Important milestone reached on road to a redefined kilogram

June 21, 2016
The NIST-4 watt balance is shown. The instrument recently took its first full measurements of Planck's constant, an important step toward redefining the kilogram. Credit: Jennifer Lauren Lee/NIST PML

In a secure vault in the suburbs of Paris, an egg-sized cylinder of metal sits in a climate-controlled room under three glass bell jars. It is the mass against which all other masses in the world are measured - by definition the quintessential kilogram.

Yet the so-called "Le Grand K" may soon be deposed from the standard-setting throne it has held for the last 127 years. Efforts are afoot in the scientific community to define mass using a fundamental constant of nature instead - a value that in theory can be measured anywhere in the universe and won't change with the smude of a fingerprint or the settling of a fleck of dust.

"The problem with the in Paris is that it's so precious that people don't want to use it," said Stephan Schlamminger, a physicist at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.

Schlamminger leads a team at NIST that has now reached an important milestone on the road to replacing the standard kilogram. The scientists have taken the first full set of measurements on a new machine, called the NIST-4, designed to measure a fundamental physical quantity called Planck's constant, or h. Planck's constant relates a quantum particle's frequency to its energy, which is turn can can be related to mass through Einstein's E=mc2.

NIST-4 is a , a high-tech scale that compares the weight of a mass to the needed to balance it. The electromagnetic force - which is created by running current through a coil of wire suspended in a - can then be used to calculate Planck's constant.

Before the world redefines the kilogram - an event currently scheduled for 2018 - multiple independent measurements of Planck's constant must agree with each other. NIST-4's first Planck's constant measurement likely meets this standard, Schlamminger said. The value, which the team reports in a paper in the journal Review of Scientific Instruments, matches with other experiments relatively well, and it has an uncertainty of only 34 parts per billion.

The team aims to get the uncertainty down to 20 parts per billion in the coming year, a goal they think they can reach by more precisely measuring how the current in the coil affects the magnetic field at the coil's location and by reducing the measurement noise.

The best watt balance measurement of Planck's constant so far comes from Canada's National Research Council, with an uncertainly of 19 parts per billion, Schlamminger said. A collaboration of scientists is also working on an alternative method for measuring h, which involves counting the atoms in an almost perfect sphere of silicon. The group has arrived at a value close to the watt balance numbers, and with an uncertainly of 20 parts per billion.

All the groups will have until July 2017 to publish new measurements of Planck's constant in order to be taken into account for the redefinition of the kilogram. The results will be fed into a computer program that will calculate a value of h that best fits all the data.

With the redefinition, h will become "fixed for all time," Schlamminger said, and the role of the watt balance will be flipped. Instead of using standard masses to measure Planck's constant, the watt balance will use the standard value of h to measure mass.

All the redefining should go on with little impact on the outside world. "It's the frustrating part about being a metrologist," Schlamminger said. "If you do your job right, nobody should notice."

Still, the undertaking is an impressive scientific feat that will make the kilogram more stable and accessible in the long run. The fact that NIST-4 is taking measurements in time for the kilogram redefinition is testament to the hard work and talent of his team, Schlamminger said. The machine, which is a successor to the NIST-3 watt balance, took just four years to build from scratch, a type of project that would usually take at least 10 years, Schlamminger noted.

And although NIST-4 will eventually displace chunks of metal as the U.S. standard for measuring mass, Schlamminger is sure "Le Grand K" will retain its mythical aura. "It's such a symbol and it has such a rich history of measurement. I don't think people will just throw it in the garbage," he said.

Explore further: NIST physicists build a watt balance using LEGO blocks to measure Planck's constant

More information: "A precise instrument to determine the Planck constant, and the future kilogram," by Darine Haddad, Frank Seifert, Leon Chao, Shisong Li, David Newell, Jon R. Pratt, Carl J. Williams and Stephan Schlamminger, Review of Scientific Instruments on June 21, 2016. DOI: 10.1063/1.4953825

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humy
4 / 5 (2) Jun 21, 2016

"...The best watt balance measurement of Planck's constant so far comes from Canada's National Research Council, with an uncertainly of 19 parts per billion
...
....
The results will be fed into a computer program that will calculate a value of h that best fits all the data.

With the redefinition, h will become "fixed for all time,"
..."

Hang on! Surely it is inevitable that technology will keep on improving with time so that eventually, at some time in the future, say, 40 years from now, a new way is found to measure Planck's constant with an uncertainly of much less than that 19 parts per billion, lets say, just 0.1 parts per billion, but then what if that new measurement doesn't agree with that h value that was "fixed for all time," by that earlier computer program!? Surely it would then be idiotic to keep it "fixed for all time," thus it shouldn't ever be regarded or said to be "fixed for all time," ?
italba
4 / 5 (1) Jun 21, 2016
I can't understand why mass isn't just defined against the Avogadro constant.
KBK
1 / 5 (3) Jun 21, 2016
Science is slowly coming to the point of accepting a holographic multiverse.

