The Meter
1983 October 21 – The 17th CGPM defines the metre as equal to the length of the path traveled by light in vacuum during a time interval of 1⁄299,792,458 of a second.[12]
2002 – The International Committee for Weights and Measures (CIPM) considers the metre to be a unit of proper length and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by general relativity to be negligible with respect to the uncertainties of realisation".[13]
The second
the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.[1]
During the 1970s it was realized that gravitational time dilation caused the second produced by each atomic clock to differ depending on its altitude. A uniform second was produced by correcting the output of each atomic clock to mean sea level (the rotating geoid), lengthening the second by about 1×10−10. This correction was applied at the beginning of 1977 and formalized in 1980. In relativistic terms, the SI second is defined as the proper time on the rotating geoid.[21]
The definition of the second was later refined at the 1997 meeting of the BIPM to include the statement
This definition refers to a caesium atom at rest at a temperature of 0 K.
. In 1983, the metre was redefined in the International System of Units (SI) as the distance traveled by light in vacuum in 1⁄299,792,458 of a second. As a result, the numerical value of c in metres per second is now fixed exactly by the definition of the metre.[4]
From what I can see the unit the second which is part of the definition of the speed of light has been updated in 1993 however the unit for the value of c has not been recalculated according to the cesium at rest at zero k.
Also the definition of the meter requires the definition of the light year, but the definition of the speed of light requires the definition of the meter, kind of a chicken before the egg scenario unless I'm missing something here
Thankfully all 3 were changed in the same year to its final value but no mention is made on why the cesium at rest at zero k changed. Did this affect the value of the second? if so then that also affects the definition of both the speed of light and the definition of the meter.
My point is that there is a strong need to have a good measure of cesium which is fundamental in our time base to have an accurate measure of the unit metre as well as the speed of light according to these SI definitions.
Now onto what your reply was they decided to choose the light year as fixed and adjust the definition of the metre instead the following below defines that, however it still does not confirm the validity of the definition of the speed of light
Because the previous definition was deemed inadequate for the needs of various experiments, the 17th CGPM in 1983 decided to redefine the metre.[132] The new (and current) definition reads: "The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second."[79] As a result of this definition, the value of the speed of light in vacuum is exactly 299,792,458 m/s[133][134] and has become a defined constant in the SI system of units.[10] Improved experimental techniques do not affect the value of the speed of light in SI units, but instead result in a more precise realization of the metre
So I ask you, we have 3 definitions one which depends on cesium the other two depend on each other as well as the cesium, after all if the second is off by a small amount then the other two are effected.
The definition of the metre requires accuracy in the constant c as well as the the unit second. the value of c is used to define the metre but also requires accuracy of the second.
You can see the source of my confusion of the twist arounds here lol.