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Newsgroups: sci.physics.relativity
Subject: Re: E = 3/4 mc? or E = mc?? The forgotten Hassenohrl 1905 work.
From: nospam@de-ster.demon.nl (J. J. Lodder)
Reply-To: jjlxa31@xs4all.nl (J. J. Lodder)
Date: Sun, 8 Dec 2024 12:30:36 +0100
References: <309fb33a3a66f01873fdc890e899a968@www.novabbs.com> <674BCF8E.822@ix.netcom.com> <674CCA90.3DD9@ix.netcom.com> <a89d71ab22cb1e3e279a59fe50ab5ebb@www.novabbs.com> <9f1cd556912a273a8946c77614611242@www.novabbs.com> <8a0014e4135992c8ec7bd3f2f1983164@www.novabbs.com> <d906fde3148d43d339b1663f1127216a@www.novabbs.com> <13877dcc9c6a6f2dd8056d8c05f0c661@www.novabbs.com> <a7d26012926823b22e139af8670cbbe7@www.novabbs.com> <df76d88c3e9729de443afca2c0cf99fa@www.novabbs.com> <2c831e6c7e0103c00fcebe8074fec8db@www.novabbs.com> <7d37d6e841cd1936217b21a5847fc507@www.novabbs.com> <7511bb1b9b748c76df265f91eaaa468a@www.novabbs.com> <67503f94$0$12915$426a74cc@news.free.fr> <3c8abe81804e4c5b6ced7aefae766c7d@www.novabbs.com> <6750b8d4$0$29710$426a74cc@news.free.fr> <2Ji4P.2$4s%.1@fx15.ams4> <6751f410$0$518$426a74cc@news.free.fr> <geD4P.802$qW31.662@fx07.ams4> <675357ca$0$28494$426a74cc@news.free.fr> <QY25P.1046$4s%.357@fx15.ams4> <iDc5P.41335$rwVa.34840@fx16.ams4>
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Paul B. Andersen <relativity@paulba.no> wrote:

> Den 07.12.2024 22:19, skrev Paul B. Andersen:
> > Den 06.12.2024 21:00, skrev J. J. Lodder:
> >> Paul B. Andersen <relativity@paulba.no> wrote:
> >>
> > 
> > According to:
> > https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf
> > (2019)
> > The SI definitions are:
> > 
> > The relevant defining constants:
> >   ??_Cs = 9192631770 Hz  (hyperfine transition frequency of Cs133)
> >   c = 299 792 458 m/s (speed of light in vacuum)
> > 
> > The relevant base units:
> > Second:
> >   1 s = 9192631770/??_Cs  1 Hz = ??_Cs/9192631770
> > 
> > Metre:
> >   1 metre = (c/299792458)s = (9192631770/299792458)?(c/??_Cs)
> > 
> > The home page of BIMP:
> > https://www.bipm.org/en/measurement-units
> > 
> > Give the exact same definitions, so I assume
> > that the definitions above are valid now.
> > 
> > 
> > https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf
> 
> >>>
> >>> If the speed of light is measured _with the meter and second
> >>> defined above_ it is obviously possible to get a result slightly
> >>> different from the defined speed of light.
> >>>
> >>> So I was not "completely, absolutely, and totally wrong".
> >>
> >> You were, and it would seem that you still are.
> >> You cannot measure the speed of light because it has a defined value.
> >> If you would think that what you are doing is a speed of light
> >> measurement you don't understand what you are doing.
> 
> Yes, I was indeed "absolutely, and totally wrong",
> but not completely wrong.
> 
> > 
> > When you have a definition of second and a definition of metre,
> > it is _obviously_ possible to measure the speed of light.
> > 
> > If you measure the speed of light in air, you would probably
> > find that v_air ≈ 2.99705e8 m/s.
> > 
> > If you measure it in vacuum on the ground, you would probably
> > get a value slightly less than 299792458 m/s because the vacuum
> > isn't perfect.
> 
> OK so far.
> 
> > 
> > If you measure it in perfect vacuum (in a space-vehicle?) you
> > would probably get the value 299792458 m/s.
> 
> You would certainly measure the value 299792458 m/s.
> 
> It is possible measure the speed of light in vacuum, but not much
> point in doing so since the result is given by definition.
> 
> > But it isn't impossible, if you had extremely precise instruments,
> > that you would measure a value slightly different from 299792458 m/s,
> > e.g. 299792458.000001 m/s.
> 
> This is indeed "completely, absolutely, and totally wrong".
> 
> I somehow thought that the "real speed" of light in vacuum
> measured before 1985 was different from 299792458 m/s.

Of course it was. The adopted value was a compromise
between the results of different teams.
BTW, you are also falling into the 'das ding an sich' trap.

> (Which it probably was, but the difference hidden in the error bar)
> And since the definition of metre only contain the defined constant c,
> i thought "the real speed" of light could be different from c.

Yes, that is where you go wrong.

> But this is utter nonsense!

Beginning to see the light?

> Now I can't understand how I could think so.
> My brain seems to be slower than it used to be. :-(
> 
> The real speed of light in vacuum is exactly c = 299792458 m/s,
> and 1 metre = (1 second/299792458)c, is derived from c,
> which means that the measured speed of light in vacuum will
> always be c.

Correct.
Perhaps I can explain the practicalities behind it in another way.
If you measure the speed of light accurately
you must of course do an error analysis.
The result of this that almost all of the error results from
the ecessary realisation of the meter standard. (in your laboratory)
So the paradoxal result is that you cannot measure the speed of light
even when there is a meter standard of some kind.

You may call whatever it is that you are doing
'a speed of light measurement',
but if you are a competent experimentalist you will understand
that what you are really doing is a meter calibraton experiment.
Hence the speed of light must be given a defined value,
for practical experimental reasons. [1]

Jan

[1] Which have not changed.
(and will not change in the forseeable future)
Meter standards are orders of magnitude less accurate
than time standards. (see why this must be?)