User:Physchim62/Speed of light constancy

Constancy
The idea that the speed of light is a physical constant comes from Maxwell's equations, which describe light as electromagnetic radiation. Maxwell's 1865 theory predicted the existence of radio waves, which were discovered by Heinrich Hertz in 1887, a strong confirmation of the theory. By the end of the nineteenth century, the idea of a single and constant speed of light was well established, particularly by the work of Albert Michelson.

Michelson pioneered the technique of interferometry for measuring lengths, a technique that relies on the idea that the speed of light is constant. He calibrated his technique against the then U.S. metal-bar standards of both the yard (made out of bronze) and the metre (made out of a platinum–iridium alloy), and also against the International Prototype Metre in France. His measurements indicated that the length of the bronze standard yards was steadily getting shorter, a result that was later confirmed by comparison with other standard yards in the Anglosphere. His results persuaded the United States to officially adopt the metric system in 1893 with the Mendenhall Order.

Some scientists have questioned why the fundamental constants of nature, including c, have the values they do, and whether they are changing as the universe evolves. These questions remain an interest of on-going research.

Constancy over time
There have been several theoretical speculations as to what might occur if the speed of light were slowly changing over time, although none has received general support within the physics community. Nevertheless, there are experimental tests which place an upper limit on any variation.

Several tests of general relativity consider a possible variation in the Newtonian gravitational constant G, following the Dirac large numbers hypothesis (1937). From the Lunar laser ranging experiment, the maximum possible change in G is 1.1 parts in 1012 per year, while analysis of more than 250,000 ranging observations of U.S. and Russian spacecraft has lowered the possible variation to 7 parts in 1014 per year. Any change in the speed of light over the timescale of these observations would appear as an apparent change in G, that is the data would not distinguish between a change in G and a change in c. In fact the maximum possible change in c that is derived from the data is one third of the maximum possible variation in G, or about 2 parts in 1014 per year.

Other methods for detecting a possible change in the speed of light look for changes in the fine structure constant α. This constant is a pure number, and so its value is independent of the system of units used to measure it. It can also be expressed as a ratio of other physical constants, one of which is the speed of light:
 * $$\alpha =\ \frac{e^2}{\hbar c \ 4 \pi \varepsilon_0}\ =\ \frac{e^2 c \mu_0}{2 h} = \frac{k_\mathrm{e} e^2}{\hbar c}$$

Single-ion optical atomic clocks to place a very stringent constraint on the present time variation of α, with a maximum possible variation (2008) of 4 parts in 1017 per year. This is consistent with data from the Oklo natural nuclear fission reactor, which suggest a possible change in α of 4.5 parts in 108 over the past 2 billion years, or 2 parts in 1017 per year.

Independence of frequency
Others have suggested that the speed of light may exhibit a small dispersion—that is, different frequencies may travel at slightly different velocities. (The invariant speed c of special relativity would the be the upper limit of the speed of light in vacuum. )

However, to date, such variations, if any, are smaller than experimental errors of observation—the measured speed of electromagnetic radiation is the same for different frequencies to within very stringent experimental limits.

A reason for light to travel at a frequency-dependent speed would be a non-zero rest mass of the photon, the carrier of the electromagnetic force or field. At present, the rest mass of the photon is taken to be zero, implying that it always travels at speed c. If the hypothetically massive photon is described by Proca theory, the experimental upper bound for its mass is about 10−57 grams. If photon mass is generated by a Higgs mechanism, the experimental upper limit is less sharp, m ≤ 10−14 eV/c2 (equivalent to of the order of 10−47 g).

Equivalence of constants
In modern physics, the "speed of light" is far more than simply the speed of propagation of electromagnetic radiation. For example In each case, the constancy of the speed of light is used as an assumption in constructing the theory: if the assumption is incorrect, the theory will be incorrect, yet the relativity and quantum theories have survived many, many experimental tests.
 * special relativity implies that there is a maximum speed which all observers will measure to be the same, and this is generally assumed to be the same as the speed of light;
 * Hermann Minkowski realized in 1907 that special relativity implies that space and time are not separate concepts, but a combined in what has come to be known as Minkowski spacetime, where the speed of light plays the role of the equivalence factor between our measurements of time and our measurements of space;
 * developments in quantum theory, especially quantum electrodynamics (QED), link the speed of light to phenomena at the atomic and sub-atomic scale, particularly through the dimensionless fine structure constant.