Strings
link the ultracold with the superhot
Perfect liquids suggest theory’s math mirrors something real
By Tom Siegfried
Shadows live in a simple world. They glide effortlessly
across any sort of surface, oblivious to the higher
dimension of space in which 3-D bodies move, collide
and sometimes block the paths of rays of light.
Shadows have no idea how important that third dimension
is, and how objects in it endow those very shadows with their
quasi-physical existence. Indeed, the laws of shadow physics all depend on the third dimension’s presence. And just
as the clueless inhabitants of the shadow world require an
extra dimension to explain how they exist and interact, reality for humans may also depend on an invisible dimension or
dimensions unknown.
Physicists, in fact, have long pondered possible higher
dimensions beyond the familiar four — three of space and
one of time — that describe ordinary experience. Such extra
dimensions have emerged as essential features in a sophisticated mathematical pastime known as superstring theory.
Believed by some theorists to be the ultimate building blocks
of all physical reality, superstrings are supposedly inaccessible to experimental study. If they exist, they would be far
too small to detect directly — enlarging a superstring to the
size of an amoeba would be the equivalent of making an ant
as big as the visible universe. Similarly, the extra dimensions
that strings require would probably be far too small to detect
by available methods.
So string theory has long remained in the physics version
of The Twilight Zone, disconnected from the ordinary world
of sight and sound. But now the extra-dimensional math has
begun to audition for Reality TV. For the first time, superstring theorists can point to a place where their formulas
help other physicists understand something they can see in
their experiments.
One such experiment generates matter in its most fiery
form — simulating the temperatures of the Big Bang itself.
Another probes matter most frigid — atoms vastly colder
than even the depths of outer space. At both extremes, matter behaves surprisingly like a liquid, contrary to all expec-
tations. More surprising still, explaining this behavior
apparently requires an extra dimension of space, something
that superstring theory conveniently provides. And so the
scientists who study hot matter, cold matter and string matter have found themselves sharing common ground in an
extra-dimensional world.
“It surprised the heck out of us two years ago when we
started realizing that this was the case,” says physicist Peter
Steinberg of Brookhaven National Laboratory on Long
Island, N. Y. “It’s a once-in-a-generation convergence of
scientific communities. None of us really saw this coming.”
Cosmic soup
Steinberg and other scientists discussed the new developments recently in Chicago at the annual meeting of the American Association for the Advancement of Science. Speakers at a
session there described the surprising confluence of different
physics fields as a sort of perfect storm, with the eye centered
on the esoteric idea of a “perfect liquid.”
Liquids are usually the Goldilocks state of matter, the not-too-hot, not-too-cold, cohesive yet shapeless assemblages of
molecules that exist only in a relatively narrow range of temperatures. Colder, and matter typically becomes solid — rigid
and crystalline. Hotter, and matter turns gaseous, with molecules flying about freely and occasionally colliding. Hotter
still, and a gas should become plasma, with electrons torn
from atoms to form an electromagnetic mélange of charged
particles, a gas with flash.
When the universe was very young, and still superhot
from the aftermath of the Big Bang, plasma should have
been the only state of matter around. And that’s what scientists at Brookhaven expected to see when they smashed gold
ions together at 99.99 percent of the speed of light using a
machine called RHIC (for Relativistic Heavy Ion Collider).
RHIC physicists thought the ion collisions would melt the
gold’s protons and neutrons into a hot plasma of quarks
and gluons at a temperature of a trillion kelvins, replicating
