# Scientists ask: Is the kilo losing weight?

##### April 24, 2011 By Elizabeth Weise

Ensuring a pound of butter is indeed a pound, or a gallon of milk a full gallon, has long been the province of government agencies that deal with weights and measures. But now it seems scientists are having a little trouble with the golf-ball-size piece of metal that is used to set the standard weight for a kilogram, or kilo.

A bunch of these prototypes have been made over the years, seven of which are kept in a triple-locked vault at the International Bureau of Weights and Measures in Sevres, with one known as the International Prototype.

The problem is that as these have been taken out and weighed, which last happened in 1990, something odd has turned up - their weights began diverging. The International Prototype, for example, weighed 50 micrograms less than the others, meaning it had lost weight, or the others were getting heavier, or they were all moving a bit - no one knows for certain. And no one knows what caused the changing weights, either.

Americans might not think the definition of a kilo affects them, but it does. Since 1893 "the pound has been defined as a derived measure of the kilo," says Richard Davis, formerly the specialist with the U.S. and then the secretary of the Consultative Committee for Mass and Related Quantities.

Not that the 50 micrograms will affect someone buying a pound of coffee in America or a kilo of potatoes in Germany. "It's a pretty small effect. It's the weight of a small grain of sand, and this has no consequence," says Michael Stock, director of the International Bureau's Electricity Department. "It's only people working at the highest levels of science who will be affected."

But to scientists, for whom very precise measurement is important, it's a big deal. So they decided to start working on a new standard based on a universal constant - a measure that relies on science principles rather than on an object whose size or other properties could change from one sample to another.

The standard for a meter, for example, is now defined as "the length of the path traveled by light in vacuum during a time interval of {99,792,458 of a second." Again, even for Americans who don't know a kilometer from a kleptomaniac, this is an issue because the official definition of a foot is exactly 0.3048 meters.

There are seven base units in the International System of Units (things like seconds, meters, degrees) and every one of them but the kilo has one of these universal constant definitions. Only the kilo is still defined by a manmade artifact - in this case a cylinder of metal made up of 90 percent platinum and 10 percent iridium that's 1.54212598 inches high by 1.54212598 inches in diameter. Which, by definition, weighs exactly 1 kilo (2.2046 pounds).

The whole system is based on a group of scientists in France who, after the revolution of 1789, started to set up a universal measurement system to get away from the hodgepodge of measurements then existing in Europe, many of which were based on things like the length of the current king's arm. "What do you do when you get a new king?" Stock says.

The system began to be adopted by the rest of the world in the 19th century when industrialization and international trade made having similar measuring systems important.

But finding a universal constant for a kilo isn't as easy as it might seem.

One suggestion was to create a precise sphere of pure silicon that weighed exactly 1 kilo, then count the number of silicon atoms it contained and define a kilo as the weight of that many silicon atoms. But while that sounds simple, it turns out to be technically very difficult.

Another idea was to base it on a relationship with an esoteric concept in physics called the Planck Constant, something even Stock had trouble expressing in layman's terms. But it would allow scientists to create a definition based on a universal constant.

Except that the experiments to establish it may be a little beyond science just yet. Groups in the United Kingdom, the United States, Switzerland and France have been doing the experiments and so far they have not come up with the same number.

"That's the problem," says Stock. "There are different results, and they don't agree."

It is unlikely that the universe is shifting under our feet, the researchers say. More that our measuring ability isn't quite up to capturing the extremely small thing being measured here.

For now, the kilo stays linked to the platinum/iridium cylinder locked away outside Paris. The meeting of the General Conference on Weights and Measures, which could adopt a new definition of the kilo, is scheduled for 2015.

Asked whether we can expect a definitive kilo by then, Stock smiles. "Probably yes, but good science takes time."

Explore further: Weighty mission for scientists: redefine the kilo

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##### beelize54
1.2 / 5 (13) Apr 24, 2011
IMO the Earth is passing through a dense cloud of neutrinos, which are destroying solar cycles and magnetic field of Earth, they're doing global warming and kilogram prototype lighter.
##### FenderFennec
5 / 5 (1) Apr 24, 2011
##### ke11yslater
3 / 5 (4) Apr 24, 2011
The metric system is taught in the US.
'Americans might not think the definition of a kilo affects them, but it does... Again, even for Americans who don't know a kilometer from a kleptomaniac'
USA TODAY, fire this tard.
##### Knobby
2.3 / 5 (3) Apr 24, 2011
So you are saying the unit of thousand is getting lighter?

