Singer, S. F. Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia.
Last reviewed:June 2020
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- Characteristics of the space around Earth
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Conceptually, a three-dimensional extent within which entities exist and have physical relationships to each other. In common usage, the term "space" has come to mean generally outer space or the region beyond the Earth (see illustration), but depending on perspective and context, "space" can have many interrelated definitions. Physically, space is that property of the universe associated with extension in three mutually perpendicular directions. From a Newtonian point of view, space may contain matter, but it exists apart from matter. Geophysically, space is that portion of the universe beyond the immediate influence of Earth and its atmosphere. From the point of view of flight, space is that region in which a vehicle cannot obtain oxygen for its engines or rely upon an atmospheric gas for support (either by buoyancy or by aerodynamic effects). This boundary is approximately 100 km (62 miles) in altitude, and is known as the Kármán line. Astronomically, space is a part of the spacetime continuum by which all events are uniquely located. Relativistically, the space and time variables of uniformly moving (inertial) reference systems are connected by the Lorentz transformations; in this context, space and time are combined into a concept known as spacetime. Gravitationally, one characteristic of space is that all bodies undergo the same acceleration in a gravitational field and therefore that inertial forces are equivalent to gravitational forces. Perceptually, space is sensed indirectly by the objects and events within it. Thus, a survey of space is more a survey of its contents. See also: Euclidean geometry; Lorentz transformations; Matter; Relativity; Spacetime
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