In such a scenario, nothing is in permanence, nothing is immutable. There can be difference, but not much in the way of permanence. No specific speed of light, no perfected Planck's constant and so on.

So not only will the idea of a perfect kilo be thrown away, but it will have to be first realized, that the perfect kilo is irrelevant. Meaningless, outside of generalized differentials of the given locale and moment.

"All matter originates and exists only by virtue of a force... We must assume behind this force the existence of a conscious and intelligent Mind. This Mind is the matrix of all matter." ---Max Planck

Which means.... that differential and data, and then cognition and intelligence... is all we have, all else is impermanence. And this does indeed also include the so called 'real world'.
matica
5 / 5 (2) Jun 21, 2016
humy: The numerical value of h will not change. The 19 parts per billion uncertainty will be contained in the ability to measure a mass of, say, one kilogram, even though the kilogram is defined exactly in terms of h. As measurement technology improves, one will be able to determine the mass of an object more accurately.

tpb
5 / 5 (1) Jun 21, 2016
This is kind of strange since the results depend on the earths gravity since that is what is being balanced out. Doesn't the force of gravity change depending on where you are on the planet as determined by height and the density of mass under you?
Is this change greater than low parts per billion?
gwrede
4.2 / 5 (5) Jun 21, 2016
A mass of 1kg is a mass of 1kg on any planet. But its _weight_ may be different on other planets. Mass and weight are not the same thing.
tpb
1 / 5 (1) Jun 21, 2016
gwrede, the point I was making is that they are measuring the electrical energy needed to balance out the "weight" of 1kg, which depends on gravity.
shavera
5 / 5 (4) Jun 21, 2016
humy: think of it like this: We define a meter to be how far light travels in precisely 1/299792548 second. No matter how much more precisely we measure portions of a second, the meter's definition is permanently fixed at that value.

We might do something similar, where we define h to be (to use simple numbers) 6.626x10^-34 J*s, where a J is kg m^2 / s^2 . (can work backwards to get kg in terms of h). No matter how much more precisely we measure mass in the future, that definition won't change. The point of having the high precision now is so that when we go to make our permanent definition, it causes the least amount of difference in the definition of a kilogram from what it currently is.
humy
5 / 5 (2) Jun 22, 2016
humy: The numerical value of h will not change. The 19 parts per billion uncertainty will be contained in the ability to measure a mass of, say, one kilogram, even though the kilogram is defined exactly in terms of h. As measurement technology improves, one will be able to determine the mass of an object more accurately.


Oh I see. I thought a kg was always defined as the mass of one cubic centimeter of water but then just now I looked it up and I found this link below that suggests they are thinking of redefining it according to h although they haven't done that yet (as far as I am aware)
https://en.wikipe...Kilogram
"...the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant. The decision was originally deferred until 2014; in 2014 it was deferred again until the next meeting..."

Personally I would like the idea better of defining a kg according to x number of carbon 12 atoms and then defining h according to the kg.
humy
5 / 5 (2) Jun 22, 2016
[ humy:...
I thought a kg was always defined as the mass of one cubic centimeter of water ...


misedit:
that should have been;
" ...defined as 1000 times the mass of one cubic centimeter of water..."
BongThePuffin
Jun 22, 2016
This comment has been removed by a moderator.
Da Schneib
not rated yet Jun 25, 2016
Good. It's always been a little weird to use a system of units where the length, duration, amount of material, electrical current, and temperature are all defined by basic physical constants, but mass is defined by some chunk of metal in someone's closet.
Eikka
5 / 5 (1) Jun 26, 2016
This is kind of strange since the results depend on the earths gravity since that is what is being balanced out.


They're not balancing the object against gravity, but against its own inertia.

The Watt balance basically subjects the object to a very precisely known force and watches how it accelerates under that force, which depends on its inertia, which depends on its mass. The trick is in generating and measuring the force to the required level of accuracy and eliminating unknown external forces.
tpb
not rated yet Jun 27, 2016
Eikka, if so, this would require precise measurement of velocity and distance moved, and air resistance if not enclosed in a vacuum. If done in one direction only and not two, would require moving the mass exactly horizontal or vertical or being able to measure the offset angle.
Measuring the power in the electromagnet to better than 20ppb is a difficult task by itself, much less measuring the velocity and distance the mass moved with time as well to better than 20ppb. Also if the distance between the mass and the electromagnet changes, the force applied will also change.
So power, time, velocity, distance, and force all together would need to be measured to better than 20ppb if the mass is actually moved.
TechnoCreed
not rated yet Jun 27, 2016
@tpq
Instead of reading Eikka's bullshitty explaination of the watt balance. Look at this short video. It will help you make some sense of it https://www.youtu...vVgsotIk

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