Weighty mission for scientists: redefine the kilo

Jan 24, 2011
The original cylinder of platinum and iridium used as the standard weight for one kilogram. Scientists said Monday they were moving closer to coming up with a non-physical definition of the kilo after discovering the metal artefact used as the international standard had shed a little weight.

Scientists said Monday they were moving closer to coming up with a non-physical definition of the kilo after discovering the metal artefact used as the international standard had shed a little weight.

Researchers caution there is still some way to go before their mission is complete, but if successful it would lead to the end of the useful life of the last manufactured object on which fundamental units of measure depend.

At the moment, the international standard for the kilo -- the equivalent of around 2.2 pounds -- is a chunk of metal, under triple lock-and-key in France since 1889.

But scientists became concerned about the cylinder of platinum and iridium housed at the International Bureau of Weights and Measures (BIPM) in Sevres, near Paris, after discovering it had mysteriously lost a tiny amount of weight.

Experts at the institute revealed in 2007 that the metal chunk is 50 micrograms -- 0.0000017 ounces -- lighter than the average of several dozen copies, meaning it had lost the equivalent of a small grain of sand.

They are now searching for a non-physical way of defining the kilo, which would bring it in line with the six other base units that make up the International System of Units (SI).

The other units are the metre, the second, the ampere, the kelvin, the mole and the candela, and none of them are now based on a physical reference object.

Experiments are focused on establishing a link between mass and the Planck constant, the fundamental unit of measurement in , to provide a new definition of the kilo.

Michael Stock, a BIPM scientist who will on Monday discuss the proposed change in London, said the metal chunk, known as the "international prototype", was coming to the end of its useful life.

"Measurements get more and more precise, and precise measurements require well-defined measurement units to express their results," he said.

He added that "our experiments are moving forward, however, it is too early to implement the new definition of the kilogram just yet."

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User comments : 22

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Nik_2213
3 / 5 (4) Jan 24, 2011
Could it be that the copies have gained weight due to handling despite the greatest care ??
fmfbrestel
2.3 / 5 (3) Jan 24, 2011
good question, I'm sure that they haven't thought of that, and are now busy checking thanks to your post.

/sarcasm
CarolinaScotsman
1.7 / 5 (3) Jan 24, 2011
Or perhaps their measuring devices have become less sensitive and therefore measure things too little? Depends on perspective. It's all an illusion anyway, though a persistant one. (and yes, I know the original quote referred to time)
Javinator
1 / 5 (1) Jan 24, 2011
Hmm... perhaps they could define it in terms of other defined constants?

A kilogram could be the mass of a specific dense substance of a particular volume at a particular temperature in a vacuum (defined in meters and kelvins which are OK as is mentioned above).

Although since we can't get a perfect vacuum I guess that wouldn't really work (since pressure in SI is defined in terms of kgs which leads to circular logic).

Maybe with a dense enough material such that its volume is not appreciably affected by changes in pressure. Or pressure could be varied linearly and its volume change could be correlated with the pressure changes to define the relationship...

What a dilly of a pickle.
Blakut
1 / 5 (1) Jan 24, 2011
Or it could be something like this: the amount of matter needed to create some energy. Let's say you get a particle and an antiparticle and annihilate them. You get some energy... then mass could be defined as the quantity necessary to get some energy x (let's say a photon of wavelength l).
nuge
1 / 5 (1) Jan 24, 2011
They could define 1kg of mass using its energy equivalence, E = mc^2 = 8.988 x 10^16 J. (i.e. "1 kg is the amount of mass that can be created using 8.988 x 10^16 J of energy")
electrodynamic
3 / 5 (2) Jan 24, 2011
They could define 1kg of mass using its energy equivalence, E = mc^2 = 8.988 x 10^16 J. (i.e. "1 kg is the amount of mass that can be created using 8.988 x 10^16 J of energy")

Makes good sense to me since mass is made from energy in the first place.
brant
2.6 / 5 (5) Jan 24, 2011
Maybe they could do science and figure out why it happened so it doesnt happen again....
Caliban
1 / 5 (1) Jan 24, 2011
Kg is a unit of weight. Weight is dependent upon several interrellated variables. I really don't see any way that you can -in a practical,operational sense- remove it from this relationship, and still be able to make use of the concept of "weight", independent of those variables.

You can, to be sure, define it as an ideal, but unless you have a way of actually reproducing the physical thing itself, then how can you use it as a real, physical reference?

My candidate for the definition? A defined volume of liquid Helium at X degrees K.

ekim
5 / 5 (1) Jan 25, 2011
I`m surprised they haven`t mentioned this -
ht(delete)tp://www.theage.com.au/news/national/making-an-exact-difference/2007/06/14/1181414466901.html
lexington
5 / 5 (6) Jan 25, 2011
Why is everybody reporting on the kilogram as a unit of weight? Like, I get that science reporters don't do actual science but I'm surprised at *none* of them care to mention that it's a unit of mass.
Argon
1 / 5 (3) Jan 25, 2011
"Scientists said Monday they were moving closer to coming up with a non-physical definition of the kilo"

Changing over to a "NON-PHYSICAL" definition of ANY standard has always been the failing "standard" model's dogmatic way of avoiding reality; starting even before 1983 with the "constant" speed of light in a "vacuum"!

