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Too_Many_Tools
09-12-2007, 05:08 PM
Thought you might find this interesting...

TMT

http://news.yahoo.com/s/ap/20070912/ap_on_re_eu/shrinking_kilogram;_ylt=AveZdkMDr01tdNMYIDpZcsZn.3 QA


Kilo prototype mysteriously loses weight By JAMEY KEATEN, Associated Press Writer
Wed Sep 12

A kilogram just isn't what it used to be.

The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight — if ever so slightly. Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.

"The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart," he said. "We don't really have a good hypothesis for it."

The kilogram's uncertainty could affect even countries that don't use the metric system — it is the ultimate weight standard for the U.S. customary system, where it equals 2.2 pounds. For scientists, the inconstant metric constant is a nuisance, threatening calculation of things like electricity generation.

"They depend on a mass measurement and it's inconvenient for them to have a definition of the kilogram which is based on some artifact," said Davis, who is American.

But don't expect the slimmed-down kilo to have any effect, other than possibly envy, on wary waistline-watchers: 50 micrograms is roughly equivalent to the weight of a fingerprint.

"For the lay person, it won't mean anything," said Davis. "The kilogram will stay the kilogram, and the weights you have in a weight set will all still be correct."

Of all the world's kilograms, only the one in Sevres really counts. It is kept in a triple-locked safe at a chateau and rarely sees the light of day — mostly for comparison with other cylinders shipped in periodically from around the world.

"It's not clear whether the original has become lighter, or the national prototypes have become heavier," said Michael Borys, a senior researcher with Germany's national measures institute in Braunschweig. "But by definition, only the original represents exactly a kilogram."

The kilogram's fluctuation shows how technological progress is leaving science's most basic measurements in its dust. The cylinder was high-tech for its day in 1889 when cast from a platinum and iridium alloy, measuring 1.54 inches in diameter and height.

At a November meeting of scientists in Paris, an advisory panel on measurements will present possible steps toward basing the kilogram and other measures — like Kelvin for temperature, and the mole for amount — on more precise calculations. Ultimately, policy makers from around the world would have to agree to any change.

Many measurements have undergone makeovers over the years. The meter was once defined as roughly the distance between scratches on a bar, a far cry from today's high-tech standard involving the distance that light travels in a vacuum.

One of the leading alternatives for a 21st-century kilogram is a sphere made out of a Silicon-28 isotope crystal, which would involve a single type of atom and have a fixed mass.

"We could obviously use a better definition," Davis said.

TGTool
09-12-2007, 05:13 PM
It probably really hasn't lost any weight. The others just have fingerprints on them. :)

A.K. Boomer
09-12-2007, 05:37 PM
What kind of material is it made out of --- It either didnt say or i missed it, im sure certain materials would matter, then again thier leaving this all up to the french? wut up wit dat?

TGTool
09-12-2007, 07:31 PM
The kilogram's fluctuation shows how technological progress is leaving science's most basic measurements in its dust. The cylinder was high-tech for its day in 1889 when cast from a platinum and iridium alloy, measuring 1.54 inches in diameter and height.



I believe they mentioned it.

Makes you wonder what it would have weighed if they machined it to metric rather than imperial dimensions though. :rolleyes:

Alistair Hosie
09-12-2007, 07:33 PM
maybe someone breathed on one before measuring it,seriously maybe they didnt have the precision tools for measuring we have today Alistair

TGTool
09-12-2007, 07:38 PM
I think the article says that they compare them on a regular basis so they must be seeing some consistent drift, improvements in measurement notwithstanding.

