User:Brews ohare/Speed of light

Speed of light set by definition
In 1983 the 17th Conférence Générale des Poids et Mesures defined the metre to be the length of the path travelled by light in vacuum during a time interval of 1 &frasl; 299 792 458 of a second. The reasons for using this definition are stated in Resolution 1.

The term 'vacuum' is used in this definition and in this article to refer to the reference state of free space. Like absolute zero, it is an idealized state that only can be approximated in the physical world. Measurements in any real-world medium, such as air or a medium perturbed by gravity must be corrected so as to relate to vacuum.

The effect of this definition is to fix the speed of light in vacuum at exactly c = 299 792 458 m/s; thus the speed of light is now "a defined constant, not to be measured again..." although "...length measurement can be continuously improved without resorting to a new definition."

It is important to separate the definition of a physical quantity itself from the definition of the unit in which it is measured. Measurement is basically comparison between the quantity being measured and the standard unit; it is a matter of ratios. Setting the speed of light to a definite numerical value when measured in the SI units of m/s simply means comparisons of length become comparisons of transit times of light. In particular, comparison of a length with the metre is a comparison of its transit time with 1/299,792,458 s.

Because the second is defined in terms of atomic transitions which can be measured with a fair degree of accuracy, the new definition allows for a more precise definition of the metre, assuming an adequate realization of "vacuum" is available. Suggested atomic transitions for determining the second, with their associated uncertainties, are tabulated.

Rather than measure a time-of-flight, one implementation of this definition is to use a recommended source with established frequency (f ), and delineate the metre in terms of the wavelength (λ) of this light as determined using the defined numerical value of c and the relationship λ = c / f. Practical realizations of the metre use recommended wavelengths of visible light in a laboratory vacuum with corrections being applied to take account of actual conditions such as diffraction, gravitation or imperfection in the vacuum.