Space

The definition of space in physics is contentious. Various concepts used to
try to define space have included:

   * the structure defined by the set of "spatial relationships" between
     objects
   * a manifold defined by a coordinate system where an object can be
     located.
   * the entity that stops all objects in the universe from touching one
     another

In classical physics, space is a three-dimensional Euclidean space where any
position can be described using three coordinates. Relativistic physics
examines spacetime rather than space; spacetime is modeled as a
four-dimensional manifold.

Philosophical questions concerning space include: Is space absolute or
purely relational? Does space have one correct geometry, or is the geometry
of space just a convention? Historical Eminences who have taken sides in
these debates include Isaac Newton (space is absolute), Gottfried Leibniz
(space is relational), and Henri PoincarŽ (spatial geometry is a
convention).

Two important thought-experiments connected with these questions are:
Newton's bucket argument and PoincarŽ's disc-world.

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Space is the relatively empty parts of the Universe, outside the atmospheres
of planets. It is sometimes called "outer space" to distinguish it from
airspace and terrestrial locations.

As Earth's atmosphere has no abrupt cut-off, but rather thins gradually with
increasing altitude, there is no definite boundary between the atmosphere
and space. In the United States, persons who travel above an altitude of 50
miles (80 kilometers) are designated as astronauts. 400,000 feet (75 miles
or 120 kilometers) marks the boundary where atmospheric effects become
noticeable during re-entry. The altitude of 100 kilometers or 62 miles is
also frequently used as the boundary between atmosphere and space.

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The term "inner space" has sometimes been used to describe the contents of
the human mind.

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In mathematics, a space is a set, usually with some additional structure.

For examples, see Euclidean space, vector space, normed vector space, Banach
space, inner product space, Hilbert space, topological space, uniform space,
and metric space.

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In some orthographies, a space is a blank area that serves as punctuation to
provide interword separation.

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