User:Brews ohare/Sandbox2

Does the definition preclude some measurements?

 * Does the definition of c as an exact value mean that any test of whether light is isotropic, dispersionless, etc. is placed beyond test, by definition? The answer is: “No. What is placed beyond test is the defined value of c, not any experimental investigations.”   For example, the special theory of relativity is based upon a number of postulates concerning properties of the speed of light. In the present SI units system where the speed of light has a defined value of c = 299,792,458 m/s exactly, nonetheless these postulates can continue to be tested as experimental technique improves.


 * To illustrate how the properties of light may continue to be examined within the 1983 decision, consider the hypothetical observation of anisotropy in the propagation of light. In the SI system of units, anisotropy would take the form of the metre having different lengths in different directions. Of course, a standard of length cannot be allowed to be uncertain, so the effect of this hypothetical anisotropy would be to change the definition of the metre, by adding a directional correction. At the same time, however, the explanation of this anisotropy would be attempted by improvements in theory, and one might conjecture that the successful explanation could involve an anisotropic propagation of light. In any event, the fundamental physical phenomenon of the propagation of light must be kept separate in our thinking from the numerical value of c in the SI system of units. (Separation of these notions might be a good reason to use the symbol c0 for the SI conversion factor instead of c, a practice recommended by the CGPM.)


 * In sum, tests of the special theory of relativity, for example, are not impeded by an exact definition of c in the SI system of units. The experiments take the form of tests of the adequacy of the definition of the metre. If there are any necessary changes in this definition due to future experimental results, they will be accompanied eventually by theoretical explanations, and these theories may indeed invoke, if need be, as yet undiscovered fundamental properties of the propagation of light.