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Re: philosophy and the vacuum



unsigned wrote:

At 11:22 AM 9/11/00 -0500, Joel Rauber wrote:

This means that the standard for the meter will be different
depending on
which vacuum you use as a reference. Which implies a
logical problem in
the definition. Unless we agree on which vacuum to use, and
then are we
using the same one that the SI guru's intend?

You asked about philosophy: It is philosophically important
to keep in
mind that physics is a natural science. It is not an exact
science like,
say, arithmetic.

I am talking about a basis for logically defining a system of units. In
this sense we may look at the situation in a gedanken-like fashion and
treat
physics as an exact science; as one does in most gedanken experiments.

<snip>


The same philosophy and the same sort of procedures apply to the
speed-of-light experiment, which is more properly called the
meter-stick-calibration experiment. Your calibration will never be
_exactly_ the same as my calibration. But we believe that
(if we do it
right) we can greatly reduce the magnitude of junk effects
(stray gas in
the cell, waveguide effects due to the walls of the cell, et
cetera) and
then place tight bounds on the magnitude of any remaining junk
effects. The "SI gurus" intended an idealization, namely
extrapolation to
the limit where the junk effects are negligible.


It depends on what you mean by a junk effect. If you mean the usual sorts
inaccuracies in laboratory experiments, like the specific examples you cite
above, then I agree. But I'm asking more about systematic effects, which
can only be removed if you know what they are; and if you have an adequate
theory available to correct for them.

It also assumes you know what the reference is and can remove the "junk"
effects relative to it. Which vacuum is the idealization that the "SI
gurus" intend? So that I can know which "junk" effects to remove? (see
comment below)


You asked in particular about the effects of the walls. So stop
philosophizing and do the calculation yourself.

For purposes of this discussion its unnecessary to carry out any
calculations. It was enough for me to take you at your word that their
there are different vacuums which will yield different values for the
"speed
of light" and hence different calibrations of the length of a meter.


If the actual uncertainty is large enough to cause you
trouble in practice,
please explain what you are doing that requires such high accuracy!


My pondering isn't related to a required accuracy type of a situation; but
rather to the definition of the unit.

Joel Rauber
Joel_Rauber@sdstate.edu