Science

Today’s Astronomy Picture of the Day:

It’s a false-color image of the Sun in ultraviolet (click for better resolution). Put together using data from TRACE, a NASA Small Explorer satellite. You’re seeing the bubbling gas of the solar corona looping along magnetic fields.

The Shaw Prize in astrophysics has been awarded to Saul Perlmutter, Adam Riess, and Brian Schmidt, for discovering the acceleration of the universe by measuring the Hubble diagram using Type Ia supernovae. The Shaw Prize is relatively new, having first been given in 2004, and is awarded in three areas: Astronomy, Mathematical Sciences, and Life Sciences and Medicine. It comes with a total of US$1 million, split between the three recipients. Competitive with, although not quite as much as, the Nobel prize…

Brian was the leader of the High-Z Supernova Search Team and Adam was lead author on their paper; Saul was the leader of the Supernova Cosmology Project and also lead author on their paper. (Get some more inside scoop from Rob Knop.) To most of us, their finding was a complete surprise, as we were all quite familiar with the fine-tuning problems associated with the cosmological constant (the most straightforward explanation for the acceleration). But by 1998, it had become impossible to deny that something fishy was going on — the universe was not the simple matter-dominated flat Einstein-de Sitter cosmology of the standard Cold Dark Matter model. In late 1997 I was asked to give a review talk at a CMB conference in Santa Barbara, on the topic of “every way to measure the cosmological parameters other than the CMB.” In assembling the talk the overall message came through loud and clear, from considerations of the age of the universe, direct measures of the mass density, and properties of large-scale structure. There were plenty of ideas floating in the air, including an open universe (the most obvious choice), warm dark matter, a mix of hot and cold dark matter, or some dramatic features in the primordial power spectrum, as well as the old standby cosmological constant. But only the last of these solved all of the problems with one fell swoop. So when the two supernova groups announced in 1998 that they had direct evidence that there really was a cosmological constant, in the form of an accelerating universe, the community was primed to believe them, which they did fairly quickly. Soon thereafter, of course, improved measurements of the CMB anisotropies indicated that the universe was spatially flat, in perfect accord with the combined supernova and matter-density measurements. If you were to plot the inferred density of both matter and cosmological constant, the constraints from the three different techniques — supernovae, matter dynamics (clusters or large-scale structure) and the CMB — the allowed regions overlapped in perfect harmony.

A preposterous universe, maybe, but I like it.

A web page presenting a scale-model hydrogen atom (via Cynical-C):

And you thought there was a lot of empty space in the solar system. Well, there’s even more nothing inside an atom. A hydrogen atom is only about a ten millionth of a millimeter in diameter, but the proton in the middle is a hundred thousand times smaller, and the electron whizzing around the outside is a thousand times smaller than THAT. The rest of the atom is empty. I tried to picture it, and I couldn’t. So I put together this page – and I still can’t picture it.

The page is scaled so that the smallest thing on it, the electron, is one pixel. That makes the proton, this big ball right next to us, a thousand pixels across, and the distance between them is… yep, fifty million pixels (not a hundred million, because we’re only showing the radius of the atom. ie: from the middle to the edge). If your monitor displays 72 pixels to the inch, then that works out to eleven miles – making this possibly the biggest page you’ve ever seen.

Okay, we all know that, science-wise, this is utterly bogus. Mostly because the proton and electron are not little spheres of fixed size, as our classical intuitions inevitably imagine them to be — they should be represented by wavefunctions, and the electron’s wavefunction in particular should be spread throughout all eleven miles. Admittedly, an eleven-mile web page that accurately represented the ground-state wavefunction of the hydrogen atom would have been harder to construct (although, hmm, maybe not impossible). And I’m not sure where the “sizes” of the particles came from. The proton really does have a size, about 1.5 x 10^-15 m, since it’s a bound state of quarks. But the electron is a point particle, as far as we know. There are various distance scales you can associate to it, but the smallest of these is the classical electron radius, which is about twice the size of the proton diameter. John Baez explains. I don’t know how to get an electron to be one thousandth the size of a proton, unless you’re using masses rather than lengths, which is a big mistake because (in the wacky world of quantum mechanics) lengths get smaller when masses get bigger.

