- noun the boundary of a black hole, at which its gravitational field is strong enough to prevent the escape of electromagnetic radiation
- The surface of a black hole, at which a distant observer sees time stop and below which he or she can see nothing. From the point of view of an observer travelling into a black hole, the event horizon is the point beyond which it is not possible to escape the hole, however powerful a rocket is used. Black holes are small – even one of 100 solar masses has an event horizon with a radius of just under 300km. This is the Schwarzschild radius.
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