Science

Via Chad Orzel, I see that the latest constraints on short-distance modifications of Newton’s inverse-square law from the Eot-Wash group at the University of Washington have now appeared in PRL. And the answer is: extra dimensions must be smaller than 0.045 millimeters (in any not-too-contrived model).

We used to think that extra dimensions must be enormously smaller than that, if they exist at all. If you have n extra compact dimensions of space, long-ranges forces like gravity and electromagnetism would go from falling off as an inverse-square law, 1/r ², to something like 1/r ²⁺ⁿ. Gravity is weak and hard to test, but electromagnetism is easy to test, and it behaves quite conventionally down to scales probed by particle accelerators.

In 1998, Arkani-Hamed, Dimopoulos and Dvali realized we could hide extra dimensions that were much larger than that, by positing a three-dimensional brane on which all of the particles of the Standard Model were confined. Then it’s easy to see why electromagnetism wouldn’t notice the extra dimensions: photons couldn’t get there! But gravity can always get there. So it became a big new project to test Newton’s law of gravity at short distances. As a separate motivation for the large-extra-dimensions idea, you could explain why gravity is so weak by imagining that it’s really not so weak at a fundamental level, but gets diluted by the extra dimensions. It all works out perfectly nicely if you have two extra dimensions of about a millimeter in size, which was happily right where the experiments hadn’t quite probed. By now, as you can see, they have been pushed there and beyond.

Which by no means implies that the experiments aren’t worth doing any more — you never know what suprises you might find in regimes where you’ve never looked. The title of the new paper tries to score some motivational points by referring to the “Dark Energy Length Scale.” This notion is a bit less concrete than the size of an extra dimension, but okay. What cosmologists have measured in the case of dark energy is an energy density, about 10^-8 ergs per cubic centimeter. But if we multiply by appropriate powers of Planck’s constant and the speed of light, we can convert this density into a length (to the -4th power), and that length turns out to be about 0.08 millimeters. Now, this little bit of dimensional analysis may or may not be connected to anything physical; they reference papers by Beane and by Dvali, Gabadadze, Kolanovic, and Nitti, speculating that this length scale actually corresponds to something important. These ideas are not completely baked, but they’re fascinating, and the important point is that we have a length scale at which stuff happens, and we don’t completely understand what’s going on, so let’s do all the experiments we can to try to dig up some clues.

The other important point about this work is that it puts to rest the vicious rumors we were hearing over a year ago, about which Eric Adelberger (leader of the Eot-Wash group) was nice enough to comment here. Namely, the rumor that they had actually found a weak repulsive force in their data. This is the kind of thing that happens all the time when you’re doing ultra-precise measurements at the very edge of what is possible; unforeseen effects creep in, and it takes time to stamp out everything that shouldn’t be there. These guys are careful, and would never jump up and down about a real effect unless they were truly convinced it was there. If I had been in charge (putting aside for the moment the fact that, if the experiment relied on my technical expertise, the lower limit on the size of extra dimensions would probably be measured in kilometers), I would probably have floated that rumor intentionally, just so people paid attention when the results did come out. Unlike me, Eric Adelberger has enormous integrity, so they just told the honest truth all along.

Chad keeps saying that these experiments don’t get enough credit, but I don’t know why he thinks that. (Chad, why do you think that?) Ever since the idea of large extra dimensions was floated in 1998, everyone working in string theory, particle physics, and cosmology has been very excited by the search for short-range forces, and most everyone knows that the Eot-Wash group is kicking butt within the field. Their 2000 paper, which pushed the limit on extra dimensions below a millimeter for the first time, has hundreds of citations, and Adelberger gets far more invitations to give colloquia and conference talks than he can possibly accept. Some influential theorists have even described the torsion-balance work as one of the most profound experiments in physics. This is not exactly a small, under-the-radar operation. We’re all looking forward to what they do next.

The internet works so that we don’t have to! This week is the big annual meeting of the American Astronomical Society in Seattle, so expect to see a series of astro-news stories pop up all through the week. The first one concerns a new result from the Cosmological Evolution Survey (COSMOS) — they’ve used weak lensing to reconstruct a three-dimensional image of where the dark matter is. Here is an image from the Nature paper by Richard Massey et al. (subscription required).

It is, needless to say, really cool. The image itself is not where the real science lies, of course; it’s spatially distorted, and very hard to show error bars in a 3-d plot. But there is definitely important science lurking in the details; for example, they seem to find dark-matter concentrations with little or no ordinary matter in the same place. It’ll take some work to figure out whether this is easily compatible with the theoretical models (one could imagine dissipative effects clearing baryons out of a region, leaving dark matter behind, in a mini-version of the Bullet Cluster), or whether we’re going to be challenged. Fun either way!

