Types of quantum field thories
Fix a timelike curve in spacetime, thought of as representing an observer. If is a hypersurface? in spacetime, then (at least if spacetime is orientable and possibly otherwise), separates spacetime into two regions, arbitrarily called the inside and outside. Then is an event horizon (relative to ) if never intersects the future of the inside of . (It follows that lies entirely outside .)
Although we have given this definition relative to an observer, we can reverse the situation, beginning with a lightlike hypersurface and noting that is a horizon relative to every observer that remains in the past of (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?.