**Event Horizon Telescope Observations as Probes for Quantum Structure of Astrophysical Black Holes**

Steven B. Giddings and Dimitrios Psaltis

[ct_accordion title=”Abstract (click to read)” text=”Abstract” icon=”” state=”closed” style=”normal” group=”group1″ ]**Abstract:** The need for a consistent quantum evolution for black holes has led to proposals that their semiclassical description is modified not just near the singularity, but at horizon or larger scales. If such modifications extend beyond the horizon, they influence regions accessible to distant observeration. Natural candidates for these modifications behave like metric fluctuations, with characteristic length and time scales set by the horizon radius. We investigate the possibility of using the Event Horizon Telescope to observe these effects, if they have a strength sufficient to make quantum evolution consistent with unitarity. We find that such quantum fluctuations can introduce a strong time dependence for the shape and size of the shadow that a black hole casts on its surrounding emission. For the black hole in the center of the Milky Way, detecting the rapid time variability of its shadow will require non-imaging timing techniques. However, for the much larger black hole in the center of the M87 galaxy, a variable black-hole shadow, if present with these parameters, would be readily observable in the individual snapshots that will be obtained by the Event Horizon Telescope.[/ct_accordion]

For more information on this work, read the paper submitted to Physical Review D. In this web page, you can find some useful animations, the stills of which appear as figures in the paper.

The discovery of Hawking radiation yields a logical contradiction when one tries to account for quantum information absorbed by a black hole: this information can’t escape, can’t be destroyed, and can’t be preserved after the black hole evaporates. This situation appears to represent a fundamental conflict between the principles underpinning local quantum field theory: the principles of quantum mechanics, relativity, and locality. Therefore, while it has long been believed that the vicinity of the horizon is well-described by classical general relativity, since curvatures are expected to be small there, many theorists who study quantum evolution of black holes have now concluded that there must be modifications to their description via local quantum field theory, and that in order to resolve the conflict, these modifications must extend at least to horizon scales. A natural candidate for these yields “soft” quantum deformations of the effective metric near a black hole.

In the near future two new approaches to probing the innermost regions of black hole spacetimes will become available, with the potential of offering probes of their quantum structure that are clean of astrophysical complexities. The first involves gravitational wave observations either of coalescing black holes (such as the initial LIGO detection of the source GW150914) or of extreme mass ratio inspirals. The second involves obtaining images of accreting black holes with horizon-scale resolution using the Event Horizon Telescope. The goal of our work is to investigate the possible signatures of soft quantum modifications to black hole metrics that could be imprinted on Event Horizon Telescope observations, and thus the sensitivity of these observations to such effects.

As a first step, we performed a number of exploratory calculations of the effect on photon trajectories of different monochromatic perturbations of the black-hole metric in order to identify the range of parameters that will introduce observable effects on the predicted images.

**Figure 1.**The trajectories of photons approaching a non-spinning black hole in plane parallel rays from the far infinity (from the right side of the animation). The frames of the animation correspond to different phases of a black hole with a single, spherically symmetric (l=m=0) metric perturbation. Photon trajectories are colored blue if (in the absence of any perturbations) they would have crossed the event horizon and are colored red, if they would have escaped to infinity. As expected, metric perturbations cause some of the blue trajectories to escape to infinity and some of the red trajectories to cross the event horizon, at different phases of the perturbation.

**Figure 2.**Same as in Figure 1 but for two phases of an l = m = 2 perturbation mode.

We then used a simple model for the plasma in the accretion flow around a black hole in order to make concrete predictions of the effects of strong, soft quantum perturbations on the image of accreting black holes.

**Figure 7.**Time-dependent images using a simple plasma model for the emission from the inner accretion flow around a perturbed black hole. The metric perurbations change in a time-dependent manner the image of the flow as well as the size, brightness, and width of the bright photon ring that surrounds the black-hole shadow.

**Figure 9.**Same as Figure 7, but for a superposition of modes. The superposition causes the shape and size of the black-hole shadow to be highly asymmetric and variable.