Scientists fear kilograms don't weigh as much as they used to

[muriel_volestrangler]

A group of experts meeting in London today want to redefine the kilogram so that it is no longer based on the mass of a solid cylinder of platinum-iridium alloy that sits beneath three layers of protective glass sealed in a locked vault in Sèvres, France.

This metal block, known as the International Prototype Kilogram, has been used since it was first registered with the International Bureau of Weights and Measures (BIPM) in 1889 as the definitive unit of mass against which all other kilograms are measured.

In the past 122 years, it has been brought out of storage just three times to calibrate the national prototype kilograms used by countries around the world. However, scientists now believe it is time to redefine the kilogram because there is evidence that the precise mass of the international prototype in Sèvres is not as constant as it should be.

"We think it is losing weight, and we don't know why," says the BIPM's Michael Stock, who is due to attend the meeting at the Royal Society in London that will look again at the kilogram. "From the three times we have had it out to make calibrations, we have had indications that it is not perfectly stable. It seems to have lost about 50 micrograms and there is no real explanation."

http://www.independent.co.uk/news/science/scientists-fear-kilograms-dont-weigh-as-much-as-they-used-to-2192481.html

They want to tie the definition to the Planck constant instead. "In practice, the kilogram would be based on the electric power needed to counter perfectly a kilogram pulled by the Earth's gravity by levitating it in mid-air. " I'm slightly surprised by that; that would seem to be very dependent on where the measurement is done, since surface gravity varies with latitude. http://www.guardian.co.uk/science/2011/jan/24/scientists-weigh-up-shrinking-kilogram|The Guardian> puts it as "scientists can relate the mass of an object to the electrical energy needed to move it, using the Planck constant", which sounds more location-independent.

[hedgehog]

1. I always get a kick out of the statement that the metric system is

more scientific than the Imperial system. How is tying the definition of a kilogram to a mass of Platinum n Paris more "scientific" than declaring an inch to be 3 barleycorns?

The metric system may be more convenient than the imperial system for some uses, and it may be the system most often used by scientists. On the other hand, I find the Imperial system very handy for some uses. My husband's foot is close enough to 12" that he can readily step off a length if a tape isn't handy.

[backscatter712]

6. IIRC, the kilogram's the last basic metric unit to be tied to a physical object.

Take the meter...

"The metre is the length of the path travelled by light in vacuum during a time interval of 1⁄299 792 458 of a second.", taken from http://en.wikipedia.org/wiki/Metre

The meter used to have a reference object - a platinum-iridium bar not so different from the platinum-iridium reference mass. Of course, the length of the reference bar varied over time, making it hard to get a precise definition of the meter, so that's why the meter's defined in terms of the distance light travels in a specific amount of time - for precision. When you want accuracy to a zillionth of a millimeter, you want the new and current meter measurement.

Now scientists are trying to figure out how to do the same thing for the kilograms that they did with the meter and the other SI units.

[hedgehog]

8. Hm, let me see, I can measure a piece of fabric by holding one end to my nose and stretching

out my arm; pretty tough to estimate the length of the path travelled by light in vacuum during a time interval of 1⁄299 792 458 of a second.....

[backscatter712]

11. Yeah, but you don't have to measure your piece of fabric down to a precision of micrometers...

The scientists in the labs, OTOH, have to means to make the measurement described using light, and can do so very accurately and repeatedly.

Different tools for different jobs.

[hedgehog]

12. Different tools for different job: exactly! The metric system can be very useful

in the proper environment. I just object to the claim that one arbitrary system if units is somehow "more scientific" than another arbitrary set.

[Recursion]

19. I'm pro-metric, but I absolutely HATE metric recipes

And I hope that even if we convert everything else, we keep using the Imperial system in the kitchen.

[hedgehog]

26. It's possible that all the imperial kitchen measurements were developed to

be in good ratio to a single egg and/or to each other. Look how often a pound of butter or a quarter pound goes along with a half cup sugar, two cups flour.

[NickB79]

34. Now imagine ordering that fabric from someone like Shaq, or Gary Coleman

Hope you either like getting way too much, or way too little, fabric on a regular basis.

