space-time


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A four-dimensional coordinate system, or reference frame, that physicists and astronomers use to describe the universe. Space-time has three spatial axes (x, y, z) and one time axis (t). A point in this reference frame is called an event, because it is something that happens in space and time. Einstein's General Theory of Relativity explains that the speed of light is constant between all reference frames, regardless of their motion relative to each other.