Science

For some reason Nature News was inspired to tweet about my old blog post on Seriously, The Laws of Physics Underlying Everyday Life Are Completely Understood. Which I mentioned on Facebook, which led to an interesting comment, which I then mentioned on Google+… but now it’s substantive enough that I feel like I am neglecting our loyal blog readers! So here is a copy of my G+ comment, and a lament that I suck at proper use of the internet.

Not sure what brought this back to life. Like the Lord of the Rings, this is part of a trilogy; don’t miss the first installment, or the exciting conclusion.

As Michael Salem points out on an alternative social-media site (rhymes with “lacebook”), some of the resistance to this really quite unobjectionable claim comes from a lack of familiarity with the idea of a “range of validity” for a theory. We tend to think of scientific theories as “right” or “wrong,” which is hardly surprising. But not correct! Theories can be “right” within a certain regime, and useless outside that regime. Newtonian gravity is perfectly good if you want to fly a rocket to the Moon. But you need to toss it out and use general relativity (which has a wider range of validity) if you want to talk about black holes. And you have to toss out GR and use quantum gravity if you want to talk about the birth of the universe.

Just because there is something we don’t understand about some phenomenon (superconductivity, cancer, consciousness) does not imply that everything we think we know might be wrong. Sometimes we can say with confidence that certain things are known, even when other things are not.

Not only do theories have ranges of validity, but in some cases (as with the Standard Model of particle physics) we know what the range is. Or at least, we know where we have tested the theory and where we can be confident it is valid. The Standard Model is valid for all the particles and interactions that constitute our everyday existence.

Today we think of ourselves and the stuff we see around us as made of electrons, protons, and neutrons, interacting through gravity, electromagnetism, and the nuclear forces. A thousand years from now, we will still think precisely that. Unless we destroy the planet, or are uploaded into computers and decide that the laws of physics outside the Matrix aren’t that interesting any more.

Alliterative title stolen shamelessly from the lovely and understanding Jennifer Ouellette, who blogs background about the hunt for new particles at Discovery News.

So here we have science, marching on. Just last week we heard that CDF, one of the big experiments at the Tevatron at Fermilab, had collected more data relevant to a mysterious bump they had previously reported around 150 GeV in collisions that produced a W boson and two jets. The new data (7.3 inverse femtobarns, up from 4.3 fb^-1 previously) made the bump look even more prominent, rather than watching it regress back down to the mean. The discrepancy is now more than 4 sigma, giving license to get just a wee bit excited that new physics might be on the loose.

Now D0, the other big experiment at the Tevatron, is ready to weigh in — and the “D” stands for “damper,” it appears. Here’s a blog post at symmetry, a link to the technical paper, and a webcast for a talk that will happen this afternoon at 4:00pm Central Time. You knew that Jester would be on the case, and he is.

But this picture tells you all you need to know.

With 4.3 fb^-1 of data analyzed, the CDF bump should be just barely visible, as indicated by the dotted line labeled “Gaussian.” But there doesn’t seem to be anything there. And it’s not just you; the collaboration estimates that the probability that there is really a bump there is less than 10^-5. Not very encouraging, really.

But still — it does seem to be there in the CDF data. So what’s going on? At this point, it’s not clear. Both experiments are extremely mature and well-understood, and the collaborations are good at what they do, so it is likely to be something very subtle at work. It still could be new physics, that is somehow playing games with us, but certainly the prospects don’t look as good today as they did yesterday. Look like science is going to have to march on a bit more before everything is clear.

The Tevatron, Fermilab’s mighty but ancient (as these things go) particle accelerator, is scheduled to be shut down at the end of this year. But the old beast might have a trick or two left yet.

Way back in April we talked about a couple of lingering anomalies in the Tevatron data that had risen to the level where theorists were intrigued enough to start building models. One of these — a forward/backward asymmetry in top-quark interactions — had been around for a while, and was taken seriously by a number of people. The other — a tiny bump near 150 GeV in the total number of events that produce a W boson and two jets — was relatively new, and was greeted by a bit of scoffing. The bump credibility took another hit when it was pointed out that it could be explained away by a simple (although completely hypothetical) systematic error — a miscalibration of the jet energies. Bump-hunting is hard, and experiments near the end of their lifetimes are more willing to share their anomalies than they would be if they knew they were going to keep going, since there’s little hope that new data will solve the problem.

But there’s some hope. The real reason to be patient rather than excited by the bump at 150 GeV was that it was a 3-sigma effect, in a game where most 3-sigma effects go away. In particle physics, we generally take a solid 3-sigma result as “evidence for” something, and require 5 sigma — a much greater deviation from the expected numbers — to declare something a “discovery.”

More data are now in! This is from the CDF experiment at Fermilab, as reported in a conference talk by Giovanni Punzi (pdf), and shared worldwide by Jester at Résonaances. There’s a reason why I mentioned Résonaances among the physics blogs above — it’s unquestionably the go-to place for new results in particle physics.

