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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."
Except for the challenges of making one, what's it useful for? You can't use it to calibrate anything, the wear and tear caused by the friction of handling would eventually change it's mass and defy it's purpose. Is the actual "finished product" good for anything else than sitting in another vault somewhere?
.: Max Romantschuk
I personally wouldn't put too much trust into a measurement that depends on gravitational acceleration for several reasons. ... and most importantly
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)
3) Your measurement will (*literally*) depend on the phase of the moon (just like tides)
Opus: the Swiss army knife of audio codec
Work it out if you really want to know but it's not a problem, even if pi were needed to make the sphere which it isn't. I remember readong somewhere a while ago (it may have been on slashdot, may have been somewhere else) that 34 decimal places of pi are sufficient to calculate the radius of the uiverse to within the width of a hydrogen atom.
It's fairly easy to prove that this is roughly correct if you look up the the two sizes. Compare the orders of magnitude.
The real purpose of the project is to produce a reproducable standard. If something was to happen to the lump of metal that currently defines a kilogram, there is no reliable way to reproduce it. You can make another lump of metal and weigh it, but even the most precise scale we have been calibrated back to the original lump of metal plus or minus the error of the machine. The problem is that the errors are cumulative. If we have to replace the lump of metal several times, it will be less and less precise. If however we can base the weight on a physical constant, then we can use that physical constant to calibrate future scales on. There will be errors based on the precision of the machine, but they are no longer cumulative. If you build a more precise machine, you get a more precise measurement.
I remember a small magazine (called Science Digest, IIRC) I read in the mid 80's. It was short little science articles, probably a couple hundred words each (at most)...not unlike RSS feeds today, perhaps.
One article was about scientists making the most perfect sphere to date out of some crystal. It was measured to be so perfectly round that if you scaled it up to the size of the earth, it's highest peak would be 12 feet higher than it's lowest point.
I'm sure the technology for this thing has improved a lot in 20 years.
Tiller's Rule: Never use a word in written form that you've only heard and never read. You will end up looking foolish.
Worse - you can't touch the Sphere of One-ness with anything.
... in a vacuum chamber ... at the bottom of a flight of stairs, in a disused lavatory with a sign on the door reading "prendre garde du léopard."
Q: May I put my greasy paws on it?
A: No. Fingerprints will alter the mass in a measureable way.
Q: White gloves?
A: Abrasive.
Q: Use a special cradle that's machined to exactly the same radius profile such that you won't scratch or deform the Sphere of One-ness?
A: Nope. That'll result in a molecular interference fit. You'll never get the two pieces apart.
So ultimately, they're building a very precise bauble that no one will ever be allowed to touch. I suspect that bouncing photons off the surface may displace an atom or two, so they'll keep it in a dark room
The 'special crystal which took three years to grow in Russia' that the article writes about is made of pure silicon-28 precisely to get around this objection.
I think the concern is that samples of silicon from different sources (consider, for example, 'depleted silicon' from the scrapyard of the Russian isotope-enrichment facility) might have different isotope distributions at the 10^-7 level, whilst good laser enrichment can ensure a really very constant isotope distribution.
The picture shows a beautiful shot of the perfect silicon sphere. Out of curiosity, I looked very closly at the scene reflected by it's surface, thinking perhaps I might get a glimpse of the photographer. However, he was nowhere to be seen.
Then I got to thinking - it should be easy to reconstruct the scene that is portrayed in the reflection from the surface of the sphere. All that is needed is to cut out the image of the silicon sphere and paste onto the surface of a three dimensional sphere. Then we could rotate it this way and that and look around the scientist's lab. So I did this - using a software simulation. I cut out the silicon sphere from the article's photo, and used it as a texture on a spherical 3D mesh, and added a little code to rotate it back and forth so that I could look around the scientist's lab. Guess what - there is no sign of the photographer! What we see is a very messy lab, with a closed door on the right. There are florescent fixtures on the ceiling that are currently turned off. There is a large window at the end of the room. I do believe that the ceiling, though it meets the left wall at the usual 90 degree angle, curves down to the wall at the right - a very unusual space, as if it was crammed into to an attic. At the extreme right of the room I believe we see a curtain hastily thrown over whatever would have been on the right side of the view. If the photographer is in the room, as he must be, I think he must be kneeling to the left of the window about three-quarters of the way back, and using a telephoto lens.
I have made available the exe that I created on my web site so that you may take a look for yourself. The code is a hasty adaptation of Microsoft's DX3D mesh tutorial "Tut_06Meshes" from the DxSDK 9.0, which is also included. You can get the zip package here. Perhaps you could modify the code to produce an even better view, but unfortunately, the resolution of the original image is really too low to get much out of it. It was a lot of fun doing this, and if you come up with a better result than me I would like to hear from you.