Still, it gives you some feeling for the instubstantiality of matter, as Geiger and Marsden long ago demonstrated. And it’s pretty cool.

In October 1984, it was announced that the Nobel Prize for Physics had been awarded to Carlo Rubbia and Simon van der Meer, for the discovery of the W and Z bosons at the UA1 experiment at CERN just the previous year. This was the capstone discovery in the establishment of the Standard Model of particle physics. The third generation of fermions had already been discovered (the tau lepton by Martin Perl in 1977, the bottom quark by Leon Lederman also in 1977), and the nature of the strong interactions had been elucidated by deep-inelastic scattering experiments at SLAC in the late 1960’s and early 1970’s. Unsuspected by many, particle physics was about to enter an extended period in which no truly surprising experimental results would emerge; subsequent particle experiments have only been able to confirm the Standard Model over and over again, including the eventual discovery of the top quark at Fermilab in 1995. (Astrophysics, of course, has provided substantial evidence for physics beyond the Standard Model, from neutrino oscillations to dark matter and dark energy.)

A month earlier, in September 1984, Michael Green and John Schwarz submitted a paper on anomaly cancellation in superstring theories. String theory had been around for a while, and it had been understood for ten years that it predicted gravity, and was a candidate “theory of everything.” But there were many such candidates, each of which had run into significant difficulties when taken seriously as a theory of quantum gravity. Most people who were paying attention had presumed that string theory would face the same fate, but the Green-Schwarz result convinced them otherwise. A brief article in Physics Today was entitled “Anomaly Cancellation Launches Superstring Bandwagon,” and theorists everywhere jumped to learn everything they could about the exciting new possibilities the theory offered.

So here we are, over twenty years later, still with no surprising new results from particle accelerators (although hopefully that will change soon), and still with strings dominating the landscape (if you will) of theoretical high-energy physics. And still, one hardly needs to mention, with no clear path to connecting string theory to low-energy phenomenology, nor indeed any likely experimental tests of any sort.

In the circumstances, it’s not surprising there would be something of a backlash against string theory. The latest manifestation of anti-stringy sentiment is in two new books aimed at popular audiences: Peter Woit‘s Not Even Wrong: The Failure of String Theory and the Continuing Challenge to Unify the Laws of Physics, and Lee Smolin’s The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next. I haven’t read either book, so I won’t presume to review them, but I think we’ve heard the core arguments expressed on this blog and elsewhere. I’m a firm believer that it’s good to have such books out there; I’m happy to let the public in on our internecine squabbles, just as I’m happy to keep them updated on tentative experimental results and speculative theoretical ideas. It seems unduly patronizing to think that we can’t reveal anything to the wider world until everyone in the community agrees on it.

But I don’t actually agree with what the books are saying. Here is the main point I want to make with this post, trite though it may be: the reason why string theory is so popular in physics departments is because, in the considered judgment of a large number of smart people, it is the most promising route to quantizing gravity and moving physics beyond the Standard Model. I don’t necessarily want to rehash the reasons why people think string theory is promising — I’m not positing an objective measurement of the relative merits, but simply an empirical observation about people’s best judgments. Rather, I just want to emphasize that, when you get right down to it, people like string theory for intellectual reasons, not socio-psycho-political ones. It’s not a Vast String Theory Conspiracy, funded by shadowy billionaires who funnel money through Princeton and Santa Barbara to brainwash naive onlookers into believing the hype. It’s trained experts who think that this is the best way to go, based on the results they have seen thus far. And — here’s the punchline — such judgments could change, if new results (experimental or theoretical) came along to suggest that there were some better idea. The way to garner support for alternative approaches is not to complain about the dominance of string theory; it’s to make the substantive case that some specific alternative is more promising. (Which people are certainly trying to do, in addition to the socio-psycho-political commentating about which I am kvetching.)

