|
Stanford University
Contact:
P.A. Moore, SLAC
(650) 926-2605; e-mail: xanadu@slac.stanford.edu
6/5/00
Physicists hope to simulate a black hole event horizon in the lab
Einstein`s General Theory of Relativity predicts the existence of black
holes -- astrophysical objects so dense that even light cannot escape from
them. The boundary around the black hole (where the light cannot escape)
is called the "event horizon." In 1974, Stephen Hawking of Cambridge
University theorized that a black hole is not entirely black, but could
actually emit "blackbody," or "thermal," radiation (the kind of radiation
that also occurs when the stove is red hot). Hawking said this radiation
has a well-defined temperature that is proportional to the gravitational
force at its event horizon.
Using high-intensity lasers, scientists hope to simulate a black hole event
horizon in a laboratory within the decade, something that has never been
done before. At the June 6 meeting of the American Astronomical Society
in Rochester, N.Y., Dr. Pisin Chen from the Department of Energy`s Stanford
Linear Accelerator Center (SLAC) at Stanford University presented a theory
that supports the possibility of such lab experiments. Chen said that an
electron under violent acceleration, such as that driven by an ultra-intense
laser, would quiver under a "heat bath" of photons that surrounds it and
thereby induce a much stronger Hawking-like radiation (often called Unruh
radiation) that theoretically could be observed in the lab.
"Hawking`s finding uncovered a deep connection between gravitation,
quantum mechanics and thermodynamics," Chen said, "and if we can simulate
this phenomenon in the lab, it will be a major step toward understanding the
nature of event horizons." Such an experiment could take place at a variety
of laboratories.
Under normal circumstances, a vacuum is a space in which there is no
matter. But at the quantum level, the vacuum is full of particles and
antiparticles that constantly appear and disappear. The Heisenberg
uncertainty principle allows these "virtual" particles and antiparticles
to emerge from the vacuum for a brief moment and disappear back into
the vacuum again without violating the energy conservation law. According
to Hawking, if a particle/antiparticle pair is created near the event horizon
of a black hole, gravity will pull one of the particles into the hole
permanently, while the other particle (or antiparticle) can escape, or be
"radiated," from the black hole. "In this way the black hole could radiate
something from nothing," said Chen.
The typical Hawking radiation temperature from solar-mass-sized black
holes is as low as 0.0001 degree Kelvin (close to absolute zero, and
radiation becomes fainter as the temperature decreases). Though of
fundamental importance in physics, Hawking radiation is very hard to
observe directly from space. One curious feature about Hawking radiation
is that the temperature is inversely proportional to the mass of the black
hole. Thus, the only black holes that might render detectable radiation
would be primordial "mini-holes" that may have formed shortly after the
Big Bang. Such black holes would have a mass of 10**15 grams but a
size smaller than an atom. The possibility of detecting such mini-holes,
however, is uncertain.
In 1976, Bill Unruh of the University of British Columbia showed that an
accelerated observer would experience a similar "heat bath" of photons
around him or her, due also to the existence of an event horizon. The
temperature of the heat bath follows the same Hawking temperature
formula, except that instead of the gravitational force, it is proportional
to the magnitude of the observer`s acceleration. Although the Unruh effect
induced by acceleration is not precisely the Hawking effect from black
holes, it nevertheless shares many common characteristics. It is therefore
an intriguing idea that the Hawking effect could be studied using violent
acceleration in the laboratory setting, since the temperature associated
with the Unruh effect can be much higher if the observer is intensely
accelerated.
Chen, whose work at SLAC is supported by the Department of Energy,
theorized that it should be possible to detect the Unruh radiation emitted
by electrons that are accelerated by ultra-intense lasers. One major
challenge with detecting Unruh radiation is that enormous accelerations
are required to produce sufficient radiation. For example, one would need
to accelerate a particle over 10**20 meters per second squared (m/sec2)
to generate a temperature of 1 degree Kelvin. It turns out that
state-of-the-art lasers can deliver pulses of less than a picosecond
(one-trillionth of a second) with petawatts (10**15 watts) of power.
These technologies can in principle accelerate electrons over 10**25
times the acceleration due to the gravity on Earth`s surface, or
10**28 m/sec2, more than two orders of magnitude higher than previous
experimental proposals.
Since the 1980s several groups have proposed experiments to detect Unruh
radiation. Unruh himself suggested that sound waves propagating in a
supersonic fluid behave similarly to quantum fields propagating in the
vicinity of a black hole. The late John Bell of the Geneva-based European
Organization for Nuclear Research (CERN) and Jon Leinaas of the University
of Oslo in Norway suggested that the known polarization effect of high-
energy electrons in circular accelerators is actually a manifestation of
the Unruh effect. Joseph Rogers of Cornell University proposed that a
magnetically confined electron in a so-called Penning trap would give
the Unruh signal. Meanwhile Eli Yablonovitch, now at the University of
California-Los Angeles, proposed that Unruh radiation would be produced
when a gas is suddenly ionized to become a plasma. In addition, Simon
Darbinyan of the Yerevan Physics Institute in Armenia and co-workers
suggested that Unruh radiation could be emitted by a beam of particles
that channel through a crystal lattice.
In all these proposed experiments, however, the Unruh signal would be
buried under much stronger background signals, a problem that Chen has
managed to circumvent. In the idea proposed by Chen, electrons are
instantly accelerated and decelerated in every cycle by a standing wave
formed by two counter-propagating, ultra-intense laser pulses. He
proposed to detect the Unruh radiation from a minute change of the known
classical Larmor radiation emitted when an electron is accelerated.
Despite the high acceleration produced in the petawatt laser, the total
Unruh radiation power is still found to be smaller than that from the
Larmor radiation. However, Chen calculated the angular distribution of
both types of radiation and found a "blind spot" (along the direction of
acceleration) where the Unruh signal dominates the Larmor signal.
The proposed Linac (an abbreviation for Linear Accelerator) Coherent
Light Source (an X-ray free electron laser, or FEL) at SLAC, and other
FEL facilities, would have the capacity for scientists to conduct such
an experiment. Construction of the Linac Coherent Light Source (LCLS)
could start as early as 2003, with completion in 2006. Petawatt-class
"table-top" lasers currently under development in various laboratories
also might be invoked for such a test.
It has yet to be seen whether this new approach proposed by Chen can
eventually provide insights into the Hawking effect. Chen admitted that
his ideas also involve several theoretical and technical assumptions that
need further testing. "Given the importance of the Hawking effect, I think
that continuing the search for Hawking-like signals in the laboratory
setting is a very worthwhile effort," he said. |
