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The reason for this is that the choice of a system of units may arbitrarily select any physical constant as its basis, making the question of which constant is undergoing change an artefact of the choice of units. [5] [6] [7] For example, in SI units, the speed of light has been given a defined value in 1983. Thus, it was meaningful to ...
Some key dimensionless quantities (thought to be constant) that are related to the speed of light (among other dimensional quantities such as ħ, e, ε 0), notably the fine-structure constant or the proton-to-electron mass ratio, could in principle have meaningful variance and their possible variation continues to be studied. [29]
The speed at which light propagates through transparent materials, such as glass or air, is less than c; similarly, the speed of electromagnetic waves in wire cables is slower 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 (n = c / v ).
A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that cannot be explained by a theory and therefore must be measured experimentally. It is distinct from a mathematical constant , which has a fixed numerical value, but does not directly involve any physical measurement.
In the June 2013 issue of the American Journal of Physics, Hankins, Rackson, and Kim repeated the Greaves et al. experiment intending to obtain with greater accuracy the one-way speed of light. [28] This experiment assumed that the signal return path to the measuring device has a constant delay, independent of the end point of the light flight ...
[5]: 127 He found the speed of light was slower in water than in air. This experiment did not determine the absolute speeds of light in water or air, only their relative speeds. The rotational speed of the mirror could not be sufficiently accurately measured to determine the absolute speeds of light in water or air.
The experiment compared the speed of light in perpendicular directions in an attempt to detect the relative motion of matter, including their laboratory, through the luminiferous aether, or "aether wind" as it was sometimes called. The result was negative, in that Michelson and Morley found no significant difference between the speed of light ...
This apparently anachronistic analysis implies that Rømer was measuring the ratio c ⁄ v, where c is the speed of light and v is the Earth's orbital velocity (strictly, the component of the Earth's orbital velocity parallel to the Earth–Jupiter vector), and indicates that the major inaccuracy of Rømer's calculations was his poor knowledge ...