While in daily use a kilogram is commonly abbreviated to kilo, in an article about international measures etc i find sloppy to omit the actual unit, especially when further on the author is talking about kilometers. In essence it is the gram that is getting lighter, a kilo on it's own is a dimensionless quantity
##### schedel
1 / 5 (1) Apr 24, 2011
You find it sloppy that the article is using a normal abbreviation, but you find it OK that a scientific report is using not standard units in the same paragraph in which it explains that they exist? It is commonly known (in whole the world, except America) that you report in meters, not inches; kilogram, not pounds
##### ennui27
not rated yet Apr 25, 2011
Will this conumdrum disappear like a standard kilo when and if the Higgs boson is observed? I would expect it would have some relation to the mass of a kilo - it does cause it, after all.
##### abqwildcat
5 / 5 (3) Apr 25, 2011
"There are seven base units in the International System of Units (things like seconds, meters, degrees)..."

Unless I'm mistaken, degrees is not an SI unit. The 7 SI base units are meter (length), kilogram (mass), second (time), Ampere (current), Kelvin (temperature), mole (amount of substance), and candela (light intensity).

Depending on whether this "degrees" is referring to a plane angle or temperature, either is problematic. Angles are considered dimensionless units since they can be expressed as the ratio of two lengths, and if we were talking about the SI measure of angularity, it would be a radian, not degree. And the SI unit of temperature is Kelvin (no degrees).

##### Husky
not rated yet Apr 25, 2011
the first tangible sign of the big rip, the universe is loosing it, or somebody ate the easter egg
##### Moebius
3.7 / 5 (3) Apr 25, 2011
But to scientists, for whom very precise measurement is important, it's a big deal. So they decided to start working on a new standard based on a universal constant - a measure that relies on science principles rather than on an object whose size or other properties could change from one sample to another.

The standard doesn't agree with us so lets just change the standard. True scientists would find out why the weights of the standards are diverging.

And for the conspiracy nut posters, only one sample is getting lighter (or all but one are getting heavier) so try and think through your nutty theories about changing universal constants before you post. You have a point but if you grow your hair longer you can hide it.
##### d_robison
not rated yet Apr 25, 2011
My last post didn't seem to go through so sorry if this turns into a double post.

We could just define one kilogram as the average of all the latest measurements (not ideal but at least its something) until we can figure out another way to define one kilogram. That way we don't have to worry about a our unit of mass changing over time.

Also I agree with Moebius, the conspiracy theory nuts need to stop posting.

@beelize54, neutrinos react EXTREMELY infrequently with matter and other particles. They are not the cause of global warming and not changing the mass of the platinum cylinder. If you were to shoot trillions (or more) of neutrinos at the platinum cylinder from point blank, they would all probably pass right through it.
##### 6_6
1 / 5 (3) Apr 25, 2011
why is this so surprising? I thought the earths magnetic poles were shifting?
##### d_robison
not rated yet Apr 25, 2011
why is this so surprising? I thought the earths magnetic poles were shifting?

What does that have to do with anything?
##### beelize54
1 / 5 (4) Apr 25, 2011
neutrinos react EXTREMELY infrequently with matter and other particles. They are not the cause of global warming and not changing the mass of the platinum cylinder. If you were to shoot trillions (or more) of neutrinos at the platinum cylinder from point blank, they would all probably pass right through it
Just because they interact so weakly, they could increase the density of vacuum significantly. IMO antineutrinos are accelerating the decay of radioactive elements and slow down fusion. It could explain many observations of radioactive decay at spaceprobes.

##### Tachyon8491
3.3 / 5 (7) Apr 25, 2011
Point is, no-one presently knows the true mass of a kilogram within the uncertainty of 50ug. It cannot be derived from base SI units unless these themselves are referred to a physical sample. More science needs to be applied to finding some plausible explanation for the primary standards losing or gaining mass over time, then a choice to be made - perhaps on averaging their difference, or if the mechanism of loss is found, to retrodict the original mass and use that. Once that has been achieved with scientific consensus, the standard mass can then be related to a suitable set of base SI units - exact dimensions of a lab-standard pure, SINGLE element, in number of wavelengths of known frequency, or exact molar mass in number of atoms derived, if that is possible.