Fortunately, I don’t have to go into details about the result, as others already have. Phil, Clifford, Rob, Angela, and Steinn have all blogged about the finding. (We’re all on a first-name basis around here.) Steinn’s post is, admittedly, pretty consise, but he wins points for breaking an even better story — Google is joining the Large Synoptic Survey Telescope consortium! Rob is even live-blogging the entire meeting, which is an heroic undertaking. (Yes, it’s true that he did bump into me up in Seattle, but I’m not there for the meeting! In fact I’m already back in LA. There are reasons to visit Seattle other than the AAS.)

Ah, I remember the good old days of ’04, when there wasn’t any competition out there in the cosmo-blogging world. Our internet is all grown up now. Sadly, Michael Bérubé is retiring from the game, which will leave the blogosphere a much poorer place. Read Sunday’s Credo for an example. Without his inspiration, I certainly wouldn’t be doing this myself.

Anyway — the COSMOS project is well worth being wowed by in its own right. It’s an ambitious undertaking; they take a two-square-degree field of the sky and beat on it with every telescope they can find — in optical, infrared, ultraviolet, X-rays, and radio waves. More than half a dozen ground-based telescopes, as well as five satellites (the Hubble Space Telescope, Spitzer infrared observatory, XMM and Chandra for X-rays, and GALEX for the ultraviolet), are joined in the effort. Here’s the abstract from one of their recent summary papers:

The Cosmic Evolution Survey (COSMOS) is designed to probe the correlated evolution of galaxies, star formation, active galactic nuclei (AGN) and dark matter (DM) with large-scale structure (LSS) over the redshift range z < 0.5 to 6. The survey includes multi-wavelength imaging and spectroscopy from X-ray to radio wavelengths covering a 2 square deg area, including HST imaging. Given the very high sensitivity and resolution of these datasets, COSMOS also provides unprecedented samples of objects at high redshift with greatly reduced cosmic variance, compared to earlier surveys. Here we provide a brief overview of the survey strategy, the characteristics of the major COSMOS datasets, and summarize the science goals.

This new dark matter map is just the beginning of fun stuff to emerge from this collaboration — stay tuned!

Update: There I go again.

“I like to think of visible matter as the olive in the martini of dark matter,” said Sean Carroll, a theoretical physicist at Caltech.

I love my job.

Wouldn’t you be just a tiny bit worried if you saw this trundling down your street?

Bee at Backreaction explains why it’s not so scary, unless you’re an elementary particle who is self-conscious about your weight. There’s a movie, too.

Scott Aaronson, well-known around these parts for thinking that a priori constraints on conversations with super-intelligent aliens are more important insights into the fundamental workings of the universe than dark energy and the holographic principle, is suffering from a bit of Stockholm syndrome. He has visited the Stanford high-energy theory group (intellectual hotbed of aggressive Landscapism), given an interesting talk on Computational Complexity and the Anthropic Principle, and discovered to his bemusement that string theorists are quite open-minded and reasonable people! When faced with an interesting new idea, they are even willing to consider it! And their objections to Loop Quantum Gravity seem to be based on physics, rather than just prejudice! Who would have thought? (Also linked from Not Even Wrong.)

So now, unable to choose sides in the Wars based on the likeability of the combatants, he’s offering his services to the highest bidder. Whoever offers him the best reimbursements, he’ll gladly shill for their viewpoint, at least temporarily. Why didn’t I think of this? Well, Scott, I can’t offer any hard cash, but I can promise that you’ll be treated even better when you visit Caltech than when you visited Stanford. (Even if you do think you are the second-funniest physics blogger.) We’re much more fun than those Northern Californians.

My real reason for blogging about this, however, is to get on the record that the phrase “The String Wars” is totally mine. I used it in an email, and George Johnson picked it up for his KITP discussions. It’s much more fun than the milqetoasty “String Debates” occasionally favored by those who prefer substantive engagement to showy fireworks. So anyone who makes any money off of this phrase, I want half.

Yeah, I already used this title once before. It’s a good title, okay? Cut me a little holiday slack here.

By way of slightly-warmed-over blogging, I present to you the slides from a talk I gave a few weeks ago at Villanova, my undergrad alma mater. The original mandate was to talk about scientific literacy to a collection of undergrads, but I didn’t know how to make that fascinating. So I took it to the next level and went a bit meta, talking about the way science works. It was at a fairly abstract level — I didn’t go into building detectors, and error bars, or anything like that — but not too highbrow philosophy-of-sciencey — I didn’t get into Kuhn vs. Popper, much less Feyerabend or the Strong Programme, although you’ll find touches here and there.