That is, after all, the way string theory itself became popular. Green and Schwarz labored for years on a relatively lonely quest to understand the theory, before they were able to demonstrate anomaly cancellation. This one result got people psyched about the theory, and off it went. It’s not a matter of impressionable young physicists docilely obeying the dictates of their elders. Read Jacques Distler’s (absolutely typical) story about how he dived into string theory as a graduate student, despite the fact that his advisor Sidney Coleman wasn’t working on it. In a completely different field, listen to Nobel-winning economist Gary Becker on the response to his ideas (via Marginal Revolution):

“There was a sea change. I began to notice it in the 1970s and 1980s. A lot of the younger people coming out of Harvard, MIT and Stanford were very interested in what I was doing, even though their faculty were mainly – not entirely – opposed to the sort of stuff I was doing.”

This is just how academics act. They are stubborn and willful (even at a charmingly young age!), and ultimately more persuaded by ideas than by hectoring from their elders. And it’s not just the charmingly young — if good ideas come along, supported by exciting results, plenty of entrenched middle-aged fogeys like myself will be happy to join the party. If you build it, they will come.

There’s no question that academic fields are heavily influenced by fads and bandwagons, and physics is no exception. But there are also built-in mechanisms that work to protect a certain amount of diversity of ideas — tenure, of course, but also the basic decentralized nature of university hiring, in which different departments will be interested in varying degrees in hiring people in certain fields. Since the nature of science is that we don’t yet know the right answers to the questions we are currently asking, different people will have incompatible intuitions about what avenues are the most promising to pursue. Some people are impressed by finite scattering amplitudes, others like covariant-looking formulations, others don’t want to stray too far from the data. The thing is, these considered judgments are the best guide we have, even if they are not always right. Green and Schwarz were lonely, but they persevered. If you want to duplicate their success, find a surprising new result! You can’t ask a department to hire people in an area they don’t think is promising, just because it serves the greater goal of diversifying the field overall. Crypto-socialist pinko though I may be in the political arena, when it comes to intellectual life I’m a firm believer in the free market of ideas, and would tend to resist affirmative-action programs for underrepresented theories.

…

The String Theory BacklashRead More »

Just left a great little workshop at the Perimeter Institute in Canada, organized by Justin Khoury. It was a focused but relaxed meeting, with plenty of opportunity for interaction; every speaker had at an hour and a half or two hours to speak, and discussion during the talks was actively encouraged. Not hard to figure out what people are interested in from looking over the talks:

• Robert Brandenberger: String Gas Cosmology. Proposed an alternative mechanism to inflation for generating cosmological perturbations.
• Me: Spontaneous Inflation and the Arrow of Time. (See a report on my talk by Yidun Wan, a graduate student who blogs at Road to Unification.) Proposed a way to make inflation respectable in the context of a multiverse, but tried not to mention the anthropic principle or the landscape.
• Rocky Kolb: Acceleration from Cosmological Perturbations. Proposed an alternative to dark energy in the form of back-reaction from cosmological perturbations. Made fun of the anthropic principle.
• Frank Wilczek: Particle Physics and Dark Matter. Used the anthropic principle (but not the string-theory landscape) to make predictions about the density of dark-matter axions.
• Burt Ovrut: The Heterotic Standard Model. Proposed a very specific compactification of string theory that gives the Minimal Supersymmetric Standard Model (and nothing else) at low energies, with stabilized moduli and a (fine-tuned) positive cosmological contant. Made fun of the landscape.
• Neil Turok: Perturbations in the Cyclic-Universe Scenario. Proposed an alternative mechanism to inflation for generating cosmological perturbations. Made fun of the landscape.
• Paul Steinhardt: Ways to Calculate the Cosmological Constant, and Ways Not To. Proposed a dynamical mechanism for obtaining a small cosmological constant. Made fun of the landscape.

The themes should be clear: cosmological perturbations, inflation and alternatives thereto, dark energy, the anthropic principle and the landscape.

