Black Holes that Don't Trap
http://focus.aps.org/story/v18/st12

black hole spacetime


Three decades ago Stephen Hawking discovered that black holes can emit
radiation, so they aren't really black. Now it seems that they may not
necessarily be holes. Theorists reporting in the 27 October PRL studied
the Hawking-like sound waves radiating from a black hole "analogue"--a
fluid flow in which the downstream current acts like a gravitational
pull and which is governed by black-hole-like equations. Surprisingly,
the Hawking waves appeared in the fluid even without the inescapable
region normally associated with black holes. The results suggest that
experimenters may have less trouble than expected creating Hawking
radiation analogues in the lab. The team speculates that real black
holes might also exist in such a state, but other experts are
skeptical.

The "point of no return" for anything venturing near a black hole is
called the event horizon--nothing leaves once it crosses this imaginary
spherical shell surrounding the hole. Hawking realized, however, that
the warped spacetime on the cusp of the event horizon would stimulate
the creation of particles, some having enough energy to break free of
the gravitational hold.

Unfortunately, the predicted radiation from real black holes is too
feeble to detect. Some researchers have therefore tried to indirectly
verify the theory with analogue black holes--solid or liquid systems
that trap sound or light in a way similar to real black holes. One
example is a quantum fluid like liquid helium with some portion moving
faster than the speed of sound. A swimmer in this supersonic region
could cry out for help, but the sound waves would never escape to the
stationary region. The equations describing such a sound trap are
similar to those for a black hole's funnel-shaped spacetime.
Theoretical studies of acoustic event horizons predict that they should
emit Hawking-like radiation from the edge of the supersonic region in
the form of quantized sound waves called phonons.

Researchers would like to observe this analogue Hawking radiation in
the lab in order to learn more about this exotic phenomenon. But no one
has succeeded so far, in part because the instabilities and turbulence
associated with a supersonic flow make such measurements difficult. But
it turns out that supersonic flows shouldn't be necessary for
generating Hawking radiation, according to the predictions of Carlos
Barcel=F3, of the Astrophysics Institute of Andaluc=EDa in Granada, Spain,
and his colleagues.

The team modeled a frictionless fluid flowing down a narrow pipe. They
imagined first that the flow speed accelerated with distance down the
pipe (perhaps because of a decrease in diameter) and second that the
maximum velocity increased with time. If the downstream flow reached
sound speed, they calculated that an upstream observer would hear a
Hawking-like hum. However, the team discovered that they could slowly
turn up the velocity--getting close to but never quite reaching
supersonic speeds--and yet the same phonon radiation came out. "It was
surprising. I was expecting to see no radiation," says co-author
Stefano Liberati of the International School of Advanced Studies in
Trieste, Italy.

The researchers speculate that a collapsing star could emit Hawking
radiation without ever forming an event horizon. From a distance, it
would still look like a black hole as long as it kept collapsing,
because light would be strongly affected by the severely warped
spacetime. But William Unruh of the University of British Columbia in
Vancouver thinks this scenario is unrealistic. To emit the expected
amount of radiation, he says, the star would have to shrink very close
to the compactness needed for an event horizon to form, so close that
the difference would not be physically meaningful.

--Michael Schirber
Michael Schirber is a freelance science writer in Montpellier, France.
Hawking Like Radiation Does Not Require a Trapped Region
Carlos Barcel=F3, Stefano Liberati, Sebastiano Sonego, and Matt Visser
Phys. Rev. Lett. (to be published)

Black Holes that Don't Trap
http://focus.aps.org/story/v18/st12

the Hawking-like sound waves radiating from a black hole "analogue"

imagined.............. (perhaps.................. The researchers speculate.

Black Holes that Don't Trap
http://focus.aps.org/story/v18/st12

The team modeled a frictionless fluid flowing down a narrow pipe. They
imagined first that the flow speed accelerated with distance down the
pipe (perhaps because of a decrease in diameter) and second that the
maximum velocity increased with time. If the downstream flow reached
sound speed, they calculated that an upstream observer would hear a
Hawking-like hum. However, the team discovered that they could slowly
turn up the velocity--getting close to but never quite reaching
supersonic speeds--and yet the same phonon radiation came out. "It was
surprising. I was expecting to see no radiation," says co-author
Stefano Liberati of the International School of Advanced Studies in
Trieste, Italy.

The researchers speculate that a collapsing star could emit Hawking
radiation without ever forming an event horizon. From a distance, it
would still look like a black hole as long as it kept collapsing,
because light would be strongly affected by the severely warped
spacetime. But William Unruh of the University of British Columbia in
Vancouver thinks this scenario is unrealistic. To emit the expected
amount of radiation, he says, the star would have to shrink very close
to the compactness needed for an event horizon to form, so close that
the difference would not be physically meaningful.