Press Release: Violent Acceleration and the Event Horizon

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Date Issued: Jun 06, 2000

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Office for Communications, Stanford Linear Accelerator Center: Telephone: 650-926-8703 Fax Number: 650-926-8793

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Pisin Chen, Stanford Linear Accelerator Center: 1 (650)926-3384, chen@slac.stanford.edu

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Stanford Linear Accelerator Center

Using high-intensity lasers, scientists hope to simulate a black hole event horizon in a laboratory, something that has never been done before. Today at the American Astronomical Society meeting in Rochester, New York, Dr. Pisin Chen, from DOE's Stanford Linear Accelerator Center, Stanford University, presented his theory. 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) which theoretically could be observed in the lab. "Hawking's finding uncovered a deep connection between gravitation, quantum mechanics, and thermodynamics," said Chen, " 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 within the decade at a variety of laboratories.

Black holes are astrophysical objects predicted by Einstein's General Theory of Relativity. They are supposed to be 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 in England made a seminal theoretical discovery 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 showed that this radiation has a well-defined temperature that is proportional to the gravitational force at its event horizon.

Dr. Pisin Chen
Stanford Linear Accelerator Center

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.

Figure 1
(select image for a larger view)

In 1976, Bill Unruh of the University of British Columbia in Canada showed that an accelerated observer would experience a similar "heat bath" of photons around him, due also to the existence of an event horizon in this case (See Figure 1). 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 with the Hawking effect. 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.

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.

Figure 2
(select image for a larger view)

In all these proposed experiments, however, the Unruh signal would be buried under the 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 proposes 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 (See Figure 2).

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.

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