The speed of light in vacuum denoted c, is a universal physical constant important in many areas of physics. Its exact value is defined as 299792458 metres per second, it is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of ​1⁄299792458 second. According to special relativity, c is the upper limit for the speed at which conventional matter and information can travel. Though this speed is most associated with light, it is the speed at which all massless particles and field perturbations travel in vacuum, including electromagnetic radiation and gravitational waves; such particles and waves travel at c regardless of the motion of the source or the inertial reference frame of the observer. Particles with nonzero rest mass can approach c, but can never reach it. In the special and general theories of relativity, c interrelates space and time, appears in the famous equation of mass–energy equivalence E = mc2.

The 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. For example, for visible light the refractive index of glass is around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200000 km/s. For many practical purposes and other electromagnetic waves will appear to propagate instantaneously, but for long distances and 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 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 by studying the apparent motion of Jupiter's moon Io. In 1865, James Clerk Maxwell proposed that light was an electromagnetic wave, therefore travelled at the speed c appearing in his theory of electromagnetism. In 1905, Albert Einstein postulated that the speed of light c 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 theory of relativity and in doing so showed that the parameter c had relevance outside of the context of light and electromagnetism. After centuries of precise measurements, in 1975 the speed of light was known to be 299792458 m/s with a measurement uncertainty of 4 parts per billion. In 1983, the metre was redefined in the International System of Units as the distance travelled by light in vacuum in 1/299792458 of a second; the speed of light in vacuum is denoted by a lowercase c, for "constant" or the Latin celeritas.

In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant shown to equal √2 times the speed of light in vacuum. The symbol V was used as an alternative symbol for the speed of light, introduced by James Clerk Maxwell in 1865. In 1894, Paul Drude redefined c with its modern meaning. Einstein used V in his original German-language papers on special relativity in 1905, but in 1907 he switched to c, which by had become the standard symbol for the speed of light. Sometimes c is used for the speed of waves in any material medium, c0 for the speed of light in vacuum; this subscripted notation, endorsed in official SI literature, has the same form as other related constants: namely, μ0 for the vacuum permeability or magnetic constant, ε0 for the vacuum permittivity or electric constant, Z0 for the impedance of free space. This article uses c for the speed of light in vacuum. Since 1983, the metre has been defined in the International System of Units as the distance light travels in vacuum in ​1⁄299792458 of a second.

This definition fixes the speed of light in vacuum at 299792458 m/s. As a dimensional physical constant, the numerical value of c is different for different unit systems. In branches of physics in which c appears such as in relativity, it is common to use systems of natural units of measurement or the geometrized unit system where c = 1. Using these units, c does not appear explicitly because multiplication or division by 1 does not affect the result; the speed at which light waves propagate in vacuum is independent both of the motion of the wave source and of the inertial frame of reference of the observer. This invariance of the speed of light was postulated by Einstein in 1905, after being motivated by Maxwell's theory of electromagnetism and the lack of evidence for the luminiferous aether, it is only possible to verify experimentally that the two-way speed of light is frame-independent, because it is impossible to measure the one-way speed of light without some convention as to how clocks at the source and at the detector sh

Sean O'Sullivan is an entrepreneur and investor, born in New York City and living in Princeton, New Jersey. He is best known for co-founding MapInfo Corporation, which popularized street maps on computers, for coining the term "Cloud Computing", he is Managing Partner at SOSV, a $525M venture capital operation. O'Sullivan has had an unusually diverse career, being described as a "renaissance man". A major advocate of economic and social development, he founded JumpStart International in 2003. JumpStart was a humanitarian engineering organization based in Baghdad and operating throughout Iraq during the post-war period of 2003-2006; as benefactor of the O’Sullivan Foundation, he has been a primary funder of organizations such as the Khan Academy and CoderDojo. As Chairman of the Irish Entrepreneurship Forum and founder of Open Ireland, he is a leader and influencer of Irish government policy in fueling economic growth and recovery in the Technology sector. In 1995, O' Sullivan founded SOSV, a venture capital and investment management firm that seeks to invest in the earliest stages of start-up companies via a family of accelerator programs.

While at University of Southern California, he wrote and directed 16 short films and worked on over 100 student film and television projects in roles ranging from director of photography to editor to sound designer. O’Sullivan’s graduate thesis is a full-length documentary about dementia in the elderly. Called String Worms at Budd Terrace, the film won the 2005 “Silver Images” award for best documentary film about elderly issues, his short film Squirrel Nuts won “Best of Show-Narrative” at the UCLA Short-takes film competition and played on national television through the Independent Film Channel. During the war in Iraq, Mr. O’Sullivan worked as a freelance photographer and cameraman for publications and broadcast media. O'Sullivan holds a Bachelor of Science in Electrical Engineering from Rensselaer Polytechnic Institute and a Master of Fine Arts in Film Production from the University of Southern California. Entrepreneur of the Year Rensselaer, 2011 Winner, “Silver Images” award 2005 for best documentary film about elderly issues Winner, “Best of Show-Narrative” for a short film entitled "Squirrel Nuts" UCLA Short-takes film competition Carma JumpStart International

The danzón-mambo is a subgenre of Cuban dance music that marked the transition from the classical danzόn to the mambo and the cha-cha-chá. It was in the context of the danzón-mambo that the Cuban dance band format called charanga reached its present form; the danzón-mambo was created by the musicians and arrangers of Antonio Arcaño's charanga, Arcaño y sus Maravillas, founded in 1937. According to Santos, The main forces behind Arcano's mambo were the Lopez brothers, Orestes... and Israel... who did most of the composing and arranging for the group, played the'cello and the string bass, respectively. Speaking, the danzón-mambo represents a further and stronger incorporation of elements of the son into the danzón; the first sections, or danzones, did not depart from the traditional danzón structure. But, the final section of the danzón-mambo was based on tumbaos and guajeos from the montuno section of the son, which created a complex, clave-oriented polyphony with strong accents on the upbeat.

In order to further reinforce the son feeling, Arcaño added the tumbadora to the traditional charanga percussion lineup of pailas and güiro. The paila player began to use a cowbell in the final section; this final section, at first called nuevo ritmo came to be called mambo. Out of the danzón-mambo came both the mambo and the cha-cha-chá; the mambo would subsequently become a genre played by American-style big bands, as such, did not pose a threat to the danzón-mambo. But, in the face of the sudden overwhelming popularity of the cha-cha-chá in the 1950s, the danzón-mambo began to disappear. However, a convention arose of playing the final section of the danzón-mambo with a cha-cha-chá rhythm, enabling the dancers to dance both the danzón and the cha-cha-chá in the course of the same composition; this became known as the danzón-cha and is the form of danzón most favored by dancers in Cuba at present. The Cuban Danzon: Its Ancestors and Descendants 1982. Various Artists. Folkways Records FW04066. De Nuevo El Monarca.

1993. Antonio Arcaño y sus Maravillas. ARTEX CD-069. "Chanchullo" Orovio, Helio. 1981. Diccionario de la Música Cubana. La Habana, Editorial Letras Cubanas. ISBN 959-10-0048-0 Santos, John. 1982. The Cuban Danzón: Its Ancestors and Descendants. New York, Folkways Records FE 4066