A newly proposed experiment promises to create a “tabletop” black hole that could prove whether information is truly lost when black holes evaporate. The idea that information could be lost this way has created a paradox in our current understanding of basic physics.
43 years ago, Stephen Hawking combined quantum field theory with Einstein’s theory of general relativity and discovered black hole evaporation. The debate over whether information is really lost during Hawking evaporation has persisted ever since. Almost all the contemporary leading theoretical physicists have participated in this “black hole war”. In quantum mechanics, the probability, or information, must be preserved before and after a physical process. The seeming loss of information as a result of the black hole evaporation therefore implies that general relativity and quantum mechanics, the two pillars of modern physics, may be in conflict.
Fig.1: Accelerating mirror mimics evaporating black hole. Left: Black hole Hawking evaporation and the trapping of the partner modes near the horizon. Right: An accelerating mirror also has a horizon and can also emit Hawking particles and trap their partner modes. The analogy between these two systems may be appreciated via Einstein’s equivalence principle.
So far investigations of this paradox have been mostly theoretical because of the difficulty of observing black holes in their later stages, when this potential contradiction is most acute. According to theory, a solar-size black hole would take 1067 years to evaporate entirely, yet our universe is only about 1010 years old. Therefore essentially all astrophysical black holes are too young to provide useful clues on the information loss paradox even if they are observed, such as that responsible for the gravitational waves observed by LIGO in 2016.
Now, in a paper that was published in Physical Review Letters on January 23 (Phys. Rev. Lett. 118, 045001 (2017); http://journals.aps.org/prl/issues/118/4), Pisin Chen, Professor of Physics and Director of the Leung Center for Cosmology ad Particle Astrophysics (LeCosPA), National Taiwan University, and Gerard Mourou, Professor and Director of International Center for Zeta-Exa-Watt Science and Technology (IZEST), École Polytechnique, conceived a laboratory black hole to simulate this evaporation. Using state-of-the-art laser and nanofabrication technologies, they plan to mimic black hole evolutions at their later stage, to reveal crucial details on how information may be preserved during black hole evaporation.
According to Einstein’s equivalence principle, an accelerating mirror moving near the speed of light shares some common features with a true black hole. In both cases, there exist an event horizon. Interacting with quantum fluctuations in vacuum near the horizon, both will emit Hawking particles and trap their partner modes (Fig.1) until the black hole evaporates entirely or the accelerating mirror suddenly stops. By then the partner modes will be released. The purpose of this proposed experiment is to see whether and how the Hawking particles and their partners are entangled and therefore how the information would be preserved.
It is known that an intense laser traversing a plasma would push the intercepting plasma electrons to its back, which is called by experts the “plasma wakefields”. Triggered by extremely intense lasers, such plasma density perturbations can be so concentrated that it can serve as a flying reflecting mirror. The authors pointed out in the paper that by properly tailoring the increase of the density of a thin-film target using nanofabrication technology, a relativistic plasma mirror would accelerate as the driving laser continues to enter higher density regions. At the time when the laser leaves the thin-film target, the plasma mirror would abruptly stop its motion, which mimics the ending of the Hawking evaporation (see Fig.2).
In addition to being published by Physical Review Letters, one of the most prestigious physics journals in the world, this paper, entitled “Accelerating Plasma Mirrors to Investigate Black Hole Information Loss Paradox”, was highlighted by PRL as “Editors’ Suggestion”. In addition, it was featured as a “Synopsis” in American Physical Society’s online magazine Physics (http://physics.aps.org) on January 23, 2017. On the average only a small percentage of PRL papers received such an honor.
An international collaboration has been formed, which consists of National Taiwan University, École Polytechnique, Kansai Photon Research Institute in Kyoto, and Shanghai Jiao Tong University, to carry out this experiment.
Fig.2: A schematic diagram of the proposed analog black hole experiment. The first, gaseous and uniform plasma target is used to prepare a high intensity x-ray pulse. The x-ray pulse will then induce an accelerating plasma mirror due to the increasing plasma density in the second target. As the mirror stops abruptly, it will release either a burst of energy or zero-point fluctuations. The entanglement between either of these signals and the Hawking photons emitted earlier is measured upstream.
Pisin Chen Gerard Mourou
ABOUT ÉCOLE POLYTECHNIQUE / École Polytechnique is the leading French institute which combines top-level research, academics, and innovation at the cutting-edge of science and technology. Its various undegraduate and graduate-level programs – Bachelor’s degree, Ingénieur Polytechnicien, Master’s, Graduate Degree, PhD program and PhD– are highly selective and promote a culture of excellence with a strong emphasis on science, anchored in humanist traditions. As a widely internationalized university, École Polytechnique offers a variety of international programs and attracts a growing number of foreign students and researchers from around the globe (currently 30% of students and 39% of faculty members).