We use the results of optical/IR monitoring of stellar orbits to show more » that the mass-to-distance ratio for Sgr A* is already known to an accuracy of ∼4%. We show that the black hole in the center of the Milky Way, Sgr A*, is the optimal target for performing this test with upcoming observations using the Event Horizon Telescope (EHT). Therefore, measuring the size of a shadow and verifying whether it is within this 4% range constitutes a null hypothesis test of general relativity. The half opening angle of a Kerr black hole shadow is always equal to (5 ± 0.2)GM/Dc, where M is the mass of the black hole and D is its distance from the Earth. Finally, we discuss physical and numerical limitations of the models, highlighting the possible importance of kinetic effects and duration of the simulations. We also find that all models with i ≥ 70° fail at least two constraints, as do all models with equal ion and electron temperature exploratory, nonthermal model sets tend to have higher 2.2 μm flux density and the population of cold electrons is limited by X-ray constraints due to the risk of bremsstrahlung overproduction. We identify a promising cluster of these models, which are MAD and have inclination i ≤ 30°. A number of models fail only the variability constraints. Light-curve variability provides a particularly severe constraint, failing nearly all strongly magnetized (magnetically arrested disk (MAD)) models and a large fraction of weakly magnetized models. All models fail more » at least one constraint. We test the models against 11 constraints drawn from EHT 230 GHz data and observations at 86 GHz, 2.2 μm, and in the X-ray. Our main approach is to compare resolved EHT data at 230 GHz and unresolved non-EHT observations from radio to X-ray wavelengths to predictions from a library of models based on time-dependent general relativistic magnetohydrodynamics simulations, including aligned, tilted, and stellar-wind-fed simulations radiative transfer is performed assuming both thermal and nonthermal electron distribution functions. In this paper we provide a first physical interpretation for the Event Horizon Telescope's (EHT) 2017 observations of Sgr A*. 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. 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. We find that such quantum more » fluctuations can introduce a strong time dependence for the shape and size of the shadow that a black hole casts on its surrounding emission. In this study, 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. Natural candidates for these modifications behave like metric fluctuations, with characteristic length and time scales set by the horizon radius. If such modifications extend beyond the horizon, they influence regions accessible to distant observeration. 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.
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