Classical and Quantum Signatures of Black Holes

About This Grant

Black holes are at the forefront of current mysteries in both astrophysics and fundamental physics. Ubiquitous in the universe, they serve as an energizing source for a variety of spectacular observational phenomena, but the underlying processes are poorly understood. Similarly, black holes are key players in thought experiments probing the apparent incompatibility between general relativity and quantum theory, the reconciliation of which is arguably the biggest open question in fundamental physics. In the black hole context, two normally disparate areas--astrophysics and fundamental physics--suddenly share many mathematical and physical threads, and simultaneous study of the two creates new opportunities and synergies. This award is concerned with the appearance and effects of a black hole on an outside observer (its "signatures"), both in real astrophysical observation and for hypothetical observers featured in thought experiments probing quantum gravity. In particular, the award studies how a black hole bends light to create a "photon ring" visible to future radio telescopes, and how a black hole passively destroys ("decoheres") quantum states and their associated information. The team will train students in STEM areas. Studies of the black hole photon ring will focus on which aspects can be observed with the next generation of telescopes, and how these aspects encode information about the black hole spin. The key challenge is that the effects of black hole spin are subtle, and easily confused with errors in modeling the astrophysical plasma (accretion flow) providing the emission. The general strategy to break such degeneracies is to consider the photon ring not in isolation, but in comparison with the main emission. Since the two originate from the same source, only affected differently by the black hole, their relative properties encode information about the black hole. The award pursues theoretical modeling focusing on the relative astrometry, thickness, and shape, aiming to discover a robust method for measuring black hole spin with near-term electromagnetic observations of supermassive black holes. Studies of decoherence by black holes will focus on obtaining precise decoherence rates and comparing those to analogous rates for ordinary bodies at finite temperature. Currently, precise rates are available only for decoherence mediated by electromagnetic or scalar fields; the award will produce analogous results for gravitationally mediated decoherence. The key challenge is that the "laboratory" hosting the quantum state must now be modeled along with the state itself, introducing technical and conceptual complications. However, it is precisely these challenges that give the gravitational problem its special interest: everything gravitates, so the black hole decoherence effect is universal and unavoidable, and may play a key role in questions of information flow in quantum gravity. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.