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Kilogram Gets Controversial; Why Not Split the Difference?
gbrumfiel writes "As Slashdot has noted, the kilogram has a problem. The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France. But the physical object appears to be getting lighter. Scientists want to replace the cylinder with a new standard based on Planck's constant, but two experiments designed to facilitate the switch keep coming up with different results. Now one researcher is proposing a solution: just average the two diverging experiments and use that value as the official definition. Not everyone thinks that averaging the two amounts to sound research: 'Deciding to just average these two results would be perfectly proper mathematics, but it would not be science,' says Michael Hart, a physicist at the University of Manchester, UK."
The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has. The obvious conclusion is that the rest of the universe is getting heavier.
When our name is on the back of your car, we're behind you all the way!
A physicist, engineer and a statistician are out hunting. Suddenly, a deer appears 50 yards away.
The physicist does some basic ballistic calculations, assuming a vacuum, lifts his rifle to a specific angle, and shoots. The bullet lands 5 yards short.
The engineer adds a fudge factor for air resistance, lifts his rifle slightly higher, and shoots. The bullet lands 5 yards long.
The statistician yells "We got him!"
If Slashdot were chemistry it would look like this:Cadaverine
Let them eat pounds!
Particles, stuff that matters.
It turns out that France imposed a Mass Tax in the last few years which means the cylinder has to cough it up for the good of the state.
On the plus (or more like the non-plus) side, the people of France are now looking fit & trim.
"There is more worth loving than we have strength to love." - Brian Jay Stanley
Why don't they just take the weight of a gram and multiply it by 1024?
Sheesh, evil *and* a jerk. -- Jade
In this case the background is that the standard for mass, unlike time or distance, cannot independently be constructed in the lab. This means that science and industry are susceptible to two issues. The first is degradation of a physical standard, in this case a hunk of metal in France. The second is that one is dependent on other to create proxies of the standard, and as a result have no true assurance of the accuracy of the standard. A suitable lab with suitable personal can masure time and distance without the need of a proxy manufactured by others, and no dependence on a fixed physical object.. There is a desire for the same to be true for mass.
Second, no one knows if the hunk of metal is shrinking, and if it is how much it is shrinking by. If the experts knew it was shrinking, then they could figure out how to at least partially correct it. The hunk of metal might not be charging at all, or it could be accreating matter. Without an independent standard, which does not apparently exists, as everything is based on the hunk of metal, all there is is guesswork.
The third is the idea that Planck's Constant is being used to create the standard. In fact Planck's constant is one two approaches. The other is to create a sphere from a silicon and use Avagadro's Constant to define the mass. The problem is that these two approaches do no lead to consistant results, with an error about an order of magnitude large than the expected error.
The issue with averaging is that while one does average within a result, and even results that are taken from similar procedures, it is unclear that averages in this case is suitable. It seems to me that the results point to an interesting area of research, and rather than just averaging, more work should be done understanding the inconsistency. If it is not random error, and not an artifact, then something really fascinating might be going on.
"She's a scientist and a lesbian. She's not going to let it slide." Orphan Black
Speaking as an experimental physiscist
ahem. 175parts per billion is 1.75e-7. For metrology that is a huge discrepancy. What is worse is that the measurements themself are a factor of 5 better, leaving no room for error.
For experiments where the physicists believe they understand them this is unacceptable, because it actually means the pysics of at least one method of both is not well enough understood, i.e. you have a systematic error. If the physics is not well understood then you don't know if the systematic error will be constant.
If the measurement will not be constant then the average will also not be constant. So an metrology institute where a reference weight should be define will need both methods and still not get any stable definition.
If you already need to afford both methods, then you can create reference weights and at the same time check if the difference between both methods is the right one and constant at your place.
Important rule in experimental physics: NEVER average over systematic mistakes. Average over random results. On systematic mistakes, the word average makes no sense
A gram is not the mass of 1 cubic centimeter of water. It is 1/1000 of the weight of that lump of metal in france!
There are a ton of posts above arguing over that, and you can't use that to define mass because it is affected by pressure. Pressure has a mass component so it ultimately becomes circular.
And that's the crux of the issue. Both results should be the same within the margin of error. The fact that they're not either indicates that the methodology is off or we simply don't understand the underlying physics well enough. I'd imagine it might be a bit of both (that a lack of understanding on the underlying physics results in incorrect methodology).
