nLab event horizon





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Fix a timelike curve γ\gamma in spacetime, thought of as representing an observer. If HH is a hypersurface in spacetime, then (at least if spacetime is orientable and possibly otherwise), HH separates spacetime into two regions, arbitrarily called the inside and outside. Then HH is an event horizon (relative to γ\gamma) if γ\gamma never intersects the future of the inside of HH. (It follows that γ\gamma lies entirely outside HH.)

Although we have given this definition relative to an observer, we can reverse the situation, beginning with a lightlike hypersurface HH and noting that HH is a horizon relative to every observer that remains in the past of HH (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 Bekenstein-Hawking 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?.


Resolved image of the direct vicinity of the event horizon of the black hole in the center of the galaxy Messier 87:

Discussion of event horizons of black holes in terms of AdS/CFT is in

  • Kyriakos Papadodimas, Suvrat Raju, An Infalling Observer in AdS/CFT (arXiv:1211.6767)

Last revised on April 10, 2019 at 14:11:19. See the history of this page for a list of all contributions to it.