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Exactly One Kilogram Of Silicon
Ed Pegg Jr writes "You may know of the importance of 299792458
for length, and 9192631770 for time. However, the official standard for weight is still a block of platinum/iridium made a hundred years ago. A group of scientists from the Avogadro Project are hoping to change that, though, by producing a perfect sphere of ultrapure silicon."
What exactly is the benefit of having an object with a mass of exactly one kilogram
To calibrate scales.
Yes.
"The lesson to be learned is not to take the comments on slashdot too literally." --Vinnie Falco, BearShare
Consistency of units we already have.
We've got cgs (centimeter-gram-second) and MKS (meter-kilogram-second) systems, and at least one more.
If you're not satisfied, make your own. And watch the rush to adopt it!
I'm partial to MKS myself; less silly multiplication by powers of ten to correct units.
Neither system is consistent in the sense I suspect the original poster meant. Both systems mix purportedly "basic" units with a different sort of unit that requires a prefix, despite the fact that it's supposed to be basic:
centimeter - gram - second
meter - kilogram - second
It's just a matter of names. Logically, either system should be all "basic" units, with no prefixes. If a meter happened to be defined as 1/100th the current length, then you'd have the "meter gram second" system for the first case, and it would make more sense.
Of course, you still have the problem that a kilo of water is a cubic decimeter, not a cubic meter. So in the MKS system, you'd have to have a "gram" weighing rather inconveniently 1000 times as much, and a meter 1/10th the size, to make this "meter gram second" system more consistent.
The real answer, of course, lies in that word "convenient". The metric system, despite the claims of some proponents, is not really particularly inherently logical. Sure, it uses powers of ten. But the units themselves were chosen completely arbitrarily. And they were chosen simply to be relatively close to units already in use at the time - yards, bushels, pounds, quarts, and so on, because those were convenient sizes people were used to, not because there was some overriding scientific reason for choosing those sizes. (1/10^7 of the distance from Paris to the North Pole, indeed. Talk about chauvinisitic... as if there were a fundamental scientific principle rooted in the location of Paris!)
A truly "scientific" system would have units sized on fundamental constants - speed of light, Planck's constant, permitivity/permissivity of free space, all those sorts of numbers. You'd know you had such a system because they would be nice round numbers rather than arbitary long strings of digits as they are in the current metric (or English) system.
And when it comes to prefixes, you could always argue that the English system is far more logical in the age of computers. The metric system goes by powers of ten, which happens to be convenient for manual calculation. But the English system goes by powers of two; e.g, 16 cups = 8 pints = 4 quarts = 2 half gallon = 1 gallon. Much nicer for computers; no roundoff error there in the floating point processor, and you can change units by a simple shift operation rather than the hugely more time-consuming multiplication by ten. Same logic, different base.
ITYM mass. SI has no unit for weight. There's the newton for force, but it is not defined in terms of gravity. It is also not a SI base unit.
But then again, I could be wrong.
1/10^7 of the distance from Paris to the North Pole,
Actually, the meter was defined as roughly 1e-7 of the distance from the equator to the North Pole along Paris's meridian, making the earth roughly 4e7 m (40,000 km) around.
Will I retire or break 10K?
as TFA says, it's to avoid having edges which can be chipped etc.
I know you're just joking, but SI (the metric system) does deal explicitly with this. You can use one prefix, at most.
Engineers are in the habit of occasionally abusing this convention--instead of using u (mu, for micro-, 10^-6) as a prefix for some units (uH, microhenries of inductance) they will use mm (millimilli, presumably, as in mmH).
As far as I know, nobody ever combines prefixes associated with exponents that cancel out (partially or fully). It's common sense, really--have you ever seen a computer advertised with 256 milligigabytes of RAM? (Yes, I know that the byte is not an SI unit, but it illustrates the absurdity.)
~Idarubicin
Water expands and contracts as the temperature and pressure around it change, even in the range where it is still liquid.
It turns out that liquid water at 1 atmosphere pressure is most dense at about 4 degrees Centigrade, where its density is 0.9999750 g/cm^3. at closer to room temperature- at 22 degrees C- its density is only .9977735 g/cm^3. It never actually gets up to 1 g/cm^3 the unit system was originally designed to use, I think because of the limits of accuracy of measurements when the current definitions of individual units were set. In defining a unit, all those significant figures are relevant, so for these purposes the above are unacceptably big differences.
Measuring mass by what the volume of water is would be more complicated and less accurate than the current system. Besides, a liter is trickier to define than a kilogram, so it'd likely be the other way around if defined in terms of each other.
"we can communicate our definition of length and time to aliens 1000 light years away (if they are listening us), but we can't tell them what we mean by 1 kilo"
And then:
what matters is some reference atomic mass and then pick up Avogadro number (based on existing 1 kilo mass) and then get rid of the existing standard.
Doesn't the use of an Avagadro number of atoms, of specified number of protons and neutrons, exactly solve the weight description problem? You've got a point about ensuring that the ensemble of atoms doesn't interract with anything chemically, spontaneously decay, or be affected by a cosmic radiation event. However the existing platinum/iridium standard weight is subject to those same effects, and is (very) slowly evaporating away anyway!
The issue you're probably thinking of is transmitting the identification of left and right to aliens, consistant with our own usage. Martin Gardner has a very accessible discussion of this in his book "The New Ambidextrous Universe", and it has deeper implications from CPT (Charge, Parity, Time) symmetry in physics. The problem wasn't found to have a solution until C.S. Wu found a violation in CPT symmetry in 1957, allowing left and right to be uniquely identified.
Ian.
A physicist is an atom's way of thinking about atoms
Not only do diamonds' surfaces oxidize, they're pretty easy to burn, and only metastable in any case.
Diamonds are *not* forever.
A good question. The reason is that a kilogram is not a mathematical object (which is pure idea). It is a scientific object (which can and should have a real world representation).
Pi does not exist in the real world. If you don't agree, show me an object in the real world that has exactly pi length, weight, or volume.
By contrast, the kilogram is an idea (an agreement really), that leads to a real world object (bar of platinum, sphere of silicon) that people can test their measuring devices against.
Ask yourself this: if you and your friend had two scales, how would you know which one is more accurate?
Answer: you would test them against a scale you agreed was more accurate.
But, in order to test for accuracy, you need a very "accurate" object. You need something that everyone agrees weighs a certain amount (say a kilogram?) And your "most accurate" scale had better exactly weigh that object as exactly one kilogram.
That's basically what calibration is: you take an object you declare to be 1 kg (or 1 g) and then you set your scale to indicate it as such. Obviously, there is more to it than that, but that's the very basics.
Science relies tremendously on these types of standards. One of the biggest (and unsung) "wins" of the 20th century was the tremendous increase in the objective standards of accuracy. Imagine trying to build a microprocessor if everything was designed in terms of hand lengths or feet lengths of the various contractors. Without increasingly tight, objective standards of measurement, modern science and technology would not exist.
Ironically enough, I'm a mathematician. I would encourage you to talk to a professional scientist or engineer and ask them about it.
I think that is actually what they are doing--saying 1 kg is N atoms of silicon. They are picking silicon since it is the closest material we can come to creating an object with precicely 10^23 atoms.
That difficulty is the only reason the kg hasn't been redefined already--a standard definition isn't very useful if you can't actually implement it.