299,792,458 meters per second (within a vacuum.)
http://en.wikipedia.org/wiki/Speed_of_lightO.K. So we have a value of 299,792,458 Meters per Second in a certain condition - a "vacuum". This is the currently stated Speed for Light, and it is considered the prime Constant, which is why the symbol for it is "c".
First, what is a Meter? According to Columbia Encyclopedia:
meter, abbr. m, fundamental unit of length in the metric system. The meter was originally defined as 1/10,000,000 of the distance between the equator and either pole; however, the original survey was inaccurate and the meter was later defined simply as the distance between two scratches on a bar made of a platinum-iridium alloy and kept at Sevres, France, near Paris. More recently, it has been defined as the distance light travels through a vacuum in 1/299,792,458 of a second. The meter is now the legal standard of length for most of the world, other standards, such as the yard, being defined in terms of the meter.
So the Speed of Light is defined using the Meter and Second, and the Meter is defined by the Speed of Light and the Second. This is Circular Logic and is not Scientific in nature. It is akin to describing a Foot as 12 inches, and then describing an inch as 1/12 of a Foot. If there is a stable base for our unshakable Speed of Light Constant, it must certainly lie in the Second.
What is a Second?
Columbia Encyclopedia:
second, abbr. sec or s, fundamental unit of time in all systems of measurement. In practical terms, the second is 1/60 of a minute, 1/3,600 of an hour, or 1/86,400 of a day. Since the length of the day varies, however, the second must be defined in more precise terms. For many years it was defined as 1/86,400 of the mean solar day (see solar time), thus eliminating seasonal variations. Because the rotation of the earth itself is not constant, the second was redefined (1956) in terms of ephemeris time (ET), which is calculated from the motions of celestial bodies in accordance with the laws of motion; 1 sec is 1/31,556,925.9747 of the length of the tropical year for 1900. In 1967 the second was redefined to be 9,192,631,770 periods of vibration of the radiation emitted at a specific wavelength by an atom of cesium-133.
So the Second is defined using a the transition between "hyperfine states" of a particular element, Cesium-133 , and these pulses are "counted" while the material is at a Ground State, at Absolute Zero.
What is Cesium-133? Cesium is an isoptope of Cesium which has 55protons 78Neutrons, with an isotopic mass of 132.905451933.(u)- "u" is the symbol for Atomic Mass Units - From Columbia Emcyclopedia :
"u" representing atomic mass unit or amu, in chemistry and physics, unit defined as exactly 1/12 the mass of an atom of carbon-12, the isotope of carbon with six protons and six neutrons in its nucleus. One amu is equal to approximately 1.66 × 10−24 grams.
So Cesium is defined using Grams, which are defined by of course the Meter and the Speed of Light. Again, this is circular logic and not scientific. That which is being defined is being used to define itself.It is also true that the method of counting these pulses of radiation from Cesium-133 involves calculations involving both the speed of light and the Meter.
How about measuring at Absolute Zero?- From Wikipedia:
Absolute zero is the theoretical temperature at which entropy would reach its minimum value. The laws of thermodynamics state that absolute zero cannot be reached because this would require a thermodynamic system to be fully removed from the rest of the universe.
An accurate measurement by this method is impossible, it requires an impossible condition.
About Vacuum from Columbia Encyclopedia:
"vacuum, theoretically, space without matter in it. A perfect vacuum has never been obtained; the best man-made vacuums contain less than 100,000 gas molecules per cc, compared to about 30 billion billion (30×1018) molecules for air at sea level. The most nearly perfect vacuum exists in intergalactic space, where it is estimated that on the average there is less than one molecule per cubic meter."
If a vacuum is an impossible condition, how can one claim to have measured light in it? This is of course not including the current theories concerning Dark Matter that suggest that this matter is everywhere. That throws an even bigger monkey wrench in the formula. There are also issues with special conditions concerning Gravity, and that a truly accurate measurement of the Speed of Light would require being removed from the Universe completely.
The Speed of Light, which is the supposed "Prime Constant" and the Bedrock of all Physics is not Scientifically defined.