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# Redefining the kilogram in terms of Planck’s constant

Redefining the kilogram in terms of Planck’s constant
The official definition of a kilogram is currently determined as the mass of a metal cylinder called the International Prototype of the Kilogram, but that could be about to change
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The official definition of a kilogram is currently determined as the mass of a metal cylinder called the International Prototype of the Kilogram, but that could be about to change
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To calculate the Planck constant, the NIST team used a Kibble balance, which suspends a weight using electromagnetic forces

The kilogram has the dubious distinction of being the only SI unit still based on a physical object; specifically, a metal cylinder kept in a vault in France. Plans are well underway to redefine the kilogram in mathematical terms instead, and to that end a team at the National Institute of Standards and Technology (NIST) has submitted a precise new calculation of a key formula.

Since 1879, the kilogram has been defined as the exact mass of the International Prototype of the Kilogram (IPK), a small cylinder made of platinum and iridium. But there are a few problems with defining a base unit in terms of a physical artefact: the IPK gathers contaminants that make it slightly heavier over time, so it needs to be regularly treated. To complicate matters, there are 40 copies around the world and they're all getting "dirty" at different rates, meaning their masses are slowly drifting out of sync.

That's obviously not something you want in a base unit that's supposed to be universal. And the discrepancies don't just affect the kilogram itself: other units such as the pound, ton or milligram are defined in terms of their relationship to the kilo, as are non-mass units like the ampere (for electric current) or the candela (luminous intensity).

A better option is to develop a new definition based on a mathematical foundation that can be calculated anywhere, and the Planck constant fits the bill. This formula allows researchers to find mass in relation to electromagnetic energy, so by finding as precise a value for it as possible, the kilogram can be redefined in terms of the official definition of the meter and the second.

NIST's new value for the Planck constant is 6.626069934 x 10-34 kg∙m2/s, with an uncertainty of 13 parts per billion. If that number makes your eyes glaze over, the important part is the end: 13 parts per billion is incredibly precise.

To measure the Planck constant, the researchers used a Kibble balance, a device that suspends a 1-kg weight with electromagnetic forces. They can calculate the constant according to the amount of electromagnetic energy it takes to balance the mass.

The team says the more precise figure comes courtesy of having 16 months' worth of measurements to draw from, as well as adjustments they'd made in how the electromagnetic field was created and measured.

These experiments join several other projects that were attempting to find the most precise value of the Planck constant, and while everyone's answers were different, they have low enough levels of uncertainty to make a case for redefining the kilogram in terms of the Planck constant.

"There needed to be three experiments with uncertainties below 50 parts per billion, and one below 20 parts per billion," says Stephan Schlamminger, lead researcher on the project. "But we have three below 20 parts per billion."

All of these measurements have been submitted for consideration by an international body, which will review them to determine the official value of Planck's constant. The official definition of a kilogram – along with the other units that depend on it – is set to be changed in November next year.

The NIST results were published in the journal Metrologia.

Source: NIST

LeahC
This sounds all good, but from my understanding, suspending an object via electromagnetic forces in order to calculate a standard unit of weight will also be effected by the vary same issues that the older method is. Weight is after all Mass x Gravity and is purely an Earth bound measurement. In space 1kg has no meaning, instead the object's Mass and resistance to movement would be more important. So to say that the calibration can be conducted anywhere is false, as Gravity is different at different altitudes (distances from the center of the earth). The other issue is the dirt on the object being suspended will surely have the same effect as dirt on the original 1 Kg cylinder. Then the medium it is being suspended in needs to be taken into account as well. I am no expert by any means and perhaps I have not understood all aspects of the article, but surely it would be better to base it on an Atomic Mass of a specific element or something a bit more universal. Soon, humans will be on Mars or one day further and this form of measurement will not be applicable anywhere other than on earth.
pmshah
A US institute that is going to define a KG eight that absolutely noone in the industry is gong to use. Great !!!
Only thing that will change is the equivalent weight in grams that is printed on "some" packaged food products.
Brian M
'To measure the Planck constant, the researchers used a Kibble balance, a device that suspends a 1-kg weight with electromagnetic forces'
So actually its based on a 1 -kg weight - Full circle then! Its not an absolute measurement as its based on a physical lump of metal (plus dirt!) which was used to calculate Planks constant.
But at least the new value will be in terms (albeit relative to an arbitrary 1 kg block of metal) of a electromagnetic force, so shouldn't change (?).
MalcolmLander
Don't confuse mass with weight. A kilogram is a unit of mass. Weight (or force) is measured as mass x acceleration. So the scientists working on this project have their basic physics correct. What a surprise!
1 kg will still be 1 kg mass on the moon. But you can't measure it using a set of bathroom scales accurately unless you recalibrate the scales to the gravitational field. On earth, they are calibrated to the average gravitational field which technically means that they measure a kilogram-force rather than a kilogram.
S Michael
I have read all of the post and they make a lot of sense. My question is this. If a piece of metal, stored in a vault gains weight because of so called "dirt". How much weight do planets gain just being in space? How much weight does our planet gain every 1000 years? Is there a tipping point that changes our obit or balance? Or does the solar winds keep the "dirt" away and keep us balanced?
Grunchy
Dibs on the platinum cylinder.
Gregg Eshelman
"...they have low enough levels of uncertainty..."
We demand rigidly defined areas of doubt and uncertainty! Vroomfondel "The Hitchhiker's Guide to the Galaxy"
Surefoot
@pmshah, the whole world uses that unit. Yes even you in USA since your medieval units are now derived off this kilogram definition.
@LeahC, the atomic mass is defined with.. the kilogram.
@Malcolm, they actually use calibrated pressure sensors under a specific apparatus that compensates for the local gravity field. It's quite big so upper weights need to be compensated since they are in an ever so slightly weaker gravity field ! Crazy stuff.
@S Michael, yes planets gain weight, from space dust and debris (meteors and such). The gain is very, very small compared to the planets mass. And yes it affects its orbit and even its spin (and precession), ever so slightly, from what i recall internal movements from the planet core and mantle affect it a lot more. We do adjust world atomic clocks every few years or so (up or down 1 second) as a result. The change over 1000 years will still be impossible to notice without very precise instruments and atomic clocks..
T N Args
Not sure what is wrong with defining a kilogram of mass as the mass of a litre of H20 at a specified temperature and pressure.
JimFox
"Not sure what is wrong with defining a kilogram of mass as the mass of a litre of H20 at a specified temperature and pressure." Very hard to measure a litre of water, either by mass or volume, to the degree of accuracy achieved by these folks [13 parts per billion]. Slippery stuff, water- meniscus effect, surface tension, atoms escaping/recombining.