Event Horizon
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.
Sources
APODs including "Event Horizon"
The Holographic Principle
18 May 2003
Is this image worth a thousand words? According to the Holographic Principle, the most information you can get from this image is about 3 x 1065 bits for a normal sized computer monitor. The Holographic Principle, yet unproven, states that there is a maximum amount of information content held by regions adjacent to any surface. Therefore, counter-intuitively, the information content inside a room depends not on the volume of the room but on the area of the bounding walls. The principle derives from the idea that the Planck length, the length scale where quantum mechanics begins to dominate classical gravity, is one side of an area that can hold only about one bit of information. The limit was first postulated by physicist Gerard 't Hooft in 1993. It can arise from generalizations from seemingly distant speculation that the information held by a black hole is determined not by its enclosed volume but by the surface area of its event horizon. The term "holographic" arises from a hologram analogy where three-dimension images are created by projecting light though a flat screen. Beware, other people looking at the above image may not claim to see 3 x 1065 bits -- they might claim to see a teapot.