Indeed. A year is defined as the period of time it takes for the Earth to orbit the Sun. Before the Earth existed, there was no definition of a year.
Measuring anything in years that pre-dates the formation of the Earth is simply an extrapolation from our current methods of time measurement...
I really don't think that's correct. If you look at the top of page 29 it says "The dalton (Da) and the unified atomic mass unit (u) are alternative names (and symbols) for the same unit, equal to 1/12 of the mass of a free carbon 12 atom, at rest and in its ground state". So the amu is not...
You've got it exactly. The Avogadro constant is expressed in the units mol-1. You might as well define "dozen" as an SI unit (symbol "doz") and specify the "dozenal constant" as 1.2 x 101 doz-1.
Not really a "unit" in the sense that it's otherwise understood - just a gigantic multiplier.
Indeed they are. And the reason why it's been possible to fix both numbers is that the Avogadro constant is unconnected with the definition of the kilogram. It is the number of elementary entities in one mole. Fixing both numbers required the breaking of the connection between the kilogram...
Is the definition of the amu going to change as the result of the SI redefinitions then? I understood that the amu was a non-SI unit accepted for use with the SI, and that its value was obtained experimentally. That's certainly what it says in the BIPM's...
Well it was, but it won't be any more. It will be a number chosen to be as close as possible to the number of atomic mass units in the gram, but it won't actually be the number of atomic mass units in the gram. It will simply be a number laid down in the definitions of the SI, with no actual...
I rather like this handy little diagram:
https://en.wikipedia.org/wiki/2019_redefinition_of_SI_base_units#/media/File:Unit_relations_in_the_new_SI.svg
I don't think Avogadro's number will have any relation to the definition of the kilogram. Avogadro's number will be used to define the mole...
All three. :-)
No, I'm just being flippant. It's a really interesting discussion but I'm too tired to contribute any more at the moment. Thanks to everyone who's taken part.
Neither of those is true.
The definition of the second goes back to around 1000 AD. It was based on the theory that the period of the Earth's rotation was constant. That period was divided into 24 hours, each divided into 60 minutes of 60 seconds each. So the original definition of the...
And therefore any such inference must be unscientific.
I have no objections to people formulating theories about what exists outside the observable universe, but it doesn't constitute "science" as I understand it.
You're getting it back to front. The units we use now were conveniently chosen so that they more or less correspond to the ones that were used previously.
Over its history the second has been defined in terms of the rotation of the Earth, in terms of the Earth's orbit round the Sun, and (since...
Well you can't measure the speed of light now, because it's a defined constant (in SI units). But lots of measurements were taken prior to the time that its value was defined - otherwise there'd be no basis for the value that's currently agreed on.
It does. And the fact that we've taken millions of measurements of the speed of light within the observable universe and always got the same result allows us to infer the speed of light within the observable universe when it's not being measured.
However, it doesn't allow us to infer anything...
That's fair enough I suppose, but it's not a scientific theory. It's a plausible hypothesis based on the Occam's razor principle, as mentioned earlier. I agree that it would be highly unlikely if the laws of physics suddenly changed at the boundaries of the portion of the universe observable...