even-numbered peaks have them out of phase. The out-of-phase oscillations are suppressed, so we expect dark matter to boost the odd-numbered peaks. This is exactly what appears to happen, as this figure indicates. At least a little bit; the data need to improve before we can be sure. But it's hard to see how a modified theory of gravity could explain this phenomenon.
Of course, perhaps a modified theory of gravity could predict gravitational forces pointing in directions other than where there is ordinary matter; you'd have to tell me the theory first before we could say for sure. MOND doesn't, though, and such a theory is even harder to imagine than one that simply fits the galaxy data.
Tomorrow I'll talk a little about modified gravity and the issues of vacuum energy and the accelerating universe.
Friday, May 21, 2004
By popular demand
I'm thinking of starting a new tradition, declaring Friday to be Narcissism Day here at Preposterous Universe. A day we can take off from thinking about important world events and profound cosmic mysteries, and just think about me.
In this spirit, I offer this pointer to my first-ever (so far as I know) appearance in the society pages of a major newspaper. (To balance things out, here's a media moment more relevant to my purported expertise. [Update: I found another one. What a media slut, hmm?]) The occasion was the Dinosaur Dinner I attended to benefit Project Exploration. Here's more proof:
L to R: Sean, State Senator Barack Obama, Shureice Kornegay, Jean Claude Francois, Project Exploration Executive Director (and expectant mother) Gabrielle Lyon, Michelle Obama. I'm the one with the martini, to nobody's surprise.
I have to admit that I didn't see any evidence of the stalker that Jack Ryan's campaign has assigned to follow around Obama twenty-four hours a day. Probably he couldn't afford a ticket. The Obama campaign has started its own blog, which is worth a visit.
Thursday, May 20, 2004
All you need to know
From the Reuters story about our raid on Chalabi's headquarters:
An opinion poll found only seven percent of Iraqis now viewed U.S. troops as "liberators," compared to 45 percent six months ago.Do you think it's improved since then?
Wednesday, May 19, 2004
Darker and darker
More evidence for an accelerating universe, this time from the Chandra X-ray satellite observatory. They observe X-rays from the hot gas in distant clusters of galaxies. A cluster is just a set of galaxies bound together by their mutual gravitational pull; but in addition to the galaxies themselves, the cluster is full of hot gas between the galaxies, not to mention dark matter. This picture is of the cluster Abell 2029; in blue you see the galaxies (visible in ordinary light) and in red the hot gas (reconstructed from the X-ray image). Knowing the properties of the gas, they can figure out the distances to the clusters. Comparing these with the redshifts (which tell us by how much the universe has expanded since the light we see left the cluster), we can reconstructe the expansion history of the universe.
The answer they get for the acceleration is consistent with our recent consensus model for cosmology, including substantial dark energy that seems to be nearly (or exactly) constant as the universe expands. So, not a dramatic overthrowing of what we already knew, but a nice confirmation. Which is very important, when the thing you're confirming is as surprising and ill-understood as the acceleration of the universe. Our previous evidence (from distances to supernovae, temperature fluctuations in the cosmic microwave background, and the dynamics of galaxies and large-scale structure) was very good, but every extra piece of evidence bolsters the case for this preposterous universe.
Chandra is named after Subrahmanyan Chandrasekhar, one of the leading theoretical astrophysicists of the 20th century. Also a longtime University of Chicago faculty member, and part of a tradition at NASA of naming its satellite observatories after famous scientists with UofC connections -- Chandra was preceded by the Hubble space telescope, named after a prominent alumnus, and the Compton gamma-ray observatory, named after another former faculty member. This tradition ended with the Spitzer infrared observatory, but that's okay because Lyman Spitzer was my grand-advisor (the Ph.D. advisor of George Field, my adviser). After that things went dramatically downhill, with the successor to Hubble being named after James Webb, a former NASA administrator.
Tuesday, May 18, 2004
Maybe I will not stop posting happy things until all the evil people go away. That might work, don't you think? From Circa75 via Atrios, a first-person account of getting a marriage license in my old home town of Cambridge.
