Philosophy of Black Holes Workshop

It is known that all wormholes violate classical energy conditions, non-negativity constraints on contractions of the stress-energy tensor. Since these conditions are violated by quantum fields, it was believed that wormholes can be constructed in the context of semiclassical gravity. But negative energies in quantum field theory are not without restriction: quantum energy inequalities (QEIs) control renormalized negative energies averaged over a geodesic. Thus, QEIs provide restrictions on the construction of wormholes. First, I will briefly discuss both ‘short’ and ‘long’ (without causality violations) wormhole solutions in the context of semiclassical gravity. Then I will present constraints on the Maldacena, Milekhin and Popov ‘long’ wormhole from the smeared and the doubled smeared null energy condition.

What can it mean to attribute a physical entropy to a vacuum region of spacetime—such as a black hole—in the semi-classical regime, where the spacetime geometry is treated classically? In this talk, I give an exposition and analysis of a number of conceptual and foundational problems attending such attributions. I address, inter alia, questions about: how one might try to justify such attributions; the type of entropy at issue (thermodynamical, Gibbsian, Boltzmannian, entanglement, holographic, ...); and what the physical significance of such attributions may be. I conclude with speculative remarks on a few larger philosophical issues about the nature of entropy and of spacetime that the attribution raises.

Bekenstein (1972, 1973)’s use of information-theoretic ideas to argue that the black hole entropy is a thermodynamic entropy has been the subject of a number of criticisms, essentially based on the idea that facts about the black hole's microstates should not concern the information we have about them (Wüthrich, 2017; Dougherty and Callender, 2017). On the other hand, David Wallace has argued that Bekenstein’s original motivation is no longer necessary: modern developments make a compelling case for black hole thermodynamics, without ever mentioning the word `information' (Wallace, 2018; Wallace 2019). But then the question arises, what role does a concept such as information play in the study of black holes? For after all, the paradox of information loss has been a constant source of debate in theoretical physics since Stephen Hawking (1976).Far from any commitment to informational immaterialism, this presentation will argue that the concept of quantum information and, more generally, the framework of enquiry provided by quantum information theory (QIT), has been of great use in the study of black holes. Once it is recognised that, from the outside, black holes are no different from other quantum systems, they are suitable for an information-theoretic analysis: the old puzzle posed by black holes can be reformulated, new questions can be asked, and answers can be sought using the tools provided by QIT. In this presentation, I will illustrate how this framework has been used by the physicists Hayden and Preskill (2007) to answer the following question: `Since black holes are ordinary quantum systems, they do not destroy information, but how exactly does the information come out?'

Hawking's discovery that black holes radiate has led to the black hole information paradox: a puzzle that has fascinated theoretical physicists for many years. Although some aspects of the paradox have been resolved through the help of holography and AdS/CFT, other aspects are still unclear. I will review the paradox and talk about how randomness can be of aid in deepening our understanding of the black hole information paradox.

We study static black holes in scalar-Gauss-Bonnet (sGB) gravity with a massive scalar field as an example of higher curvature gravity. The scalar mass introduces an additional scale and leads to a strong suppression of the scalar field beyond its Compton wavelength. We numerically compute sGB black hole spacetimes and scalar configurations and also compare with perturbative results for small couplings, where we focus on a dilatonic coupling function. We analyze the constraints on the parameters from requiring the curvature singularity to be located inside the black hole horizon $r_h$ and the relation to the regularity condition for the scalar field. For scalar field masses $m r_h \gtrsim 10^{-1}$, this leads to a new and currently most stringent bound on sGB coupling constant $α$ of $α/r_h^2 \sim 10^{-1}$ in the context of stellar mass black holes. Lastly, we look at several properties of the black hole configurations relevant for further work on observational consequences, including the scalar monopole charge, Arnowitt Deser Misner mass, curvature invariants and the frequencies of the innermost stable circular orbit and light ring.