A pound of gold, a pound of feathers

February 4, 2005 3:35 PM

How much does a kilogram weigh? Well, less than it used to. That's one of the reasons why Germany's PTB (Physical and Technical Institute) has been attempting to create a virtual kilogram. This was news a while back when they first decided to use silicon. Now they're going to try it with bismuth. (Details in German, English via Babelfish.)
posted by sninky-chan (24 comments total)

I think they're barking up the wrong tree.

Why not define a specific mass as the quantity needed to 'bend' light to a certain extent? No need to measure atoms/molecules that way - just their cumulative effect.

Also known as gravitational lensing, more here.

Great post, made me think (even if I got it wrong). Thanks!
posted by Jos Bleau at 3:52 PM on February 4, 2005

Why not define a specific mass as the quantity needed to 'bend' light to a certain extent? No need to measure atoms/molecules that way - just their cumulative effect.

Because gravity is a very weak force - and so difficult to observe unless very large amounts of mass are involved.
posted by kickingtheground at 4:00 PM on February 4, 2005

Wow, I had no idea our weight standard was still defined like that. I found this as a reason why: "This one physical standard is still used because scientists can weigh objects very accurately. Weight standards in other countries can be adjusted to the Paris standard kilogram with an accuracy of one part per hundred million. So far, no one has figured out how to define the kilogram in any other way that can be reproduced with better accuracy than this."

Why not define a specific mass as the quantity needed to 'bend' light to a certain extent? No need to measure atoms/molecules that way - just their cumulative effect.

1) You would have to isolate the mass you want to measure from any other mass, which is impossible.
2) I don't think you could even measure the minute amount of bending that a kilogram or even a million kilograms would do to light.
Interesting idea, though.
posted by Bort at 4:00 PM on February 4, 2005

Actually, on second thought, I'm not sure reason #1 is accurate.
posted by Bort at 4:02 PM on February 4, 2005

Glad you liked it, Jos Bleau, and thanks for the nice words on my first FPP.

Even if gravitational lensing can even be measured on smaller masses (this part of the aforementioned Wikipedia article leads me to believe it's not impossible), I think Bort's first reason does hold water -- for smaller masses, seems to me you'd really have to take care that you weren't on top of the Canadian shield as opposed to in Death Valley when you do your measurement.
posted by sninky-chan at 4:13 PM on February 4, 2005

Hey, remember the episode of the John Larroquette show where they accidentally bent The Inch? Then, in an attempt to fix it, they discovered that it had been off the entire time, so then all the rulers in the country had to be changed.

Cool post. Sorry I only have stupid comments to give.
posted by Sticherbeast at 4:19 PM on February 4, 2005

Pedant Mode: A kilogram weighs nothing, unless a kilogram of some matter is being accelerated, in which case, then that matter would have weight.

The gram is the SI unit of mass, which is constant, regardless of acceleration.
posted by eriko at 4:21 PM on February 4, 2005

Changing rulers is a slam dunk.
posted by weapons-grade pandemonium at 4:29 PM on February 4, 2005

It's getting the masses to follow that's tricky.
posted by weapons-grade pandemonium at 4:31 PM on February 4, 2005

And here I had foolishly believed that a Kilogram was defined as the mass of one Liter of water.


(Pedantically speaking, we misuse it as a measure of weight by measuring the effects of gravity on that mass. Unless you do a lot of measuring things in space or on other planets, same-diff.)
posted by ilsa at 4:38 PM on February 4, 2005

Pedantically speaking, we misuse it as a measure of weight by measuring the effects of gravity on that mass.

Correct. We also tend to use the pound (a measure of weight) as a measure of mass as well. The unit of mass in the US system is the slug.
posted by eriko at 4:54 PM on February 4, 2005

The gram is the SI unit of mass, which is constant, regardless of acceleration.