A sign poking up from the crowd saying "YAY" caught my eye. Some people had "Toto, we're not in Kansas, Welcome to Equality" signs for the Phelps folks, but the crowd had grown so large you couldn't tell if they were still there.Try to read the whole thing without getting choked up, I dare you.
A little light amusement
From Matt Stoller by way of The Poor Man, one of the funniest things you'll ever read: the 2000 Republican Party Platform. Probably it wasn't so amusing at the time, but age does wonders. Here, just a few choice excerpts:
"The arrogance, inconsistency, and unreliability of the administration's diplomacy have undermined American alliances, alienated friends, and emboldened our adversaries."Here at Preposterous, we aim to provide entertaining distractions from the relentlessly depressing real world. Good to know that the Republican National Committee is collaborating in the effort.
Monday, May 17, 2004
Studies link black to white, up to down
You wonder why people get confused by science stories in the press? Two studies on the efficacy of Atkins-like low-carb/high-protein diets were recently reported in the Annals of Internal Medicine. If you visited the Google News page devoted to coverage of these stories (here is the page itself, although the content may shift with time), these are the first six headlines you would have seen, without any editing on my part:
Sometimes, if the medium is not actually the message, it nevertheless garbles the message so much as to be counterproductive. In particular, the need for a short and punchy headline forces distortion, not just oversimplification, of the story being reported. (Let's face it, would you click first on the story from the Minneapolis Star Tribune?)
You can't blame science reporters, who have a tough job and don't write headlines anyway. A daily newspaper is just not an effective way to teach science. The news cycle demands that results be packaged in both catchy and timely ways, whereas the actual way that science is done is more often characterized by a gradual emergence of consensus. Not that I know what the proper remedy is, other than to teach students to be more aware and science-literate by the time they finish high school, so can they take simple headlines with a grain of salt.
Marriage in Massachusetts
A couple of the many couples getting married in Massachusetts today (Associated Press photos). Julie and Hillary Goodridge, lead plaintiffs in the case to allow same-sex couples to marry, getting their marriage license:
John Mirthes and Rick Reynolds chat with volunteer Sian Robertson as they wait to file their intent to marry:
I'm told that scenes like this are going to be the end of civilization as we know it. But with all the other images we've been seeing in the news lately, these made me feel good.
(Related: Career choices explained.)
Friday, May 14, 2004
Josh Marshall has an excerpt from a Washington Times excerpt from Bill Sammon's new book, Misunderestimated: The President Battles Terrorism, John Kerry and the Bush Haters. Here is my own excerpt of Josh's excerpt:
"I get the newspapers — the New York Times, The Washington Times, The Washington Post and USA Today — those are the four papers delivered," he said. "I can scan a front page, and if there is a particular story of interest, I'll skim it."Remember, this is a pro-Bush book, reprinted in a pro-Bush newspaper.
You want to know why liberals are really so angry? You can't parody this guy! His reality exceeds our best attempts at humorous exaggeration.
I missed poem on your blog day. Here's a belated entry by Kate Ryan, recent winner of the Ruth Lilly Prize.
THE OTHER SHOE
Thursday, May 13, 2004
... here at the new & improved blogspot. At first it wouldn't let me publish my last post, and now it's repeating it over and over. Hopefully we'll return to normal soon.
Some reactions to my list of misconceptions about cosmology. Chad Orzel at Uncertain Principles has misconceptions about quantum mechanics and thermodynamics. It's a very good list, even if he does say that vacuum energy is useless. It's useless in the sense that it cannot be made to do thermodynamic work (because the vacuum energy is spread absolutely uniformly), but in another sense it's quite useful: it makes the universe accelerate, thereby giving cosmologists something deep to think about. (And occasionally get them a job.)
Chris C Mooney notes that a planetarium show at the Smithsonian doesn't even mention the Big Bang, although it's supposed to be a tour of the universe. So perhaps one of the sources of misconceptions is that we aren't clear enough about what we actually do think people need to know? Right off the top of my head, here are some facts about cosmology I think every educated person should know:
Wednesday, May 12, 2004
We are happy to report that this blog is the first page to come up in a Google search for preposterous. Out of 275,000 pages, that's not too shabby.
