Making Connections

(An edited version of this is going to run in the BNL Monday Memo tomorrow. I just wanted to post it here because...well, because I had written it already! And I post the book cover on the right because...well, because I loved that show as a young PBS viewer and because...well, because this is why I got into the science biz to begin with -- to find unexpected connections! And here's the press rundown -- thanks Clifford!)

At the annual meeting of the American Association for the Advancement of Science last week in Chicago, the sessions covered a wide range of scientific fields, from climate change to the intersections of mathematics with origami. Of course for me, the highlight was the symposium that I organized with Bill Zajc, professor at Columbia University and former spokesperson for the PHENIX experiment at RHIC.

Titled “Quest for the Perfect Liquid: Connections Between Heavy Ions, String Theory, and Cold Atoms,” the session covered the emerging relevance of the physics done at RHIC to other subfields of physics, ones that were never thought to be related.

It included speakers both from within the RHIC community, as well as from fields who turn out to have a closer connection to RHIC physics than had been realized. It turns out that physicists from three separate fields are all intensely interested in the physics of strongly-coupled liquids, which flow so easily that they are called “perfect” liquids.

PHENIX spokesperson Barbara Jacak of Story Brook University presented the major results from RHIC, which has been providing collisions since 2000. She paid particular attention to the main features of the medium formed in heavy ion collisions at RHIC: that it flows like a near-perfect liquid, a property intimately connected to its ability to stop the motion of fast-moving quarks (both light and heavy). Using these data, RHIC scientists are able to determine a particular ratio -- that of viscosity to entropy density -- to be quite small.

John Thomas, an atomic physicist from Duke University, presented his experimental results on ultracold atomic gases. In these experiments, clouds of atoms are released from optical traps, and their expansion is visualized by laser flash-imaging techniques. An external magnetic field affects the coupling of the atoms and can be tuned to put the system in a strongly-coupled “universal” regime. Here, the system acts in a way similar to RHIC collisions, expanding asymmetrically according to the laws of fluid flow. The viscosity to entropy density ratio has also been measured here and is as low (or maybe lower!) than that found at RHIC.

Rounding out the presenters was (notable blogger) Clifford Johnson of the University of Southern California. He described how the mathematical techniques of string theory are elucidating the properties of these strongly interacting near-perfect liquids, based on the ideas of string theory. These techniques are used to draw a connection between a strongly-coupled quark-gluon liquid in our world of three spatial dimensions and a gravity theory living in four spatial dimensions with a black hole sitting deep in the fifth dimension! While this scenario sounds strange, it provides one of the few concrete predictions arising from string theory calculations -- that the ratio of viscosity to entropy density has a lower limit, a value which seems to be observed by both RHIC and the ultracold atomic physics experiments.

The serendipitous convergence of these three separate fields has been very exciting -- and useful -- for everyone involved, giving all of us the strong sense that we are only starting to grasp its implications. And it is quite striking that the tools of string theory have provided a sharp, testable prediction for the first time -- one that seems to be borne out by experiments from completely different fields.

Our symposium was attended by over 120 people, ranging from high-school students to scientists in a variety of physics subdisciplines, showing the wide interest this emerging field of science has attracted. It was subsequently covered by several physics blogs.

Symmetry magazine’s assistant editor Glenda Chui served as a discussant along with Bill, and they both addressed questions to the speakers following the talks.

Chui herself posted a blog (http://www.symmetrymagazine.org/breaking/2009/02/16/a-first-string-theory-predicts-an-experimental-result/) a few days later, which was picked up by digg.com. The interest in Chui’s blog was so great that it overloaded Symmetry's server, necessitating a major upgrade. The comments to the piece show that connecting experimental data to string theory is controversial, especially if this is used to argue for the “reality” of string theory. But such debate is fitting for the kind of forefront science we do here at Brookhaven with RHIC.