Roundest Object In the World Created

holy_calamity writes "An international research group has created the most perfect spheres ever made, in a bid to pin down a definition of the kilogram. It should be possible to count exactly the number of atoms in one of the roughly 9cm silicon spheres to define the unit. Currently the kilogram is defined only by a 120-year-old lump of platinum in Paris, but its mass is changing relative to copies held elsewhere. Other SI units have more systematic definitions."

Comments

Re:Wishing...

[maxume]

CmdrTaco did. Look at the dept. line.

What's the problem?

[Dolda2000]

I've never really understood the problem with creating a more stringent definition of the kilogram. Other SI units are measured in measurable quantities, such as the second being defined in terms of cycles of radiation from Caesium atoms. Why cannot the kilogram just as easily be defined as the mass of a certain number of atoms of one or another kind?

Of course, I'm no experimental physicist, but if I were to guess, I might suggest the fact that the binding energy (and thus the mass) might change with force-field fluctuations in the vicinity, but I think that problem should be solvable by defining the proper environment for measuring.

Does anyone know?

Re:What's the problem?

[nkh]

Where I was studying a few years ago, I had a teacher who was working with a french laboratory to create a standard for the kilogram. It was supposed to work with a machine to record the pressure applied to it (some kind of scale as far as I understood) and a bunch of lasers to measure everything. It was the first and only time I've heard about someone trying to standardize the kilogram.

Re:What's the problem?

[icegreentea]

That's exactly what the TFA says. They've created two spheres made out of pure crystalline silicon (so now they've lined them up). The sphere is nearly perfect (its about 95mm in diameter, has small scale variations of 0.3nm, and deviations from perfect cruve of 60-70nm). Now they're going to independently measure 60 000 points of the surface of the sphere to map out all of the surface variations, and then they're going to measure spacing using X-ray crystallography.

Also, nitpicking but... one kilogram = 10^15 picograms, always.

Re:What's the problem?

[multi io]

The theoretical model would include physical things -- namely, silicon atoms. Just like the theoretical model for the definition of the second includes the speed of light. If silicon atoms are too difficult to handle theoretically, why not use, say, electrons, as in 1kg=weight of 1.1xxxe30 electrons (at rest)?

Re:What's the problem?

[phobos13013]

We are getting measures mixed up here; they are not measuring the number of atoms for the kilogram, as that is not a measure of mass. They are measuring the number of atoms to make Avogardo's constant exact and tying it to the kilogram! They will define a specific number of atoms in a certain amount of the substance then saying that the kilogram is defined as the mass contained in X number of atoms!

In fact, this change in the kilogram is coupled with a change in avogadro's constant to make one immutable and the other exact!

Re:What's the problem?

[MrNaz]

That's in fact what this is doing. The silicone balls are intended to be calibrated according to a fixed value of Avogadro's number, rather than the Avogadgro number being defined according to the standard. Given this, it would be possible for a researcher anywhere in the world to recreate their own standard.

However, given that milling carbon into a monocrystalline structure is expensive (think 1kg diamond), they are using silicon instead. Thus, the KG would be defined in terms of the number of silicon atoms. They have yet to decide (as far as I know) whether to use naturally occurring silicon, or to remove all the isotopes and only use 28Si. This would, effectively, create a new number, redefining Avogadro's number as:

Avogadros number = New Constant * (mass(12C) / mass(28Si))

sphere

[Elisanre]

Is there a reason for it to be a sphere? Easier to determin the weight?

just add water

[krystar]

why not just define a kg as 1 Liter of pure H2O at 4deg C?....it is that way anyway.

Re:Metric...

[odourpreventer]

It gets worse. US standards are based on metric standards. (For instance, the inch is defined as 25.4 mm.) You're basically using a French system!

Gravity Probe B has more Perfect Spheres

[wooferhound]

I think that Gravity Probe B has the most perfect spheres and they are much smaller that the Kilogram sphere.

Kilogram Silicon Spheres
"If you were to blow up our spheres to the size of the Earth, you would see a small ripple in the smoothness of about 12 to 15 mm, and a variation of only 3 to 5 metres in the roundness"

Gravity Probe B Spheres
http://www.nasa.gov/mission_pages/gpb/index.html
"If these ping pong-sized balls of fused quartz and silicon were the size of the Earth, the elevation of the entire surface would vary by no more than 12 feet"

I can't believe no one has posted this...