Inflating the universe
Sometimes you have to love NASA. It's the only organization I know of (although I'm sure there are countless others) that measures the success of its research programs by how many column inches are devoted to them in newspapers worldwide. As a byproduct, they've become very good at getting out their message in interesting ways.
This is by way of prelude to describing what I received in my mailbox yesterday: a WMAP beach ball (pictured at right). WMAP is the Wilkinson Microwave Background Anisotropy Probe, a satellite that has measured the tiny temperature fluctuations in the cosmic microwave background to unprecedented precision. The statistical properties of these fluctuations depend in interesting ways on the parameters describing our universe (such as the amount of dark matter and dark energy, or the overall geometry of space), so the WMAP results have provided a treasure trove of information for cosmologists. The cosmic microwave background radiation provides a picture of the universe when it first became transparent, at an age of about 379,000 years; it's kind of amazing we can extrapolate our current theories back that far and come even close to the right answer, much less get things spot-on.
The beach ball is a playful public-relations gimmick, which I'm all in favor of. Other types of scientists could learn a lot about outreach from these folks.
Tuesday, May 11, 2004
I notice that I'm actually writing much less about politics in this blog than I originally expected to. Part of that is because there are few things I have to say that aren't already being said more eloquently in some other blog. Another part is that the situation right now is so depressing and outrageous on multiple levels that there's an overwhelming temptation to forget about light and just throw heat.
So instead you will get some warmed-over cosmology. In particular, I was asked a while back to come up with a list of "most prevalent misconceptions about cosmology." I'm not sure among whom they should be prevalent; some of my colleagues have way-out ideas, but I'm not going to go about setting them all straight. Anyway, here was my suggested list of misconceptions. Comments welcome as to more misconceptions, although they have to be arguably prevalent -- not just your own personal misconceptions, we could be here a long time.
Monday, May 10, 2004
Any physicist knows the most common responses when you first tell someone what you do for a living -- "I hated physics in high school" being the consensus pick for number one. Which is not inconsistent with the fact that people are fascinated by the actual physics that we do, whether it's studying dark energy or the physics of crumpling paper. Our education system, for whatever reasons, tends to scare people away from science more than it draws them in.
Which is why the work of Project Exploration is so wonderful. Founded and run by Paul Sereno and Gabrielle Lyon, PE works to get children (especially girls and inner-city kids) interested in science by using one of the greatest draws we have: dinosaurs. Paul is an celebrated paleontologist who is our best living approximation to Indiana Jones; his wife Gabe is a professional educator who is really the soul of PE. In the short time they've been in operation, they've already made a tangible difference in a lot of people's lives; as just one measure, almost all of the children who work with PE end up going to college, while it's a good bet that almost none of them would have if it hadn't been for the project.
Their latest brilliant idea is to display dinosaurs in a good approximation of their natural habitat -- the Giants exhibition shows fossils and exhibits amidst the plant life at the Garfield Park Conservatory. It's an impressive exhibit, very worth checking out if your're in Chicago. I visited Saturday night for the Fourth Annual Dinosaur Dinner, a gala benefit for PE. It was great fun, including a benefit auction of items like a dinosaur-femur bench and a dinner with Paul and Gabe. (This is my new standard for success in life: when I can auction off dinner with myself in a reasonable expectation that someone other than my Mom would bid for it.)
Gabe is interested in expanding the purview of Project Exploration to include other types of science. We both think it would be fun next year to have a Dark Energy Dinner, where everyone comes dressed in black. Watch this space for updates to see if it will come to pass.
The other celebrity I got to meet at the dinner was Barak Obama, our Democratic nominee for US Senate from Illinois. In a thirty-second conversation, he came off as extremely intelligent and engaging (which is his job, I suppose). I mentioned that I had endorsed him on my blog, and he was curious about the blogging process -- how much time it took, etc. It's about time we get someone in government who has a UofC affiliation but is not a crazy neoconservative, so I'm rooting hard for him.
