We show how Event Horizon Telescope (EHT) observations of the supermassive object at the center of M87 can constrain deviations from General Relativity (GR) in a relatively model-independent way. We focus on the class of theories whose deviations from GR modify black holes into alternative compact objects whose properties approach those of an ordinary black hole sufficiently far from the would-be event horizon. We examine this class for two reasons: ($i$) they tend to reproduce black-hole expectations for astrophysical accretion disks (and so do not undermine the evidence linking black holes to active galactic nuclei); ($ii$) they lend themselves to a robust effective-field-theory treatment that expands in powers of $\ell/r$, where $\ell$ is the fundamental length scale that sets the distance over which deviations from GR are significant and $r$ is a measure of distance from the would-be horizon. At leading order the observational impact of these types of theories arise as modifications to the transmission and reflection coefficients of modes as they approach the horizon. We show how EHT observations can constrain this reflection coefficient, assuming only that the deviations from GR are small enough to be treated perturbatively. Our preliminary analysis indicates that such reflection coefficients can already be constrained to be less than of order 10\% (corresponding to $\ell \lesssim 100 \mu m$), and so can rule out some benchmark cases used when seeking black-hole echoes. The precise bounds depend on the black hole spin, as well as on detailed properties of the reflection coefficient (such as its dependence on angular direction).

中文翻译：

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用事件视界望远镜约束基础物理学

我们展示了事件视界望远镜 (EHT) 对 M87 中心超大质量物体的观测如何以相对独立于模型的方式限制对广义相对论 (GR) 的偏差。我们关注一类理论，它们与 GR 的偏差将黑洞修改为替代的致密物体，其特性接近距离可能的事件视界足够远的普通黑洞的特性。我们研究这个类有两个原因：（$i$）它们倾向于重现对天体物理吸积盘的黑洞预期（因此不会破坏将黑洞与活动星系核联系起来的证据）；($ii$) 它们适用于强大的有效场理论处理，该处理扩展了 $\ell/r$ 的幂，其中 $\ell$ 是基本长度尺度，它设置了与 GR 的偏差显着的距离，而 $r$ 是与可能的地平线的距离度量。这些类型的理论的观测影响首先出现在模式接近地平线时对传输和反射系数的修改。我们展示了 EHT 观测如何约束这个反射系数，假设与 GR 的偏差小到足以进行微扰处理。我们的初步分析表明，这样的反射系数已经可以被限制为小于 10\%（对应于 $\ell\lesssim 100 \mu m$），因此可以排除一些在寻找黑洞时使用的基准情况回声。精确的界限取决于黑洞自旋，