# Redefinitions of SI units

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1303560.  Fri Nov 16, 2018 5:37 am

The BBC is reporting today that the kilogram is due to be redefined "in terms of an electric current". Is this correct? My understanding is that several SI base units are due to be redefined by fixing the values of fundamental physical constants, as follows:

The second (unit of time) will continue to be defined more or less as now, in terms of properties of the caesium atom.

The metre (unit of length) will continue to be defined as now, with reference to the definition of the second and a fixed value for the speed of light.

The kilogram (unit of mass) will now be defined in terms of the metre and the second, and a fixed value for the Planck constant.

The ampere (unit of electric current) will now be defined in terms of the second, and a fixed value for the elementary charge.

The kelvin (unit of temperature) will now be defined in terms of the metre, kilogram and second, and a fixed value for the Boltzmann constant.

The mole (unit of amount of substance) will be defined in terms of a fixed value for the Avogadro constant.

The candela (unit of luminous intensity) will continue to be defined
as now.

https://en.wikipedia.org/wiki/Proposed_redefinition_of_SI_base_units#Impact_on_base_unit_definitions

So the definitions of the kilogram and ampere would appear to be independent of each other, as in this diagram. Have I misunderstood something or has the BBC?

EDIT: This from the BBC site:

 Quote: There is a quantity that relates weight to electrical current, called Planck's constant - named after the German physicist Max Planck and denoted by the symbol h.

https://www.bbc.co.uk/news/science-environment-46143399

The dimensions of Planck's constant are m^2 kg s^-1. How does it relate to electrical current?

[Edited to correct error in definition of mole]

Last edited by GuyBarry on Mon Nov 19, 2018 10:55 am; edited 1 time in total

1303597.  Fri Nov 16, 2018 9:30 am

 GuyBarry wrote: The BBC is reporting today that the kilogram is due to be redefined "in terms of an electric current". Is this correct?

Not really, but kind of.

 GuyBarry wrote: Have I misunderstood something or has the BBC?

I think the BBC is trying to explain it in simple terms that a layman would understand. Since Planck's constant will mean little to most people, the BBC reporter seems to have latched on to the part of the press release that says the kg can now be defined in terms of a Kibble balance which uses a certain current and voltage to very accurately measure weight.

 GuyBarry wrote: The dimensions of Planck's constant are m^2 kg s^-1. How does it relate to electrical current?

Dimensions can be played around with. Although they're derived units rather than base units, Planck's constant can also be defined in units of "J s " or "eV s". The latter, electron volt-seconds, relates to an electric current, albeit tangentially.

More info on the new SI definitions here in the BIPM press release.

1303620.  Fri Nov 16, 2018 11:57 am

 dr.bob wrote: Since Planck's constant will mean little to most people, the BBC reporter seems to have latched on to the part of the press release that says the kg can now be defined in terms of a Kibble balance which uses a certain current and voltage to very accurately measure weight.

But the definition of the volt is dependent on the definition of the kilogram:

1 V = 1 ‎kg m^2 s^-3 A^-1

So if you use voltage in determining the value of the kilogram, you're defining the kilogram in terms of itself.

 1303668.  Fri Nov 16, 2018 6:47 pm I used to have Planck's constant always at my fingertips, but life moves on. I did recently use it to calculate the wavelength of a cricket ball, just for old time's sake. The SI system (tautology alert) was originally labelled the 'mks' system (metres-kilogrammes-seconds), or based on it, which superseded the 'cgs system' (centimetres-grammes-seconds). It is very clever, but I feel more comfortable with the FFF system.

 1303722.  Sat Nov 17, 2018 12:29 pm If I adjust my bathroom scales, will I be lighter or heavier?

 1303736.  Sat Nov 17, 2018 1:21 pm All I want to know is whether I have to adjust my recipes or not.

 1303737.  Sat Nov 17, 2018 1:33 pm No one will have to adjust anything for everyday practical purposes. These are changes that are only relevant to scientists carrying out experiments to high levels of precision. I don't understand why this has become a mainstream media story at all, to be honest. The reporting suggests that only the kilogram is being redefined. In actual fact the whole basis of the SI is being redefined in such a way as to fix the value of certain fundamental physical constants, as was done with the speed of light back in 1983. We had a discussion on the forum a while back where it was suggested that it might be possible to "un-fix" the speed of light if experimental evidence pointed in that direction. This change strikes me as further indication that scientific thinking is moving in the opposite direction.