Perhaps like Mae West - "I used to be Snow White, but I drifted." Who knows what those things are doing in the dark when they're not out being measured.

dp
09-12-2007, 07:42 PM
Makes you wonder what it would have weighed if they machined it to metric rather than imperial dimensions though. :rolleyes:

It wouldn't matter what its physical dimensions are, it would still weigh a kilogram. That's the problem - it has lost or gained mass, not really known, but no matter, it still weighs a kilogram. If it lost half its mass it would still weigh a kilogram. When you grab the kilogram standard down off the shelf and put it on a scale you have to adjust the scale until it reads one kg. A quandry? Sure. A paradox? Not really. A problem? Of course.

madman
09-12-2007, 09:04 PM
Maybe a Big Rats Gnawin on it ??

toastydeath
09-12-2007, 09:43 PM
This just illustrates the need to move to natural constants for our standards. They already did it with the two most important ones, time and length, and now the remaining prototype standards just have to follow suit.

Our measurement apparatuses are plenty accurate, but they aren't constant. Like back when Kr-86 was the standard for length. Interferometers were capable of higher resolution than the Kr-86 source, but there's drift over time in an interferometer, and they have to be referenced back against a primary standard periodically. The Kr-86 is stable forever, but you can't measure it all that accurately. Mass and force are the same way.

The big problem, like many people have already posted, is that you can't know with any absolute certainty what is going on. You don't have any standard unit, so all you can do is use comparative devices. Like comparing two parts with a dial test indicator with arbitrary graduations; if you don't have a gage block stack as a reference, all you can say is "hey, this one is less big than this one."

I've seen pictures of the kilo prototype and the vault, and believe me, there aren't any fingerprints or rats gnawing on the thing.

andy_b
09-12-2007, 10:06 PM
maybe dark matter is sticking to the rest of them. since you can't see it, how would you know it's there?

andy b.

littlelocos
09-12-2007, 11:19 PM
This just illustrates the need to move to natural constants for our standards. They already did it with the two most important ones, time and length, and now the remaining prototype standards just have to follow suit.




Toasty,
At the National Institute of Standards and Technology in Gaithersburg, MD, they have been working on this issue for years. One, the Electronic Kilogram, is especially interesting.

To give you an idea of the level of accuracy they are measuring/comparing mass, the scientists there can sense changes in the mass of the Kg with variations of temperature and humidity as these conditions change the buoyancy of the air. The Kg mass at NIST is kept in a room whose air temperature is controlled to +/-0.01C and +/-1% RH to help negate the effect.

In another lab, they are using a smaller version of the electronic Kg to measure the force needed to pull a single atom from the surface of a test sample.

See this link to an article on the E-Kg
http://www.eeel.nist.gov/817/news/pressreleases/electrokilogram.htm

and this link to other efforts to redefine the Kilogram. (including a perfect sphere of crystaline silicone.)
http://www.nist.gov/public_affairs/newsfromnist_redef_kilogram.htm

Enjoy the articles,
Todd.

Evan
09-12-2007, 11:24 PM
So, what do you replace a mass standard with except another mass standard? BTW, 50 micrograms is a lot. One of the meds I take has a starting dose of 125 micrograms. That's an easy amount to measure.

I think that some sort contamination was present when the masses were originally compared and has been evaporating slowly since. I bet if they look at the materials used to clean it that they will find that those materials have become more pure over the years and that some unsuspected impurity used to be present in greater quantity than it is now. Perhaps something like a minute trace of an oil or wax dissolved in the solvents used to clean the artifact was left behind after cleaning. If that is the case it will be very difficult to verify unless they still have some samples of the earlier cleaning solutions.

toastydeath
09-12-2007, 11:50 PM
You replace the mass standard with a value that can be experimentally determined using universal constants.

For example:

The second is some ridiculous number of hyperfine level oscillations in a cesium-133 atom in its ground state. That's going to be the same anywhere, and anyone who really wants to can generate their own primary time standard can do so. There's no reliance on some artifact for accuracy, and your resolution of the second is limited only by your resolution in the experiment.

