> I would not equate the notion of a change of c with time with that of a
> change of the conversion factor between calories and joules. One can define
> two independent units for length, for instance, inch and centimeter. It then
> becomes an experimental problem to find out how many centimeters fit into
> what has been taken to be an inch. This is a laborious experiment but it add
> nothing to our understanding of nature. Therefore, people decided to have a
> single unit of length and define all other lengths in terms of it. For
> instance, one inch = 2.5400000000000000000000000000....cm, exactly. Before
> we knew that heat was another form of energy, we had separate units. Now we
> know that calories and joules denote units of energy and so the calorie has
> been defined in terms of the joule, one calorie = 4.184 joules, exactly.
> However, the constancy of the speed of light (independent of the speeds of
> the observers) is a true indication of what goes on in nature.
I'm sort of taking a debating position here rather than being
dogmaric but I think there's a good case for this: c is a speed basic
to the structure of relativistic space-time, & can be viewed as simply
the conversion between directions in one direction in spacetime & those
in another. We can simply take c = 1 as theorists often do. It
also seems to be the case that there are some entities - e.g., light -
which travel at this speed. _But maybe they don't_. The photon may
have a rest mass. (The best observational limit on it is, I think, still
the one Ron Burman & I established ~20 years ago, ~ 10^-58 g.) _If_ the
photon rest mass is non-zero, c shouldn't be called "the speed of
light".
George Murphy