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Perfect Silicon Sphere to Redefine the Kilogram
Posted by
CowboyNeal
on Fri Jun 15, 2007 06:29 AM
from the weights-and-measures dept.
from the weights-and-measures dept.
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."
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alternate theories (Score:5, Informative)
I found some alternate theories that are also attempting to precisely measure the kilogram at everything2 [everything2.com]. They look pretty interesting, here's a small excerpt:
Hey wait, TFA skims over what they're going to do with the Silicon ball once its made. Again, from everything1:
Re:alternate theories (Score:5, Insightful)
If you have a lump of anything of a known mass, why bother with the rest?
Parent
Re: (Score:3, Insightful)
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?"
Re:alternate theories (Score:5, Funny)
Parent
Re:alternate theories (Score:4, Funny)
Parent
Re:alternate theories (Score:4, Insightful)
Actually, the discussion involves using EM Flux as a way to calibrate mass measurements.
Or, using the duck analogy further - then finding (or making) a duck that precisely matches the needed measurements...
Regardless, the goal here is to get a reliable way to reproduce accurate mass meausre, without having a chunk of known mass available at or available to the reproduction site.
Parent
Re:alternate theories (Score:5, Insightful)
Mass = how much matter there is in an object.
Weight = how much pull does a particular gravity (like Earth's g) has on that quantity of matter.
That's why you could be floating (weightless) in a space ship without having lost any of your fingers or other parts of your body (mass)
Parent
Re:alternate theories (Score:5, Interesting)
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)
Parent
Re:alternate theories (Score:5, Informative)
Parent
The real reason they are changing it (Score:4, Funny)
Ah yes... (Score:5, Funny)
Re:Ah yes... (Score:4, Informative)
They don't. The idea isn't to make the new sphere weigh a kilogram. The idea is to redefine the kilogram in terms of the weight of an atom of silicon (i.e. 602383623523895723945743 atoms of Si-14 weigh exactly 14 grams). The idea of the ultrapure and ultraround Si sphere is that (a) you can measure the lattice spacing of the Si atoms in it using x-ray crystallography, so you know how far apart the Si atoms are, and (b) you can measure the diameter of your ultraround sphere very accurately, so you can calculate its volume very accurately. Given these two, you can calculate with very small error bars how many atoms of Si there are in the sphere, and given the definition of the kg in terms of how many atoms of Si make up a kg you can calculate exactly how much the sphere weighs.
You can then stick it on your balance that needs calibrating, and twiddle the dials until the balance thinks that the sphere weighs the same as the calculated weight.
Parent
I always thought that (Score:4, Insightful)
If they've already defined the metre using constants, isn't something like this the best way of defining a kilogram.
Re:I always thought that (Score:5, Informative)
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.
Parent
What's it useful for? (Score:4, Interesting)
Re: (Score:3, Interesting)
It's hierarchical. You use the standard kilogram to calibrate other, slightly less exalted standard kilograms. So the one kept in London and the one in New York and the one in Tokyo get calibrated against the one in Paris. Then you calibrate actual working weights against those.
Re:What's it useful for? (Score:4, Informative)
Parent
Re: (Score:3, Interesting)
The idea behind this is that rather than having a absolute reference mass in a vault in Paris, we can create new reference masses at will, so we can have additional ones in London, one in New York, one i
Okay geeks... (Score:4, Funny)
Dan East
This is about measuring the Paris kilo (Score:5, Informative)
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.
perfect, well-rounded, bouncy (Score:5, Funny)
SI horsepower (Score:5, Funny)
One horsepower is the power of the reference horse in an archive in Paris.
single isotope (Score:3, Informative)
This particular effort is a very interesting set of challenges. It requires the use of single isotope of silicon; calibrations for distance and roundness, and a sophisticated means to to count the atoms. This last step requires the silicon to be perfectly crystalline.
