An event horizon is the boundary around a black hole beyond which nothing—not even light—can escape. It marks the point where the escape velocity equals the speed of light.
From an outside observer's perspective, objects falling toward the horizon appear to slow down and redden due to gravitational time dilation and redshift, fading gradually but never seeming to cross the boundary.
In contrast, an object falling into the hole would cross the horizon in a finite time according to its own clock and would not experience any dramatic effects at that exact boundary in large black holes—no ‘wall’ or sudden force at the horizon itself.
The radius of a non-rotating black hole’s event horizon is called the Schwarzschild radius, which depends on the mass of the black hole: larger mass equals a larger horizon.
Once past the event horizon, all future paths lead inward—toward the singularity. Light cones tip inward, making escape physically impossible, trapping matter and information within.
The Event Horizon Telescope produced the first direct images of black hole shadows, observing the silhouette of the M87* and Sagittarius A* supermassive black holes, providing strong evidence for the existence and properties of event horizons.
Event horizons are not material surfaces but mathematical boundaries derived from spacetime geometry. Quantum effects near horizons—such as Hawking radiation—cause black holes to slowly evaporate over extremely long timescales.
1 July 2013
What would it look like to orbit a black hole? Since the strong gravity of the black hole can significantly alter light paths, conditions would indeed look strange. For one thing, the entire sky would be visible, since even stars behind the black hole would have their light bent to the observer's eye. For another, the sky near the black hole would appear significantly distorted, with more and more images of the entire sky visible increasingly near the black hole. Most visually striking, perhaps, is the outermost sky image completely contained inside an easily discernible circle known as the Einstein ring. Orbiting a black hole, as shown in the above scientifically-accurate computer-created illustrative video, will show stars that pass nearly directly behind the black hole as zipping around rapidly near the Einstein ring. Although star images near the Einstein ring may appear to move faster than light, no star is actually moving that quickly. The above video is part of a sequence of videos visually exploring the space near a black hole's event horizon. (Disclosure: Video creator Robert Nemiroff is an editor for APOD.)