On Friday, 22 March 2019 11:11:33 UTC, whisky-dave wrote:
On Thursday, 21 March 2019 22:39:22 UTC, wrote:
On Thursday, 21 March 2019 12:36:15 UTC, whisky-dave wrote:
On Wednesday, 20 March 2019 17:18:23 UTC, wrote:
On Wednesday, 20 March 2019 11:43:48 UTC, whisky-dave wrote:


It has not been possible to measure the speed of light since 1986 or earlier;

yes it has , it;s been in physics books long before then.

I think that you did not pay enough attention to my "since" above.

It made little sense. are you saying the egyptians knew the speed of light,

No, I'm not saying that. But the Egyptians have always been on the fringe of Western scientific culture, so they would have known the speed of light soon after we did.


What's 1986 got to do with it, or are yuo saying the speed of light changed in 1986 ?

Which part of that do you want to be answered? s/or/and.

(1) Not a lot; I should have written "1983". The Web page "https://en.wikipedia.org/wiki/Metre" contains : "21 October 1983 €“ The 17th CGPM defines the metre as the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second." I don't know offhand the date on which that took effect.

(( Before that change, "However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1 650 763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum." ))

(2) At each of those changes, the speed of light changed, because the definition of speed incorporated the definition of the metre, which changed by an unknown amount as near zero as could be managed on the available information. It is possible, in principle, to measure, with greater precision than available in 1983, the wavelength of that line in terms of the current metre and/or the frequency in terms of the current second, and that would yield the value of the change in the speed of light.

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(c) Dr. S. Lartius, UK. Gmail: dr.s.lartius@ |