Same with distance. The meter is some fraction of the distance light travels in one second, which works out to some number of wavelengths of light at some frequency, which can be directly measured by an interferometer to a very close value. Again, no physical standard is necessary and can be reproduced by anyone with enough time and money on their hands.

littlelocos
09-13-2007, 12:04 AM
So, what do you replace a mass standard with except another mass standard? BTW, 50 micrograms is a lot.

... two contenders are combinations of well-known electrical standards and counting the number of silicon atoms in an artifact grown an atom at a time. Both of these are described in the links above.

I am pretty sure the AP article is misquoting the dimension. 50 micrograms is HUGE at the level they are talking about.

This seems to be a better article. It focuses on the work being done at NPL in Great Britain. (although it gives the drift rate at 50 milligrams per century, ...argh)

http://pubs.acs.org/cen/science/83/8329sci1.html


Todd.

Too_Many_Tools
09-13-2007, 12:54 AM
This just illustrates the need to move to natural constants for our standards. They already did it with the two most important ones, time and length, and now the remaining prototype standards just have to follow suit.

Our measurement apparatuses are plenty accurate, but they aren't constant. Like back when Kr-86 was the standard for length. Interferometers were capable of higher resolution than the Kr-86 source, but there's drift over time in an interferometer, and they have to be referenced back against a primary standard periodically. The Kr-86 is stable forever, but you can't measure it all that accurately. Mass and force are the same way.

The big problem, like many people have already posted, is that you can't know with any absolute certainty what is going on. You don't have any standard unit, so all you can do is use comparative devices. Like comparing two parts with a dial test indicator with arbitrary graduations; if you don't have a gage block stack as a reference, all you can say is "hey, this one is less big than this one."

I've seen pictures of the kilo prototype and the vault, and believe me, there aren't any fingerprints or rats gnawing on the thing.


The link I supplied has a photo of the standard.

TMT

Too_Many_Tools
09-13-2007, 12:58 AM
This just illustrates the need to move to natural constants for our standards. They already did it with the two most important ones, time and length, and now the remaining prototype standards just have to follow suit.

Our measurement apparatuses are plenty accurate, but they aren't constant. Like back when Kr-86 was the standard for length. Interferometers were capable of higher resolution than the Kr-86 source, but there's drift over time in an interferometer, and they have to be referenced back against a primary standard periodically. The Kr-86 is stable forever, but you can't measure it all that accurately. Mass and force are the same way.

The big problem, like many people have already posted, is that you can't know with any absolute certainty what is going on. You don't have any standard unit, so all you can do is use comparative devices. Like comparing two parts with a dial test indicator with arbitrary graduations; if you don't have a gage block stack as a reference, all you can say is "hey, this one is less big than this one."

I've seen pictures of the kilo prototype and the vault, and believe me, there aren't any fingerprints or rats gnawing on the thing.


It recalls the saying....

"The man who owns two clocks never knows what time it really is."

TMT

dp
09-13-2007, 01:18 AM
Mass standards need to be replicatable no matter the environment. Frinstance, send a robot to Alpha Centauri, third stone. First thing you need to do is calculate local gravity because everything in the time/space continuum depends on understanding local gravity. Do you use a known quantity like a block of platinum and iridium alloy, or go with an electronic mass dealy that blows a fuse while passing Rigel 7? I'd go with the block. And besides, any lump of metal is on topic.

toastydeath
09-13-2007, 02:02 AM
You don't need a mass standard while exploring Alpha Centauri. You only need a reference of some sort, calibrated to a known standard. Primary and secondary standards (even experimentally determined standards) are far too fragile to be calavanting about outside of expensive laboratories. People don't keep yard and meter masters in shops. We have gage blocks, micrometers, dial indicators, et cetera which can all be traced back a several step long path to some kind of primary standard. Those are all more rugged, and more useful, than a master prototype for day to day operations.

Master standards are not a matter of wide application or ease of reproduction. Standards are designed for the bleeding edge of metrological accuracy, and they sacrifice whatever is necessary to gain that. The needs of even ultramodern lights out manufacturing facilities, space exploration, et cetera, are not taken into account. None of those people are going to ever see a master. Once you have the bleeding edge of accuracy, you can then design and calibrate sturdier devices for specific applications.

