Speed of light

The speed of light in a vacuum is defined to be 299,792,458 metres per second (1,079,252,848.8 km/h, which is approximately 186,282.4 miles per second, or 670,616,629.38 miles per hour). The speed of light is denoted by the letter c, reputedly from the Latin celeritas, "speed", and also known as Einstein's constant. Note that this speed is a definition, not a measurement; in fact, the fundamental SI unit of distance, the metre, is defined in terms of the speed of light and the second. The speed of light through a transparent medium (that is, not in vacuum) is less than c; the ratio of c to this speed is called the refractive index of the medium. "Speed of light" is sometimes abbreviated SOL.

Overview

According to standard modern physical theory, all electromagnetic radiation, including visible light, propagates (or moves) at a constant speed in a vacuum, commonly known as the speed of light, which is a physical constant denoted as c. This speed c is also the speed of propagation of gravity in the theory of general relativity.

Related Topics:
Physical theory - Electromagnetic radiation - Visible light - Physical constant - Gravity - General relativity

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One consequence of the laws of electromagnetism (such as Maxwell's equations) is that the speed c of electromagnetic radiation does not depend on the velocity of the object emitting the radiation; thus for instance the light emitted from a rapidly moving light source would travel at the same speed as the light coming from a stationary light source (although the colour, frequency, energy, and momentum of the light will be shifted, which is called the relativistic Doppler effect). If one combines this observation with the principle of relativity, one concludes that all observers will measure the speed of light in vacuum as being the same, regardless of the reference frame of the observer or the velocity of the object emitting the light. Because of this, one can view c as a fundamental physical constant. This fact can then be used as a basis for the theory of special relativity. It is worth noting that it is the constant speed c, rather than light itself, which is fundamental to special relativity; thus if light is somehow manipulated to travel at more or less than c, this will not directly affect the theory of special relativity.

Related Topics:
Maxwell's equations - Relativistic Doppler effect - Principle of relativity - Reference frame - Physical constant - Special relativity

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Observers travelling at large velocities will find that distances and times are distorted ("dilated") in accordance with the Lorentz transforms; however, the transforms distort times and distances in such a way that the speed of light remains constant. A person travelling near the speed of light would also find that colours of lights ahead were blue shifted and of those behind were red shifted.

Related Topics:
Lorentz transforms - Blue shift - Red shift

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If information could travel faster than c in one reference frame, causality would be violated: in some other reference frames, the information would be received before it had been sent, so the 'cause' could be observed after the 'effect'. Due to special relativity's time dilation, the ratio between an external observer's perceived time and the time perceived by an observer moving closer and closer to the speed of light approaches zero. If something could move faster than light, this ratio would not be a real number. Such a violation of causality has never been observed.

Related Topics:
Causality - Time dilation - Real number

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To put it another way, information propagates to and from a point from regions defined by a light cone. The interval AB in the diagram to the right is 'time-like' (that is, there is a frame of reference in which event A and event B occur at the same location in space, separated only by their occurring at different times, and if A precedes B in that frame then A precedes B in all frames: there is no frame of reference in which event A and event B occur simultaneously). Thus, it is hypothetically possible for matter (or information) to travel from A to B, so there can be a causal relationship (with A the 'cause' and B the 'effect').

Related Topics:
Light cone - Interval - Time-like

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On the other hand, the interval AC in the diagram to the right is 'space-like' (that is, there is a frame of reference in which event A and event C occur simultaneously, separated only in space; see simultaneity). However, there are also frames in which A precedes C (as shown) or in which C precedes A. Barring some way of travelling faster than light, it is not possible for any matter (or information) to travel from A to C or from C to A. Thus there is no causal connection between A and C.

Related Topics:
Space-like - Simultaneity - Faster than light

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According to the currently prevailing definition, adopted in 1983, the speed of light is exactly 299,792,458 metres per second (approximately 3 × 108 metres per second, or about thirty centimetres (one foot) per nanosecond). The value of c defines the

Related Topics:
1983 - Centimetres - Foot - Nanosecond

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permittivity of free space (epsilon_0) in SI units as:

Related Topics:
Permittivity - SI

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: arepsilon_0 = 10^{7}/4pi c^2 quad mathrm{(in~ A^2, s^4, kg^{-1}, m^{-3}, , or , F , m^{-1})}

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The permeability of free space (mu_0) is not dependent on c and is defined in SI units as:

Related Topics:
Permeability - SI

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: mu_0 = 4,pi, 10^{-7} quad mathrm{(in~ kg, m, s^{-2}, A^{-2}, , or , N , A^{-2})}.

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These constants appear in Maxwell's equations, which describe electromagnetism, and are related by:

Related Topics:
Maxwell's equations - Electromagnetism

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:c= rac {1} {sqrt{arepsilon_0mu_0}}

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Astronomical distances are sometimes measured in light years (the distance that light would travel in one year, roughly 9.46 × 1012 kilometres or about 5.88 × 1012 miles) especially in popularised texts.

Related Topics:
Astronomical - Light years - 9.46 × 10¹²

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