It’s remarkable how polarizing the whole idea of the string-theory landscape and the anthropic principle really is. It’s not a simple split of string theorists vs. cosmologists vs. everyone else; there are string theorists who love the lanscape, as well as ones who hate it, and likewise for cosmologists or anyone else paying attention. I’ve been arguing that the landscape/multiverse might very well exist and is interesting to think about, but that it’s absolutely impossible right now (and might always be) to use it to calculate anything, or even to sensibly re-calibrate our notions of what is “natural.” I was happy to learn that Paul Steinhardt and Neil Turok are basically in agreement with this view, and are even writing a paper that attempts to make it crystal clear that the landscape does not correctly predict the cosmological constant ala Weinberg. In fact, if we’re allowed to take it seriously at all, it makes quite a strong and vividly different prediction altogether: the cosmological constant should be quite large (many times the matter density, although presumably not at the Planck scale), and we should live in a single lonely galaxy in an empty universe dominated by vacuum energy. Their paper is in preparation, and I hope to say more about it when it comes out. In the meantime, there is serious and hard work to be done to understand the generation and evolution of cosmological perturbations, so it hasn’t all devolved into a shouting match over whether talking about unobservable parts of the universe should count as science.

I’ve been meaning to post about the claim that experimenters have demonstrated that the proton/electron mass ratio is changing with time. Although it’s a fascinating discovery if true, there’s something that doesn’t quite smell right about it. So I hit on the idea of first posting about the idea of physics claims not “smelling” right more generally. But then I thought that such a post would necessarily involve a careful exposition of one particular example. So it’s time for the story of the Screwy Universe.

In April 1997, while a postdoc at the Institute for Theoretical Physics at UC Santa Barbara, I received an email from George Field, who had been my Ph.D. advisor. He was suggesting that I take a look at a news article that had appeared on the front page of the New York Times. George is one of my favorite people in the whole world, and I owe whatever success I may have had as a scientist to his insightful guidance in my early career. But okay, I was busy, and didn’t immediately look at the article — lots of crazy stuff appears in the NYT, after all.

But George wrote again, gently suggesting that I really should take a look at this article, which I finally did. And it was indeed striking. Two scientists, Borge Nodland of the University of Rochester and John Ralston of the University of Kansas, were claiming that they had detected a violation of a fundamental principle of modern cosmology — isotropy, the idea that space looks the same in every direction. In particular, they had considered the polarization of radio waves coming from distant quasars, and looked for a rotation of the polarization angle as the waves traveled through space. And they had found evidence of just such a rotation! If N&R were right, there was a preferred direction in the cosmos — along that direction, polarized radio waves would gently corkscrew as they traveled through space, while in the opposite direction they would twist the other way. Completely contrary, of course, to our conventional expectations, which are that (1) polarized waves maintain their polarization angles in empty space, rather than rotating, and (2) every direction in the sky is basically equivalent to every other direction.

Clearly important stuff. But for George and me this hit particularly close to home, as we had previously collaborated with particle theorist Roman Jackiw on a very similar-sounding project, looking for gentle rotations in the polarization of distant sources (and not finding any). In fact, this work with George and Roman was the topic of my first published paper. Our motivation was to test Lorentz invariance by searching for the effects of a constant vector field spread throughout spacetime. It turns out that such a vector can couple to ordinary electromagnetism, but only in certain specified ways. We showed that, if the vector pointed mostly in the time direction of spacetime, its effect would be to uniformly rotate the observed polarization of distant radio sources; we then searched for such an effect in the existing data, and didn’t find any. My job as the beginning graduate student was to look in the literature for measurements of the polarization angles and redshifts of as many galaxies as I could find. I managed to scrape up 160 such galaxies, which was enough to put a good limit on the effect we were looking for. (I should say that, as a nervous beginning graduate student, George was extremely intimidating because of his formidable intellect and amazing accomplishments, but in other circumstances one would recognize that he was extremely gentle and easygoing. Roman, on the other hand, was intimidating, period. But also fantastically smart, and an excellent collaborator once one calmed down and got into the science.)

At the time, anxious young ingenue that I was, I was somewhat worried that writing my first paper on a topic as outlandish as Lorentz violation might spell the premature end of my career. Nowadays, of course, it is all the rage, and we are proud pioneers.

So the news of Nodand and Ralston’s work had a personal resonance — it sounded like they were investigating something similar. And then I noticed in the NYT story — 160 radio galaxies! These guys were using the very data I had typed in as a first-year grad student. (Although, as it later turned out, they were careful enough to check everything, and had found a few typos.) In fact they had basically done exactly the same thing that we had done, except that they had considered a Lorentz-violating vector field that was pointing in a spatial direction instead of in the time direction. As a result, they were asking whether there was a direction-dependent rotation of polarizations — clockwise if you looked at one side of the sky, counter-clockwise if you looked at the other — rather than a uniform one across the sky. And, remarkably, they seemed to be saying that there was such a rotation!

