Roundest objects in the world created
[A]n international group of engineers and craftsmen has . . . built a pair of nearly perfect spheres that are thought to be the roundest objects in the world.
The unusual balls, discussed last week at the SPIE Astronomical Telescopes and Instrumentation conference in France, were created as an answer to the “kilogram problem”.
High purity Si-28, and they hope to count the number of atoms to define the kilogram. With no danger to the pergium miners on Janus VI.
But even if all of the Avogadro Project’s research teams arrive at the same number of silicon atoms in each sphere, it’s far from clear that the International Committee for Weights and Measures will take up their definition.
[…]
These million-dollar spheres may be the roundest in the world, but will they be round enough?
One of the issues with international standards is that individual standards labs each want to be able to realize the standard. It’s not only a matter of having the absolute best measurement. If you make it too technologically advanced or involved, so it’s only within the grasp of a few labs, it’s not likely to be adopted.
Is the kilogram only definable by an artifact (versus ab initio measurement arising from fundamental constants)? Disaster! As with Euclid trying to navigate the globe, there would be at least one sour founding postulate upon which physics rests. There is worse! What if mass is indeterminate, depending upon configuration?
http://arxiv.org/abs/0801.4566
Chiral Gravity in Three Dimensions
http://arxiv.org/abs/0806.3082
Anomalous CMB polarization and gravitational chirality
http://arxiv.org/abs/0806.2821
Fermions in Loop Quantum Cosmology and the Role of Parity
http://arxiv.org/abs/0805.1294
Parity-violating macroscopic force between chiral molecules and source mass
http://www.mazepath.com/uncleal/qz4.pdf
That delicious disaster is testable in existing apparatus. Somebody should look.
Does the mass definition imply that gravitational mass and inertial mass are exactly the same? I know that we generally assume that, but I have to admit that I don’t know if it’s been unequivocally proven that the two masses are exactly the same in all cases. (Then again, we’re not mathematicians — just saying “we don’t know why, but every observation has shown they’re the same” is generally good enough for physicists.)
Also, do the labs really need to have an artifact kilogram?
It seems to me that it would be easier to have a small known fixed mass, then define the kilogram in terms of a large whole number of those masses. For example, take the kilogram to be exactly 1000 x (mass of 1/12 mole carbon-12 atoms). Avogadro’s number isn’t fixed, but isn’t its uncertainly strictly because we don’t have a fixed kilogram anyway?
You could make pretty closely approximate artifacts if you ever needed to do a physical comparison, and you avoid the problem of forcing a comparison to some chunk of metal someone threw together a hundred years ago.
Why won’t this work? If it were such an ingenious idea someone much smarter than me would have thought of it already, so there must be some problem with this idea.
Coming up with the definition is only part of the battle. Realizing it, i.e. making the measurement of it, so you can transfer that information (“traceability”), is a larger part. So you need to count the number of atoms in your sample. How precisely and accurately can you do that?
The “need” to have the standard is where the politics comes in. National pride, and the distaste for having to rely on some other country for it, because you aren’t up to the task. Among other considerations. It’s part of the reasoning for funding science projects in general.
@Uncle Al — it depends what you mean by an artifact. The Kelvin depends on getting pure water.