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.