4 ms·

It will be always be arbitrary anyway. Say we used the wavelength of some special light as unit for length... Why did we choose that light, and not something el

It will be always be arbitrary anyway. Say we used the wavelength of some special light as unit for length... Why did we choose that light, and not something else? The point with the kilogram redifinition is that it's currently tied to some piece of metal stored in Paris somewhere. We can physically lose the definition.

pintxo 8y ago
And we actually do: „Though the IPK, the current primary artefact, and its replicas are stored in carefully controlled laboratory conditions, their masses have been subject to fluctuation as a result of poorly understood factors, possibly including handling, cleaning and contamination. The IPK has diverged from its replicas by 50 μg since their manufacture late in the 19th century.“ https://en.m.wikipedia.org/wiki/Kilogram https://en.m.wikipedia.org/wiki/Kilogram
krastanov 8y ago
We do use resonant frequency of certain atoms to define units, but it is not arbitrary, rather it was a transition chosen for it's extremely small width and ease of measurement. It is quite a bit less arbitrary than using the size of our planet or of an artifact we have made. And we are getting even better, with definitions based on fundamental constants now, independent of a particular measurement experiment. This is as non-arbitrary as it can get (we will continue using a conversion factor to make the unit human scaled, but that is all).
- Double_a_92 8y ago
  But one meter is still the length we are used to. So we got the resonant frequency of some atoms, which is arbitrary because they just happened to be relatively easy to measure for us... Multiplied by some arbitrary factor so the unit reaches the usual length. That doesn't seem very fundamental to me. Could you imagine proposing that to some intergalactical economic union as new standard?
  - ElBarto 8y ago
    The resonant frequency of the atom is not arbitrary, it is a physical constant. That's the key and the point: It allows to measure exactly 1 second anywhere in the universe because the resonant frequency of that atom is the same anywhere in the universe. Compare with a previous definition of the second as a fraction of the solar day... A solar day isn't in fact constant and, in any case, once you leave earth it means nothing at all.
- ElBarto 8y ago
  The definitions are still arbitrary. The important change is that they are arbitrary numbers of a fundamental physical constant instead of being completely arbitrary. That means that the unit may be exactly measured anywhere in the universe based on the definition only without the need of any artifact.
nraynaud 8y ago
Yeah, but there are interesting criteria: an abstract definition for theoretical re-construction of reference artifacts(I don’t know if anyone has ever actually re-built a reference without comparing it to a previously existing reference during development), using integers in the definition where it makes sense, etc.

pintxo 8y ago

And we actually do: „Though the IPK, the current primary artefact, and its replicas are stored in carefully controlled laboratory conditions, their masses have been subject to fluctuation as a result of poorly understood factors, possibly including handling, cleaning and contamination. The IPK has diverged from its replicas by 50 μg since their manufacture late in the 19th century.“ https://en.m.wikipedia.org/wiki/Kilogram https://en.m.wikipedia.org/wiki/Kilogram

krastanov 8y ago

We do use resonant frequency of certain atoms to define units, but it is not arbitrary, rather it was a transition chosen for it's extremely small width and ease of measurement. It is quite a bit less arbitrary than using the size of our planet or of an artifact we have made. And we are getting even better, with definitions based on fundamental constants now, independent of a particular measurement experiment. This is as non-arbitrary as it can get (we will continue using a conversion factor to make the unit human scaled, but that is all).

Double_a_92 8y ago
But one meter is still the length we are used to. So we got the resonant frequency of some atoms, which is arbitrary because they just happened to be relatively easy to measure for us... Multiplied by some arbitrary factor so the unit reaches the usual length. That doesn't seem very fundamental to me. Could you imagine proposing that to some intergalactical economic union as new standard?
- ElBarto 8y ago
  The resonant frequency of the atom is not arbitrary, it is a physical constant. That's the key and the point: It allows to measure exactly 1 second anywhere in the universe because the resonant frequency of that atom is the same anywhere in the universe. Compare with a previous definition of the second as a fraction of the solar day... A solar day isn't in fact constant and, in any case, once you leave earth it means nothing at all.
ElBarto 8y ago
The definitions are still arbitrary. The important change is that they are arbitrary numbers of a fundamental physical constant instead of being completely arbitrary. That means that the unit may be exactly measured anywhere in the universe based on the definition only without the need of any artifact.

nraynaud 8y ago

Yeah, but there are interesting criteria: an abstract definition for theoretical re-construction of reference artifacts(I don’t know if anyone has ever actually re-built a reference without comparing it to a previously existing reference during development), using integers in the definition where it makes sense, etc.