A First: String Theory Predicts an Experimental Result
Note: This is a February 2009 article written by Glennda Chui that originally appeared in the online publication symmetry breaking.
One of the biggest criticisms of string theory is that its predictions can’t be tested experimentally–a requirement for any solid scientific idea.
That’s not true anymore.
At a AAAS session on Sunday, physicists said string theory is making important contributions to the study of two extreme forms of matter –one heated to trillions of degrees, the other chilled to near-absolute zero. In both cases the matter became a “perfect liquid” that ripples and flows freely, like water. String theorists analyzed the results by applying what they had learned from pondering how a black hole might behave in five dimensions. Then they went on to calculate just how free-flowing these liquids might be, predictions that the experimenters are using to guide the next stage of their work.
“It’s really a surprising, I would say serendipitous, once-in-a-generation convergence of scientific communities,” says Peter Steinberg, a nuclear physicist at Brookhaven National Laboratory and one of the organizers of the panel. “None of us saw this coming.” (Full disclosure: Peter invited me to be a discussant for the session, which meant I got to take in all the talks and then ask the panel whatever I wanted. Sweet!)
Not to say that string theory has been proved. Clifford Johnson of the University of Southern California, the string theorist on the panel, was very clear about that. All the arguments about whether nature is composed of unimaginably tiny vibrating strings and multiple dimensions, and whether this will eventually explain the basic workings of the universe, are still unresolved.
“We’re still very far from getting string theory in good enough shape to really understand all those questions,” he said. “But what is really encouraging is when that tool box you’ve been working on to gear up to understand those questions, when you find a way of making that toolbox useful in some other experiments. That tells you that your tool is a robust tool that may be on the right track. So we haven’t proven that reality is all about string theory or however you want to put it, but we certainly have indeed found a place, it seems, where string theory has been a useful guide and has made been making some modest but sharp and testable predictions in the lab.”
The tale begins in 2002, when researchers in John Thomas’s JETLab group at Duke University announced that they had created a super-cooled gas of lithium 6 atoms that behaved like a fluid; see their paper here (subscription required.) They did this, Thomas explained, by trapping about 300 million lithium 6 atoms in a tiny, cigar-shaped bowl of laser light. At this point the atoms look like a little red ball, visible in a photo he flashed on the screen. Then they hit the ball of atoms with a carbon-dioxide laser beam. The atoms started banging into each other and quickly evaporated. This cools them–something we’re all familiar with from getting chilly as our sweat dries–until they reach a temperature of about a billionth of a degree above absolute zero.
At this point the blob of atoms began acting strangely. Laser flash photos showed that it expanded but only in one direction, and in a way characteristic of flowing liquid. In technical terms, they had created the first strongly interacting Fermi gas.
The gas’s super-fluid behavior, Thomas said, is similar to what takes place in superconducting materials, which conduct electrical current with perfect efficiency. Today’s superconductors operate only at relatively cold temperatures, and scientists have been working for decades to create one that operates at room temperature. The behavior of the lithium gas is analogous to that of a superconductor that could operate at temperatures of thousands of degrees, making this work of great interest to condensed-matter physicists.
Flash forward three years to Brookhaven National Laboratory, where physicists had been studying head-on collisions of gold nuclei at RHIC, the Relativistic Heavy Ion Collider. As Steinberg put it, the collision of 400 of these nuclei produces 10,000 particles, primarily quarks and gluons, in a tiny drop of matter that may be the long-sought quark-gluon plasma.
The quark-gluon plasma is an idea that stems from the curious nature of the strong force, which binds quarks together to make protons and neutrons. The strong force is carried by the gluon particle. Unlike most of the forces we’re familiar with, it’s like a rubber band: it becomes stronger when you try to pull the quarks far apart. If you squeeze quarks close enough together, the rubber band part of the force melts away and the quarks and gluons are able to freely interact with each other.
In contrast to Duke’s super-cold lithium blobs, the RHIC plasma was super-hot, as in trillions of degrees, said RHIC collaborator Barbara Jacak of Stony Brook University. The last time the universe was that hot was one microsecond after the big bang, and so physicists are hoping their experiments at RHIC will shed light on the state of the universe at that long-ago instant.
Weirdly, the hot RHIC plasma also flowed like a liquid; the lab’s 2005 announcement of the discovery described it as the most perfect liquid ever observed, with virtually no viscosity–the quality that makes honey flow more slowly than milk.
The fact that the plasma behaved like a liquid surprised scientists, who had expected it to take the form of a gas. While the particles in these fluids are independent, they are also strongly coupled, meaning that each one is tied very tightly to nearby things. As Steinberg puts it, “The system moves in concert. You don’t think of it as particles; you think of it as a stuff.” Johnson describes it as a form of emergent behavior that is akin to the wetness of water. Individual particles in water don’t have any property that could be called wetness. Wetness only arises when very many molecules are present.
But are these liquids really perfect? Enter the string theorists, who are quite at home in multiple dimensions and bring a whole new vantage point to the question.
“The goal is try to understand what are essentially new phases of matter that are showing up at these laboratories,” Johnson said. “It’s exciting. It’s novel. It’s not often we create new phases of matter in the world,” phases that are thought to naturally exist only just after the big bang or in the cores of compact stars.
The string theory analysis starts not from the viewpoint of quarks and gluons, but from quantum black holes–a theoretical form of black hole that is very tiny and, unlike its massive star-gobbling cousin, has never been observed in nature.
Suppose, Johnson said, you had a quantum black hole in a five-dimensional universe, in which there are four dimensions of space and one of time. If you were to build a box around that black hole, the holographic principle states that you could understand all of its internal physics from a perch on this surrounding wall. The holographic principle is so called because it’s akin to creating a 3-D image on a two-dimensional sheet. String theory along with the holographic principle provides a view of quantum gravity that allows you to look inside the black hole and understand its internal physics.
It turns out that “this physics that lives on these walls really resembles the physics we’re seeing in the experiments. That’s the exciting thing,” Johnson said. String theory provides a kind of dictionary that translates between our four-dimensional world, where the experiments take place, and the five-dimensional world in which theorists envision the quantum black hole.
As for the black hole and the extra dimensions, Johnson said he’s agnostic as to whether they exist or not. He thinks of them as a tool, one that allowed string theorists to calculate the ratio of viscosity to the fluid’s entropy, a measure of its disorder. String theory predicts that this ratio is naturally very low for the two experimental “perfect liquids.” The experimenters are now closing in on that value, which will reveal just how perfect the liquids are.
“Why does it work so well? What are the prospects for more success? These are things we are still trying to understand,” Johnson said. “This is also a very powerful test of the tools that come from string theory. We’ve been working in isolation for a long time, not knowing whether these tools necessarily are anything to do with experimental physics.”
This collaboration between string theorists and experimentalists in two fields of physics came about by happenstance. Perhaps, the panelists said, there are ways to foster other such discussions and collaborations, which don’t naturally occur in the world of science, where specialists tend to huddle only with others of their own kind.