Defining the Event Horizon
The event horizon is a boundary in spacetime around a black hole beyond which events cannot affect an outside observer. It signifies the 'point of no return' where the gravitational pull becomes so immense that everything, including light, is irrevocably drawn inwards.
Nature of the Boundary
Contrary to a physical surface, the event horizon is a conceptual boundary. It's defined by the region where the escape velocity required to leave the black hole's gravitational field exceeds the speed of light. Once anything crosses this threshold, it is permanently trapped.
Practical Implications of Crossing
If an object were to cross an event horizon, it would experience no immediate physical sensation at the moment of crossing. However, from that point on, all paths lead deeper into the black hole, and no information or signal could ever be transmitted back to the external universe.
Significance in Black Hole Physics
The event horizon is fundamental to understanding black holes, dictating their effective 'size' and the limits of their interaction with the cosmos. It represents the ultimate frontier for testing the most extreme predictions of Albert Einstein's theory of general relativity.