Friday, May 07, 2004
Nature has a feature known as "concepts essays," in which they ask highly respected (or at least "willing") scientists to write short reflective pieces about specific concept of importance to their field. The idea is to go slightly beyond a standard pedagogical introduction to a subject and allow for the kind of discussions that scientists might have around coffee but would never put into a journal article. (I.e. it's an old-media version of a blog.)
I was invited to write such an essay about dark matter, which has now appeared. (That's a pdf version on my site; there is an html version on the Nature site, but it's not as pretty and might require registration.) I couldn't help but mention dark energy as well, so in the final version the "concept" includes both dark matter and dark energy. (Here's a very short intro to both subjects.)
The idiosyncratic angle I chose to take was to ask how interesting the dark sector could be -- in particular, whether there could be interactions between dark matter particles (or dark matter and ordinary matter, or dark matter and dark energy) that might allow some sort of structures to form, even intelligent life. We might ask, for example, whether there could be some weakly-coupled massless abelian gauge boson that mediates interactions between dark matter particles: "dark light."
As I say in the article, probably not. We don't know as much as we would like about the distribution of dark matter, but we do know something, and it appears to be much more smoothly distributed than the ordinary matter in the universe. See for example this computer reconstruction of the dark matter density in a cluster of galaxies, using gravitational lensing. The simple explanation for this smoothness is that dark matter is probably collisionless. When atoms of ordinary matter bump together, they can emit light and cool, and this dissipation process allows the ordinary stuff to condense into the center of galaxies. But the interactions that give rise to dissipation are exactly those necessary for making structures and life. Of course, all we have are upper limits; it's still possible that there is life out there in the dark matter, but characterized by much larger sizes and much longer timescales than anything in our experience. Perhaps a dark heartbeat takes millions of years to complete.
Now I am wondering whether the goofy illustration chosen by the editors to accompany the article (shown above) might feature the most dramatic decolletage ever to appear in a major scientific journal. Anyone have any other candidates?
Thursday, May 06, 2004
The World Series
Ed Brayton at Dispatches from the Culture Wars mentions something I should have known (and tells some gripping stories in the process): the World Series of Poker (WSOP) is going on in Las Vegas this very moment. Like lots of people, I played some five-card draw with family and friends when I was young. But I never became really interested in poker until last summer, when I read James McManus' Positively Fifth Street. Talk about a gripping story: McManus is a writer and amateur poker player, who was given the assignment by Harper's of writing a story about the WSOP. Being an impetuous type, he took his expense-and-advance money and used it to enter a satellite tournament (a cheaper way of trying to play your way into the main event, rather than just ponying up the $10,000 entrance fee). Remarkably, he won the satellite, and more remarkably, he kept on winning -- all the way to the final table, where he finished fifth and took home over $200,000. (The book is fascinating and annoying at the same time, due to the authors self-absorption. If you want a more balanced view of the world of professional poker players, try The Biggest Game in Town by A. Alvarez.)
To gauge my ignorance, I didn't even know that real poker players didn't play five-card draw, but rather Texas Hold-Em. It's a simple game at heart: everyone gets two cards dealt face-down that only they can see. Then five cards are dealt face-up in the middle of the table; each player makes the best five-card poker hand that they can, using their own cards and the five on the table. The complications only arise in the betting process, which happens after the first two cards are dealt and after each card thereafter.
Not so complicated, right? But of course it's incredibly complex when you get into it. The secret of the allure (and challenge) of poker is that it's a game of incomplete information, the kind game theorists love to think about. You know the cards you already have, and you (should) know the probabilities of various further cards coming your way, but you have to infer your opponents' hands from tiny hints (their bets, their positions at the table, their personal styles, etc). Texas Hold-Em is so popular because it manages to accurately hit the mark between "enough information to devise a consistently winning strategy" and "not enough information to do much more than guess." The charm in such games is that there is no perfect strategy, in the sense that there is no algorithm guaranteed to win in the long run against any other algorithm. The best poker players (and there are a good number of people who earn their living from poker, so it's by no means "gambling") are able to use different algorithms against different opponents, as the situation warrants.
