December 18, 2012 | 6
The whole point of an explanation is to reduce something you don’t know to something you do. By that standard, you don’t gain much by explaining anything in terms of black holes. Appealing to the most mysterious objects known to science as an explanation sounds like using one mystery to explain another.
Yet this is precisely what physicists have been doing to make sense of high-temperature superconductors and plasmas of nuclear particles. Both of these states of matter are about as un-black-hole-like as you can imagine. They don’t suck you to your death—indeed, the force of gravity plays no role in them at all—and they don’t split open the very foundations of physics. They are hard to understand in much the same way Earth’s climate is: the laws governing their constituents are perfectly well-known, but there are just so damned many constituents.
In the course of studying black holes, however, string theorists have discovered unexpected parallels, or “dualities,” between gravitational systems and non-gravitational ones. These correspondences may be purely mathematical or may reflect deeper physical linkages, but either way, you can leverage your knowledge of one domain to solve problems in another. In the January issue of Sci Am, Harvard physicist Subir Sachdev describes how to take analyses of gravitational phenomena and apply them to otherwise intractable problems regarding superconductors. Sabine Hossenfelder at Backreaction blogged on this topic recently, too, although she presumed a comfort level with vector fields and critical points.
But what about running the dualities in the other direction, using laboratory measurements of extreme materials to probe exotic gravitational physics? At an afternoon coffee-and-cookie break this spring at the Kavli Institute for Theoretical Physics, string theorist Ramy Brustein of Ben-Gurion University in Israel told me a way to do just that. He and Joey Medved of Rhodes University in South Africa have since written up their proposal. An expert on nuclear plasmas, Raju Venugopalan at the Brookhaven National Lab, likes the idea of returning the favor that string theorists have paid his subject area. “Can these experiments be used to learn about aspects of gravity?” Venugopalan wonders. “That would just be a phenomenon.”
The experiments in question entail smashing gold or lead nuclei together to create plasmas of quarks and gluons. When Brookhaven’s RHIC accelerator, following up earlier discoveries at CERN, first created these plasmas in 2005, physicists were flummoxed. They’d predicted the plasmas would behave like a gas, since quarks and gluons interact only weakly under the conditions that RHIC achieved. But the particulate debris betrayed pressure gradients that a gas cannot sustain. The plasmas must actually be liquid. Evidently the sheer number of particles compensated for the inherent weakness of their interactions.
Theorists were at a loss to calculate basic parameters of the fluid, such as viscosity—loosely speaking, the friction of fluid flow. The best they could manage was a rough argument based on Heisenberg’s uncertainty principle. Viscosity depends on the energy of the fluid’s constituent particles and the average time between successive particle collisions, and the uncertainty principle relates these two quantities, thereby implying a minimum possible value to the viscosity (as explained here). Even a so-called superfluid can’t evade Dr. Heisenberg’s strictures. A gas actually has a fairly large viscosity, since its particles are spaced farther apart and collide less frequently than those in a liquid. (A technical note: by “viscosity,” I really mean the ratio of viscosity to density.)
But what exactly the minimum value should be, theorists couldn’t tell, until Dam Son of the University of Washington and his colleagues applied duality. They equated the viscosity of a fluid to gravitational waves caroming off a black hole in higher-dimensional space—which, even for a physicist, is not an analogy that springs to mind. “That was a big surprise,” Brustein says. “The fact you can calculate hydrodynamical parameters from gravity was not understood.” The answer: 1/4π, in the appropriate units. The viscosity measured by RHIC comes close. Water, some 400 times more viscous, is molasses in comparison.
Surprisingly, the minimum value is the same for all fluids, whatever they are made of. Through the logic of duality, this universality has a simple explanation: Viscosity is equivalent to a gravitational phenomenon, and according to Einstein’s general theory of relativity, gravitation is blind to compositional details.
This is the line of reasoning Brustein hopes to flip around. The way he tells the story, it all started on an extended visit to CERN during the snowy winter in Europe two years ago. Brustein was out shoveling his driveway in the French village of Thoiry and got talking to his neighbor. Turned out the neighbor was the technical director of the ALICE experiment, which is CERN’s answer to RHIC. Not long after, Brustein bumped into the ALICE team leader at a formal dinner. Clearly it was meant to be. Some months later, Brustein sat with ALICE scientists in the CERN cafeteria and sketched out his ideas on a napkin (see photo above). Even if they don’t work out, Brustein has at least checked off two items on physicists’ list of 1000 things to do before you die: (1) napkin sketch, and (2) CERN cafeteria, a storied hangout where scientists have come up with such ideas as the World Wide Web.
Brustein’s insight was that viscosity is not the only fluid property you can measure. Shape is another. If the duality is valid, viscosity and shape will be related in a way that pins down the corresponding theory of gravity. “He’s looking for new observables that are a bit more discriminating than viscosity,” Venugopalan says.
For instance, if Einstein’s general relativity governs the gravitational dual, the minimum viscosity will equal 1/4π and the plasma should be spherically symmetrical. Nuclear physicists would not expect an ephemeral roiling fireball to have such symmetry, so this counts as a strong and significant prediction. “It’s an actual way of proving the quark-gluon plasma has a gravitational dual,” Brustein says. Things get even more interesting if Einstein’s theory is only an approximation to a deeper theory, as string theory holds. Then the viscosity value will differ from 1/4π and may no longer be universal among substances; the plasma shape will gain some angular structure (a quadrupolar correlation function, to be technical). So the experiment is able to probe post-Einsteinian physics.
To be sure, these measurements would not probe the law of gravity that governs our universe, but only the law of gravity that is implicit in the plasma dynamics. That is to say, the plasma’s fluid behavior can be thought of as related to some hypothetical universe where gravity acts a certain way. That universe may or may not be a model for ours. What the measurements would do, however, is test the general concept of duality, which currently has the status of a conjecture, and validate it as a tool in the search for a unified theory.
Brustein’s biggest challenge is not the physics per se; it is to persuade RHIC and ALICE experimentalists to take the data he needs. Typically experimentalists measure just the numbers of particles coming out in different directions, rather than the details of the particles’ energy and momentum. Venugopolan cautions: “Though I appreciate where Brustein is coming from and it would be indeed great if one can make an empirical determination of these questions, there are a large number of nontrivial issues to resolve before one gets there.” Particle experimentalists are busy people these days and have no shortage of ideas for what to look for. So Brustein might have to eat a lot more cookies and shovel more driveways to convince them.
Napkin sketch courtesy of Ramy Brustein