Re: E = 3/4 mc? or E = mc?? The forgotten Hassenohrl 1905 work.

Paul B. Andersen <relativity@paulba.no> wrote:

Den 06.12.2024 21:00, skrev J. J. Lodder:

Paul B. Andersen <relativity@paulba.no> wrote:
 

Den 05.12.2024 19:42, skrev J. J. Lodder:

Paul B. Andersen <relativity@paulba.no> wrote:>

So if the speed of light, measured with instruments with better
precision than they had in 1983 is found to be 299792458.000001 m/s,
then that only means that the real speed of light (measured with
SI metre and SI second) is different from the defined one.

>
Note: measured with SI metre and SI second.
>

>
So this is completely, absolutely, and totally wrong.
Such a result does not mean that the speed of light
is off its defined value,
it means that your meter standard is off,
and that you must use your measurement result to recalibrate it.
(so that the speed of light comes out to its defined value)

 
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

Certainly. Letters are merely letters, you should know how to read.
You should have read on to the section on the realisations of those
units.
===
Avec un tel système d'unités,
il n'existe en principe aucune limite concernant l'exactitude avec
laquelle une unité peut
être réalisée. L'exception reste la seconde pour laquelle la transition
micro-onde du césium
doit être conservée, pour le moment, comme base de la définition.
===

Translating: All units may be developed in the future
to ever inceasing accuracy, except for the second,
which is the defining basis of the system.

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.

 
When you have a definition of second and a definition of metre,
it is _obviously_ possible to measure the speed of light.

So you persist in being utterly wrong about it.
All meter standards are based on the defined speed of light.
So they cannot be used 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.

True, but irrelevant.

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.

Again, irrelevant.
Of course you can measure -other- speeds.

If you measure it in perfect vacuum (in a space-vehicle?) you
would probably get the value 299792458 m/s.

In that case you would be an incompetent idiot
who doesn't know what he is doing.
(which is calibrating a local meter standard)

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.
 
However, so precise instruments hardly exists, and probably never will.
So I don't think this ever will be a real problem needing a fix.

Definitions can never be fixed by experiments.
Only people can do that, by agreeing on another one.

But my point is:
It is possible to measure the speed of light even if it exists
a defined constant c = 299792458 m/s

OK, I give up on you. It would seem that you will never get it.

If you are claiming otherwise, you are simply wrong.
 

>
You wrote:

In fact, the kind of experiments that used to be called
'speed of light measurements' (so before 1983)
are still being done routinely today, at places like NIST, or BIPM.
The difference is that nowadays, precisely the same kind of measurements
are called 'calibration of a (secudary) meter standard',
or 'calibration of a frequency standard'.

 
Calibration of a frequency standard is just that, and not
a 'speed of light measurements'.

Do you have any idea of how these things were done?
(and still are)

Is any such recalibration of the meter ever done?

 
Of course, routinely, on a day to day basis.
Guess there are whole departments devoted to it.
(it is a subtle art)
The results are published nowadays as a list of frequencies
of prefered optical frequency standards.
(measuring the frequency of an optical frequency standard
  and calibrating a secondary meter standard are just two different ways
  of saying the same thing)
And remember, there is no longer such a thing as -the- meter.
It is a secondary unit, and any convenient secondary standard will do.

 
In:
https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf
 
I read:
https://www.bipm.org/en/cipm-mra
 
"The CIPM has adopted various secondary representations of
  the second, based on a selected number of spectral lines of atoms,
  ions or molecules. The unperturbed frequencies of these lines can
  be determined with a relative uncertainty not lower than that of
  the realization of the second based on the 133Cs hyperfine transition
  frequency, but some can be reproduced with superior stability."

Yes, of course, many of them, such as rubidium clocks,
hydrogen masers, etc.
The difference is that the Cesium standard is exact, by definition.
All others have error bars on them.

This is how I interpret this:
The second is still defined by "the unperturbed ground state
hyperfine transition frequency of the caesium 133 atom"
   ??_Cs = 9192631770 Hz by definition.
 
But practical realisations of this frequency standard,
that is an atomic frequency standard based on Cs133 is
not immune to perturbation, a magnetic field may affect it.

Certainly, it takes a very competent experimentalist to get it right.
Well, that is what standards labs are for.

So there exist more stable frequency standards than Cs,
and some  are extremely more stable.
But the frequencies of these standards are still defined
by ??_Cs. 1 hz = ??_Cs/9192631770
This is "Calibration of a frequency standard".

They are more stable only in terms of themselves.
Their true frequency is not known to comparable accurately.
(to far less than the stability of the Cesium standard, actually)

The "secondary representations of second"
don't change the duration of a second
and the "secondary representations of metre"
don't change the length of a metre.

The secondary representations of the metre -are- the metre. [1]
There is no primary meter, so no 'the length of the meter,
('ding an sich' error again)

Jan

[1] Practicalities: secondary meter standards are less accurate than the
second by several orders of magnitude.