4 ms·

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://

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 https://en.wikipedia.org/wiki/SI_base_unit

icebraining 8y ago
This will still have some arbitrary constant, much like the meter and the second.
- analog31 8y ago
  Indeed, it's not the arbitrariness that's going away, but the need for special artifacts such as the kg standard.
  - JdeBP 8y ago
    One can be even more specific: the need for the last remaining special artefact that is the kg standard.
- Patient0 8y ago
  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).
dpcx 8y ago
Sadly that list is based on data from the 2005 standard. Several of these have since been redefined.
- jobigoud 8y ago
  Can you point to one in particular? We can update the article if it needs to.
  - guipsp 8y ago
    I found this: https://www.bipm.org/en/publications/si-brochure/ https://www.bipm.org/en/publications/si-brochure/ Also includes the (currently in draft) 9th edition.
  - JdeBP 8y ago
    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 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_base_units#Second https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...
- JdeBP 8y ago
  It's inline with the 8th edition of the SI Brochure, published in 2014. It's not inline with the draft 9th edition.
Double_a_92 8y ago
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
moftz 8y ago
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.

icebraining 8y ago

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

analog31 8y ago
Indeed, it's not the arbitrariness that's going away, but the need for special artifacts such as the kg standard.
- JdeBP 8y ago
  One can be even more specific: the need for the last remaining special artefact that is the kg standard.
Patient0 8y ago
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).

dpcx 8y ago

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

jobigoud 8y ago
Can you point to one in particular? We can update the article if it needs to.
- guipsp 8y ago
  I found this: https://www.bipm.org/en/publications/si-brochure/ https://www.bipm.org/en/publications/si-brochure/ Also includes the (currently in draft) 9th edition.
- JdeBP 8y ago
  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 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_base_units#Second https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_ba...
JdeBP 8y ago
It's inline with the 8th edition of the SI Brochure, published in 2014. It's not inline with the draft 9th edition.

Double_a_92 8y ago

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

moftz 8y ago

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.