1303889.  Mon Nov 19, 2018 9:18 am

GuyBarry wrote:
 dr.bob wrote: Since Planck's constant will mean little to most people, the BBC reporter seems to have latched on to the part of the press release that says the kg can now be defined in terms of a Kibble balance which uses a certain current and voltage to very accurately measure weight.

But the definition of the volt is dependent on the definition of the kilogram:

1 V = 1 ‎kg m^2 s^-3 A^-1

So if you use voltage in determining the value of the kilogram, you're defining the kilogram in terms of itself.

Sorry, my mistake. I shouldn't have said that the kg can be defined using a Kibble balance. Having re-read the press release, it clearly states that a Kibble balance can be used to realize the kg.

As you point out in the original post, the kg is defined solely by using Planck's constant. The press release mentions the Kibble balance as a practical way this new definition can be used by experimenters to find out how heavy things are.

 GuyBarry wrote: We had a discussion on the forum a while back where it was suggested that it might be possible to "un-fix" the speed of light if experimental evidence pointed in that direction. This change strikes me as further indication that scientific thinking is moving in the opposite direction.

I thought the discussion we had was concerning this very implementation of redefined SI units. I don't see how changing from "in the near future we're almost certainly going to decide to implement this thing" to "we've decided to implement this thing" can be classed as "further indication" of something.

 1303890.  Mon Nov 19, 2018 9:26 am

 1303892.  Mon Nov 19, 2018 9:32 am I don't particularly want to revisit the earlier discussion since it all got rather heated, but the point I was making on that occasion was that if you build a fixed value for a physical constant into your system of units, then it becomes impossible to determine the value of that constant experimentally while working within that system of units - all you can do is verify the defined value. Just as it's currently impossible to frame the hypothesis that the speed of light is variable using SI units, from next May it will become impossible to frame the hypothesis that Planck's constant (or the Boltzmann constant, Avogadro constant or elementary charge) is variable. They will become "axioms" of the system.

 1303897.  Mon Nov 19, 2018 10:11 am I'm aware of the discussion, and also have little interest in revisiting it. My point is that we were discussing the implementation of this new definition of SI units, so I fail to see how doing the thing we were discussing is a further indication of anything.

 1303898.  Mon Nov 19, 2018 10:39 am We weren't actually, we were talking about whether it would be possible to reverse the 1983 decision to fix the speed of light. It seems quite clear to me now that it would be impossible to retain our current system of SI units under the assumption of a non-constant speed of light. Not only would a change in the speed of light affect the definition of the metre, it would also affect the definition of the kilogram (which will now depend on the metre) and the kelvin (which depends on the metre and kilogram).

1303983.  Tue Nov 20, 2018 8:56 am

 GuyBarry wrote: We weren't actually, we were talking about whether it would be possible to reverse the 1983 decision to fix the speed of light.

Certainly the majority of the discussion focussed around the definition of the speed of light. However, right at the start of the discssion, you pointed out that the other SI units were to be redefined in terms of physical constants that would be set in stone. That was back in March, and is the very thing you are now describing as "further indication" of the direction of scientific thinking.

 GuyBarry wrote: It seems quite clear to me now that it would be impossible to retain our current system of SI units under the assumption of a non-constant speed of light.

Two things here:

1) Experimental physicists don't "assume" anything. They observe and measure.

2) I pointed out in the original discussion that the SI system of units has been redefined several times in its lifetime. I recognise that we disagreed as to whether it would be possible to make an experimental observation that would invalidate the proposed definitions. However I hope you would agree that, if it was felt necessary, the BIPM would be prepared to redefine the definition of SI units again.

1303998.  Tue Nov 20, 2018 10:43 am

dr.bob wrote:
 GuyBarry wrote: We weren't actually, we were talking about whether it would be possible to reverse the 1983 decision to fix the speed of light.

Certainly the majority of the discussion focussed around the definition of the speed of light. However, right at the start of the discssion, you pointed out that the other SI units were to be redefined in terms of physical constants that would be set in stone. That was back in March, and is the very thing you are now describing as "further indication" of the direction of scientific thinking.

Back in March it was just a proposal - now it's a resolution.

Quote:
 GuyBarry wrote: It seems quite clear to me now that it would be impossible to retain our current system of SI units under the assumption of a non-constant speed of light.

Two things here:

1) Experimental physicists don't "assume" anything. They observe and measure.

You can only measure relative to a standard. The current standard assumes a constant speed of light. There are very good theoretical reasons for making this assumption but it's still only an assumption.