Besides, the only thing masters are really used for is a point of comparison with other laboratories to see the results and comparative experimental resolutions of various laboratories. From a practical, day to day sense, very few people care about master standards, how they're defined, or when to use them. Shops and 99.999% of businesses only care that the gage blocks are checked periodically by someone who knows what they're doing, and that everyone's micrometer matches up with the blocks.

Too_Many_Tools -

The "too many clocks" problem unfortunately also applies to the micrometers in any given inspection department. =(

dp
09-13-2007, 03:00 AM
From a practical, day to day sense, very few people care about master standards, how they're defined, or when to use them.

I take it you don't work in computing where nanoseconds are money. There's a reason places like this (http://www.ntp.org/) exist. You couldn't run google or ebay without it.

toastydeath
09-13-2007, 04:41 AM
I take it you don't work in computing where nanoseconds are money. There's a reason places like this (http://www.ntp.org/) exist. You couldn't run google or ebay without it.

You just took a left turn, for no apparent reason, with a quip that has nothing to do with what's being discussed. I am trying to take it on good faith that you understand what master standard is for, and why you wouldn't want to go to Alpha Centauri with one, and why Ebay really doesn't want one.

Primary standards and the working units that come from them are two different subjects, which you seem to really want to blend together. They are only the same on a superficial level.

The ability to COUNT in nanoseconds or measure in ten millionths has nothing to do with a primary standard, which have to do with defining the unit you're going to count and divide. It doesn't matter if you have a device that can divide some arbitrary amount of time into 10^-800000 units or that it's accurate for billions of years if you can't precisely define what that arbitrary amount of time should be (i.e. what a second actually is).

Yes, counting in nanoseconds is important, as is measuring to millionths and further. But people who do both of these things do NOT own, operate, maintain, or care about atomic clocks and experimental interferometry.

This whole thread is about primary standards. The primary derivation of a unit of measure - that's what the kilogram problem is about. Ebay doesn't have to do this, NIST and NPL do. And whoever goes to Alpha Centauri won't have to worry, either. What businesses want is an agreeable unit of sufficient resolution. They really DON'T care about the day to day particulars of a standard, how you define the unit, or anything else as long as it is 'the same' every time, and it is there every time they wake up in the morning. They don't care about the operation of master standards, as long as they know everyone winds up using the same unit when all the laboratory voodoo is over with.

A quick example, using gauge blocks, an interferometer set up to determine an inch, and a micrometer. The interferometer would determine the distance that is precisely an inch. Now, what does a shop do with that? All you've got is a machine that says "This is an inch." You've still got to lap a block, or calibrate another interferometer (a very different interferometer) from that primary standard experiment so that you can do something with it. Say you lap a block. Now you have something you can make many of, not lose any accuracy, and can set other things from. This is a secondary standard. This is something shops can buy, and check micrometers with, or set a dial indicator, or any number of things.

Time works the exact same way. You not only need the timecodes from the atomic clocks saying what a second is, you need your device capable of dividing that amount of time accurately. Now, Ebay begins caring once you have your divisions. The second's definition, by itself, is useless to businesses.

And the definition is what a primary standard is for. Not the division.

And actually, I did work in computer science and networking.

Evan
09-13-2007, 05:25 AM
Quote:
Originally Posted by Evan
So, what do you replace a mass standard with except another mass standard? BTW, 50 micrograms is a lot.


... two contenders are combinations of well-known electrical standards and counting the number of silicon atoms in an artifact grown an atom at a time. Both of these are described in the links above.



A mass standard cannot be defined in terms of electrical phenomena. The concept of mass falls squarely in the Einsteinian arena. Counting atoms is impractical at best and horrendously difficult to verify, if it can be done at all. Ultimately, the only way to replace a physical mass standard it to define a unit of mass in terms of the total energy content. E=mc^2 is the relation. This requires a complete characterization of the environment including effects such as frame dragging.