I'm sure that professional game theorists have analyzed poker to death, but I haven't ever seen any technical work on the subject myself. David Sklansky has written a book called Theory of Poker, but it doesn't get into all the fun game-theory aspects. (For actually learning poker strategy, the acknowledged classic is Super-System by Doyle Brunson et al.)
I have played a few times in casinos, in Los Angeles and (once, late at night, in the midst of a cross-country trip from California to Chicago) at the Bellagio in Vegas. But you can play at any time online; my favorite site is ultimatebet.com, although there are several alternative sites that I haven't looked into closely. One of the nicest features of poker is that it is a perfect meritocracy; anyone can do it, and your success depends only on your own skill, not on help from anyone else. At the casino you will sit down at a typical table with people older and younger than you, men and women, blacks, whites, and Asians, gays and straights, extroverts and introverts. Some of the world's best players grew up as pampered bourgoisie, others were Vietnamese boat people. For some reason I haven't been playing that much lately; I fear my poker time has been taken over by blogging. (Neither one of which is very lucrative, at my level of skill.)
Wednesday, May 05, 2004
An apparent rupture in the spacetime continuum has been noted over at archy and Pandagon. In one of the questions asked to President Bush on his current "bus" tour, the interlocutor referred to an apparent acausal propagation of economic hardship:
In 1998, due to the impending recession, I started living the American nightmare.John McKay at archy wonders whether there could be a quantum-mechanical explanation for how anyone could be suffering through the effects of a recession that wouldn't begin for another two years. But I think it is more likely that the explanation requires closed timelike curves. You see, in relativity it is impossible to move faster than the speed of light; physical particles are therefore confined to move along "timelike" trajectories that move inexorably forward in time. But in general relativity spacetime is curved; it is therefore conceivable that a timelike curve can loop back and intersect itself in the past, in a kind of time machine. Bush's questioner had obviously found such a closed timelike curve, lived through the horrors of the recent recession, gone backwards in time, and cowered in fear as he lived once more through 1998 with the ever-present knowledge that the economy would tumble just about the time Bush was elected. Of course, it is probably necessary to violate some of the laws of physics to actually create closed timelike curves in the real world; this is the content of Hawking's Chronology Protection Conjecture. But the administration has never let the laws of nature get in the way of their plans.
Or, I suppose, maybe the bus-tour audiences are not completely representative samples of the local population; one might even suspect that they are carefully selected to be sympathetic. But that would make me a wacky conspiracy theorist. Besides, there was one actual question, when Bush was asked about a cut in federal funding to local health services. His priceless response, as reported on NPR:
Well, that's what happens when you're trying to cut the deficit in half.Such an answer cannot be explained simply by closed timelike curves; we need to invoke parallel universes.
Monday, May 03, 2004
Life and the forces of nature
One of the most profound experiments in physics is one you may never have heard of. It's the torsion-balance experiment at the University of Washington (and others like it elsewhere in the world).
This group, led by Eric Adelberger, has recently garnered attention for testing Newton's inverse-square law of gravity down to a tenth of a millimeter. This experiment is interesting because there are good (or at least plausible) reasons to suspect that Newtonian gravity actually breaks down at around a millimeter. In particular, models with large extra dimensions of space can unify the scales of quantum gravity and particle physics if there are two extra dimensions about a millimeter in size. The Washington group has placed significant constraints on this fascinating idea.
But the profound experiment I'm referring to is the one about "testing the equivalence principle." The Equivalence Principle is Einstein's idea that you can't tell, if you are sitting in a sealed laboratory, whether your lab is on the surface of a gravitating body or accelerating through space at uniform acceleration. So if the EP is right, uncharged bodies should all fall at the same rate in a gravitational field, just as they would in an accelerating rocket.
