**An event horizon is a concept related to black holes, a theoretical prediction of general relativity**. It is defined as the boundary surface beyond which no event can affect an external observer. **According to the theory of relativity, space and time form a single complex called space-time, which is deformed by the presence of mass or energy**.

According to a definition given by Roger Penrose, the event horizon in a black hole is a particular surface of space-time that separates places from which signals can escape from those from which no signals can escape. In a much more general sense, if by event we mean an observable phenomenon, identified by the four space-time coordinates**, an event horizon can be defined as a region of space-time beyond which it ceases to be possible to observe the phenomenon**.

In the case of Schwarzschild black holes, the event horizon is a spherical surface surrounding a singularity at the center of the sphere; the latter is a point at which the density would be infinite and the laws of physics, according to the theory of general relativity, lose their meaning.

**The singularity may not be necessary, according to some theories of quantum gravity**, which postulate space-time as an entity with a physical reality, and not just a mere mathematical concept, divided into discrete elements with a diameter of a Planck length.

In other words, space-time would have, according to the aforementioned theory, a physically active, not passive role and its intimate structure would be made up of real atoms which would form a dense network in constant evolution.

Under normal conditions, the atomic structure of space-time would not be perceived, which would appear to be a mathematical continuum and the Universe would be described by general relativity, but at distances in the order of the Planck length, things would change radically: the quantum and gravitational effects would assume comparable intensities.

It would be as if the space assumed its own physical personality and interacted with energy in an active way. A very common mistake is to imagine the event horizon of a black hole as a more or less spherical static surface.

What is better to keep in mind, however, is that it is a horizon to all intents and purposes, or rather something that cannot be reached and which moves away as an observer approaches.