[Recursion]

21. Even then the meter wasn't "defined by" the bar

It was defined as a certain percentage of the circumference of the earth at the equator.

[backscatter712]

27. Very true, and the circumference of the Earth definition also had a lot of slop.

IIRC, a meter was defined as 1/10,000,000th of the distance between the North Pole and the equator. Of course, that distance varies, depending on where you draw the line, so they refined the definition to draw the line straight through Paris. Of course, then, errors were introduced due to surveying problems - narrowing down the position of the North Pole and the equator turned out to be a problem, and then there are seismic shifts that throw off the measurements too. Oh, and the Earth itself is not perfectly round, so that throws things off too.

That's why they came up with the current definition based on the distance traveled by light in a vacuum over a defined fraction of a second. Much more precise in scientific applications where they want a huge amount of precision.

I think the platinum-iridium meter bar was the next try after giving up on the fraction of the circumference of the Earth definition, before they found that like with the reference kilogram, there were problems with accuracy.

[DissedByBush]

7. The pound is defined by the kilogram

So however screwed up the kilo is, the pound is just a derivation of screwed up.

What I like best about metric is how everything fits together.

What's a gallon of water weigh? I don't know, let me look it up.

How about a liter of water? That's easy, by definition, one kilogram.

How big is a liter? Well, one liter is 1,000 milliliters, one cubic centimeter is one milliliter, so 10x10x10 centimeters.

[hedgehog]

9. The metric system is easier to use for those six math papers everyone got

the first week of school where you had to convert all the units. I used a slide rule to do my homework in statics and mechanics using imperial units, and the conversion factors became second nature. What kills me is this; I know what a slug is and what a pound is, but what does it mean when someone tells me something weighs a kilogram?

[Gregorian]

13. The pound is defined by the slug.

The slug is mass.

[Recursion]

20. Well that's one system, there's also one with the pound as mass and the poundal as force

Part of the problem is the units were defined before that distinction was fully understood

[Gregorian]

32. The pound-mass is also defined by the slug.

And I remember how absolutely impossible that conversion was. I did it one time. What a stupid country we are. We'd rather fight than switch.

http://en.wikipedia.org/wiki/Slug_%28mass%29

F = m·a/gc = w·a/g

It looks simple, but when one is deriving it from scrtach I remember it as being horribly tough.

[Heywood J]

10. "My car gets forty rods to the hogshead and that's the way I likes it!" (NT)

[bigmonkey]

18. What terrible mileage!

:-)

[Recursion]

22. Why I love Google

http://www.google.com/search?q=40+rods+per+hogshead+in+miles+per+gallon

[Pancho Sanza]

30. What's the equivalent in furlongs per fortnight?

:D

[Heywood J]

36. I don't know, it's hard to get fortnights into the tank.

I've heard they tend to eat the rubber hoses on older vehicles.

[Towlie]

14. But what does a cubic meter of Chinese soup weigh?

[leahcim]

16. 3 barleycorns

How is tying the definition of a kilogram to a mass of Platinum n Paris more "scientific" than declaring an inch to be 3 barleycorns?

If barleycorns were all the same length as measured to some ridiculous accuracy, I'd agree with you. But they aren't. 50 micrograms off the standard kilogram is disturbing, but it's much less than the variation one would get from measuring agricultural produce for your standards needs.

Imperial is defined in terms of metric now, so one has the luxury of picking which system one wants to work in without taking a hit in precision. But before metric was around, traditional standards were ad hoc, and inconsistent between countries, regions within a country, and even different industries within a region. The cleaned up "traditional units" we have now, with precise and universal definitions, are only possible thanks to the metric system forming a common baseline for everything.

[jberryhill]

23. Centimeters would be fine if they were 2.54 times larger

[krispos42]

25. It's much easier to use from a physics standpoint

Mass and energy and making units cancel out while doing multiplication and division.


