The term space has two general meanings. First, it refers to the three-dimensional extension in which all things exist and move. We sometimes speak about outer space as everything that exists outside our own solar system. But the term space in astronomy and in everyday conversation can also refer to everything that makes up the universe, including our own solar system and Earth.

Mathematicians also speak about space in an abstract sense and try to determine properties that can be attributed to it. Although they most commonly refer to three-dimensional space, no mathematical reason exists not to study two-dimensional, one-dimensional, four-dimensional, or even n-dimensional (an unlimited number of dimensions) space.

Space-time continuum

One of the most important scientific discoveries of the twentieth century had to do with the nature of space. Traditionally, both scientists and nonscientists thought of space and time as being two different and generally unrelated phenomena. A person might describe where he or she is in terms of three-dimensional space: at the corner of Lithia Way and East Main Street in Ashland, for example. Or he or she might say what time it is: 4:00 P.M. on April 14.

What the great German-born American physicist Albert Einstein (1879–1955) showed was that space and time are really part of the same way of describing the universe. Instead of talking about space or time, one needed to talk about one's place on the space-time continuum. That is, we move about in four dimensions, the three physical dimensions with which we are familiar and a fourth dimension—the dimension of time.

Einstein's conception of space-time dramatically altered the way scientists thought about many aspects of the physical world. For example, it suggested a new way of defining gravity. Instead of being a force between two objects, Einstein said, gravity must be thought of in terms of irregularities in the space-time continuum of the universe. As objects pass through these irregularities, they exhibit behaviors that correspond almost exactly to the effects that we once knew as gravitational attraction.

[ See also Big bang theory ; Cosmology ; Relativity ; Time ]

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