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Fix a timelike curve $\gamma$ in spacetime, thought of as representing an observer. If $H$ is a hypersurface? in spacetime, then (at least if spacetime is orientable and possibly otherwise), $H$ separates spacetime into two regions, arbitrarily called the inside and outside. Then $H$ is an event horizon (relative to $\gamma$) if $\gamma$ never intersects the future of the inside of $H$. (It follows that $\gamma$ lies entirely outside $H$.)
Although we have given this definition relative to an observer, we can reverse the situation, beginning with a lightlike hypersurface $H$ and noting that $H$ is a horizon relative to every observer that remains in the past of $H$ (of which, unlike for a spacelike hypersurface, there are typically many). This includes the horizon around a black hole and the future light cone of any event.
Theoretically, the observer outside an event horizon will observe BekensteinHawking entropy at the horizon and Hawking radiation? from it. In the case of an observer accelerating to remain outside a future light cone, this is the Unruh effect?.
Discussion of event horizons of black holes in terms of AdS/CFT is in
Last revised on August 27, 2018 at 02:07:49. See the history of this page for a list of all contributions to it.