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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.
(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.
But this is utter nonsense!
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.
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