This story may not be as mundane as a contaminated cleaning solution. There are some problems nibbling at our understanding of gravity and it's effects at the level of macroscopic masses.

This may be in some way related. It's not yet possible to say.


Something strange is happening in the outer reaches of our solar system. The Pioneer 10 and 11 spacecraft (http://planetary.org/explore/topics/past_missions/pioneer_10_and_11.html) are not where they are supposed to be.
The Pioneer 10 and 11 spacecraft (http://planetary.org/explore/topics/past_missions/pioneer_10_and_11.html) are not where they are supposed to be. These missions, launched in 1972 and 1973, have covered hundreds of millions of kilometers, heading toward the edge of our solar system. But something is holding them back. Each year, they fall behind in their projected travel by about 5,000 kilometers (3,000 miles).

Jet Propulsion Laboratory scientist John Anderson and his colleagues have been searching for an explanation since 1980. But as of yet, they have found nothing conclusive; no spacecraft behavior or previously unknown property of the outer solar system can explain the deceleration of the Pioneer spacecraft. Scientists are being forced to consider the unthinkable: something may be wrong with our understanding of the laws of physics (http://planetary.org/programs/projects/pioneer_anomaly/update_20050510b.html).

http://planetary.org/programs/projects/pioneer_anomaly/

andy_b
09-13-2007, 10:01 AM
This story may not be as mundane as a contaminated cleaning solution. There are some problems nibbling at our understanding of gravity and it's effects at the level of macroscopic masses.

This may be in some way related. It's not yet possible to say.

that was what i was also wondering. plus, how would any standard developed here on Earth behave as it was flying about the galaxy at some insane speed (the actual standard, or the attempt to reproduce the standard). and then there's still that dark matter problem.

andy b.

A.K. Boomer
09-13-2007, 11:18 AM
Gravitational Changes are occuring right on our own planet, What do we call sea level anymore? the old ratings or the new, im sure these weights are measured in the most stringent conditions including at the same altitude to the inch (or MM) but what good is that if your entire medium has changed around you, the planet is also constantly picking up "space weight" (meteors and dust and such) Its also condensing and getting smaller --- If we had some kind of foolproof scale that did not deviate its not hard to imagine one of these weights weighing differently in a thousand years then what it weighs today ------- and it wouldnt be the weights fault -- because as contradictory as it sounds it actually still weighs the same.


Perhaps this archaic system needs to be abolished, and we need to "weigh" things out in space where everything is weightless, By accelerating the "kilo" to a predetermined perfect clocked speed and then abruptly stopping it we can measure how much mass it has without this flimsy fluctuating thing that we not only call a constant but label it "gravity"...

Just going to the extremes to get a little perspective...

Fasttrack
09-13-2007, 12:46 PM
Hold up a sec lets not confuse force with mass ... changes in gravity aren't going to change the mass of the standard. I interperted the section:

"Of all the world's kilograms, only the one in Sevres really counts. It is kept in a triple-locked safe at a chateau and rarely sees the light of day — mostly for comparison with other cylinders shipped in periodically from around the world."

to mean that they are taking two artifacts and comparing them on a balance at the same location. A change of gravity isn't going to be so localized that a diference of 10" is going to make a difference. So even if the samples "weigh" differently in china than they do in france, its how they compare to each other at some location that matters. Thats the whole point of mass - its the same in space as it is on earth.


Incidently E=mc^2 is the rest mass energy, the total energy of a body (minus any potential energy) is given by E=mc^2/sqrt[1-v^2/c^2] or E=mc^2(gamma)

But it can be very difficult to measure energy with any certainty ... in fact, that uncertainty is given by (delta)E(delta)t= h/4pi where (delta)t is the amount of time required to make the measurment.

