Perfect Silicon Sphere to Redefine the Kilogram

MrCreosote writes "The Age reports optical specialists at CSIRO are helping create a new standard for the kilogram, based on a precise number of atoms in a perfect sphere of silicon. This will replace the International Prototype, a lump of metal alloy in a vault in Paris."

Comments

alternate theories

[arun_s]

I found some alternate theories that are also attempting to precisely measure the kilogram at everything2. They look pretty interesting, here's a small excerpt:

Superconducting levitation

This method works along essentially the same principles as the Watt Balance. In it, a superconductor of a known mass is placed within a superconducting coil. By running current through the coil, a magnetic field is generated that causes the superconducting mass to levitate. By levitating it at different positions and measuring the current required to do so, the magnetic flux can be calculated. Magnetic flux relates directly to Planck's constant, and because the force generated by the magnetically-induced levitation and the downward force of gravity must be equal, Planck's constant can thus be precisely related to the kilogram.

Hey wait, TFA skims over what they're going to do with the Silicon ball once its made. Again, from everything1:

X-ray interferometry is used to determine the distance between lattice planes in the silicon crystal, permitting physicists to determine, as closely as possible, the number of atoms in these spheres. Currently, a measurement accuracy of one part in 10^7 is possible, after considering all of the various sorts of error introduced in the process, but it is hoped that ten times this accuracy will be possible within five years.

Re:alternate theories

[JanneM]

This method works along essentially the same principles as the Watt Balance. In it, a superconductor of a known mass is placed within a superconducting coil.

If you have a lump of anything of a known mass, why bother with the rest?

Re:alternate theories

[jmv]

I personally wouldn't put too much trust into a measurement that depends on gravitational acceleration for several reasons.
1) It means you can't move the setup somewhere else easily because gravity is location-dependent
2) Events like the 2004 tsunami has a slight (but measurable) effect on the Earth's rotation and hence on the acceleration (because of centrifugal force) ... and most importantly
3) Your measurement will (*literally*) depend on the phase of the moon (just like tides)

Re:alternate theories

[_Eric]

You're mixing up mass (an amount of matter) and weight (the gravitational force felt by matter). The kilogram can be used anywhere. Only using a device based on absolute measurement of weight (spring based scale) will render the device dependent on the the local gravity field. Yet, true enough, this is how most of modern electronic scales work (they could still weight a known internal mass for calibration to work that around, but I don't know if or how this is actually done).

Re:alternate theories

[hanshotfirst]

Duck Measurer: "I put a duck on one side of the scale, and use weights (lumps of known mass) on the other side to determine the mass of the duck."
Some Guy: "Umm, but you already know the mass of the weights, why are you bothering?"

To see if they float, of course.

Re:"perfect" sphere

[Anonymous Coward]

>May God have mercy on your soul if you ever attempt to call a woman a physical object to her face.
Especially if he compares her to a perfect sphere.

Re:"perfect" sphere

[CommunistHamster]

It makes the calculations simpler.

This is about measuring the Paris kilo

[femto]

The CSIRO project is about determining how many silicon atoms are equivalent in mass to the current standard kilogram. Once that number is established the actual kilogram in Paris is redundant. If it gets lost or destroyed we can reconstruct the kilogram by counting out 'n' silicon atoms. It also means anyone can construct their own kilogram by counting out 'n' silicon atoms, without having to go to Paris to do a comparison.

It is a separate (but related) project to figure out the second part of the project: how to easily count out 'n' silicon atoms, so creating a universally available standard. One way might be to make a silicon sphere, like the CSIRO, but most people don't have the ability to do that.

Re:I always thought that

[at0mjack]

The main problem with this as a definition is that water expands and contracts with temperature. So, if you wanted to define the kilogram in terms of a volume of water, you need to specify the temperature at which you are making the measurement. Temperature isn't something you can measure with very high precision (parts per million or parts per billion), so you end up with unavoidably large errors. As a result this is useless as a basic standard, the essence of which is that you should be able to repeat the standard measurement and get the same answer to N decimal places.

perfect, well-rounded, bouncy

[siddesu]

silicon spheres will define the standard ... will they be coming in pairs by any chance?

SI horsepower

[Bromskloss]

One horsepower is the power of the reference horse in an archive in Paris.