Science

The Planck Surveyor satellite, a European mission to observe the cosmic microwave background (and various things that get in the way), has released its first science results. 25 papers in all!

I haven’t absorbed all the goodness as yet, so I’ll just point you to more interesting resources — e.g. blog posts by Peter Coles or Andrew Jaffe, or this BBC article if you prefer your media more mainstream. Note that these are not, for the most part, results about the cosmic microwave background and all the yummy cosmological goodness one hopes to derive therefrom. There’s a lot about dust in our own galaxy, as well as infrared emission from some of the very earliest galaxies in the universe. (Much of this is relevant, of course, to straightening out possible anomalies in the actual CMB.)

CMB results are expected circa January 2013. That’s when I’ll win my bet with Max Tegmark.

NASA doesn’t have nearly enough money to do what it wants to do. Well, nothing unusual about that. We’ve talked recently about the constraints that budgetary realities are putting on astronomers’ ambitions — here, here, here. Now it’s chickens-coming-home-to-roost time, apparently. Dennis Overbye has an article in the Times (via Brian Schmidt) about how cost overruns on the James Webb Space Telescope — the giant multipurpose infrared satellite into which basket NASA is putting many of its eggs — are forcing dark energy onto the back burner.

The current NASA vision for dark energy is a mission called WFIRST (Wide-Field Infrared Survey Telescope), which grew out of JDEM (the Joint Dark Energy Mission), which was in turn descended from SNAP (Supernova Acceleration Probe). WFIRST would try to use three different techniques to constrain dark energy parameters — weak lensing, baryon acoustic oscillations, and supernovae. It would also be able to search for exoplanets using microlensing, just as a bonus. But cost overruns on JWST have left NASA with very little money to do ambitious (or even not-very-ambitious) new missions, so WFIRST is now up in the air, despite being judged the highest priority by the National Academy Decadal Survey.

It looks like the U.S. might try to stay in the dark-energy game by funding a 20% share in Euclid, a planned mission by the European Space Agency. Meanwhile, techniques that try to measure parameters of dark energy without leaving the ground are continuing to improve. So maybe it will end up not being a big deal, and we’ll learn what we need to know anyway. Or maybe we’ll miss out on the opportunity for a transformative discovery. The only thing we know for certain is that it’s not easy to make these tough choices when it comes to planning missions over the course of decades.

I was looking at Google maps at a location near Chicago, so I scooted over to take a look at Fermilab (map). As always, I was struck first by the sheer beauty of the arrangement, and next by how wonderful it is that we human beings would undertake such a massive project just to better understand the laws of nature. And finally, of course, by the irony that it takes something this big to examine particles on very small scales. Blame the wave nature of matter for that: to look at short distances, you need high energies, and that means a whomping big accelerator.

This moved me to take a look at other giant scientific facilities. Unfortunately CERN puts its accelerators underground, so the Large Hadron Collider doesn’t mark the landscape with enormous circles. Here’s SLAC (map), the largest linear accelerator in the world and claimed to be the world’s straightest object.

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Scientific Artifacts from the SkyRead More »

Here’s an excellent article in Popular Science about the Pioneer anomaly. (Via Dan Vergano on Twitter.) The Pioneer spacecraft, launched in the early 1970’s, have been moseying through the outer regions of the Solar System for quite some time now. But a careful analysis of tracking data indicated that the acceleration of the two spacecraft didn’t quite match what we’d expect from gravity; there appears to be an anomalous acceleration, nearly constant over time and pointing toward the Sun. Many new-physics explanations have been proposed, but it’s always been a difficult scenario to master; it’s very hard to imagine a new force that would account for the Pioneer data but not also show up in observations of the outer planets. (The Voyager spacecraft aren’t as useful for this purpose, as they are guided by tiny thrusters that overwhelm the signal, while the Pioneers float freely and are pointed using gyroscopes.)

The most likely explanation has always been that we didn’t completely understand the spacecraft, or the tracking system. Indeed, it’s been recognized for a while that a small imbalance in how the spacecraft radiated heat could account for the acceleration — but that imbalance didn’t seem to be supported by what we knew about the vessels. That may be changing, however. The Popular Science article is a little cagey, but it mentions a new and unprecedentedly thorough analysis by Viktor Toth and Slava Turshyev that should be coming out soon. Here is as much as they would let on:

Five years have passed. Using the telemetry data, the two scientists created an extremely elaborate “finite element” 3-D computer model of each Pioneer spacecraft, in which the thermal properties of 100,000 positions on their surfaces are independently tracked for the duration of the 30-year mission. Everything there is to know about heat conduction across the spacecraft’s surfaces, as well as the way that heat flow and temperature declined over time as the power of the generators lessened, they know. The results of the telemetry analysis? “The heat recoil force accounts for part of the acceleration,” said Turyshev. They wouldn’t tell me how significant a part. (Turyshev: “We’d like to publish that in the scientific literature.”) But according to Toth, “You can take it to the bank that whatever remains of the anomaly after accounting for that thermal acceleration, it will at most be much less than the canonical value of 8.74 x 10^-10 m/s², and then, mind you, all those wonderful numerical coincidences people talk about are destroyed.”

