Astronomical tests for quantum black hole structure

Abstract

Quantum modifications to black holes on scales comparable to the horizon size, or even more radical physics, are apparently needed to reconcile the existence of black holes with the principles of quantum mechanics. This piece gives an overview of some possible observational tests for such departures from a classical description of black holes, via gravitational wave detection and very long baseline interferometry. (Invited comment for Nature Astronomy.) * giddings@ucsb.edu 1 arXiv:1703.03387v2 [gr-qc] Oct 2017Black holes present a profound challenge to our current foundations of physics, and an exciting era of astronomy is just opening where gravitational wave observation, represented by the recent LIGO detections [1], and very long baseline interferometry (VLBI), which is on the verge of resolving the immediate environs of super-massive black holes [2], may provide important hints about the new principles of physics needed.While the near-horizon region of a black hole is intrinsically interesting and has not been directly probed before, the conventional expectation has been that physics here, for astrophysical black holes, will be governed by classical general relativity (GR), and so the new observations should simply confirm GR's validity. After all, the spacetime curvature near such a horizona basic measure of the local gravity -would be tiny as compared to the values expected to be associated with any quantum behavior of spacetime.However, Hawking's discovery that black holes evaporate [3] has shaken this view to the core, with many quantum physicists now concluding that this result implies significant modifications to spacetime near black holes, and perhaps even further away. In fact, an attempt to give a logically consistent description of the evolution of black holes has lead to a foundational crisis in physics, which is apparently difficult to resolve without modifications to conventional physics not just at very short distance scales, as is expected in quantum gravity, but on macroscopic scales comparable to horizon radii of large black holes.The problem arises when considering information that falls into a black hole. In our most basic framework for currently established physics, quantum field theory, information cannot propagate faster than light, so then cannot escape. Therefore if a black hole evaporates away and disappears, as Hawking predicted, it destroys the information that fell in. This violates a bedrock principle of quantum mechanics, unitarity. Physicists have struggled with this issue for forty years [3], without finding a logically consistent resolution respecting current principles of physics. This problem appears to represent a profound conflict between the principles of relativity, the principles of quantum mechanics, and the principle of locality, which are the foundational principles of quantum field theory -our most fundamental current description of reality.Apparently something must give. Attempts to modify quantum mechanics have led to nonsensical conclusions, so many no...

Mixing of gravitational wave echoes

“…However, Cardoso et al have pointed out that the ringdown waveform detected is related only with the light ring of the post-merger object rather than the horizon [3], see [4] for a review. Thus although the GW events observed are compatible with the BHs predicted in General Relativity (GR) [5,6], it is still possible that the new physics might present near the horizon [7,8].…”

“…Defining the operator A = ∂ 2 ∂x 2 + ω 2 , we have A −1Ψ = g(x, x ′ )Ψdx ′ , where Green's function g(x, x ′ ) satisfying the boundary conditions (8) and (9) is…”

First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

“…A defining feature of black holes is their event horizon, a one-way causal boundary in spacetime from which not even light can escape (Schwarzschild 1916). The production of black holes is generic in GR (Penrose 1965), and more than a century after Schwarzschild, they remain at the heart of fundamental questions in unifying GR with quantum physics (Hawking 1976;Giddings 2017).…”

Astrophysical Black Holes

“…Einstein's theory of general relativity has been extensively tested in weak gravitational fields [118], while the strong gravity regime is still largely unexplored. The spacetime around astrophysical black holes should be well described by the Kerr solution, but deviations from standard predictions can be possible if general relativity is not the correct theory of gravity, in the presence of exotic fields, or in the case of macroscopic quantum gravity effects [119][120][121].…”