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 coordinatess. 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 Poincar� (spatial geometry is a convention).

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

Space is the relatively empty parts of the Universe, outside the atmospheress 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.

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