College of Engineering

Physics Colloquium: Complementarity, and the Quantum Mechanics of Black Holes

Date(s): 11/28/2012 4:30 PM - 11/28/20125:30 PM
Location: Seng 102
Contact: Robert Fisher 508-999-8353

David Kagan

UMass Dartmouth Physics Department

Title: Complementarity, and the Quantum Mechanics of Black Holes


Before the 1970s, physicists thought that black holes were truly black---that is---they gave off no radiation. How could they? Once within the event horizon, nothing---not even radiative light---could escape. This picture changed due to the pioneering work of Jacob Bekenstein and Stephen Hawking. Quantum mechanics implied that black holes would indeed radiate.

But radiating black holes lead to an "information paradox". Quantum mechanics requires that information about a system is conserved---probabilities must always add up to 1. However, if black holes are able to radiate away, then won't they carry the information that has fallen in with them? This is the paradox.

If you try to resolve this paradox by positing that the lost information is actually preserved in the black hole's radiation, this leads to yet another apparent conflict with quantum mechanics. However, as I will discuss, an updated version of Niels Bohr's concept of "complementarity" helps us avoid these conflicts. If these ideas are correct, they help reveal some profound aspects of the nature of spacetime and observers in a quantum theory of gravity.

Bio : David Kagan received his Ph.D. in theoretical physics from Cambridge University in 2007 where he studied as a Kellett scholar and NSF Graduate Fellow. He subsequently was a postdoctoral Science Fellow at Columbia University where he pursued a number of research projects in string theory. As a Science Fellow, he participated in teaching and developing Frontiers of Science, a course ranging over numerous cutting edge topics in the sciences, from particle physics to neuroscience. David is now a member of the faculty at UMass-Dartmouth where he continues to enjoy his dual passions of teaching and research.

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