On Tue, 19 Jul 2016 01:25 am, Ian Kelly wrote: > On Mon, Jul 18, 2016 at 3:29 AM, Steven D'Aprano > <steve+comp.lang.pyt...@pearwood.info> wrote: >> On Monday 18 July 2016 14:16, Rustom Mody wrote: >>> In short one could think of inexact and exact — in scheme's intended >>> semantics — as better called scientific (or science-ic) and mathematic >>> numbers. >> >> I don't think so. "Science" uses both experimentally-derived numbers >> (e.g. G, c, the mass of the electron) and numbers known exactly (√2, e, >> π). > > Off-topic, c being a fundamental constant is actually in the latter > category. Its *exact* value is 299792458 m/s. > > The length of the meter, on the other hand, is defined as the distance > traveled by light in a vacuum in 1/299792458 seconds and is subject to > the precision of measurements.
You're right, of course, but that's a post-facto redefinition of both c and the metre. Historically, the metre was defined first, then people spent a lot of time measuring the speed of light (I believe it was Galileo who made the first known attempt). Eventually, physicists decided that it was easier to declare by fiat that c = 299792458 m/s EXACTLY, and use that definition to imply a standard metre, rather than keep a standard metre and use that to measure c. That makes it a matter of convenience and practicality rather than purity. It also reflects the current theoretical paradigm that sees c as a constant and both distance and time subject to distortion. Had special and general relativity been disproven, or never invented, it's unlikely that we would have declared c to be more fundamental than distance. (We haven't declared that G is exactly such-and-such a value, and used that to define the kilogram.) -- Steven “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough, things got worse. -- https://mail.python.org/mailman/listinfo/python-list
