The speed of light (usually denoted c) is a physical constant, the speed at which electromagnetic radiation, such as light, travels in free space (that is, in a perfect vacuum). Its value is 299,792,458 metres per second. This constant is significant in the understanding and study of astronomy, space travel and other fields.
According to the theory of special relativity, c is an important constant connecting space and time in the unified structure of spacetime, and its square is the constant of proportionality between mass and energy (E = mc2). In any inertial frame of reference, independently of the relative velocity of the emitter and the observer, c is the speed of all massless particles and associated fields, including all electromagnetic radiation in free space, and it is believed to be the speed of gravity and of gravitational waves. It is an upper bound on the speed at which energy, matter, and information can travel, as surpassing it would be equivalent to traveling backwards in time; its finite value is a limiting factor in the speed of operation of electronic devices.
For much of human history, it was not known whether light was transmitted instantaneously or simply very quickly. In the 17th century, Ole RÃ¸mer first demonstrated that it traveled at a finite speed by studying the apparent motion of Jupiter's moon Io; using these observations, Christiaan Huygens estimated the speed of light to be about 220,000 kilometres per second. Since then, scientists have devised increasingly sophisticated techniques to improve the precision of measurement. By 1975, the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion (4 Ã— 10âˆ’9), mostly due to the uncertainty in the length of the metre. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1â„299,792,458 of a second. As a result, the numerical value of c in metres per second is now a fixed, exact value by definition.
The actual speed at which light propagates through transparent materials, such as glass or air, is less than c; the ratio between c and the speed v at which light travels in a material is called the refractive index n of the material (n = c/v). For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travel at c/1.5 â‰ˆ 200,000 km/s; the refractive index of air is about 1.0003, so the speed of light in air is very close to c.
Gravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation causes dispersed matter to coalesce, thus accounting for the existence of the Earth, the Sun, and most of the macroscopic objects in the universe. It is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth; for the formation of tides; for convection, by which fluid flow occurs under the influence of a density gradient and gravity; for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena observed on Earth. Modern physics describes gravitation using the general theory of relativity, in which gravitation is a consequence of the curvature of spacetime which governs the motion of inertial objects. The simpler Newton's law of universal gravitation provides an accurate approximation for most calculations.
A classical field theory is a physical theory that describes the study of how one or more physical fields interact with matter. The word 'classical' is used in contrast to those field theories that incorporate quantum mechanics (quantum field theories).
A physical field can be thought of as the assignment of a physical quantity at each point of space and time (usually in a continuous manner). For example, on weather forecasts, the wind velocity during a day over a country is described by assigning a vector at each point of space (with moving arrows representing the change in wind velocity during the day). From the mathematical viewpoint, classical fields are described by sections of fiber bundles (covariant classical field theory). The term 'classical field theory' is commonly reserved for describing those physical theories that describe electromagnetism and gravitation, two of the fundamental forces of nature.
Descriptions of physical fields were given before the advent of relativity theory and then revised in light of this theory. Consequently, classical field theories are usually categorised as non-relativistic and relativistic.