Space and time

In physics, spacetime is any mathematical model that combines space and time into a single interwoven continuum.

Until the turn of the 20th century, the assumption had been that the 3D geometry of the universe (its description in terms of locations, shapes, distances, and directions) was distinct from time (the measurement of when events occur within the universe). However, Albert Einstein's 1905 special theory of relativity postulated that the speed of light through empty space has one definite value—a constant—that is independent of the motion of the light source. Einstein's equations precisely described important consequences of this fact: both the shape of space and the measurement of time simultaneously change for observers who have different relative velocities, which is to say, for observers who have different "inertial frames of reference."

Einstein's theory was framed in terms of kinematics (the study of moving bodies), and showed how measurements of space and time varied for observers in different reference frames. His theory was a breakthrough advance over Lorentz's 1904 theory of electromagnetic phenomena and Poincaré's electrodynamic theory. Although these theories included equations identical to those that Einstein introduced (i.e. the Lorentz transformation), they were essentially ad hoc models proposed to explain the results of various experiments—including the famous Michelson–Morley interferometer experiment—that were extremely difficult to fit into existing paradigms.

In 1908, Hermann Minkowski, expanding upon Einstein's work, presented a geometric interpretation of special relativity that fused time and the three spatial dimensions of space into a single four-dimensional continuum—what mathematicians call a 4‑dimensional "manifold." A key feature of this interpretation is the definition of a "spacetime interval" that combines distance and time. Although measurements of distance and time between events differ among observers, the spacetime interval is independent of the inertial frame of reference in which they are recorded. The resultant "spacetime" came to be known as Minkowski space.

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