Measurement is itself a very interesting study bordering on metaphysics and philosophy. The desire to measure things has been at the heart of a lot of scientific investigation, economics and other areas of study. Ref "Abstract Measurement Theory" by Louis Narens https://mitpress.mit.edu/catalog/item/default.asp
Exam (Score:5, Funny)
Re: Using the Sphere of One-ness (Score:5, Interesting)
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
Parent
I can think of a better material. (Score:4, Funny)
The standard kilogram has been losing mass (Score:4, Informative)
Wikipedia - Kilogram
http://en.wikipedia.org/wiki/Kilogram [wikipedia.org]
Slashdot: The Changing Definition of 'Kilogram'
http://science.slashdot.org/article.pl?sid=03/05/
It should be a perfect cube (Score:3, Funny)
"Zut alors! Pierre, le sphere parfait - ou est-ce que tu le placer?
"C'etait sur le table, Jean-Claude"
"Merde, il avait roller sous le sofa encore!"
Where's the photographer? (Score:5, Interesting)
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 [tropicalcoder.com]. 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.
Re:"perfect" sphere (Score:5, Insightful)
The kilogram will not change, only a proposed scientific definition of it.
The sphere doesn't mean -anything- except that it'll weight exactly a kilogram and be amazingly round.
There's either a lot of media spin, or someone's attempt to get his work recognized and used. From what I can see, there's not a single soul that has dedicated to USING this new scientific definition, other than those directly involved with the project.
Parent
Re:"perfect" sphere (Score:5, Informative)
It's important enough for laboratories in Germany, Italy, Belgium, Japan, Australia and USA to invest a great deal of time and effort.
The spheres are being made by CSIRO's Centre for Precision Optics. They've been making precision spheres for research since the late '80s, and have all the recognition they need from anyone who has a clue.
Have a look here; http://www.tip.csiro.au/IMP/Optical/spheres.htm [csiro.au]. It might help you understand the project better.
Parent
Re:"perfect" sphere (Score:5, Informative)
No, it's widely accepted as a necessary step towards being able to define the unit of mass in terms of a specific number of carbon 12 atoms. Look, it would be a lot better for this discussion if you made the effort to learn what the project was for.
Just because you personally don't understand it doesn't make it "media spin" or otherwise redundant. There's more information here http://www.npl.co.uk/mass/avogadro.html [npl.co.uk], including an FAQ which might clear up some of your misconceptions.
Parent
Re: (Score:3, Interesting)
Re: (Score:3, Funny)
The whole notion of "silicon balls" sounds fake to me!
Re: (Score:3, Interesting)
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 l
Re: (Score:3, Funny)
Re:"perfect" sphere (Score:5, Funny)
Especially if he compares her to a perfect sphere.
Parent
Re:"perfect" sphere (Score:5, Funny)
Parent
Re:"perfect" sphere (Score:4, Funny)
Reminds me of a story - a friend had gotten a boob-job and we were all out for dinner one night. Another common friend of ours hadn't known this and the first time he saw her, he burst out - "You've grown three dimensionally!"
Parent
Re:don't need to create it to define it (Score:5, Insightful)
It's a lot easier to measure a large object than a small one and multiply it, since a small error will also multiply out. What I don't get is how they intend to build an exact number of atoms into the sphere. You would need some other exact measurement, like number of electrons for calculating precise electrolysis procedures.
Parent
Re:First of all (Score:4, Informative)
Second, if that rusty lump in Paris defines what a kilogram is, in no way is this sphere gonna change that.
That's wrong. The lump is not rusty, because the lump is platinum-iridium which is quite unreactive so that corrosion ("rust") won't affect the material. Corrosion alters the weight, you know.
Second, it can change the definition. The metre used to be a platinum rod in Paris, now it is defined in how much distance light does in a certain (very short) time. Here it will be that the kilogram will be defined as N silicium atoms. (Where N is a very large number) Scientists do not like definitions based on objects, they prefer definitions based on universal constants. All this could of course be read in the article....
Parent
Trying to be helpful -- do not flame please (Score:5, Informative)
TODAY: 1kg = the mass of the "rusty lump" in Paris. We don't know how many atoms of Ir and Pt the "rusty lump" have. So, if the "rusty lump" changes mass (and it changes with time because of being rustier all the time) AND because the "rusty lump" is used to calibrate scales all over the world, the kilogram is effectively changing with time. This is BAD.
