Issue of
June 7, 2000

Black holes -- in the laboratory?
Physicists hope to simulate 'event horizon'

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 1015 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 1020 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 (1015 watts) of power. These technologies can in principle accelerate electrons over 1025 times the acceleration due to the gravity on Earth's surface, or 1028m/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. SR