But I didn’t believe it, not for a second. True, we hadn’t carefully placed a limit on such an effect, but I was convinced that I would have noticed it in the course of playing around with the data. Not to mention, there was no good theoretical reason to suspect that such an effect might exist. In short, it didn’t smell right.

As it turns out, Nodland and Ralston had simply made a mistake. …

The Screwy UniverseRead More »

Well, not all of it. Some of the Moon’s pale fire is actually snatched from cosmic rays, as seen in the Astronomy Picture of the Day from last Friday.

This is an image of the Moon in gamma rays, taken by NASA’s EGRET telescope. The gamma rays are produced by cosmic rays (which aren’t electromagnetic radiation at all, but mostly high-energy protons) striking the lunar surface. There is no equivalent process for the Sun, and in fact the Moon is much brighter than the Sun in gamma rays.

The Sun has some tricks of its own, of course. The Moon picture reminded me a bit of this one:

They’re both circular false-color blobs, so I suppose the resemblance isn’t so surprising. But this is an image of the Sun in neutrinos, reconstructed using data from the Super-Kamiokande neutrino detector in Japan. (Yes, the one that was essentially destroyed in a freak accident. But it’s now back online, and meanwhile I’m sure Koshiba’s Nobel Prize was some consolation.) The Sun, of course, makes its own neutrinos, but it’s amazing that we can actually image a celestial object using something other than photons!

Besides photons, cosmic rays, and neutrinos, there aren’t that many ways we get to observe the universe. I’m looking forward to the first images of either the Sun or Moon in gravitational waves.

Update: As Alex R. mentions in the comments, Ray Davis passed away on Wednesday. He was the pioneer in solar-neutrino measurments, overseeing the Homestake mine experiment, and shared the Nobel with Koshiba.

I was asked the other day whether Alan Guth should expect to win the Nobel Prize for inflation, now that WMAP has found tentative evidence for a slight “tilt” in the primordial perturbations, just as we might expect from inflation. At the moment I’m leaning toward “not yet,” but it started me thinking about which cosmology discoveries have yet to be honored by Nobels but should be at some point. (After the 2004 prize for asymptotic freedom, there aren’t really any completely obvious particle-physics prizes lurking out there, although prizes for color, spontaneous symmetry breaking, and CP violation would be quite warranted.)

There are two discoveries that are obviously Nobel-worthy: the temperature anisotropies in the cosmic microwave background, and the acceleration of the universe. One that is a bit less obvious, but still extremely strong, is dark matter. In each case, however, it is not precisely clear which people should actually get the prize, given the constraints: (1) laureates should be individuals, not collaborations, (2) prizes are giving to living people, not posthumously, and (3) at most three people can share one prize.

The 1992 observation of CMB anisotropies by NASA’s COBE satellite was the first step in a revolution in how cosmology is done, one that has come to dominate a lot of current research. Subsequent measurements by other experiments have obviously led to great improvements in precision, and most importantly extended our understanding of the anisotropies to smaller length scales, but I think the initial finding deserves the Nobel. So to whom should the prize be awarded? On purely scientific grounds, it seems to me that there was an obvious three-way prize that should have been given a while ago, to David Wilkinson, John Mather, and George Smoot. Wilkinson was the grandfather of the project, and was the leading CMB experimentalist for decades. Mather was the Project Manager for the satellite itself (as well as the Principal Investigator for the FIRAS instrument that measured the blackbody spectrum), while Smoot was the PI for the DMR instrument that actually measured the anisotropies. Unfortunately, Wilkinson passed away in 2002. Another complicating factor is that there were various intra-collaboration squabbles, leading to books by both Smoot and Mather that weren’t always completely complimentary toward each other. Still, background noise like that shouldn’t get in the way of great science, and these guys definitely deserve the Nobel.