Well, since we're being pendant, the BIPM seems to be the group to say what the SI units are, and they say the unit of mass is the kilogram. :)
posted by Bort at 5:01 PM on February 4, 2005

And here I had foolishly believed that a Kilogram was defined as the mass of one Liter of water.

Hmmmm. Good point. A little research shows that basically the "prototype" weight existed before the liter definition and therefore won out. "In 1950, the CIPM declared that 1 liter = 1.000028 cubic decimeters was the best conversion." So a liter isn't really 1000 cubic centimeters!

It would seem I've also hit upon another fundamental unit of measurement. 1 Nerd = someone that researches and discusses the intricacies of measurement units at 8:00 on a Friday night.
posted by Bort at 5:16 PM on February 4, 2005

eriko: The pound has been a unit of mass defined exactly in terms of the kilogram since 1893, although the definition has changed twice. And Bort is right, the SI unit of mass is the kilogram, not the gram.

Bort: In 1964, the CIPM abrogated the separate definition of the liter. It really is 1000 cubic centimeters now!
posted by grouse at 5:18 PM on February 4, 2005

*gazes down at pot belly for a few minutes, shrugs*
posted by quonsar at 6:55 PM on February 4, 2005

Fascinating stuff, thanks sninky-chan!
posted by carter at 7:39 PM on February 4, 2005

I think they're barking up the wrong tree.
Why not define a specific mass as the quantity needed to 'bend' light to a certain extent? No need to measure atoms/molecules that way - just their cumulative effect.
Also known as gravitational lensing, more here.

I just love it when random people think they know more about physics than physicists who have been studying it their whole lives.
posted by aerify at 7:46 PM on February 4, 2005

This just proves how your metric system blows!

(joke)
posted by ParisParamus at 7:47 PM on February 4, 2005

That snark was unwarranted, aerify, especially since Jos Bleau stated made me think (even if I got it wrong).
posted by trharlan at 8:23 PM on February 4, 2005

Sometimes my liberal arts friends call science black magic... Articles like this force me to agree. Though I agree with the necessity, you have to laugh at the ridiculousness.
posted by estelahe at 9:52 PM on February 4, 2005

Hey, quonsar, lighten up! You just lost like, 8 kilograms!
posted by graventy at 10:56 PM on February 4, 2005

I just love it when random people think they know more about physics than physicists who have been studying it their whole lives.
posted by aerify at 7:46 PM PST on February 4

Your conclusion that Jos Bleau believes himself to be more knowledgable than professional physicists is erroneous -- in the same post you quoted he acknowledges the possibility that he may be incorrect in his assertion that lensing is a good way to define mass, implicitly deferring to those who might be able to elaborate on why he is correct or incorrect -- namely, the professional physicists.

I personally vote for a unit of mass based on a certain number of atoms -- for example, 1 kilogram = (250/3)*6.0221415*(10^23) atoms of Carbon-12 (I am aware that I'm rounding off Avogadro's number, but since 1 mole is defined in terms of a kilogram currently, a choice needs to be made). Obviously, precise measurement would be difficult, but not nearly so much as using gravitational lensing to determine a mass (particularly since while atoms are well-understood, some interactions which could cause changes in lens measurements are still ambiguous -- for example, if the gravitational constant were changing, a kilogram would change over the millenia).
posted by j.edwards at 2:49 AM on February 5, 2005

looking at some of these rules and odd conversions makes me think the metric system really isn't so perfect afterall.
posted by MrBobaFett at 7:41 AM on February 5, 2005

Planck Units are nice elegant units. Physicists used various similar units that make certain equations nicer, like removing the electrical permittivity constant from the front of the Coulomb force, until the pervasive force that was engineering forced everything into SI units. Also, if I recall rightly, the issue with measuring gravity is just one of sensitivity. You can very precisely measure distance changes with an interferometer, which can measure distance changes on the order of the wavelength of light used in it. To measure the changes from such effects as gravitational waves, a huge path length of the laser is required, and from that lots of error is visible.
posted by apathy0o0 at 8:05 PM on February 5, 2005