Space and Time
Introduction
Space and time are two fundamental concepts in physics, defining the very fabric of the universe. They are interconnected in a four-dimensional structure known as spacetime, a concept central to both general relativity and modern cosmology.
Space
Space, in its simplest definition, refers to the three-dimensional expanse in which objects and events occur and have relative position and direction. It is a fundamental concept in several areas of physics, including quantum mechanics, astrophysics, and particle physics.
Physical Space
Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum known as spacetime. The concept of space is considered to be of fundamental importance to an understanding of the physical universe.
Outer Space
Outer space, also simply called space, refers to the vast expanse that exists beyond the Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays.
Time
Time is a concept that refers to the progression of events from the past through the present into the future. It is a fundamental part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects.
Time in Physics
In physics, time is a scalar quantity and, like length, mass, and charge, is usually described as a fundamental quantity. Time can be combined mathematically with other physical quantities to derive other concepts such as motion, kinetic energy and fields.
Arrow of Time
The arrow of time, or time's arrow, is a concept developed in 1927 by the British astronomer Arthur Eddington, and is an unsolved general physics question. It points from the past to the future, in the direction of increasing entropy, or chaos.
Spacetime
Spacetime is any mathematical model which fuses the three dimensions of space and the one dimension of time into a single four-dimensional manifold. The spacetime of our universe is usually interpreted from a Euclidean space perspective, which regards space as consisting of three dimensions, and time as consisting of one dimension, the 'fourth dimension'.
General Relativity and Spacetime
In Einstein's theory of general relativity, spacetime is a four-dimensional, non-Euclidean manifold without a pre-defined privileged time direction. This differs from classical mechanics, where the use of Euclidean space and absolute time are appropriate.