To that end, the fact that a scientist is trying to sort of whitewash the discussion to get out a set definition of a kilogram is disturbing to me. It'd be, in my opinion, possibly in the same vein as redefining light to avoid the clear particle/wave duality. If anything, I would hope that this discrepancy of results would spark even more research because it opens up a great opportunity to better understand our universe. Worrying about setting the definition of a kilogram seems a bit more moot, especially if it turns out that such a thing is impossible using the tools being suggested (ie, that one of the metrics isn't cosmologically fixed).
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Mass bends space-time, right? So why not define it as a certain amount of curvature - say the mass needed to bend a light beam in vacuo by some measurable amount, divided by a chosen constant to give 1kg according to the theory.
Math is: When there's this room... with only one person in it... and then two people leave that room... now you have to wait until another person goes back in before it's actually empty.
It is a definition with a physical representation. Which is obviously showing changes over time.
And it really shouldn't as it is the physical representation that is being actively used by our society - not the definition.
The point of this article is that they are trying to create a new definition based on a process that would produce an identical physical representation EVERY time the process is run - and the results of which wouldn't change over time.
And failing.
Now, as every scale in the world is NOT calibrated to that prototype kilogram, but to a copy, of a copy, of a copy... Those errors accumulate.
Until one day measurements of some toxic substance/medicinal drug/anything requiring milligram measurement start being significantly lighter/heavier than they should be in the given sample.
Cause we're not talking homeopathy here.
Mit der Dummheit kämpfen Götter selbst vergebens
Averaging the results of two experimental measurements is not Science, whereas averaging the results of many hundreds of measurements to determine global temperature anomaly is.
Averaging the results of different experiments that consistently give different answers is not correct. Averaging multiple measurements of the same experiment is.
How about an analogy?
You have two rulers, which do not agree. If you measure something with both and then average the result, you get a wrong result - unless the two rulers happen to be out by the same amount in different directions. If one ruler is correct, you've just broken your result.
If you use one ruler then you can average the measurements you take with it, because it's a fair assumption that each measurement will be out by a random amount in a random direction. Then your average is as correct as your ruler.
I'm not at all sure you are serious, but enough people seriously hold this opinion that it is worth responding.
A good system of units needs:
1) Base units which are well defined and independently reconstructible (i.e. a suitably equipped lab can calibrate their equipment purely from the definition of the units.)
2) Logically constructed compound units (e.g. units of force are derived from the units of mass, time and distance.)
3) Logically constructed convenience units (e.g. kilometres for use for distances which would be an inconveniently large number of metres.)
4) To be widely used.
The initial choice of your base units is largely arbitrary - whether it was a from a not-very-accurate measure of a king's foot size or from a not-very-accurate measure of the Earth's circumference. Item (1) can be satisfied equally well (or, in the case of mass, badly) by the metric or imperial systems. The definition of the metre has long since changed from the size of the Earth to quantities measurable in a lab (as has the definition of the foot.)
The SI system (based on metric measures) beats the imperial system hands down on items 2 and 3, and because of this now has a large advantage also on item 4.
Item 2: In Imperial you might measure (heat) energy in BTU and mechanical energy in some mixture of foot-pounds-seconds, but then you need a conversion factor to compare the two. Such conversion factors are never needed in SI.
Item 3: Imperial also messes up the convenience units by having lots of weird conversion factors (e.g. an acre is (I think) a furlong by a chain. How many square feet is that? How many ounces in a ton?*) Metric uses convenience units constructed from base units via consistently named factors of 10 or 1000.
You can't use the current problems with the kilogram as a reason to prefer imperial to metric, as imperial will be just as prone to exactly the same problems. The (UK) Imperial pound is similarly defined by the mass of a unique artifact. In the US, it is defined relative to the kilogram. Mass is the last base unit which doesn't satisfy requirement (1), and the efforts to fix this are what has triggered this entire debate.
One could go a step further, and define your fundamental units in terms of fundamental physical constants (i.e. the Plank mass, Plank time and Plank distance, charge on an electron, etc.) In such a system of units, the speed of light is 1, the formula for the energy of a photon doesn't need a constant in it etc. In practice, we can't use such a system, because we can't measure (in particular) the universal gravitational constant G with sufficient accuracy. Every time we got a better measure of G, our entire system of units would need to be updated. (I.e. with current technology, this system can't satisfy requirement (1) above.)
* And how many different sorts of ounces and tons are there? It is quite a few.
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