Evan
09-13-2007, 12:52 PM
There is no detectable difference between gravity and acceleration according to our present understanding. If that understanding is wrong we don't know how it is wrong or what is needed to correct our understanding. We have no indication that gravity is changing on this planet or any other. The effect of gravity varies according to numerous factors including latitude, altitude, mass concentrations and rate of rotation of the Earth. The amount of acceleration produced by the mass of the Earth doesn't change because of that.

Fasttrack
09-13-2007, 12:54 PM
I should have thought that it was far more probable that the masses were contaminated with another material that is either decaying (thus the mass in france is decreasing as it radiates) or the replicas are contaminated with another material that is corroding (thus gaining mass). Although that would require .000003125 moles of oxygen. I don't know whether that is probable or not considering the available surface area...

Evan
09-13-2007, 12:57 PM
Incidently E=mc^2 is the rest mass energy, the total energy of a body (minus any potential energy) is given by E=mc^2/sqrt[1-v^2/c^2] or E=mc^2(gamma)

Yep. Guilty of oversimplification. It's worse than that, too. You need to define and characterize frames of reference for that to be meaningful. Also, so called "relativistic mass" isn't the same as rest mass. It's why a photon has momentum. It has relativistic mass from the kinetic energy of motion but still has no rest mass.

Fasttrack
09-13-2007, 12:59 PM
There is no detectable difference between gravity and acceleration according to our present understanding. If that understanding is wrong we don't know how it is wrong or what is needed to correct our understanding. We have no indication that gravity is changing on this planet or any other. The effect of gravity varies according to numerous factors including latitude, altitude, mass concentrations and rate of rotation of the Earth. The amount of acceleration produced by the mass of the Earth doesn't change because of that.




Right ... i think.

The acceleration due to gravity does change depending upon where you are in a gravitational field. There have been rather mundane expierements that prove weights are different in the rocky mountains than they are on the plains, and are different than they are on airplanes, and etc.

The universal gravitational constant is, we believe, still constant. G=6.673X10^-11 m^3s^-2kg^-1

Although that is really just an estimation - as revealed by the funky shifts in mercury's perhelion.

Evan
09-13-2007, 01:03 PM
The acceleration due to gravity does change depending upon where you are in a gravitational field.

Yes, of course, which is part of my point. However, the total acceleration that is produced by a certain amount of matter doesn't vary with time as far as we know. We may be wrong.

Fasttrack
09-13-2007, 01:05 PM
Yep. Guilty of oversimplification. It's worse than that, too. You need to define and characterize frames of reference for that to be meaningful. Also, so called "relativistic mass" isn't the same as rest mass. It's why a photon has momentum. It has relativistic mass from the kinetic energy of motion but still has no rest mass.




True, although Einstein himself said that mass is a poor measurement because "no clear definition can be given. It is better to introduce no other mass concept than the 'rest mass' m."

Thats why in quantum mechanics today, mass isnt used. The idea of relativistic mass is as valid mathmatically as relativistic momentum, but momemntum is an easier quantity to deal with. Thus mass is taken to be relativistically invariant, while it is momentum that changes. Incidently, instead of seeing E= blah, a more common expression is

E^2=(mc^2)^2 + p^2c^2

Ironically that expression actually makes alot of quantum mechanics easier...


Yes, of course, which is part of my point. However, the total acceleration that is produced by a certain amount of matter doesn't vary with time as far as we know. We may be wrong.



Oh gotcha - thats what i thought you said but i wasn't sure. :)

dp
09-13-2007, 02:33 PM
You just took a left turn, for no apparent reason, with a quip that has nothing to do with what's being discussed. I am trying to take it on good faith that you understand what master standard is for, and why you wouldn't want to go to Alpha Centauri with one, and why Ebay really doesn't want one.

