TL;DR
This paper examines the fundamental speed at which gravitational effects propagate in general relativity, revealing that under certain conditions, gravity's signal is causal and does not exceed light speed, with implications for black hole information.
Contribution
It clarifies the conditions under which the gravitational signal is causal, superluminal, or semi-superluminal, and discusses the implications for the speed of gravity in different spacetime classes.
Findings
Gravitational signals can be causal, superluminal, or semi-superluminal.
In globally hyperbolic spacetimes, superluminal effects coincide with semi-superluminal.
Under certain conditions, the speed of gravity does not exceed the speed of light.
Abstract
The question is discussed of what is the speed of gravity (at the fundamental non-perturbative level). The question is important, if nowhere else, in discussing the problem of information "lost" in black holes. It turns out that the duly defined "gravitational signal" generally may be causal, superluminal and "semi-superluminal". In the class of globally hyperbolic spacetimes the two last varieties coincide. And if some (often imposed, but not always satisfied) conditions hold, the signals may be \emph{only} causal. In this sense the speed of gravity does not exceed the speed of light.
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