we have two different
explain without knowing about objects in
the third dimension, quark physics makes
kinds of systems
more sense using the 4-D math. Quarks
capturing the same
can be viewed, for instance, as the end-
kind of physics.…
points of strings that vibrate in an extra
String theory provides
dimension, and that explains how they can
be so strongly coupled. Precisely the same
us with a dictionary
math can then also describe the collec-
tive behavior of the cold lithium atoms. As
Johnson points out, viscosity is all about
how neighboring pieces of a fluid communicate with each other. With an extra
UNIVERSIT Y OF SOUTHERN CALIFORNIA dimension, that communication can take
place as disturbances in the higher dimen-
sional space, explaining the perfect liquid behavior.
suggest, there is a limit to how low the resistance to flow, or viscosity, can go. A liquid
with that lowest possible viscosity earns the
label “perfect,” and both the hot RHIC soup
and the cold lithium cloud turn out to be
nearly as close to perfect as possible.
This formula for perfection is actually a
ratio of viscosity to entropy — a measure of
disorder that depends on the system’s tem-
perature. For a perfect liquid, the viscosity-
entropy ratio is a very small number (about
0.08 in units derived from certain fundamental constants). For ordinary water, the
ratio is 380 times higher than that theoretical minimum; liquid helium’s ratio is only 0.7,
still about nine times higher than perfection. But both RHIC’s
soup and the lithium atoms approach the theoretical limit
even more closely. Cold lithium’s ratio is less than 0.5, and the
quark-gluon soup is in the neighborhood of 0.2.
Not only does string theory predict the perfect liquid limit
for the viscosity-entropy ratio, string math also offers an
explanation for how the cold and hot worlds can be so similar. Both systems can be described as something like a
shadow world sitting in a higher dimension. Strongly coupled particles are linked by ripples traveling through the
extra dimension, says Steinberg, of Brookhaven.
String math describing such ripples stems from an idea
called the holographic principle, used by string theorists to
describe certain kinds of black holes. A black hole’s entropy
depends on its surface area — as though all the information in its three-dimensional interior is stored on its two-dimensional surface. (The “holographic” label is an allusion
to ordinary holograms, where 3-D images are coated on a
2-D surface, like an emblem on a credit card.) The holographic principle has value because in some cases the math
for a complex 3-D system (neglecting time) can be too hard
to solve, but the equivalent 4-D math provides simpler equations to describe the same phenomena.
“The point is that we have two different kinds of systems
capturing the same kind of physics,” says string theorist
Clifford Johnson of the University of Southern California
in Los Angeles. “String theory provides us with a dictionary
that translates between these two systems.”
One of the two systems is a realm of four spatial dimensions
where the string math describes gravity and quantum theory;
the other is the 3-D world of quarks and gluons. Usually the
math for describing each of these systems looks very different.
But string theory’s extra dimension allows the math to be transformed in ways that show the two systems to actually be equivalent — in technical terms, the systems are “dual” to each other.
“The bottom line is we can exploit all this, because we can
use … easy computations in the gravity system to compute
hard-to-compute things in the dual system,” Johnson said at
the Chicago meeting.
Strings strike back
In recent years it has become popular to criticize string
theory as out of touch with reality. Popular books have been
written by scientists, some prominent and others not so
prominent, arguing that string theory makes no predictions
that experiment can test, that its fundamental objects can’t
be observed, that physicists have wasted their time on an
enterprise that isn’t even scientific to begin with.
Such arguments leave an impression of utter unfamiliarity
with the history of science. In times past, the same kinds of
aspersions were cast against quarks, neutrinos, even the very
existence of atoms. Superstrings are in good company. And
string theory’s limit on how low viscosity can go now seems
to have established that string math does indeed mirror
something real in nature. “This may well be the first prediction from string theory to be validated by experiment,”
Steinberg writes in a recent paper ( arxiv.org/abs/0903.1474).
Superstrings’ success with perfect liquids does not, of
course, establish that the whole theory is the correct description of the universe. Much work remains to figure out how
much of reality string theory actually captures beyond the
realm of perfect liquids. But the usefulness of superstring
math in these instances argues strongly that those equations
capture something true. Establishing that truth for certain
will still not be easy.
It’s not surprising, of course, that such groundbreaking
science should be difficult and controversial. Advances in
physics today are naturally much tougher to achieve than they
used to be — because the problems remaining to be solved are
precisely those that have resisted solution for so long.
“A new truth always has to contend with many difficul-
ties,” the German physicist Max Planck said decades ago. “If
it were not so, it would have been discovered much sooner.”
s View slides from the AAAS session “Quest for the Perfect
Liquid” at www.bnl.gov/aaas09/perfectliquid.asp