From: nospam@de-ster.demon.nl
Tom Roberts wrote:
> On 7/31/17 7/31/17 9:10 AM, Jos Bergervoet wrote:
> > On 7/31/2017 1:35 PM, Thomas Koenig wrote:
> >> Time certainly doesn't equal length,
> >
> > Then height doesn't equal width. And you might just
> > as well use the 'fathom' to measure the former and
> > the 'furlong' for the latter.
>
> Well said!
Actually there is a tribe, called 'pilots' who do just that.
(feet for altitude, nautical miles for horizontal distance)
Yet everybody trusts them to get everything right.
It would be natural for a physicist turned pilot
to give altitude and distance different dimensions too,
for these are obviously very different concepts.
> > More reasonable is to compute time^2-length^2 (the
> > metric in our universe). Having the same units in
> > that case saves you applying a conversion constant.
>
> This would also avoid common confusion about "the speed of light" -- in
> MKS that conversion constant is not really the (vacuum) speed of light,
> but rather is related to a symmetry of the metric.
Neither of the two. It is just a defined conversion constant.
The actual value is obtained by counting periods of a *stationary* wave.
Metric, speed of light and all that
is theoretical talk of no concern to metrology.
> We conflate them by using the same symbol,
> c, for both, because it "just so happens" that this constant is equal to the
> vacuum speed of light.
>
> Numerically these two meanings of "c" are the same [#]; physically they are
> quite different. This has bewildered many a student....
Tell them (like Jackson does) that the photon might have
a very small but non-zero mass.
That makes to conceptual difference immediately obvious,
Jan
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