Most researchers think of them as microscopic pinpricks, the remnants of
stars that have collapsed under their own weight. But over the past couple of
years, a number of mavericks have proposed that black holes are actually
extended bodies, made up of an exotic state of matter that congeals, like a
liquid turning to ice, during the collapse. The idea offers a provocative way of
thinking about quantum gravity, which would unify Einstein's general theory of
relativity with quantum mechanics.
In the textbook picture, the pinprick (or
singularity) is surrounded by an event horizon. The horizon is not a physical
surface, merely a conceptual one, and although it marks the point of no return
for material plummeting toward the singularity, relativity says that nothing
special happens there; the laws of physics are the same everywhere. For quantum
mechanics, though, the event horizon is deeply paradoxical. It allows
information to be lost from our world, an act that quantum theory forbids.
"What you have been taught in school is almost certainly wrong, because
classical black hole spacetimes are inconsistent with quantum mechanics,"
says physicist George Chapline of Lawrence Livermore National Laboratory.
The new conceptions of black holes eliminate the event horizon altogether.
The basic idea is that there does, in fact, exist a force that could halt the
collapse of a star when all else fails. That force is gravity itself. In matter
with certain properties, gravity switches from being an attractive force to a
repulsive force. Such a material, going by the name "dark energy," is
thought to be driving the acceleration of cosmic expansion.
Last year physicists Pawel O. Mazur of the University of South Carolina and
Emil Mottola of Los Alamos National Laboratory reasoned that a pocket of the
stuff might freeze out, like ice crystals, during the collapse of a star. The
result, which they call a gravastar, would look like fried ice cream: a crust of
dense but otherwise ordinary matter stabilized by a curious interior. The crust
replaces what would have been the event horizon.
Another proposal goes further. It conjectures not only that dark energy would
freeze out but that relativity would break down altogether. The idea comes from
a dark-horse contender for quantum gravity, the proponents of which are struck
by the resemblance between the basic laws of physics and the behavior of fluids
and solids (also known as condensed matter). In many ways, the equations of
sound propagation through a moving fluid are a dead ringer for general
relativity; sound waves can get trapped in the fluid much as light gets trapped
in a black hole. Maybe spacetime is literally a kind of fluid.
What makes this approach so interesting is that the behavior of condensed
matter is collective. The details of individual molecules hardly matter; the
system's properties emerge from the act of aggregation. When water freezes, the
molecules do not change, but the collective behavior does, and the laws that
apply to liquids no longer do. Under the right conditions, a fluid can turn into
a superfluid, governed by quantum mechanics even on macroscopic scales. Chapline,
along with physicists Evan Hohlfeld, Robert B. Laughlin and David I. Santiago of
Stanford University, has proposed that a similar process happens at event
horizons. The equations of relativity fail, and new laws emerge. "If one
thinks of spacetime as a superfluid, then it is very natural that in fact
something physical does happen at the event horizon--that is, the classical
event horizon is replaced by a quantum phase transition," Chapline says.
