# Re: The speed of light

Michael S. Lorrey (mike@lorrey.com)
Mon, 19 Jul 1999 15:28:18 -0400

"Eliezer S. Yudkowsky" wrote:
>
> 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).

Rotation is irrelevant. A the key is that light travels that many meters in a second no matter what your inertial frame of reference. Thus it is an objective constant to anchor to. The constant no longer is 'meter', but second. Second, as a unit of measurement is based on a constant I am guessing is Planck Time, which is not dependent on the speed of light.

Mike Lorrey