Doesn’t look good for people who prefer to imagine that wild new physics is responsible. Not that they will go away — the power of wishful thinking is strong. You can already hear them staking out territory, even before the new report comes out:

Other physicists are more combative. “Heat? That’s simply not the right explanation. They are wrong,” commented Johan Masreliez, an independent researcher in Washington who supports the expanding spacetime model of cosmology, for which it is crucial that the value of the Pioneer anomaly equals c times H. “But then I’m biased,” he added.

Even if the new analysis gives a very sensible and believable account of the Pioneer anomaly in terms of very ordinary physics, expect the true believers to hang on for years to come. The rest of us will move on — at least until the next exciting anomaly pops up.

Also: big props to Natalie Wolchover, who wrote the PopSci piece. Very measured tone, carefully researched and well-written.

This year we give thanks for an idea that is absolutely crucial to how our understanding of nature progresses: effective field theory. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, and conservation of momentum.)

“Effective field theory” is a technical term within quantum field theory, but it is associated with a more informal notion of extremely wide applicability. Namely: if we imagine dividing the world into “what happens at very short, microscopic distances” and “what happens at longer, macroscopic distances,” then it is possible to consistently describe the macroscopic world without referring to (or even understanding) the microscopic world. This is not always true, of course — our macroscopic descriptions have very specific domains of applicability, past which the microscopic details begin to matter — but it’s true very often, for a wide variety of situations with direct physical relevance.

The most basic examples are thermodynamics and fluid mechanics. You can talk about gasses and liquids very well without having any idea that they are made of atoms and molecules. Once you get deep into the details, we start talking about effects for which the atomic granularity really matters; but there is a very definite and useful regime in which it is simply irrelevant that air and water are “really” made of discrete units rather than being continuous fluids. Fluid mechanics is the “effective field theory of molecules” in the macroscopic domain.

How awesome is that? If it weren’t for the idea of effective field theory, it’s hard to imagine how we would ever make progress in physics. You wouldn’t be able to talk about atmospheric science without knowing all the details of microscopic physics (known in the trade as the ultraviolet completion), all the way down to the Planck scale! Fortunately, the universe is much more kind to us.

In particle physics, this idea is absolutely central. Protons, neutrons, and pions constitute an effective field theory that describes how quarks and gluons behave over sufficiently large distances. Another great example comes from Enrico Fermi’s theory of the weak interactions. Back in the 1930’s, Fermi proposed a theory that made use of the new “neutrino” particle. It involved processes that looked like this interaction of a proton plus electron converting into a neutron plus neutrino.

Nowadays we know better. What’s really going on is that the proton is made of two up quarks and a down quark, while the neutron is made of two downs and an up. The electron exchanges a W boson with one of the quarks, converting into an electron neutrino in the process.

But the miracle is: it doesn’t matter. Knowing that the weak interactions are “really” carried by W bosons is completely irrelevant, as long as we are concerned only with large distances. In quantum mechanics, large distances correspond to low energies. (Remember that the energy of a wave decreases as its wavelength increases; quantum mechanics is all about waves.) So for low-energy processes, the effective field theory provided by Fermi is all you need to know about the weak interactions.

The universe is kind, but that kindness comes at a price. Sometimes you want to care about the microscopic realm — for example, if you’re a physicist trying to figure out what is going on down there. When we look at spacetime on length scales of 10^-33 centimeters, do we see vibrating strings, or noncommuting matrices, or spin networks, or what? Hard to tell, because it makes no difference at all to the large-distance/low-energy physics we can actually observe.

That’s okay. A world described by a succession of effective field theories of ever-higher resolution helps us make sense of the world, while leaving physicists plenty of puzzles to think about. Very deserving of our thanksgiving.

There was some excitement last week about a Maxwell’s-Demon-type experiment conducted by Shoichi Toyabe and collaborators in Japan. (Costly Nature Physics article here; free arxiv version here.) It’s a great result, worth making a fuss about. But some commentators spun it as “converting information into energy.” That’s not quite right — it’s more like “using information to extract energy from a heat bath.”

Say you have a box of gas with a certain temperature at maximum entropy — thermodynamic equilibrium. That is, the gas is smoothly spread throughout the box. (We can safely ignore gravity.) There’s certainly energy in there, but it’s not very useful. Indeed, one way of thinking about entropy is as a measure of how useless a certain amount of energy is. If we have a low-entropy configuration, we can extract useful work from the energy inside, such as pushing a piston. If we have a high-entropy configuration, the energy is useless; there’s nothing we can do to consistently extract it.

Here’s an example from my book. Consider two pistons with the same number of gas particles inside, with the same total energy. But the top container is in a low-entropy state with all the gas on one side of the piston; the bottom container is in a high-entropy state with the gas equally spread out.

You see the difference — from the top configuration we can extract useful work by simply allowing the piston to expand. In the process, the total energy of the gas goes down (it cools off). But in the bottom piston, nothing’s going to happen. There’s just as much energy inside there, but we can’t get it out because it’s in a high-entropy state.

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Using Information to Extract EnergyRead More »