The Kilogramme Will Cease To Be A Physical Entity

One of the most illuminating high school courses no doubt for many readers as much as for your scribe, was the series of physics lessons during which the SI units were explained. That glorious sense of having the order of the universe unlocked into an interlocking series of units whose definitions could all be derived in terms of a series of base units was mind-blowing in those early teen years, and even though the explanations might have been at a for-the-children level that has been blown out of the water by later tiers of learning it’s still a bedrock that will serve an engineer or scientist life-long.

The definitions of the SI base units have evolved with scientific advancement to the point at which they are no longer tied to their original physical entity definitions. Of all the base units though there is still one that has resisted the urge to move away from the physical: the kilogramme (giving it its French spelling to preserve context) is still defined in terms of a metal cylinder in a laboratory just outside Paris. Kg diehards have not much time left to cling onto their platinum-iridium alloy though, for a new definition has been adopted in which it is derived from Planck’s Constant. From next May this will become the official kilogram, at which point concerns over microscopic erosion of the metal standard become irrelevant, and an SI kilogram can be replicated by any laboratory with the means to do so.

The piece of apparatus that makes this definition possible is the Kibble balance, a balance in which the force required to overcome the effect of gravitational force on a given mass is measured in terms of the electrical power required to do so. The gravitational force at a given point can be measured accurately and is defined in terms of the other SI units, while the electrical power can be derived from a Josephson junction, a superconducting junction whose current is defined in terms of Planck’s constant. As a result, the kilogram can be measured solely in terms of the constant and other SI units, consigning the metal cylinder to history.

This high-end metrology and physics make for interesting reading, but it’s fairly obvious that the de facto kilogram we all use will not change. Our everyday measures of everything from sugar to PLA filament will be the same today as they will be next May. But that’s not the point, everyday measurements do not need the extreme accuracy and reproducibility of a laboratory. The point of it all comes in as yet unforseen applications, as an example would the ability to synchronise timing to create GPS or digital radio have been possible were the second to be still defined in terms of astronomical movements rather than atomic states?

Standard kilogramme replica picture: Japs 88 [CC BY-SA 3.0]

72 thoughts on “The Kilogramme Will Cease To Be A Physical Entity”

1. davidelang says:

doesn’t this require that gravity be constant? and we know that gravity is not, because we have satellites up designed to measure the difference in gravity as they orbit the earth. If it’s detectable at orbital distances, how can you have it be consistent enough for a standard at ground level?

I’d be surprised if I was the first person to think of this, but it does seem like a hole is the scheme.

1. Gravity needs to be constant from one side of the apparatus to the other. This is typically approximately true.

2. Andrei T. says:

From Wikipedia:
The weight of the kilogram is then used to compute the mass of the kilogram by accurately determining the local gravitational acceleration. (See gravimeter.) This will define the mass of a kilogram in terms of a current and a voltage, as described below.

3. Artenz says:

The solution is to measure the local gravity at the place where you set up the apparatus. One of the methods is to measure how quickly objects fall in a vacuum.

1. R says:

thats’ right, they measure gravity across and around the testing location so they can have error analysis on every variable involved in the experiment. In setups so precise there are very few assumptions.

2. Leithoa says:

I would expect a pendulum of sufficient size (a meter or so) would be easier and more accurate than dropping masses in a vacuum.

1. For a very short time the _yard_ was defined as being exactly the length of a seconds pendulum. However this has two problems. 1) The gravitational field varies slightly as you travel around the earth and 2) Any future human colonies in other star systems would need to take a reference earth-sized mass with them to calibrate their yard sticks.

1. Leithoa says:

Measuring local gravity is the whole point. A long pendulum is just a lot easier to construct than a kibble balance of equal precision.

2. TheRegnirps. says:

And period varies with amplitude. Maybe they used an isochronous pendulum. If a simple string pendulum is constrained – by a cycloid of course – the period is independent of amplitude. https://en.wikipedia.org/wiki/Tautochrone_curve

2. Luke says:

– friction in the pivot
– air resistance
– coriolis force
– measuring the exact center of mass for the pendulum
– local gravity

Those are the practical fudges for the pendulum. Dropping a mass in a vacuum chamber eliminates a whole bunch of those variables.

2. They could take this opportunity to drop the spurious SI prefix (kilo) from the base unit of mass. Perhaps name it after the inventor of the means of measuring it. Buying 5 Kibbles of kibble would be fun.

1. Cliff Claven says:

Actually, it wan’t invented by someone named kibble, it was invented by a pet food company to mass trace nutrients added to kibble during the manufacturing process.

