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Thursday, March 12, 2009

Black holes and decoherence

New paper!

The black hole information problem (Wikipedia entry) is considered one of the deepest puzzles in theoretical physics. It is a "respectable" area of research with connections to string theory, quantum gravity and quantum information theory. In contrast, quantum foundations -- for example, the measurement problem (see, e.g., the Wikipedia or Stanford encyclopedia of philosophy entry) -- is considered a fringe activity by many working physicists. I've never understood this perspective, unless one claims that quantum foundations are somehow not testable experimentally and therefore more properly in the domain of philosophy. In our paper we argue that experiments relevant to black hole information are actually harder than those which address foundational questions in quantum mechanics. In fact, both types of experiments hinge on the ability to detect or manipulate macroscopic superpositions -- i.e., Schrodinger's Cat states. If the measurement problem and wavefunction collapse belong in the realm of philosophy, then so does the black hole information problem.

As John Bell said, "I am a Quantum Engineer, but on Sundays I have principles"! Bell famously hid his interest in quantum foundations for many years, afraid of the opprobrium of his colleagues at CERN and elsewhere. The first question he would ask of others who approached him about research on such topics was "Do you have a permanent job?" Bell's Theorem has been called "the most profound discovery of science."

For people interested in this subject, some recommended reading.

Against 'Measurement' by J.S. Bell. This paper originated from a lecture he gave in Erice at the summer school (Ettore Majorana) in 1989. I was a student there just the following year, so I narrowly missed meeting him in person, something I will always regret! Bell's discussion of measurement following that of K. Gottfried (pp. 35-38; when does the pure state become a mixture?) is closest to the formulation in our paper.

A nice biographical article on Bell.

A book of essays in honor of Bell: Quantum [Un]speakables, edited by R. Bertlmann and A. Zeilinger, which contains fascinating material about his intellectual development, the Bell inequalities, their theoretical origins and experimental tests.


Black holes, information and decoherence

Stephen D. H. Hsu, David Reeb

We investigate the experimental capabilities required to test whether black holes destroy information. We show that an experiment capable of illuminating the information puzzle must necessarily be able to detect or manipulate macroscopic superpositions (i.e., Everett branches). Hence, it could also address the fundamental question of decoherence versus wavefunction collapse.

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