Is it supposed to be the mass of one liter of water? (I know it is defined as the mass of a particular artifact, but is that what that artifact is supposed to represent?)
Thanks,
Rob
I was taught that a cubic decimeter of water at its greatest density is equal to one kilogram.
Likewise, one cubic centimeter of water = 1 gram
One cubic meter of water = 1 tonne.
I love the metric system!
Oops, this was supposed to be in GQ.
Rob
Moved from Great Debates.
The chunk of metal used to formally define the Kilogram was machined so that it would be close to the weight of 1,000 cubic centimeters of water at 4 degrees. I’m not really sure why they haven’t taken the obvious step of just using that as the definition. Perhaps water doesn’t have as fixed a density as I’d think at a set temperature?
Yeah I never got that either. Why not just say a kilo is this much pure water at this temp at this pressure…?
I mean we can’t exactly count the molecules here
More likely it’s hard to measure the volume of water to the necessary degree of accuracy, and ensure the purity of same. May as well use an imperishable standard lump (well, as imperishable as can be managed - nothing’s 100.0000000000000%) and be done with it.
Oh shit. The Hortas are not going to like this.
They have been working for years on trying to find a standard that did not depend on the kilogram in Paris. They replaced the meter stick years ago, but finding something that has the required accuracy is pretty damn hard. The sphere is a good start, but ultimately, it’s just another object.
What they want is some way to measure it without needing an artifact. For instance, the standard meter is defined now as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.” This allows people to create their own meter sticks without having to measure them against any standard.
Except a clock…
How do you think Iridium feels about the change?
That old Kg standard made up 90% of its reason for being.
And the second, in turn, is defined in terms of a particular oscillation mode of a cesium atom of a particular isotope. This does, in a sense, rely on an artifact, but all atoms of the same isotope are identical, so it’s an artifact that any lab can study independently.
There are groups working on techniques to precisely count out large number of atoms, with the hope of defining a kilogram as the mass of so-and-so many atoms of carbon-12, but so far, this is not yet as precise as comparisons to a standard lump of material in a vault somewhere.
Yes, but you know your clock to be accurate because you’ve used the definition of a second, the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom, to build it.
I used to quote this number during presentations I put on concerning the frequency of quartz crystal units. When asked what the hell it meant, I could only shrug and say that I had no idea, nor did anyone I’ve ever talked to. For our purposes, I always said, one second was equal to one sixtieth part of a minute. No one ever quibbled.
How does one isolate a single atom, determine the two hyperfine levels of that atom, and make whatever measurements one must make in order to determine what the hell one second actually is? Perhaps even more importantly, how does one determine which atom one needs to isolate? There are so many things I wish I knew and so little time left to even begin.
In order: you don’t, you use the equations of atomic structure and energy from quantum mechanics, and you use a spectrometer to measure the radiation emitted by a large assembly of cesium-133 atoms as they make that particular transition when you induce it with a magnetic field.
The choice of that particular transition from that particular atom is largely arbitrary, since it serves only to identify photons of a particular frequency. Any quantum mechanically specified transition would serve the same purpose. We could just as solidly define the second as 2.4967x10[sup]15[/sup] periods of the radiation corresponding to the Lyman-alpha transition of neutral hydrogen atoms (in the absence of any fine or hyperfine splitting).
The choice of that particular transition of that particular atom was probably because, for some reason, that one was easier to measure precisely than, say, the Lyman alpha of hydrogen. What reasons those are exactly, I don’t know, nor probably would anyone except experimentalists working in that particular field.
Chronos - yeah, probably.
I should note a correction: the cesium-133 transition used to define the second is taken from a hyperfine correction to the ground state, with no external magnetic field present. The extremely small separation between the energy levels is produced by internal magnetic fields, i.e. those of the subatomic particles. So you would just get a bunch of cesium-133, make it as cold as possible, and wait. Once you record the radiation, you correct for the remaining temperature and for various other effects like time dilation (Earth’s gravity will change the measurement depending on altitude).
At this level, I think you also have to compensate by time compression. IIRC it you put an atomic clock on a plane flying east and another plane flying west you will get a slightly different answer when the return to their starting location.
I just goggled it and here is a description.
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html#c2
This part (“A team of scientists based in Germany measured the [Avogadro] constant by counting the atoms in a painstakingly crafted one-kilogram sphere of silicon-28.”) of that article is hilarious. So the standard kilogram is something for which the number of atoms have been counted?