Spacetime is a mathematical model used in physics that merges the three dimensions of space and the one dimension of time into a single four-dimensional continuum. Spacetime diagrams are practical tools for visualizing and understanding relativistic effects, including how observers perceive the location and time of events.
Historically, scientists believed that space's three-dimensional geometry and time were distinct from each other. However, through the application of the Lorentz transformation and the special theory of relativity, new understandings arose, blurring the traditional distinctions between space and time.
In 1908, Hermann Minkowski offered a geometric interpretation of special relativity, fusing time and the three spatial dimensions into a single four-dimensional continuum, now known as Minkowski space. This interpretation became essential to the development of the general theory of relativity, where spacetime is curved by mass and energy.
Classical mechanics treats time as a universal, space-independent measurement. In this context, time is perceived to pass at a constant rate, unaffected by the observer's motion or external factors. It also assumes that space follows the same logical rules as common sense.
In special relativity, however, time cannot be separated from the three dimensions of space, as the rate at which time passes for an object depends on the object's velocity relative to the observer. General relativity extends this idea and explains how gravitational fields can slow the passage of time for an object viewed by an observer outside of the field.
In regular space, a position is denoted by three dimensions – X, Y, and Z in a Cartesian system. But in spacetime, an event's position is defined by four numbers: the three-dimensional location in space and the position in time. Thus, spacetime is four-dimensional.
In spacetime, a particle's path can be perceived as a series of events strung together to form a timeline, termed as the particle's "world line."
The concepts of space and time were first combined into spacetime by Henri Poincaré in the late 1800s. He argued that the simultaneity of two events is a convention, and the rate at which time passes depends on the speed of light.
In 1905, Albert Einstein studied special relativity from a kinematics perspective. He asserted that the entire theory can be built around two core principles: the principle of relativity and the principle of the constancy of light speed.
Hermann Minkowski, Einstein's former mathematics professor, had also developed the elements of special relativity by the time Einstein published his findings in 1905. Minkowski later presented a geometric interpretation of spacetime, which significantly influenced Einstein's general theory of relativity.
In special relativity, the spacetime concept is tied closely to the idea of spacetime intervals, which have analogous qualities to the spacetime used in spherical, hyperbolic, or conformal geometries. Minkowski's spacetime interpretation simplified the shift to general relativity, leading to modern recognition of the curved spacetime of general relativity as Minkowski spacetime.