Whenever a "constant" doesn't consist almost completely exclusively of an exact number (that which can be counted) then it fails to meet the definition of being "constant", unless you want to "count" man's imagination as a "constant" (which seems to be the REAL standard of this world anyway and should not be)! What ever happened to significant figures in physical science?

Without standards based upon measurable (countable) physical properties the physical sciences cease to be physical sciences and soon become (and maybe already have become, very sadly) pseudoscience!
antialias
5 / 5 (6) Jan 25, 2011
Hmm... perhaps they could define it in terms of other defined constants?

Not really, because then it wouldn't be needed at all.

A kilogram could be the mass of a specific dense substance of a particular volume at a particular temperature in a vacuum (defined in meters and kelvins which are OK as is mentioned above).

You are aware that that is exactly what the platin iridium block is?

Kg is a unit of weight
Kg is a unit of MASS. Weight is a force measured in Newtons (only in colloquial use do we use kg to denote weight because we assume 1g as a universal acceleration for everyday purposes).
LordOfRuin
5 / 5 (1) Jan 25, 2011
Perhaps the difference is down to a change in the local gravity. How do you weigh something without letting gravitational variances effect the result? Oh, actually, I can think of a few, but anyway, no need for me to give up a good idea, just because it's wrong.
antialias
5 / 5 (5) Jan 25, 2011
The gravitational acceleration (g) is not the same everywhere. The earth is flatter at the poles. (From my own experience: Schools in Florida did use g=9.76 m/s^2 while in germany we used g=9.81 m/s^2).

This does affect the weight but not the mass. The measurements done were most certainly adjusted for this (also it would be highly unlikely that all other copies of the original kilo are located north of Paris.)
ZephirAWT
Jan 25, 2011
This comment has been removed by a moderator.
Javinator
not rated yet Jan 25, 2011
You are aware that that is exactly what the platin iridium block is?


Nope. I'm slow apparently. It's obvious from the picture that you're right.

Still... Mass is the amount of something there is. So is a mole. They're interchangeable (with different experimentally determined constants) in chemistry in equations such as the ideal gas law and when calculating heats of reactions, etc.

If the mole is adequately defined as is mentioned in the article above, could they not define a kg as a number of moles of a specific substance?
antialias
not rated yet Jan 25, 2011
Well, the standard kilogram is a defined substance with a defined density (i.e. at a defined temperature/pressure). This means it contains a defined number of atoms. So yes: the current standard is exactly what you suggest it should be.

Before the platinum iridium block the kg was defined via a certain volume of water. But liquids are very susceptible to volume (i.e. density) changes due changes in temperature. Therefore reproducing the exact number of moles somewhere else was rather hard to do.
Javinator
not rated yet Jan 25, 2011
Reproduction is not the issue though is it? If they can define a mole, is a kg just not a conversion factor of that? (ie. 1kg = 83.3 mol of C-12 or something similar)
Quantum_Conundrum
not rated yet Jan 25, 2011
But scientists became concerned about the cylinder of platinum and iridium housed at the International Bureau of Weights and Measures (BIPM) in Sevres, near Paris, after discovering it had mysteriously lost a tiny amount of weight.


There are several ways this could be explained.

There could be trace quantities of a radioactive element present, which over time may result in some lost mass, but happening too slowly to detect radiation via instruments.

The amount of lost mass corresponds to roughly 1/10 of a mole of electrons, assuming beta decay....

it wouldn't be distinguishable between random background noise.

IN 120 years is lost 50 micrograms.

This would change the mass of the object, but the nuclear change would need a mass spectrometer to tell the difference, which unfortunately would require destroying the standard to test this theory...
Ethelred
not rated yet Jan 27, 2011
Several people seem to have missed that the Standard block is lighter than the copies as opposed to something without iridium for instance. So Zephyr continues its tradition of not thinking.

The key is HOW they tested. I am certain they used a BALANCE, as no other way makes any sense, which is a measure of mass and is unaffected by changes in gravity as long as there is enough for the balance to move. The only test that makes any sense is to put the original on one pan and one copy on the other pan and then switch pans and do it again. For each copy.

This subtracts out any overall effect such as gravity or dark matter or any other silliness. Somehow the original has lost mass in COMPARISON, in the SAME location, to the copies. Either that or the someone botched the copies when they were made. For instance they could have failed to swap the weights on the balance.

Ethelred
hooloovoo
5 / 5 (1) Jan 30, 2011
Doesn't really matter whether the main unit has lost mass, the copies gained mass, gravitation has changed in Paris, or anything else. If they can't get an accurate and reliable reading then it isn't good enough. The standard kilogram has been an embarrassment for a while, it's about time they got around to fixing it!
Moebius
not rated yet Jan 30, 2011
Its been under lock and key since 1889 so I assume it was made then. How pure can it be if it was made back then? It should have been remade since then to modern standards of purity, who knows what's really in it since they can't touch it. And if that's the standard and it isn't the same as the copies, how do we know how much a kilo is now?