Speed of light
According to standard modern physical theory, the speed of light propagation (and all other electromagnetic radiation) in vacuum is a physical constant (notated as c). Regardless of the reference frame of an observer or the velocity of the object emitting the light, every observer will obtain the same value for the speed of light upon measurement. This value is
or about thirty centimetres in a nanosecond. This is the maximal speed of any particle or information, which has been confirmed to a high degree of accuracy by experiment and observation in our "neighbourhood" (on a universal scale) in space and time.
Albert Einstein developed the theory of relativity by applying the consequences of the above to classical mechanics. Experimental confirmations of the theory of relativity indirectly confirm that the velocity of light has a constant magnitude.
Since the speed of light in vacuum is constant, one may measure time and distance in terms of c. Both the SI unit of length and SI unit of time have been defined in terms of wavelengths and cycles of light; currently, the meter is defined as the distance travelled by light in a certain amount of time: this relies on the constancy of the velocity of light for all observers. Distances in physical experiment or astronomy are commonly measured in in light seconds, light minutes, or light year.
Refraction
In passing through materials, light is slowed to less than c, by the ratio called the refractive index of the material. The speed of light in air is only slightly less than c. Denser media such as water and glass can slow light much more, to fractions such as 3/4 and 2/3 of c. On the microscopic scale this is caused by continual absorption and re-emission of the photons that compose the light by the atoms or molecules through which it is passing.
"Faster-than-light" experiments
Recent experimental evidence shows that is is possible for the group velocity of light to exceed c. One experiment made the group velocity of laser beams travel for extremely short distances through caesium atoms at 300 times c. However, it is not possible to use this technique to transfer information faster than c; the product of the group velocity and the velocity of information transfer is equal to the normal speed of light in the material squared.
The speed of light may also appear to be exceeded in some phenomena involving evanescent waves. Again, it is not possible that information is transmited faster than c.
History
The speed of light was first measured in 1676 by the young Danish astronomer Ole Roemer, who was studying the motions of Jupiter's moons. A plaque at the Observatory of Paris, where Roemer happened to be working, commemorates what was, in effect, the first measurement of a universal quantity made on this planet. Roemer published his result, which was within about ten percent of being correct, in Journal des Scavans of that year. Galileo seems to have been the first person to suspect that light might have a finite speed and attempt to measure it. He had written about his unsuccessful attempt using lanterns flashed from hill to hill outside Florence some decades earlier.
It is a bizarre coincidence that the average speed of the earth in its orbit is very close to one tenthousandth of this, actually within less than a percent. This gives a hint as to how Roemer measured light's speed. He was recording eclipses of Jupiter's moon Io: every day or two Io would go into Jupiter's shadow and later emerge from it. Roemer could see Io blink off and then later blink on, if Jupiter happened to be visible. Io's orbit seemed to be a kind of distant clock, but one which Roemer discovered ran fast while Earth was approaching Jupiter and slow while it was receeding from the giant planet. Roemer measured the cumulative effect: by how much it eventually got ahead and then eventually fell behind. He explained the measured variation by positing a finite velocity for light.
External Links and References
Group Velocity experiment
Java applet demonstrating group velocity information limits