WHAT THE GUYS ARE DOING: they are trying to make the most perfect silicon sphere possible that weights the same as the "rusty lump". Once they get to do that, they will count the atoms of silicon on the sphere, using interferometry. Suppose the # of atoms of the shpere is M.
WHAT WILL WE GOT THEN: 1kg = M atoms of silicon. This definition will never change, and if the silicon spheres rust or break or change weight by any circunstances, we make new ones with M atoms and we have a forever-constant definition of a kilogram. This is GOOD.
Got it? They did the a similar thing with the meter -- the original was a rod roughly 1m in size, then they did some measurements and said (*) "oh, one meter is the length that the light takes 1/299,792,458 of a second to go through in vacuum." and now they can do as many calibrating rods as needed, provided they make them the length that the light takes 1/299,792,458 of a second to go thru.
(*) actually the meter [wikipedia.org] had an intermediate definition of "1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum", but the new definition has the advantage of setting the light speed at exactly 299,792,458 m/s.
Parent
Re: (Score:3, Informative)
Re:Huh? (Score:5, Informative)
Parent
Re:Huh? (Score:5, Informative)
You don't. That's just the way we've done it in the past. I read a really interesting article a couple months ago in American Scientist magazine called An Exact Value for Avogadro's Number [americanscientist.org] that addresses exactly this question. In the past, Avogadro's Number (6.02andchange x 10^23) was defined experimentally, based on the reference kilogram. These scientists propose reversing that -- defining the number absolutely, based on the number of atoms of a particular element that fit within a sphere of a certain size. It's sort of similar to what they're doing with the silicon sphere, but it's all done on paper, rather than by actually manufacturing an artifact.
The advantage of this, they say, is that the number will remain constant and not be affected over time as refinements in building and measuring such "reference kilograms" change the accepted mass of a kilogram. They make several other arguments, as well, but it's much better if you just read the article.
Parent
Re:Huh? (Score:5, Insightful)
Ok, people are giving snarky answers here, but I'll try to give you a more straight answer.
The only way we have to keep a standard unit is to have an object with that unit and call that the standard. Let's say you were building some sort of a scale that would measure weight in kilograms; you'd have to calibrate it first. This means that you'd have to find an existing weight that was one kilogram, put it on the scale, and mark that this weight is a kilogram. But then how do you find a 1 kilogram weight? You have to measure it on some scale that's already calibrated correctly. This chain continues, and has to end somewhere.
So the two questions I anticipate are:
To answer the first question, a scale would be harder to maintain accurately. It could break, and calibrations don't hold forever. You'd have to re-calibrate it every so often, and how do you do that without an object known to be exactly 1kg?
The answer to the second question (which I imagine might have been your question all along) is a little more complicated. Let's imagine that we have no exact 1kg object stored anywhere that we use as the standard. So one guy in a lab is using an iron ball as his 1kg weight, calibrating scales with it, and selling scales to others. The iron ball slowly rusts over time, and the weight of the ball changes a little. Someone takes one of the scales calibrated with the rusty balls and does the same thing, but this time with his own hunk of iron, but the environmental conditions in this guy's lab aren't as controlled, and he tends to get water condensation on his iron ball, meaning it rusts faster and each calibration varies depending on how much water has collected.
Now, imagine it keeps on like this for 75 years, with different guys selling scales, getting their original measure from someone else, and then using their less-than-perfect means to continue calibrating and making scales. After 75 years, there are some drastically different "kilograms" floating around I buy a scale, measure out 1 kilogram, take it to a different scale and get 1.5 kilograms, while another says .75 kilograms. In this case, who's kilogram is "correct"? When the issue was raised, people would say, "Oh, if only we had a standard "kilogram" to compare them to!"
And so we have someone keep a physical reference object under very controlled conditions and of materials that will prevent corrosion or other corruption to the material.
Parent
Re: (Score:3, Informative)
The kg is a unit of mass, not weight.
This also affects the pound (Score:3, Informative)
Re: (Score:3, Informative)
Re: (Score:3, Informative)
lbf = lbm * (accel due to gravity) / g-sub-c
constant = 32.174 lbm*ft*s^-2*lbf^-1
That way, one lbm weighs very close to 1 lbf.
http://en.wikipedia.org/wiki/Pound-force [wikipedia.org]