What is "speed of light"

n. (context physics English) The speed of electromagnetic radiation in a perfect vacuum: exactly 299,792,458 metres per second by definition.

n. the speed at which light travels in a vacuum; the constancy and universality of the speed of light is recognized by defining it to be exactly 299,792,458 meters per second [syn: light speed, c]

The speed of light in vacuum, commonly denoted , is a universal physical constant important in many areas of physics. Its exact value is (approximately ), since the length of the metre is defined from this constant and the international standard for time. According to special relativity, is the maximum speed at which all matter and hence information in the universe can travel. It is the speed at which all massless particles and changes of the associated fields (including electromagnetic radiation such as light and gravitational waves) travel in vacuum. Such particles and waves travel at regardless of the motion of the source or the inertial reference frame of the observer. In the theory of relativity, interrelates space and time, and also appears in the famous equation of mass–energy equivalence .

The speed at which light propagates through transparent materials, such as glass or air, is less than ; similarly, the speed of radio waves in wire cables is slower than . The ratio between and the speed at which light travels in a material is called the refractive index of the material . For example, for visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at ; the refractive index of air for visible light is about 1.0003, so the speed of light in air is about (about slower than ).

For many practical purposes, light and other electromagnetic waves will appear to propagate instantaneously, but for long distances and very sensitive measurements, their finite speed has noticeable effects. In communicating with distant space probes, it can take minutes to hours for a message to get from Earth to the spacecraft, or vice versa. The light seen from stars left them many years ago, allowing the study of the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. The speed of light can be used with time of flight measurements to measure large distances to high precision.

Ole Rømer first demonstrated in 1676 that light travels at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, and therefore travelled at the speed appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light with respect to any inertial frame is a constant and is independent of the motion of the light source. He explored the consequences of that postulate by deriving the special theory of relativity and in doing so showed that the parameter had relevance outside of the context of light and electromagnetism.

After centuries of increasingly precise measurements, in 1975 the speed of light was known to be with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/ of a second. As a result, the numerical value of in metres per second is now fixed exactly by the definition of the metre.

The speed of light is a physical constant, the rate at which light travels in a vacuum.

Speed of Light may also refer to:

• Speed of light (cellular automaton), the greatest rate of information propagation in a cellular automaton
• "Speed of Light", a song by the band Orchestral Manoeuvres in the Dark, from their 1991 album Sugar Tax
• "Speed of Light", a song by guitarist Joe Satriani, from his 1993 album Time Machine
• "Speed of Light", a song by the band Stratovarius, from their 1996 album Episode
• "Speed of Light", a song by the band van Canto, from their 2008 album Hero
• "Speed of Light", a single by South Korea boy group Speed
• Speed of Light (album), a 2009 album by Corbin Bleu
• " Speed of Light", a song by the band Iron Maiden, from their 2015 album The Book of Souls

In Conway's Game of Life (and related cellular automata), the speed of light is a propagation rate across the grid of exactly one step (either horizontally, vertically or diagonally) per generation. In a single generation, a cell can only influence its nearest neighbours, and so the speed of light (by analogy with the speed of light in physics) is the maximum rate at which information can propagate. It is therefore an upper bound to the speed at which any pattern can move.

Speed of Light is the second solo studio album of actor and pop singer Corbin Bleu. It was released on March 10, 2009 by Hollywood Records.

The album's first single is "Moments That Matter". In December 2008, Bleu performed the song at Kids' Inaugural Concert: We Are the Future.

"Speed Of Light" is the second single by South Korea boy group Speed. The single album by promotional singles "슬플약속 (That's My Fault)".

"Speed of Light" is a song by the English heavy metal band Iron Maiden from their sixteenth studio album, The Book of Souls. The song was released as a music video on 14 August 2015 and was also made available as a digital download and issued as a single-track CD exclusive to Best Buy in the US.