Event horizon: an area in space-time that is impenetrable in one direction with respect to cause-effect relations.

No event on the inside of an event horizon can have any effects on its outside. Event horizons result from the finite speed of light, which according to Special Relativity Theory is the greatest possible speed for any cause-effect relations. Event horizons typically surround black holes. But we also consider the margin of the observable universe (the Hubble volume) to be an event horizon; however, in this case the event horizon does not occupy a fixed area of space but expands at the speed of light.

We, too, are surrounded by an event horizon that constantly emanates from us, departs from us at the speed of light, and possesses the shape of a cone within space-time.

For reasons of simplicity, we shall here represent space as a two-dimensional plane. Time progresses toward the top of the plane. The point in the center represents an event in the present — for example, the position of an observer at a certain time. A light signal emitted by the observer is transmitted in space-time at the speed of light, adopting the shape of a cone that is open at the top. The slope of the cone is a result of the speed of light.

Events outside of this cone of light cannot be observed or affected in principle from the standpoint of present time since any such interaction would have to be faster than the speed of light. This is why we cannot do anything on Earth that would have an immediate effect on the surface of the moon. Thus, an event that takes place on Earth at a certain moment can only affect the moon at a later moment separated from it by the time it takes for a light ray to travel from the one body to the other. Only then do the two events, cause and effect, both lie within the cone. Thus, the cone of light marked as "future" in the diagram is an event horizon that separates the accessible and observable inner area from the outside area that can be neither observed nor causally affected.

Conversely, any event is observable or subject to causal influence only from the standpoint of events within its "past" cone of light. If the sun exploded at this very moment we would not notice, for the explosion itself would be nothing but a point on the plane of the present beyond our own event horizon. Only when the explosion enters the cone of light of the past, which would take about nine minutes owing to the sun's distance from us, would we experience the effect.

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