The first direct evidence for the accelerating universe came from two groups: the Supernova Cosmology Project and the High-Z Supernova Team. The issues of priority are a bit complicated, but both groups certainly deserve substantial credit in discovering this surprising and enormously influential result. The SCP is an easier case: they started first, and were clearly led by Saul Perlmutter, who is a shoo-in for the Nobel. The High-Z team was a bit more democratic, and started second but actually went on record first with the claim that the universe was accelerating. Their PI was Brian Schmidt (full disclosure: my old grad-school officemate); the first author on the discovery paper was Adam Riess; and their spiritual leader was Robert Kirshner (most of the team members were either students or postdocs of Bob’s at one point or another). Hard to construct a sensible prize from that mess, but if I were in charge of the universe I might give 50% of the prize to Perlmutter and 25% each to Schmidt and Riess, and feel really bad about not including Kirshner. But the discovery is clearly worthy of a Nobel, and I likely won’t complain with whatever way they choose to divvy up the award.

Then we get into murkier waters, I think. The idea of dark matter is one of the most influential and important in modern cosmology, and a Nobel would be perfectly appropriate. You might complain that we haven’t actually discovered dark matter yet, which is certainly true and relevant; but one way or another, something is going on with the dynamics of galaxies and clusters that is above and beyond what our current theories predict, and that empirical fact is hugely important. It was first pointed out by the late Fritz Zwicky in the 1930’s, comparing the velocities of galaxies in the Coma cluster to their total mass. But the field matured immensely with Vera Rubin‘s measurements of the rotation curves of spiral galaxies, giving direct evidence that the gravitational force fell off more slowly than the distribution of visible matter could account for. Rubin absolutely deserves the prize, in my opinion. Then there is the more specific cold dark matter idea, which is a specific model for the nature of dark matter and its role in galaxy formation; credit for that is more diffuse (although the paper by Blumenthal, Faber, Primack and Rees was obviously influential), and we’re less sure that the basic idea is right, so I don’t see any need for a prize there quite yet. I think it would be great to give a joint prize to Rubin and someone else, perhaps Wendy Freedman for measuring the Hubble constant, or Jim Peebles for developing physical cosmology.

Then we get to inflation, which is a sticky issue in various ways. There is absolutely no question that inflation has been one of the most, arguably the most, influential idea in cosmology in the last several decades. There is a great deal of discussion about who gets credit for it, since a number of papers discussed very similar-sounding ideas; but it was clearly Alan Guth‘s 1981 paper that put the story together in the right way. However, Guth’s model (“old inflation”) didn’t quite work, and the two follow-up papers by Andrei Linde and by Andreas Albrecht and Paul Steinhardt (“new inflation”) actually showed that the idea was plausible. That’s a total of four people, you’ll notice. Because Andy Albrecht was a graduate student at the time of his inflation paper with Steinhardt, and because both Linde and Steinhardt have gone on to write many more influential papers about inflation, credit is sometimes informally given to “Guth, Linde, Steinhardt and others…”, which is a little unfair.

But more importantly, we don’t know whether inflation is right. There is no question that it has made a number of predictions that have been dramatically verified: the universe is spatially flat, there is a spectrum of adiabatic and Gaussian primordial density perturbations, and that spectrum is nearly scale-free although not necessarily precisely so. And these predictions were by no means guaranteed in advance; models in which the perturbations were generated by cosmic strings, for example, were quite viable in the 1980’s, but have now been ruled out by CMB anisotropy observations.

Still, the idea that some non-inflationary mechanism set the initial conditions for the Big Bang still seems plausible to me, even if I don’t know what that mechanism would be. The predictions from inflation have been sharp, but they have not been the kinds of things that we couldn’t imagine getting from any other model. If we were to find evidence for gravitational-wave perturbations in the polarization of the CMB, of the type inflation could easily explain, then I might be convinced; but it’s quite possible that the gravity wave are really there but at a level too tiny to ever be observed.

So I’m somewhat torn. Inflation is a compelling and ingenious and influential idea, and it should be recognized. But the Nobel committee doesn’t like to give out prizes unless they’re completely sure that the discovery/theory to which they’re being given has no chance of being wrong. I’m not sure how to elevate inflation from the status of “probably on the right track” to the status of “correct beyond a reasonable doubt.” In the meantime, if the Nobel committee decides to take a risk and give Alan Guth the prize, you won’t hear any complaints from me.