John Clark wrote:
> OK, but how long is a second? Is it the time it takes light to travel
> 299,792,458 meters? This sort of thing could give circular definitions
> a bad name.
The thing is, the definition of light is inherently circular. Once you've defined a "meter", you can only define the speed of light by reference to a "second", and you've automatically defined a "second" by reference to the speed of light. After all, there's really no such thing as separate units of "space" and "time". If you rotate a unit of time (a "second") by 90 degrees, it becomes a unit of space (299,792,458 meters).
Trying to have a non-circular definition is like trying to have a non-circular way to convert units of "height" into units of "width". They're the same thing. You may live in some weirdo fantasy universe where you define height and width by entirely different measures, so that "height" is measured in inches by reference to the height of Nelson's Column, while "width" is measured in meters by reference to the circumference of the planet, and you use special sloping rulers to convert between them - but the fact is, height and width are the same thing, and no conversions are necessary; the debate about how to "define" these measurements in terms of each other is silly, you're just taking too many measurements.
The speed of light is equal to one. Any unit of time defines a unit of length, and vice versa; you just multiply by the speed of light, which could as easily be phrased as "one second per 299,792,458 meters".
-- firstname.lastname@example.org Eliezer S. Yudkowsky http://pobox.com/~sentience/tmol-faq/meaningoflife.html Running on BeOS Typing in Dvorak Programming with Patterns Voting for Libertarians Heading for Singularity There Is A Better Way