Doesn't much matter for everyday stuff, though. Every fall, a man gets into his pickup truck. He starts the 4.6L 210-horsepower V-8 and drives 23 miles on 235/70R16 tires to his favoring hunting spot. Once there, he loads his 7x57mm rifle with a 24-inch barrel, uncaps his 3-9x40mm telescopic sight with the 1-inch diameter tube, and goes deer hunting. After dropping a nice deer with a 140-grain bullet, he'll drive back into town. Along the way, he'll stop by the convenience store and buy a quarter-pounder cheeseburger, a pint of chili, and a half-liter bottle of Pepsi.

:shrug:

[TheMadMonk]

35. And my natural stride is within 0.5% of a metre.

(Paced off over a measured 100m)

Which still has nothing to do with the price of tea in China.

The convenience of the SI units over Imperial is that they are orderly.

3 barleycorns to the inch, (thankyou) 12 inches to the foot, 3 feet to the yard, 22 yards to the chain, 10 chains to the furlong and 8 furlongs to the mile. 4 inches to the hand slips in there too.

And that's the easy sequence to memorise.

Three type of ounces. (fluid, weight, and troy), a truly ludicrous number of "bushels".

How many gills in a hogshead? Define a cord of wood.

Do you know under what circumstances there are 1000 miles in an hour?

Not that long ago, using the same yardstick to sell two differing fabrics, or the same fabric in two differing locations, could see you shorter by a head in very swift order.

SI units meant that even greater than the convenience to boffins, was the simple fact that merchants could "blindly" trade through brokers and intermediaries and be sure that they were in fact getting what they believed were paying for.

That some of the units are are Francocentric is a simple accident of history. Napoleon had the clout to make it stick long enough to become established.

[benld74]

2. I KNEW I was paying too much for my stuff!!!!

[ZombieHorde]

4. Funny. nt

[Pancho Sanza]

3. Until we figure a way to count quarks, all mass 'standards' are subject to error/change.

Proton decay is one possibility and there's no way to detect, much less compensate for it. I have one question though...how can they claim the "standard" has changed...that's oxymoronic because it is by definition a quantity that -must- remain constant???

In any case, it seems to me it ought to be tied to the centripetal force of a whirling mass at a given distance from a center of rotation outside of measurable gravitational forces. Don't ask me how to set it up though...and in any case all these things are marginally precise, barely accurate and far from eternal. (Think about a place you can identify as "here" right now...and "where" it will "be" in a billion years. It sure will be somewhere else. :D)

[Boojatta]

5. "how can they claim the 'standard' has changed"

Maybe it hasn't changed. Maybe everything else in the universe has gotten slightly more massive so that the standard kilogram merely appears to have gotten less massive.

[Pancho Sanza]

29. Nice point but I think it's unlikely.

:hi:
:-)

[muriel_volestrangler]

15. If, when they compare the national standard kilograms, those agree with each other better

than with the international standard kilogram, over time, it may indicate the 'standard' one is losing mass that the others aren't.

Your method would need a standard way of measuring a force without reference to a mass. Unfortunately, the ampere is defined by the force between two wires in terms of Newtons - which is derived from the definition of a kilogram. So you can't measure that force just in terms of electromagnetic force, without it becoming a circular definition, I think. Measuring the force by the extension or compression of a standard object would probably be less reproducible than the current comparison with a standard object - I don't think you could specify how to manufacture another object with precisely the same elasticity at the accuracy needed.

[dimbear]

17. Not to worry. It isn't the kilogram bar that's dieting. G is getting tired

of doing all that pulling.

Probably due to the expansion of the universe in general, things getting generally thin everywhere.

[Festivito]

24. More moisture in the air. /nt

[caraher]

28. Regarding the proposed new definition...

muriel_volestrangler writes,

"In practice, the kilogram would be based on the electric power needed to counter perfectly a kilogram pulled by the Earth's gravity by levitating it in mid-air. " I'm slightly surprised by that; that would seem to be very dependent on where the measurement is done, since surface gravity varies with latitude.

From what Wikipedia says about the technique, I would guess there's a further calibration step that involves a precision measurement of local g. Otherwise you're right, if you just treated g as a constant your precision would get clobbered by local variations.

[phantom power]

31. proton decay!

:rofl:

[Recursion]

33. The Higgs boson is getting rich by stealing platinum

Now I see its dastardly scheme...