And the anomaly is now — almost five sigma! It didn’t go away with more data, it became more prominent. It would be very hard at this point to simply attribute it to an energy miscalibration or something like that; if it is a systematic error, it’s a subtle one. But it doesn’t look like an error; it looks like a signal.

Of course, it’s still very possible that it will go away. These things usually do. But when an interesting result is pushing five sigma, it’s perfectly okay to get a bit excited and start wondering what’s going on. One of the nice things about this bump is that it’s not very hard to come up with models that can explain it — all you need is a neutral boson, similar to the well-known Z boson of the weak interactions, with a mass near 150 GeV. This kind of idea is so well-known in the trade that it already has a name — the Z’ boson, imaginatively enough.

Except it’s not that simple, of course — where would be the fun? When you start mindlessly adding new particles to the Standard Model, you have to check consistency with all sorts of experimental constraints. In particular, a naive Z’ boson would sometimes decay into leptons as well as quarks (the jets mentioned above). In that case, it would have been seen long ago in LEP, the electron-positron collider at CERN that previously lived in what is now the LHC’s tunnel. So what you really need is a “leptophobic” Z’, one that decays into quarks but not into leptons.

Or something along those lines, or something completely different. See Résonaances once again for the lay of the theoretical land. Yes, there are possible explanations within supersymmetry; and yes, there are explanations that have nothing to do with supersymmetry.

If this is real — still a very, very, big if — it’s the beginning of the “beyond the Standard Model era” in collider particle physics. Things aren’t going to snap into place overnight; there will be false starts, mysteries, and sudden epiphanies. That’s where the real fun is in science.

Update: Note that the very preliminary word from the LHC is that they don’t yet see the same bump that CDF does. But from a glance at the figure it doesn’t look like they have nearly as much data yet, so that’s probably not surprising. The LHC has seen incredible jumps in luminosity recently, however, so they should be able to do a proper check before too long.

When physicists are asked about “parallel worlds” or ideas along those lines, they have to be careful to distinguish among different interpretations of that idea. There is the “multiverse” of inflationary cosmology, the “many worlds” or “branches of the wave function” of quantum mechanics, and “parallel branes” of string theory. Increasingly, however, people are wondering whether the first two concepts might actually represent the same underlying idea. (I think the branes are still a truly distinct notion.)

At first blush it seems crazy — or at least that was my own initial reaction. When cosmologists talk about “the multiverse,” it’s a slightly poetic term. We really just mean different regions of spacetime, far away so that we can’t observe them, but nevertheless still part of what one might reasonably want to call “the universe.” In inflationary cosmology, however, these different regions can be relatively self-contained — “pocket universes,” as Alan Guth calls them. When you combine this with string theory, the emergent local laws of physics in the different pocket universes can be very different; they can have different particles, different forces, even different numbers of dimensions. So there is a good reason to think about them as separate universes, even if they’re all part of the same underlying spacetime.

The situation in quantum mechanics is superficially entirely different. Think of Schrödinger’s Cat. Quantum mechanics describes reality in terms of wave functions, which assign numbers (amplitudes) to all the various possibilities of what we can see when we make an observation. The cat is neither alive nor dead; it is in a superposition of alive + dead. At least, until we observe it. In the simplistic Copenhagen interpretation, at the moment of observation the wave function “collapses” onto one actual possibility. We see either an alive cat or a dead cat; the other possibility has simply ceased to exist. In the Many Worlds or Everett interpretation, both possibilities continue to exist, but “we” (the macroscopic observers) are split into two, one that observes a live cat and one that observes a dead one. There are now two of us, both equally real, never to come back into contact.

These two ideas sound utterly different. In the cosmological multiverse, the other universes are simply far away; in quantum mechanics, they’re right here, but in different possibility spaces (i.e. different parts of Hilbert space, if you want to get technical). But some physicists have been musing for a while that they might actually be the same, and now there are a couple of new papers by brave thinkers from the Bay Area that make this idea explicit.

Physical Theories, Eternal Inflation, and Quantum Universe, Yasunori Nomura

The Multiverse Interpretation of Quantum Mechanics, Raphael Bousso and Leonard Susskind

Related ideas have been discussed recently under the rubric of “how to do quantum mechanics in an infinitely big universe”; see papers by Don Page and another by Anthony Aguirre, David Layzer, and Max Tegmark. But these two new ones go explicitly for the “multiverse = many-worlds” theme.

After reading these papers I’ve gone from a confused skeptic to a tentative believer. This happened for a very common reason: I realized that these ideas fit very well with other ideas I’ve been thinking about myself! So I’m going to try to explain a bit about what is going on. However, for better or for worse, my interpretation of these papers is strongly colored by my own ideas. So I’m going to explain what I think has a chance of being true; I believe it’s pretty close to what is being proposed in these papers, but don’t hold the authors responsible for anything silly that I end up saying. …

Are Many Worlds and the Multiverse the Same Idea?Read More »