So far, the UW experiments have not detected any violations of the EP. But if they did, you wouldn't conclude that Einstein was wrong; instead you would guess that the bodies you were using weren't really "uncharged." In other words, you would have discovered a fifth force. The best limits right now on such fifth forces are that they are less than one-trillionth the strength of gravity, if they exist at all; that's incredibly weak. The fact that there is no noticeable fifth force is one of the most profound facts of physics.
We know of four forces in nature: gravitation, electromagnetism, and the strong and weak nuclear forces. The latter two only operate over very short ranges (atomic scales and below), leaving only gravity and E&M as forces relevant to our daily lives. ("Electricity" and "magnetism" are two different manifestations of the same force.) This is the deep and astonishing fact: everything we directly see around us can be accounted for by some simple forms of matter (electrons, atomic nuclei) interacting through just those two forces. Fortunately for us, they can interact in extremely intricate ways.
Scientists like to talk about what they are currently doing research on, which by construction tends to be speculative ideas at the boundaries of our ignorance. What can easily get lost is an appreciation for how much we actually know beyond any reasonable doubt, and how little wriggle room there is. ESP, astrology, and other paranormal phenomena provide excellent examples of ideas that simply can't work. When scientists criticize these ideas, they often start talking about blind tests and repeatability and so forth. All well and good, but the fact is that these ideas have no chance of being right even before we test them directly. There is no way for the human brain to send out a signal that would read a mind or bend a spoon, nor is there any way for the planet Venus to influence your love life. Any such influence would have to be communicated by one of the forces of nature, and there are only two possibilities: gravitation and electromagnetism. In either case the size of the force would be easily detectable, and we haven't detected it.
It would be great to find a new long-range force, and there are certainly models that predict them. But even if one were found, it would be so tremendously weak that we need all of our best technology to notice its effects at all; there is no way for such a force to push around human beings (even delicate parts of their brains) in meaningful ways. This isn't to say that there's no room left for mysteries; figuring out how electrons and nuclei interact through two simple forces to create all of human culture and the rest of the visible world leaves more than enough unanswered questions for generations to come.
Sunday, May 02, 2004
Von & Fred
We saw a fantastic concert at the HotHouse on Friday featuring Von Freeman and Fred Anderson, two legendary Chicago tenor saxophone players. Both of them have spent their lives playing in Chicago, rather than moving to New York and doing a lot of recording; consequently, either could reasonably claim the title of "most underappreciated living saxophonist." The occasion was Fred's 75th birthday, making him the younger of the two, as Von is going on 82. And they can both play like nobody's business.
Von I know very well, and it's a surprise that I've gotten this far in the blog without ever mentioning him -- I'll have to rectify that at length sometime soon. But I had never heard Fred Anderson live, and it was quite an experience. Von is the consummate showman, telling stories and flirting with the crowd, and one of the most compelling features of his music is how he mixes beautifully accessible melody and harmony with exciting and challenging free-jazz explorations. Fred, in contrast, is all intensity and concentration. Listening to Von is like driving in a convertible through mountain roads where surprising vistas can suddenly appear around the corner, whereas listening to Fred is like taking the bullet train. Or maybe jumping out of an airplane.
Fred's band is just a trio -- him, bass, and drums. He stands up on stage, short and stooped to begin with, and gradually leans forward as he begins to play. Pretty soon his upper body is nearly horizontal, as he is surrounding the sax and blowing with fierce determination. The notes come quickly and relentlessly, as he spins out impossibly long lines without noticeable pause. I was talking with Michael Raynor, Von's drummer, who was scoping out the room to see how people were reacting. We agreed that anyone could enjoy Von's style, but you would appreciate it much more if you were really into the music; but to enjoy Fred you needed to be into it from the start. I am definitely into it, and I'll have to make sure to drop by Fred's club, the Velvet Lounge, more often. The only regret was that Von and Fred played separate sets, rather than jamming together; that I would have definitely loved to see.
Fred has a new CD just out; Von has one coming in July. It's a great pleasure to listen to these two men who give so much to their music, their fellow musicians, and their city.