[carlcmc]

They should have visited this guy's website

http://www.kyokyo-u.ac.jp/youkyou/4/english4.htm?

making spherical mud balls. I've had this bookmarked in del.icio.us for a long time

Now the measurement relies on a retiree

[Phurge]

from TFA "To shape the spheres, the Australian Center for Precision Optics pulled optical engineer Achim Leistner out of retirement. Leistner, who has been creating precision spheres for decades, considers these final two to be his masterpieces"

Great. What happens when this guy kicks the bucket?

Is crystal growth really the reason why?

[wisebabo]

Is this really the reason why it's a sphere? Crystals don't PRECISELY grow into a sphere do they? Won't they still need to shave or polish it to get it to the exact radius? And then they'll need to calculate the number of atoms using Pi, an irrational number!

Why don't they make it a cube and find a length that is close enough (cubed) to give them the approx. right number of atoms and then make THAT the standard? They'll then have an EXACT number of atoms making up each length. It should be easier to cut or shave off the requisite number of atoms to maintain it, a (perfectly) flat surface seems much easier to maintain than a 3D curved surface. In fact if they make it just a little too small they could probably even ADD to the cube in single atomic layers using vapor deposition!

Obviously brighter minds than mine have thought this through more thoroughly, so really, I'm curious: why is it a sphere?

By the way, maybe this is a good use for the ISS, to keep the 1kg reference MASS somewhere it won't be distorted by gravity, not kept at any particular country for measurement and you can keep it in a high quality vacuum for free! (A little expensive to get to though).

Re:Changing Mass?

[Shados]

Its shrinking. Losing incredibly small pieces over long periods of times. No object can realistically stay -exactly- the same forever.

Re:Wishing...

[chewedtoothpick]

Honestly, as a married man I don't understand why anyone would think that...

My first thought was of more efficient ball bearings. Such perfect ball bearings alone could reduce world-wide energy usage by a large percentage. Technology like this is the truly "green" tech that we need to proliferate in addition to the other forms we are currently working on.

A cubes edges might chip easily

[spineboy]

For the specification they are trying to achieve, even a little chip from a corner would be a tremendous error. It's a lot tougher to damage a sphere that way. Of course you can damage both from dropping them.

I too think eventually this physical weight will be replaced by a known voltage/wattage on a scale to counteract a force, although for practicality purposes, having a physical object is probably much easier to use in daily situations.

Re:Energy is Rounder than Matter

[srjh]

That's actually the reasoning behind the Watt Balance another potential method for defining the kilogram.

Keep in mind that the Joule is a composite SI unit, and is itself dependent on the mass of the kilogram. Unless you can calibrate the energy measurements (exactly what they are trying to do) you end up with a circular definition.

Re:Wishing...

[Red Flayer]

The single guys think of boobies.

The married guy thinks of balls.

I'm not sure what kind of commentary that is on our social structures...

I, for one, am married, and that means I think of boobies *more* often. Of course, now, some of the time I'm wondering how much milk they hold. Which doesn't really help when we're discussing solid spheres.

Re:And you call yourself a man!

[somersault]

Well, I mean from a purely physical point of view, the world is a lot more interesting (though sadly sometimes in a negative way, as you point out) because of physical differences. I find amusement in different breast shapes. I'm sure I'd be happy with one set of breasts attached to a special someone, but while I'm single I can honestly say that I quite enjoy the variety!

I'm sure you do love your wife's breasts, but that is presumably mostly because they are a part of your wife and you love her. They also are 'your' personal set of breasts so aesthetically you will also come to find them even more pleasing because of this.

People subconsciously come to prefer things that they own - they tested it on people with short term memory loss, getting them to rate some paintings on an aesthetic scale, then 'gave' them one of the paintings, came back later when the people had forgotten about the whole thing, and then asked them again to rate the paintings, and people rated the ones they were given as higher than before. I can't find a reference for this (I have a feeling it was in Robert Anton Wilson's Prometheus Rising but despite that being about the human brain I'm not sure how it fits into that context, so maybe I read it elsewhere), so take it with a pinch of salt if you will.

Personally I can vouch for that theory though, as I never used to find medium-smaller breasts interesting until I went out with someone who had fairly small breasts. Yes, she had a freakin awesome ass, but I learned to love her breasts too. I also tend to find women more attractive if I like their personality, and consider even good looking women to be 'ugly' overall if they are evil bitches.