Sorry - wasn't my intent to offend. Mass and time are related and interdependant, and in today's world they have commercial importance, particularly in power generation and computing. And if a one-way human mission were sent to some far star they definitely would need to have some basic standards with them and I expect a good calibrated known mass standard would be part of that kit.

littlelocos
09-13-2007, 04:35 PM
A mass standard cannot be defined in terms of electrical phenomena. The concept of mass falls squarely in the Einsteinian arena. Counting atoms is impractical at best and horrendously difficult to verify, if it can be done at all. Ultimately, the only way to replace a physical mass standard it to define a unit of mass in terms of the total energy content. E=mc^2 is the relation. This requires a complete characterization of the environment including effects such as frame dragging.

Evan,
I would suggest you read the articles I referenced, not just blow off about it. If you would like to argue it with a scientist, I would suggest you contact NIST to discuss. All I can say about it is -- it works very well and does what they say.

Over and out,
Todd.

aostling
09-13-2007, 06:26 PM
BTW, 50 micrograms is a lot. That's an easy amount to measure.


Yes, if I calculate correctly, 50 micrograms of platinum has the volume of a cube 0.005" on a side.

The Prototype Kilogram contains 10% iridium. I always thought that osmium was the densest element, but Wikipedia states:



The measured density of iridium is only slightly lower than that of osmium, which is often listed as the most dense element known. However, calculations of density from the space lattice may produce more reliable data for these elements than actual measurements and give a density of 22650 kg/m³ for iridium versus 22610 kg/m³ for osmium. Definitive selection between the two is therefore not possible at this time.

Evan
09-13-2007, 06:34 PM
Evan,
I would suggest you read the articles I referenced, not just blow off about it. If you would like to argue it with a scientist, I would suggest you contact NIST to discuss. All I can say about it is -- it works very well and does what they say.

I suggest you read the article. They aren't defining the kilogram in terms of electrical phenomena. They are using a mass artifact to calibrate what amounts to a very accurate scale. The value produced is still traceable back to the artifact made of matter. It changes nothing except that they won't need to constantly refer to the artifact. The definition is still based on the artifact, not some other principle. There isn't any way to define a quantity of matter except by using a quantity of matter as a reference.

From the article you linked:


The NIST watt balance is a two-story-high apparatus designed to redefine mass in terms of fundamental physics and quantum standards. It measures the force required to balance a 1-kilogram mass artifact against the pull of Earth’s gravity, as well as two electrical values (see graphic below). These measurements are used to determine the relationship between mechanical and electrical power, which can be combined with several equations to define the kilogram in terms of basic properties of nature.

That isn't very well worded. What they are doing is calibrating a reproducible method of producing a force equivalent to the mass artifact. Once they do that they can throw away the artifact but it is still the original source of the definition of the unit of mass.

littlelocos
09-13-2007, 08:08 PM
Evan,
Cool. Thanks.

Still a difficult concept to understand -- and doubly difficult to explain. The machine is also called a "Watt Balance" which is a little easier for me to comprehend.

When touring the labs with visitors, some of the chatter about "whose kilogram artifact is most accurate" can get rather entertaining, especially when THE kilogram will always reside in Paris. The issue is "What if the French kilogram (a primary standard and the definition of the kg) changes and the US kilogram (a secondary International standard) stays the same?" Sounds frustrating.

Thanks again,
Todd.

Evan
09-13-2007, 10:20 PM
The issue is "What if the French kilogram (a primary standard and the definition of the kg) changes and the US kilogram (a secondary International standard) stays the same?" Sounds frustrating.

Similar has happened before. The inch was redefined back in the 50's if I recall correctly to be exactly 25.4 millimeters. Any really accurate inch measuring device made before that isn't correct since that change. Not to worry though, the difference is extremely small.

I take issue with the constant reference in the article to "defining the kilogram by properties of nature". They cannot and the reason I stated so is because the property of having mass, inertia, momentum and weight isn't explained by the standard model. It is only explained by the theories of relativity. Until someone develops a unified theory that includes gravity and quantum mechanics we have no knowledge of any first principles of electromagnetic theory that can define the acceleration produced by a quantity of matter. Quantum mechanics has nothing to say about gravity or how much something should weigh.