To bring things down to earth (relatively speaking), most of the talk consisted of an extended look at the battle between “dark matter” and “modified gravity.” It goes all the way back to Leverrier and the discovery of Neptune, whose existence was inferred via its gravitational tug on the orbit of Uranus. Neptune was the first successful prediction of dark matter — some unseen substance whose existence is revealed by its gravitational influence. Leverrier tried again with the similarly-discrepant orbit of Mercury, positing a planet called Vulcan; but this time it turned out that gravity itself was the culprit, after Einstein showed that general relativity correctly accounted for the precession of Mercury’s orbit. So the lesson from history is — different ideas work in different circumstances. Keep an open mind until the data come down on one side or another. (And once they do, admit it.)

Today, of course, we’re dealing with an analogous problem, given that 25% of the universe is apparently some kind of dark matter that doesn’t fit into the Standard Model of particle physics, and 70% is some kind of dark energy that is even more mysterious. Modified gravity might be at work here as well, and I talked about the prospects.

Along the way, I drew out some of the lessons about how science works that these various investigations have taught us. I intentionally did not try to wrap it all up with a neat bow into a catch-all philosophy of science, as I think the reality is kind of messy, and it’s worth admitting that. The closest I came was the famous quote from Professor Rumsfeld, previously shared. This led to a series of cautionary homilies warning against misuse of the hypothesis-testing nature of scientific inquiry. The truth is, scientific knowledge is inevitably tentative, not metaphysically certain. But that doesn’t mean that anything goes — some things we really do understand! So I cautioned against various mistakes, using perpetual-motion machines, Intelligent Design, and What the Bleep Do We Know as good examples of what not to do.

Today we give thanks for the Lagrangian of the Standard Model of Particle Physics, after electroweak symmetry breaking, with no explicit Higgs boson.

It’s served us well for three decades, withstanding every challenge that particle accelerators could think to throw at it. But it’s going to be blown out of the water in a couple of years. Still, a pretty good run. Thanks, Standard Model Lagrangian!

New Scientist has asked over 70 of the world’s most brilliant and charismatic and modest scientists to forecast what might be the big breakthroughs in their fields over the next 50 years. Some of the many examples that might be of interest to CV readers:

• Alex Vilenkin thinks we might find cosmic strings.
• Gerard ‘t Hooft imagines a deterministic theory that would supercede quantum mechanics.
• Lisa Randall hopes that the LHC will tell us something about the fundamental nature of spacetime.
• Edward Witten thinks that string theory will be fertile, and is excited about extra-solar planets.
• Steven Weinberg would like to see a theory of everything.
• Max Tegmark will be printing T-shirts emblazoned with the aforementioned TOE.
• David Deutsch looks forward to working quantum computers.
• Rocky Kolb and Kip Thorne both predict that we’ll find gravitational waves from inflation.
• Martin Rees wants to know if there was one Big Bang, or many.
• Richard Gott imagines a colony on Mars.
• Lawrence Krauss prevaricates about dark energy.
• Frank Wilczek actually steps up to the plate, predicting superintelligent computers and abundant solar power.
• Steven Pinker thinks it’s all just a trick to make him look foolish.

Hey, wait a minute — even I’m in there! Who knew? Here’s my prognostication:

The most significant breakthrough in cosmology in the next 50 years will be that we finally understand the big bang.

In recent years, the big bang model – the idea that our universe has expanded and cooled over billions of years from an initially hot, dense state – has been confirmed and elaborated in spectacular detail. But the big bang itself, the moment of purportedly infinite temperature and density at the very beginning, remains a mystery. On the basis of observational data, we can say with confidence what the universe was doing 1 second later, but our best theories all break down at the actual moment of the bang.

There is good reason to hope that this will change. The inflationary universe scenario takes us back to a tiny fraction of a second after the bang. To go back further we need to understand quantum gravity, and ideas from string theory are giving us hope that this goal is obtainable. New ways of collecting data about dark matter, dark energy and primordial perturbations allow us to test models of the earliest times. The decades to come might very well be when the human race finally figures out where it all came from.

[Here you can imagine some suitably aw-shucks paragraph in which I appear to be vaguely embarassed at all this talk of “brilliance,” which might be appropriate in describing Weinberg and Witten and ‘t Hooft but certainly doesn’t apply to little old me, who would never have made the cut if it weren’t for my blogging hobby, although I’m not quite sure how Max got in there either, and hey, if anyone wants to protest that I certainly do belong, that’s what comment sections are for. Don’t have time to construct it just now, but you know how it would go.]

Anyone else want to predict what the biggest breakthrough in the next 50 years will be?