1. alexcat3 says:

Wikipedia: “The principle that is used in the Kibble balance was proposed by Bryan Kibble of the UK National Physical Laboratory (NPL) in 1975 for measurement of the gyromagnetic ratio.[9]”
If you are right, you should edit Wikipedia.

2. What do you mean? Kilo means “multiplied by thousand”, and it seems pretty standard and well-put to the case to me.

1. Yes, kilo _does_ mean x1000, But the kg is the _base_ unit. So it shouldn’t have a prefix.

1. “Gram is the base unit”–random 10-year-old.

1. Luke says:

gram used to be the base unit with the CGS system, but then we changed to the MKS system in the 70’s.

Kilograms are just too big to be a practical base unit. If we changed the name to get rid of the “k” then what would be the new gram would need a prefix m- in front of it, and you’d just push the problem into having to measure every day objects in milli-somethings.

3. milentije89 says:

From the May 2019. not only the kilogramme will be changed but a lot more units (amper is one of them). I had a pleasure to speak about this with one German scientist who was a part of “working group” in charge of this for the past several years.
These changes will not have a significant impact to everyday measuremets, but will make some work to those who are working in laboratories on some precise equipment (volt, as disseminated unit, will be changed by 0.1 ppm or something like that if I remember correctly).

1. Jefferson Allan says:

I’m just trying to imagine the kit you’d need to measure a voltage down to 0.1ppm. Yikes.

4. How accurately do we know Planck’s constant? Or, by setting the value of the kg, are we now defining the value precisely?

1. James Churchill (pelrun) says:

The value of planck’s constant is now set by definition – they picked a value close to the measured value and fixed it. That means 1kg doesn’t change appreciably, but the error is now moved to the physical objects that the constant is used to measure, rather than the other way around.

2. Scaramouche says:

By measuring it, we’ve now changed it.

5. Missed opportunity title:

Kill the Kilogram!
:o)

1. Avin A. Laff says:

Apples and oranges. Pound is a unit of force, kilogram is a unit of mass.

1. That depends on your unit system.
You can make a consistent system where the pound is a unit of force, and one such system uses the “slug” as the unit of mass.
Or, alternatively, the pound can be a unit of mass, and one such system uses the “poundal” as the unit of force.

6. 1 Litre of water = 1Kg, yeah? – does this throw up other complications?

1. JDX says:

Complications include temperature and purity of the water, and evaporation,

1. Luke says:

And making the measuring cup to hold exactly 1 liters to the molecule.

2. Brian says:

The temperature would not change its mass, only its volume would change.

1. Shannon says:

And since the question was about relating mass and volume that’s pretty important.

2. Bunsen says:

With what fractions of deuterium, oxygen-17, and oxygen-18? And how do you verify those?

1. Leithoa says:

The fractions are well known and averaged into molar masses. If you were really concerned you could set up a still to only get the precise isotope blend you want. Natural fractionation from the water cycle is fairly well mapped out but (inter)national standards bodies could easily make water of arbitrary isotope blends. I’m sure those exist already; at commensurate expense.

3. billtheplatypus says:

Ultimately this boils down to the mass of a certain number of electrons, protons, and neutrons. From what I heard, and alternative method for redefining the kilogram was accurately determining Avagadro’s number. Then a kilogram could be defined as the mass of x neutrons/protons. It seems however, that the kibble balance won out.

1. One competing standard for the kg was a Silicon sphere of exact size, you might have seen it on YouTube and the Internet. This ties mass to length, which in turn comes from the speed of light and the second.

1. Brian says:

But then you are tied to a physical item that can’t be duplicated. It would be impossible to make an identical sphere with the same number of atoms in it. So it really needs to be based on some other non-physical constant.

1. Luke says:

You wouldn’t need an identical item, just one that can be measured exactly. The point of the silicon sphere was to define how many atoms of silicon there are in the volume of the sphere and how much they weigh.

After that, any sphere of silicon would do as long as you could measure its exact volume. The physical kilogram standards aren’t exactly a kilogram either – they’re offset by some amount, which is known and accounted for.

4. yourboss says:

haha… no, all the purported benefits of this silly system are fabrications and lies.

1. Shannon says:

Sure, one of the benefits is fabrication. We’re able to fabricate things with greater accuracy than every before.

7. Elmer Nickerson says:

” an SI kilogram can be replicated by any laboratory with the means to do so.” – That’s a bit of a tautology.

1. CityZen says:

Well, that “means” no longer needs to include a crack ninja team to go to Paris and swipe the Grand Kg (without altering it).

1. Nate B says:

I’d watch that movie.

2. light beams says:

Not at all, just like how you can measure 1 second by counting oscillations in a caesium gas, now you can measure a kg by comparing to a voltage source. This means that we could explain a kg to aliens over a radio and they could make exactly the same mass.

