simultanitet

Simultanitet refers to the condition of two or more events sharing the same temporal coordinate, i.e., happening at the same time, within a given reference frame. In everyday language it can be used loosely to describe coincident timing. In scientific usage it is defined within a specified frame of reference or coordinate system, and may differ between observers.

In physics, especially in the theory of relativity, simultaneity is not absolute. The relativity of simultaneity states that whether two spatially separated events occur at the same time depends on the observer's state of motion. A classic illustration uses a moving train and an observer on a platform; signals emitted from the ends of the train are observed at different times by different observers. Synchronization of clocks via light signals is a standard method to define simultaneity in a frame, but the definition itself is conventional and relative.

In philosophy, simultaneity is discussed in relation to the structure of time, causality, and the debate between presentism and eternalism. Some theories treat simultaneity as a primitive, relational property of events; others treat it as an emergent feature of how we segment time.

In other fields, simultaneity appears in data analysis and signal processing, where multiple streams are aligned to a common time axis, or in linguistics and cognitive science when modeling the perception of simultaneity in sensory input or the timing of joint actions.

Origin and usage: The word derives from Latin simul "at the same time" and is cognate with the English simultaneity and the French simultanéité. In Scandinavian languages, simultanitet is common.