Personally I am now interested in bringing a class action against my chewing gum manufacturer who must have been short-changing me by at least 23 femtograms per chew...
##### d_robison
5 / 5 (1) Apr 26, 2011
neutrinos react EXTREMELY infrequently with matter and other particles. They are not the cause of global warming and not changing the mass of the platinum cylinder. If you were to shoot trillions (or more) of neutrinos at the platinum cylinder from point blank, they would all probably pass right through it
Just because they interact so weakly, they could increase the density of vacuum significantly. IMO antineutrinos are accelerating the decay of radioactive elements and slow down fusion. It could explain many observations of radioactive decay at spaceprobes.

I don't think you understand the infrequency of interaction. A beam of trillions neutrinos could easily pass through the entire Earth without a single reaction.
##### ZephirAWT
Apr 26, 2011
This comment has been removed by a moderator.
##### ZephirAWT
Apr 26, 2011
This comment has been removed by a moderator.
##### d_robison
5 / 5 (2) Apr 26, 2011
We should realize, the slow neutrinos are virtually undetectable for us...The layer of neutrinos around massive objects may be responsible for the main portion of what we are calling the gravitational lensing and the frame drag effect (Lense-Thirring effect).

Opinion is not fact. Also gravitational lensing was predicted by Albert Einstein's General Theory of Relativity (and has been observed tens of thousands of times since then). Making something a lot more complex than it needs to be is terrible science.

The neutrino distribution can affect the centre of mass of solar system (around which the solar plasma is circulating)...and we are facing global climate change period.

I'm not sure where to even begin to respond to this. It is just a bunch of information (much of it inaccurate) mashed together to form some haphazard theory.
##### unknownorgin
1 / 5 (2) Apr 28, 2011
The prototype was last weighed in 1990 so many things that could change the weight but not the mass have happened. The sun is now just out of solar minimum . What was the exact position of the moon? . The earths rotational speed may have changed. Perhaps some hydrogen or other gas was emmitted from the weight. maybe a large aircraft was overhead or some geological change underground. Mars and earth orbited by each other for several months so thier orbits have changed slightly. At some point there is no such thing as a standard weight because gravity is subject to outside forces. The mass and inertia of an object will stay the same if it is always moving at the same relative velocity so this would be a better standard to measure.
##### d_robison
5 / 5 (3) Apr 28, 2011
The prototype was last weighed in 1990 so many things that could change the weight but not the mass have happened. The sun is now just out of solar minimum . What was the exact position of the moon? . The earths rotational speed may have changed. Perhaps some hydrogen or other gas was emmitted from the weight. maybe a large aircraft was overhead or some geological change underground. Mars and earth orbited by each other for several months so thier orbits have changed slightly. At some point there is no such thing as a standard weight because gravity is subject to outside forces. The mass and inertia of an object will stay the same if it is always moving at the same relative velocity so this would be a better standard to measure.

These things could effect the weight of the object, but not on the order of 10 micrograms. Most of the effects, especially the Mars/large aircraft would be negligible.
##### Moebius
not rated yet Apr 30, 2011
The article may say weight but my guess is they are using some sort of balance scale to weigh these things, not a spring scale. That is mass not weight and gravity independent with a balance scale. It requires a counterweight which itself may have changed which is why they don't know if the standards are getting lighter or heavier.
##### Feldagast
not rated yet May 01, 2011
Did they weigh them all with the same scale? exactly how did they calibrate the scale since it requires the standard to be used to set it? Were these all weighed the same day, within minutes of each other? Just questions I would ask them but not covered in the article. No doubt they need to set some measure that is unchangeable compared to a man made lump sitting in a vault, if that lump changes how would you know? What do you compare it to when it is the standard? Having a lump of metal as a set standard is fine for taking it around and calibrating scales, but eventually you have to compare that lump to something to make sure it hasn't changed but what do you compare it to?
##### Quantum_Conundrum
not rated yet May 01, 2011
When you're talking about absurdly tiny changes like this, it could be affected by anything at all. Invisible dents and scrapes could remove some of the material, or contaminate it with additional mass.
##### d_robison
not rated yet May 02, 2011
When you're talking about absurdly tiny changes like this, it could be affected by anything at all. Invisible dents and scrapes could remove some of the material, or contaminate it with additional mass.

Very true, at that scale even breathing on or close to the scale can change the measurement. I'm honestly not too concerned about it, they could just stick with the current value and stop measuring the rod until they find a new standard unit of mass.