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

On Sat, 7 Dec 2024 21:19:50 +0000, Paul B. Andersen wrote:

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:
  Δν_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/Δν_Cs  1 Hz = Δν_Cs/9192631770
>
Metre:
  1 metre = (c/299792458)s = (9192631770/299792458)⋅(c/Δν_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.

>
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.
>
If you measure it in perfect vacuum (in a space-vehicle?) you
would probably get the value 299792458 m/s.
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.
>
But my point is:
It is possible to measure the speed of light even if it exists
a defined constant c = 299792458 m/s
>
If you are claiming otherwise, you are simply wrong.

In any measurement of the speed of light, one uses a local "best
representation" (i.e. secondary standard) of the meter and a local
best representation of the second to standardize our measurement
instrumentation. These practical realizations of the units of length
and time may be in fact superior in reproducibility than the materials
and methods cited in the primary definitions of the units of length
and time.
Using such calibrated instrumentation, it is obviously possible to
perform a speed of light measurement, and as Paul stated, we are not
doing anything stupid if we get a result differing from 299792458 m/s.
======================================================================
Δv measurements of the speed of light (for example, MMX measurements
testing for anisotropies in v) are fundamentally different in
principle than direct measurements v of the speed of light, and can
be conducted without especially precise local representations of
length and time.