Friday 28 January 2011

A Kilogram is a Kilogram is....Oops, Wait...

Just read an interesting article about attempts to standardize the Kilogram in terms of universal constants... that seems not to work..

"Since 1889, shortly after SI units were adopted, the kilogram has been defined as the mass of a cylinder made of platinum and iridium that is locked in a vault at the BIPM."
OK, seems safe enough, but ; "the kilogram's mass relative to several identical copies seems to be decreasing ever so slightly. The shift is troubling because there is no way to tell whether the copies are getting heavier, or the original is getting lighter."

The longstanding plan has been to replace the venerable cylinder with a kilogram defined in terms of a fundamental constant of nature. Fundamental constants are unchanging, [yeah? that's what you said about the mass of a cylinder made of platinum and iridium.] and a definition based on them would make the kilogram as fixed as the laws of the Universe." [which as you are about to discover, may be as flexible as we want them to be]

AHA, but WHICH law... it seems there is trouble in "paradisio scientifica". Two methods are being used; a watt balance which will give a kilogram as a function of Planck's constant, and the other is based on
the number of atoms in a sphere of Silicon, which would give the Kg mass in terms of Avogadro’s number.
BUT... "In recent years, each method has taken measurements accurate to around 30 parts per billion (in relative uncertainty); within reach of the most accurate measurements of the platinum–iridium cylinder. But each experiment's best measurements diverge from each other by around 175 parts per billion, a quantity far larger than metrologists have been prepared to accept."

Ok, that difference won't be noticeable in your next bag of jasmine rice, but this week at the Royal Society in London the former head of the mass division at the International Bureau of Weights and Measures France, suggested that we compromise on the two values, then back calculate to reset Planck's constant and Avogadro’s number...
Surprisingly, not every one liked that idea...

"Let's see, what is Planck's constant on a Tuesday?"


Steven Colyer said...
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Steven Colyer said...

There are only two physical constants that we are sure are fundamental, Planck's constant h and the speed of light in vacuum, c. Many feel there should be at least one more but what that would be, and even if there is another is controversial. I lean toward the electric charge e, but I am no expert in the subject.

We can only probe so deep and "defining" things like "mass", "charge" or "spin" becomes problematic (other than how to recognize and manipulate them) as we can only probe so deep and have no idea what these things actually look like at the smallest levels. (Currently, treating the fundamental particles such as quarks and electrons as zero-dimensional point particles seems to work fine ... if not fully satisfying).

There are of course, an incredible number of mathematical constants.

Nate said...

The fact that we can't even discern whether the standard kilo is getting heavier or lighter seems to place some serious doubt on our ability to devise and validate a superior method of measuring this. We might be out of our depth here.

Could the kilo be defined in terms of the speed of light?