 Quote: 2) I pointed out in the original discussion that the SI system of units has been redefined several times in its lifetime. I recognise that we disagreed as to whether it would be possible to make an experimental observation that would invalidate the proposed definitions.

If you're operating within any coherent system of units, then of course it's impossible to make an experimental observation that invalidates the definitions of that system of units. If it were possible to do so, the system of units would be inconsistent with itself.

Take the redefinition of the kilogram that's just been agreed to. Up until now it's been defined as the mass of the International Prototype Kilogram in Paris. There were a number of national prototype kilograms with which its mass was periodically compared, and over the years fluctuations were found that suggested either that the national prototypes were gaining mass or that the IPK was losing mass. But within the SI, it was impossible to frame the hypothesis that the IPK was losing mass, because the mass of the IPK was 1 kg by definition. That's why national laboratories were asked to investigate ways of breaking the link between the kilogram and the IPK, as long ago as 1999.

It has taken twenty years to agree a definition of the kilogram that breaks this link. Once the new definitions come into force, it will be possible to make statements like "the mass of the IPK is falling by 2×10^-8 kilograms per annum" which would be meaningless under the outgoing definitions.

 Quote: However I hope you would agree that, if it was felt necessary, the BIPM would be prepared to redefine the definition of SI units again.

Of course it would, and I'm sure it will do so again in the future. My point is that the trend is now clearly away from "explicit-unit" definitions and towards "explicit-constant" definitions. The "explicit-constant" approach, which until now has only been used in the definition of the metre, is now being extended to the entire SI. I think this represents a fundamental change in philosophy, and not one that can be easily reversed.

1304003.  Tue Nov 20, 2018 11:32 am

 GuyBarry wrote: Back in March it was just a proposal - now it's a resolution.

That's a disingenuous thing to say.

The new definitions were agreed to in principle at the 24th General Conference on Weights and Measures (CGPM) in 2011 with the proviso that further work was necessary to improve the data on which the new system was based. At the next CGPM meeting in 2014, it was noted that:

 Quote: the data do not yet appear to be sufficiently robust for the CGPM to adopt the revised SI at its 25th meeting

The date of the next CGPM was set for November 2018. However, in October last year, the International Committee for Weights and Measures (CIPM) published this document which clearly states:

 Quote: The CIPM welcomed recommendations regarding the redefinition of the SI from its CCs. The CIPM noted that the agreed conditions for the redefinition have been met and decided to submit Draft Resolution A to the 26th meeting of the CGPM and to undertake all other necessary steps to proceed with the planned redefinition of the kilogram, ampere, kelvin and mole.

Now, if you want to be stupidly picky, you can try and claim that the new system remained just a proposal until all the final paperwork was completed. However, I think most people here would agree that, since it was already accepted in principle, once the agreed conditions had been met it definitely passed from being just a "proposal" to something that was definitely going to happen.

 GuyBarry wrote: You can only measure relative to a standard. The current standard assumes a constant speed of light. There are very good theoretical reasons for making this assumption but it's still only an assumption.

It really isn't. You mentioned something similar last time we had this discussion, and I'll repeat what I said then:

Physics is completely governed by data. The fact that they've fixed the value of the speed of light is based on decades of experimental data.

It's not an assumption.

 GuyBarry wrote: If you're operating within any coherent system of units, then of course it's impossible to make an experimental observation that invalidates the definitions of that system of units.

And yet they did, as you point out.

 GuyBarry wrote: Up until now it's been defined as the mass of the International Prototype Kilogram in Paris. There were a number of national prototype kilograms with which its mass was periodically compared, and over the years fluctuations were found that suggested either that the national prototypes were gaining mass or that the IPK was losing mass. But within the SI, it was impossible to frame the hypothesis that the IPK was losing mass, because the mass of the IPK was 1 kg by definition.

And here we have the clear difference between the theoretical perfect world that exists in principle, and the messy, normal world that practical scientists deal with pragmatically every day. As you point out, it was clear that the IPK was losing mass. This was well known despite, according to you, being theoretically impossible.

 GuyBarry wrote: Once the new definitions come into force, it will be possible to make statements like "the mass of the IPK is falling by 2×10^-8 kilograms per annum" which would be meaningless under the outgoing definitions.

And yet they said exactly that. Here's an article from 2011 in which Dr Terry Quinn, Director of the BIPM, is quoted as saying:

 Quote: Measurements over the past century have shown that the international prototype has lost around 50 micrograms

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