Editing A Brief History of Time (section)

=== Chapter 2: Space and Time ===
Hawking describes the evolution of scientific thought regarding the nature of [[space]] and [[time]]. He starts with the Aristotelian idea that the naturally preferred state of a body is to be at rest, and it can only be moved by [[force]], implying that heavier objects will fall faster. However, Galileo experimentally disproved Aristotle's theory by observing the motion of objects of different weights and concluding that all objects would fall at the same rate. This led to [[Newton's laws of motion]] and [[Newton's law of universal gravitation| gravity]]. However, Newton's laws implied that there is no such thing as absolute state of rest or [[absolute space]]: whether an object is 'at rest' or 'in motion' depends on the observer's [[inertial frame of reference]].

Hawking describes the classical belief in absolute time, that observers in motion will measure the same time. However, Hawking writes that this commonsense notion does not work at or near the [[speed of light]]. That light travels at a finite speed was discovered by [[Ole Rømer]] through his observations of [[Jupiter]] and its moons. Scottish scientist [[James Clerk Maxwell]]'s [[Maxwell's equations| equations]] unifying electricity and magnetism predicted the existence of waves moving at a fixed speed, the same speed that had been measured for light. Physicists believed that light must travel through a [[luminiferous aether]], and that the speed of light was relative to that of the aether. The [[Michelson–Morley experiment]], designed to detect the speed of light through the aether, got a null result. Michelson and Morley found that the speed of light was constant regardless of the motion of the source or the observer. In 1905, [[Albert Einstein]] argued that the aether is superfluous if we abandon absolute time. His [[special theory of relativity]] is based on two postulates: the laws of physics are the same for all observers moving relative to one another, and the speed of light is a universal constant.

Remarkable consequences follow. Mass and energy are related by the equation <math>E = mc^2</math>, which means that an infinite amount of energy is needed for any object with mass to travel at the speed of light (c = 3×10⁸m/s). It follows that no material body can travel at or beyond the speed of light. A 4-dimensional [[spacetime]] is described, in which 'space' and 'time' are intrinsically linked. The motion of an object through space inevitably impacts the way in which it experiences time. 

In 1915, Einstein published [[general relativity]], which explains gravity as the curvature of spacetime. Matter and energy (including light) follow [[Geodesics in general relativity|geodesics]]. Einstein's theory of gravity predicts a dynamic universe.