Talk:Speed of light

The defintion of the metre

The article makes the statement "noticing that the uncertainty in the determination of the meter was about as large as the uncertainty in the measurements of the speed of light, recommended the definition...". This statement is misleading. Two things are involved, not one.

The first is that the definition of length was changed to transit time of light. That is a logical change of viewpoint.

The second thing is that this change of definition was seen to be practical from a metrology standpoint.

The logical switch makes c a defined quantity, the accuracy issues make the change of stance to a defined value a practical advantage.

In principle a switch in definition to measure length as time of transit could have been implemented at any time, and even could be based upon (say) the speed of sound in air under specified conditions. However, that was not done for metrology reasons: it would be a hassle to prepare the standard conditions for the standard speed of sound. Because the speed of light is theoretically a universal constant (whatever its value may be) preparation of free space where this universal value is obtained is considered to be a simple task that any observer can manage without hassle. The improved accuracy in time measurement means that time of flight has an accuracy advantage (assuming the standard speed of light is readily reproduced) over counting fringes of a standard wavelength of light. John R. Brews 14:19, 29 November 2010 (UTC)