Press Release: Violent Acceleration and the Event Horizon
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Date Issued: Jun 06, 2000
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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 to the vacuum again, without violating the energy conservation law. According to Hawking, if a particle and 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 more and more faint 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 the primordial "mini-holes" that may have been formed shortly after the Big Bang. Such black holes would have a mass of 1015 grams, smaller than the size of an atom. The possibility of detecting such mini-holes, however, is uncertain.
Pisin Chen, whose work at SLAC is supported by the Department of Energy, showed theoretically 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 a sufficient amount of radiation. For example, one would need to accelerate a particle over 1020 m/sec2 to generate a temperature at 1 degree Kelvin. It turns out that state-of-the-art lasers can deliver sub-picosecond pulses 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 proposed experiments to detect the Unruh radiation. Unruh himself suggested that sound waves propagating in a supersonic fluid behave similarly to the 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 into a plasma. In addition, Simon Darbinyan of the Yerevan Physics Institute in Armenia and co-workers suggested that the Unruh radiation could be emitted by a beam of particles that channel through a crystal lattice.
The proposed Linac Coherent Light Source (an X-ray free electron laser) at SLAC, and other FEL facilities, would have the capacity of conducting such an experiment. Construction of the LCLS could start as early as 2003, with completion in 2006. Petawatt-class "table-top" lasers currently under development in various laboratories might also be invoked for such a test.
It is yet to be seen whether this new approach proposed by Chen can eventually provide insights into the very important Hawking effect. Chen admits 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," said Chen.