August 20, 2009

The kilogram. Really, *the* kilogram.

It's a particular cylinder. Don't sneeze on it. Be careful washing it. Dislodge a molecule and you throw off all the weights in the world.

20 comments:

Scott M said...

Isn't the meter based on a particular wavelength that's coming out of a particular emitter in a particular lab?

...or is it just the length of the Queen Mum's arm?

Crimso said...

It's precisely this sort of problem that lead me to preferring atomic mass units. In addition, can a physicist enlighten us as to whether the simple act of moving it changes (at least temporarily) its mass (if only by an incredibly small amount) due to relativistic effects?

Also, it occurs to me that the stimulus package alone was on the order of the square root of Avogadro's number. Put in those terms, it sounds like a lot! Good thing it's not.

Read a paper once by Oliver Lowry. He reminisces about his life in science, and how they used a quartz fiber to weigh nanogram (thats billionths of a gram for you cokeheads) quantities of materials. They did it by measuring the deflection of the quartz fiber as the material was placed on it. They needed standards for that too.

Crimso said...

Meter is now defined based on speed of light, but was indeed once based on a wavelength (or frequency) measure.

Largo said...

I remember my physics prof once saying that during WWII, it was a rather significant concern to both sides in the conflict that military action in the region of these weights and measures be moderated.

Whether this concern was limited to the scientific establishment or not, he did not say.

WV: botimp: a computer process intent on making mischief with other people online. (See: daemon.)

[Does it count when the word changes because of the preview?]

Largo said...

"It has been discovered (empirically) that the standard metre at Sevres, and the diurnal or annual motion of the earth do not give sufficient accuracy... Therefore, today, the physical units of length and time are based on atomic structures: the wavelength of a certain spectral line, and the period of another one."
-- M.D. Stafleu. Time and Again - A Systematic Analysis of the Foundations of Physics.

(A very cool book.)

traditionalguy said...

It is comfoting to just hear that there are still people on earth that care about whether a truth exists in a measureable way. That concept has not done well in public discourse during the past 40 years.

Chris said...

Seems so charmingly French. Philosophers have talked a lot about the standard meter bar since Kripke's Naming and Necessity.

rhhardin said...

A pint's a pound the world around.

kynefski said...

"The good news is that the change is extremely small, around 50 micrograms."

Hey, man, any tripper will tell you that 50 micrograms ain't extremely small. Ain't gargantuan, but...

Ron said...

Here I thought this whole post was about drugs...

Paul Zrimsek said...

Do not taunt Happy Fun Kilogram.

Bissage said...

The shape of a cylinder is surely the most practical. Still, it would be way cool if the kilogram were shaped like a butterfly flapping its wings.

Crimso said...

"Still, it would be way cool if the kilogram were shaped like a butterfly flapping its wings."

Or Tony Montana...

Original Mike said...

There's a contradiction in the article. It says the kilogram has only been taken out 3 times, but it also says: "possible that the kilogram is getting lighter. Periodic washings, for example, may have removed microscopic quantities of metal from its surface." Which is it?

I think it's the latter. I read an article in the journal Science awhile back that said the kilogram was polished periodically to keep it from accumulating matter (I don't remember the mechanism: oxidation, or absorption of air molecules) but it was a balancing act between polishing too much vs. too little. The article went on to say this was the reponsibility of one guy, who had developed the knack to polish just enough. However, the guy was nearing retirement, and none of his apprentices seemed to be able to get it down right.

The topic of the article was the same as the one Ann linked to, replacing a physical object with a more fundamental measurement. There are two competing methods, the Watt balance and another method in which a LOT of individual atoms of silicon are counted out and weighed. Both methods are horrendously challenging. I wonder if the other method has been dropped or if the author of this piece didn't do his homework.

Zach said...

It's precisely this sort of problem that lead me to preferring atomic mass units. In addition, can a physicist enlighten us as to whether the simple act of moving it changes (at least temporarily) its mass (if only by an incredibly small amount) due to relativistic effects?

Atomic mass units don't help in this case, because you'd like to use your standard to measure everyday things, and there's no good way to compare a brick, say, to a carbon 12 atom. The reason you can define a meter in terms of the speed of light is because you can measure distances very precisely by using frequency-stabilized laser beams.

Some people call m/sqrt(1-v^2/c^2) the "relativistic mass," but it's not widely used because it's not very useful -- two observers moving at different velocities will get different values.

T J Sawyer said...
This comment has been removed by the author.
Zach said...

To stick up for NPR:

A microgram is a millionth of a gram. A kilogram is 1000 grams. So a microgram is a billionth of a kilogram.

Bryan C said...

One of the things that annoys me about the metric system is the way some folks seem to feel that it represents some divine, universal system of truth. It's not. It's handy and appropriate for a lot of things but not so handy for others. And the basic units are just as fundamentally arbitrary as the old English measures were. It's just that instead of some historical king's forearm, we now have a benchmark against the speed of light or a shiny metal cylinder.

It also bugs me, a little, that our universal system of measurement is dependent upon either unique physical objects or complex measurements that are nearly impossible to replicate without a high-technology infrastructure. I mean, say World War Z leaves civilization in shambles. How do the scattered survivors rebuilding the world determine what a "real" meter is supposed to be?

Cedarford said...

What doomed the English system of measurement was simply that it was not base 10. Napoleon's metric system was arbitrary. Distance measurement based on one planets distance from one arbitrary point to another, then mass based on one cubic centimeter or a common, but arbitrary substance (water), with temperature also based on when that common substance freezes or boils on the Earth's surface at NOP, NOT.

But it was all base 10, making calculations infinitely easier than other arbitrary systems.

Based on odd, off-base ten key points. A mile 5,280 feet. A mess of base 2, 4, 12, 1:2.2, base 16 measurements of weight, volume.

The mess extended to measurements of energy - BTUs, Rads, and so on.

Mass should be amenable to an improved standard. Simply, what a mass should be mathematically as an expression of AMU equivalent in number to a gram, and then what force is needed to electrostatically separate two such masses with a given opposite charge.

But for 99.9% of work, the standard would not have to be employed. Just used to create referent masses.

blake said...

I remember a guy lecturing me on how the meter was flawed precisely because it wasn't based on a fundamental constant, but on this hunk of mass sitting in Paris.

But that somehow imperial units were. I don't remember what he said, but I'm pretty sure the Freemasons were behind it.