Perfect silicon sphere to redefine the kilogram

[apothecary]

http://www.theage.com.au/news/national/mak...1414466901.html

Perfect silicon sphere to redefine the kilogram

Chee Chee Leung

June 15, 2007

SECURELY tucked away inside a French vault is a lump of metal known as the International Prototype. A mixture of platinum and iridium, it was made in the 1880s to define the mass of a kilogram.

But work by a team of Australians could help pave the way for the retirement of this century- old prototype, as weight and measurement experts across the globe work towards a more scientific definition of the kilogram.

The project requires the development of perfect silicon spheres, and optical engineers at CSIRO's Australian Centre for Precision Optics — considered world leaders in the craft — are doing their part.

Scientists will use the spheres to determine how many silicon atoms make up a kilogram, and this will be used as the new definition — bringing the kilogram into line with other base units such as the metre and the second, which are all defined by physical constants.

"It's really an atom-counting exercise … and we'll come up with a new definition of the kilogram based on atoms, rather than based on the thing in Paris," explained Walter Giardini, of Australia's National Measurement Institute.

CSIRO's optical engineers will form two perfect spheres from a 20-centimetre cylinder of exceptionally pure silicon that arrived in Australia last night. The silicon, which has taken three years to produce, was made in Russia and grown into a near-perfect crystal in Germany.

The precision optics centre, located in the Sydney suburb of Lindfield, has already made about a dozen spheres for what is known as the Avogadro Project — with the most perfect sphere so far just 35 nanometres away from being perfectly round.

This means the diameter of the sphere varied by an average of only 35 millionths of a millimetre, making it a top contender for the title of the roundest object in the world.

A spherical shape was chosen for the project because it has no edges that might be damaged, and the volume can be calculated by using its diameter.

Optical engineer Katie Green, who will be involved in the precise cutting, grinding and polishing of the spheres, said it was exciting to be a part of a high-profile international project.

"It's probably going to take around three months' work, start to finish," she said. "It's been a number of years waiting for this material to be completed, so we're definitely looking forward to seeing it in the flesh, so to speak."

After the completion of the spheres, the silicon objects will be sent around the world to be measured and analysed by scientists.

[Anodyne]

Fuck, I remember seeing this on "beyond 2000" (that's going a while back, isn't it?) - they were using one of the rejects for a demonstration, lawn-bowls style. It's taken them a while to perfect, no? I wonder how long ago that was?

[ThunderIdeal]

was that the same process though? as i recall they were using their hands to slowly wear down the sphere as it spun around. that doesn't seem very precise.

[Auxin]

you might be surprised how precise tricks like that can be... I routinely use a stick to shave metal off mirror-surface parts in increments of 0.0002", thats 0.005 mm

[ThunderIdeal]

is it possible to make a hollow sphere, with a mirror on the inside?

even if it's not near-perfect, although that would be best.

i want to find out what happens when you get inside a mirror sphere with a torch, and the light just keeps bouncing around.

[Auxin]

similar concept is used in lazer technology... the short answer is you would not want to be in there

as far as I know all known substances do absorb some light tho, so it wouldnt build up too hideously intense

[teonanacatl]

lol yeah only got howmuch energy you start with and a torch dont kill knowone

would be cool to get inside a spherical mirror

[ThunderIdeal]

i want the long answer. damnit. i want the video footage. why has nobody found time to do this?

a torch would take a little while to reach skin-burning intensity but maybe it would be harmful to your health anyway... like actually living the last part of 2001 space odyssey.

[occidentalis]

I've been hearing about this problem with the standard for the kilogram - but I don't get it. 1L of pure water under standard conditions weighs exactly 1 kilogram. What's wrong with using that as the standard? Much cheaper than a perfect ball of silicon, easily replaceable, and replicable in every country.

[ThunderIdeal]

i don't get that either. apart from the need to strictly define 'standard conditions' whats the problem? perhaps measuring a perfect liter poses great difficulty.

[Auxin]

Water is insanely difficult to get ultra-pure and also almost impossible to maintain at high purity... and if you try to measure it you loose some. Just all around impractical for a universal standard meant to last a hundred+ years.