1. Janusz says:

I am sure that aliens are using their own number system based on odd numbers (like base 13 or base 17) and would have pragmatic solution of 13kg=1odd kg and 7 odd kg= 1 secodd kg and 19 secodd kg = pragma kg. 2.37 pragma kg is roughly…

1. Shannon says:

A prime numeric base? How irritating that would be. Maybe we shouldn’t talk to those aliens.

8. Planck’s constant seems like a somewhat arbitrary and extremely small number, I somewhat doubt that it has the cosmic and/or atomic significance often ascribed to it.

1. it seems more logical to just equate the voltage and/or current used to the equivalent of a gram.

1. James Churchill (pelrun) says:

Except how are *those* values defined? I’m not sure I want to armchair scientist this when literally the most experienced scientists in the field, the ones who *actually* have to use the definition practically, have just voted on a different approach.

1. THEODORE P HUNTINGTON says:

By the quantity of electrons and/or the voltage- as I understand it that is the purpose of the new method to measure a mass (and weight) using electromagnetism- which is perhaps thought to be more accurate than gravity-only scales. I have no problem with the method, or majority rule, but just using Planck’s constant which to me seems arbitrary. The value is in the measuring tool, not some platinum brick sitting in a case in Iceland used as a theoretical reference for the love. Personally, submission to a small group of “experts” is foolish- experts have been wrong many many times- in particular in this bizarro single-bullet/19 hijacker conspiracy theory age- there simply is a lot of corruption in what little windows we see in front of us. The method of many corrupted insiders is to carry on a ceaseless effort to snow the blind outsider public with holier than though arrogant know-it-all snow-them-with abstract math and hope that they never invent a nanocamera or device that hears thought-audio- and pretty much surrenders to authority- just sayin’

1. THEODORE P HUNTINGTON says:

we should really measure mass in light particles, hydrogen atoms, and I see no problem with “grams” or “drams”- they are all fine- like the rise of Christianity and FitzGerald’s theory of space contraction that grew into the theories of relativity- newer is not always better! How many other times have we seen not “new and improved” but “new and worsened”?

2. James Churchill (pelrun) says:

Aaand now you’re off with the fairies.

3. Shannon says:

“just saying” a load of nonsense.

9. RW ver 0.0.1 says:

If there’s 1.2 x 10^-8 uncertainty in the value of Plank’s constant though, does that mean a nanogram may or may not actually exist? :-p

1. Ren says:

That has something to do with Heisenberg’s Uncertainty Principle, right?
In other words, by trying to weigh it, (which is a form of observation) you’ve just changed it…

1. Leithoa says:

Not in this case.
This is making light of the uncertainty in the measurement / definition. Essentially the nanogram is below the ‘noise floor’ of the definition of Planck’s constant and by extension this definition of the kilogram.

1. James Churchill (pelrun) says:

Is that uncertainty in the constant, or uncertainty in the standard used to measure it? :D

Anyway, there’s zero uncertainty in the value now. PI IS EXACTLY THREE!

1. That was very interesting to watch, thank you.

Worth watching from the beginning:
The penalty for an uncalibrated instrument in the Ancient Egypt was death.

Better recalibrate the bloody DMM!
:o)

1. Ren says:

I worked in a calibration lab back in 1989. We had our lab standards sent off to be adjusted to the 1990 Volt.

1. Nate B says:

Here I thought the Volt was introduced in the 2011 model year!

10. So the metric system is now in the year 3018, while the Imperialists are still in 1824?

1. Scaramouche says:

That’s right. If you give these metric bastards an inch, they’ll take a kilometer.

11. Zerg says:

So IOW unless you’re a pointy headed type working in a multi-million dollar state of the art lab, this is all pretty much irrelevant to the rest of us.

1. Shannon says:

As is the case with most news, yes. It’s really interesting though, isn’t it.

12. bwmetz says:

My dog is already protesting her new kibble balanced diet.

13. Lufo says:

The beginning of the end of humanity as we know it …

How humanity will, after a catastrophic massive event that has wiped 3/4 of civilization, will “measure” and agree in 1 kilogramme???
With a physical weight is easy … with Planks is not that easy.

Is absurd but it is a possibility nonetheless.

1. davidelang says:

why would people in those conditions care about a universal ‘Kg’? they will probably develop new measurements that are more directly applicable and conversions to metric or imperial measurements will matter only to the extent that they are interacting with ‘pre disaster’ measuring artifacts or books.

14. One contending standard for the kg was a Silicon circle of correct size, you may have seen it on YouTube and the Web. This binds mass to length, which thus originates from the speed of light and the second.

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