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Comment by jihadjihad | Hacker News Reader

I think this is good. Not a physicist but a lot of the SI definitions seem contrived and arbitrary [1] rather than based on physical fundamentals.

[1] https://en.wikipedia.org/wiki/SI_base_unit

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They were created well before the natural units or atomic units were discovered so they are going to based off of arbitrary measurements that were repeatable by scientists of the past. SI is just as arbitrary as imperial but at least it has the advantage of base-10 for the prefixes. The problem is that you can't just change the kilogram to whatever nice natural unit you want overnight so even moving to nice universal constants would still give you some weird coefficients.

This will still have some arbitrary constant, much like the meter and the second.

Indeed, it's not the arbitrariness that's going away, but the need for special artifacts such as the kg standard.

One can be even more specific: the need for the last remaining special artefact that is the kg standard.

Fun fact: the metre was was originally defined to be 1/10 millionth the distance from the pole to the equator (so that the circumference of the Earth would be 40 million metres).

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.

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

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).

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?

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.

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.

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.

Sadly that list is based on data from the 2005 standard. Several of these have since been redefined.

Can you point to one in particular? We can update the article if it needs to.

I found this: https://www.bipm.org/en/publications/si-brochure/

Also includes the (currently in draft) 9th edition.

The definition of the second has changed between the 8th edition of the SI Brochure last updated in 2014, with which Wikipedia agrees:

> The second is 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.

and the draft 9th edition:

> The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency ∆νCs, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1 .

The draft 9th edition incorporates the work to redefine several of the base units in terms of fundamental physical constants and dispenses with the last remaining reference artefact. The changes come up for a vote next month.

* https://www.bipm.org/en/measurement-units/rev-si/#when

So Wikipedia could include the concrete wording of what the redefinitions are currently intended to be. And it does.

* https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...

It's inline with the 8th edition of the SI Brochure, published in 2014. It's not inline with the draft 9th edition.

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