Editing T-symmetry (section)

===Black holes===
The laws of gravity seem to be time reversal invariant in classical mechanics; however, specific solutions need not be.

An object can cross through the [[event horizon]] of a [[black hole]] from the outside, and then fall rapidly to the central region where our understanding of physics breaks down. Since within a black hole the forward light-cone is directed towards the center and the backward light-cone is directed outward, it is not even possible to define time-reversal in the usual manner. The only way anything can escape from a black hole is as [[Hawking radiation]].

The time reversal of a black hole would be a hypothetical object known as a [[white hole]]. From the outside they appear similar. While a black hole has a beginning and is inescapable, a white hole has an ending and cannot be entered. The forward light-cones of a white hole are directed outward; and its backward light-cones are directed towards the center.

The event horizon of a black hole may be thought of as a surface moving outward at the local speed of light and is just on the edge between escaping and falling back. The event horizon of a white hole is a surface moving inward at the local speed of light and is just on the edge between being swept outward and succeeding in reaching the center. They are two different kinds of horizons—the horizon of a white hole is like the horizon of a black hole turned inside-out.

The modern view of black hole irreversibility is to relate it to the second law of thermodynamics, since black holes are viewed as [[Black hole thermodynamics|thermodynamic objects]]. For example, according to the [[gauge–gravity duality]] conjecture, all microscopic processes in a black hole are reversible, and only the collective behavior is irreversible, as in any other macroscopic, thermal system.{{Citation needed|date=April 2010}}