Fasttrack
09-13-2007, 11:00 PM
Well even general relativity falls short. It doesn't account for mass - just the effects of mass on other particles.

The standard model is inherently relativistic, though. I think it would be wise, for the sake of others, to differentiate between special and general relativity. Special relativity is a part of the standard model as a necessity. Also it might be wise to note that the standard model and quantum mechanics are not the same thing, just interelated. The standard model describes matter and three of the four forces while quantum mechanics is primarily concerned with the wave function, uncertainty and quanta.

Incidently, supersymmetry is one likely theory that accounts for mass. It calls for a higgs field and the higgs scalar boson that mediates the field and it acts, in an over-simplified way, like balls rolling over sawdust. Some are hard marbles and they zip right through the patch of sawdust without slowing down while a sticky ball rolls through the sawdust and slows down as it gains mass. Like i said a way over-simplified view, but you all get the idea.

The nice thing about supersymmetry is that, in order for the math to work out, the mass has to be there where as the standard model the masses have to be figured in by hand later or the math explodes leaving you with such things as 0/0 or infinity/infinity etc. Plus it also explains all the other phenomenon that string theory accounts for.

Yankee1
09-13-2007, 11:17 PM
How about oxidation due to oxygen molecule collisions. or electron transfers due to close proximity to a Less noble metal and humidity?
Chuck

Evan
09-13-2007, 11:50 PM
The standard model describes matter and three of the four forces

I would consider quantum mechanics to be an integral part of the Standard Model today. Funny thing about it is that it describes matter in great detail but cannot explain why it is massive and has gravity.

General Relativity sort of skips around that as well merely explaining that matter curves space in a way that produces the effects of gravitation. It is internally and externally consistent but we are left with the fact that we don't really know why there is gravity or why gravity is unipolar. Or is it?

Fasttrack
09-14-2007, 12:35 AM
Right, thats where the higgs scalar boson, theoretically, comes in. I guess i should have more clearly emphasized that the standard model describes matter with quantities such as spin, quantum state, and etc rather than the more conventional "mass".

Since special relativity is as much of an integral part of the standard model as quantum mechanics i just thought it appropriate to differentiate between the two. The standard model is to quantum mechanics what newtons laws are to physics. Not interchangeable in usage, but definantly an integral part as you put it. I was looking for the term "integral" earlier but for some reason i just couldn't come up with it. Oh well, at least i'm not a writer!





p.s. sometimes i wish i could spell; sorry for any glaring errors. :)

<edit> Evan, you exaclty summarized what interested me in the super-symetric model. I was fascinated at how perfectly it was able to describe why something has mass, which is a really awsome thing to be able to explain, imo. Its like being able to explain what "happy" is!

guynamedbathgate
09-14-2007, 12:33 PM
perhaps they didnt realize not to slide the weight back onto the shelf. Always start with the simplest answer first. we will speculate about dark matter later.

dp
09-14-2007, 12:43 PM
I would consider quantum mechanics to be an integral part of the Standard Model today. Funny thing about it is that it describes matter in great detail but cannot explain why it is massive and has gravity.

General Relativity sort of skips around that as well merely explaining that matter curves space in a way that produces the effects of gravitation. It is internally and externally consistent but we are left with the fact that we don't really know why there is gravity or why gravity is unipolar. Or is it?

Because we don't know what or why gravity is we don't know what or why time is. But we've managed to find ways to use or waste some of each :)

halac
09-25-2007, 07:55 PM
Saw this article on the Space Daily website concerning the kilo standard.

http://www.spacedaily.com/reports/A_Better_Definition_For_The_Kilogram_Scientists_Pr opose_A_Precise_Number_Of_Carbon_Atoms_999.html