Experts Suggest Replacing Definition of Kilogram 844
fenimor writes "The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon. International team of scientists suggest replacing the kilogram artifact -- a cylinder of platinum-iridium alloy about the size of a plum --with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms. They propose to adopt either one of two definitions for the kilogram by selecting a specific value for either the Planck constant or the Avogadro number."
The last time this was mentioned (Score:4, Informative)
You might find some additional background information about this effort in an earlier Slashdot article about this topic [slashdot.org], posted in May 2003.
Re:artifact (Score:3, Informative)
Re:Anyone Else? (Score:5, Informative)
Re:artifact (Score:2, Informative)
c = 299,792,458 m/s
This [nist.gov]
It's true that it was once defined that way, however, it has been redefined.
Re:artifact (Score:2, Informative)
Thus, the meter is not defined by a physical artifact.
Re:artifact (Score:5, Informative)
Metre for Length
Kilogram (what this article is about) for Mass
Second for time
Ampere for current
Kelvin for temperature
Mole for amount
Candela for "Luminous intensity"
All the others are built up and defined from these, so these must be well defined. Change what exactly a Kg is changed more than just mass - it changes everything dependant upon it. Hence, these things must be got right.
The definition of second changes every now and then though, and I think the metre has changed a few times, too. I wrote a bit about the second here [f2s.com], in my AS-Level Physics coursework, if anyone want s a simplifed read.
(Wiki [wikipedia.org])
I don't see how this topics is maths, by the way.
Density (Score:2, Informative)
The pressure part really kills using water as a definition, because it has a mass component. Circular definitions are a no-no.
Re:How about (Score:5, Informative)
Re:Anyone Else? (Score:2, Informative)
Re:Redundant definition? (Score:3, Informative)
See NIST Special Publication 811 (1995 ed.), _Guide for the Use of the International System of Units (SI)_ by Barry N. Taylor (NIST is the National Institute of Standards and Technology, the successor agency to the National Bureau of Standards):
In commercial and everyday use, and especially in common parlance, weight is usually used as a synonym for mass. Thus the SI unit of the quantity weight used in this sense is the kilogram (kg) and the verb "to weigh" means "to determine the mass of" or "to have a mass of".
Examples: the child's weight is 23 kg the briefcase weighs 6 kg Net wt. 227 g
Re:Hmm... (Score:3, Informative)
Huh? The units of Planck's constant are energy times time (eg., J s).
Re:1L of water == 1kg? (Score:2, Informative)
Re:What? (Score:2, Informative)
(N = kg m s^-2)
Force (Score:2, Informative)
Re:Just wait. (Score:3, Informative)
Bhutan is a devout BUDDHIST country, you hoser. And they have TVs now, although when I first read about that (more than 5 years ago), the Bhutanese were quite reasonably shy about appearing on it. Hence they had to cajole some poor sod to read the national news in a pained monologue. If you see some of the recent Bhutanese movies, though, it appears things are changing fast. I have no idea how they weigh their TVs and Buddhas, though.
On topic, I think it's great that they're using Avocado's Number to define the kilo. So let's see, 6.022 x 10^23 avocados would weight ~1.8 x 10^23 kilos, or 28 Earths (this measurement based on the standard platinum-iridium avocado that was shipped to me from Paris, which is significantly larger than a plum, but was free to ship, since any attempt to weigh it would cause fatal recursion).
Re:Hmm... (Score:3, Informative)
No. Planck's constant gives the amount of energy carried by (and hence gives a meaning to the momentum of) a photon of a certain frequency. Its units are Joule-seconds, which is not a unit of energy. Since the frequency of a photon can be arbitrarily low, so can its energy.
Re:How about (Score:1, Informative)
Re:And in other news... (Score:1, Informative)
Re:I suggest (Score:5, Informative)
There are two common systems of units, mks (meter-kilogam-second) and cgs (centimeter-gram-second). The mks system is now more often referred to as the SI. In the cgs system, the gram is a base unit. In any case, what you're referring to is utterly trivial and/or irrelevant when it comes to the real work of defining the units. Any definition of the gram suffices to define the kilogram, and vice-versa.
Because Avogadro's number is JUST an artifact of the definition of the (kilo)gram, not a fundamental constant - it's (been originally) defined as the number of atoms in 12 grams (or, whatever, 0.012 kilogram) of Carbon-12.
It's happened before that they've changed things around so that something different was considered to be the more fundamental quantity: the speed of light used to be a measured quantity, but now it has a defined value. The whole issue is that as techniques change, you want to base your system of units on the things that can be most accurately measured (and reproduced) with the latest techniques.
Now, basing the definition of the kilogram (might I suggest they also change that basic to gram instead of kilogram... please) on Planck's constant somehow would be a MUCH better ideea. However, the value of that constant [i.e. 6.6260693111111 * 10^-34 and so on] makes it pretty wierd to work with unless you multiply it with 9 [to get exactly 5.96346238 * 10^-33 which makes more sense somehow].
I'm not sure where the <joke> tags belong here. Anyhow, giving h a defined value would be very much like the step they took when they gave c a defined value -- they did it because when techniques changed to the point where c was one of the most accurately measurable things in nature.
Re:I wonder... (Score:5, Informative)
Re:My thoughts (Score:3, Informative)
Re:Just wait. (Score:3, Informative)
Also, we were the only ones sane enough to base our unit of volume/capacity on the cube of our linear standard (1 gal US = 231 in^3, as it's been since the 1800's or so). Both the British gallon and the SI liter both had ugly/cumbersome definitions involving a sample of perfect water at a given temperature, pressure and local gravitational acceleration (the French got the litre right from the first, but then they broke it, and then it was fixed again, meaning the definition of "liter" has changed by tens of microleters over the past century).
* These are actually numbers agreed upon by the foot/pound using world in 1959. Before that, in the US, the number of kilograms in a pound had more digits and a foot was 1200/3937 m.
Re:I wonder... (Score:5, Informative)
Re:May I be the first to ask... (Score:2, Informative)
Since photons, by defination, cannot go slower than the speed of light, the amount of mass they have is the amount of mass they would have if stopped. Or, to put it another way, it's the amount of mass they would have if you could measure their mass while traveling at the speed of light.
Which is why it's called 'virtual mass', as you can't actually do either of those. All you can do is calculate the mass from the energy of the photon. (Which, as another post pointed out, is set by the frequency.)
It works in a hierarchy (Score:5, Informative)
Where you actually need to use them directly, sure.
To give a real world example of how the standards work in practice... I used to write software for a company in the metrology (high precision measurement) business. They made machines that are used, for example, in quality control at the end of production lines. The gauges on the most popular machines gave accurate readings with resolutions of say 1-10m.
Those machines were calibrated from reference artifacts. These were themselves checked for accuracy on still higher precision equipment. (How they actually manufacture something so close to physical perfection is an interesting area in itself...)
Ultimately, there were white room areas with very careful decontamination procedures in place that were used almost exclusively for calibrating the company's most precise equipment and checking their reference artifacts.
From there, you were one step removed from the national standards laboratories. At that level the formal scientific definitions are just fine.
In other words, you work from major standards labs that can use the precise definitions effectively, and propagate the information (with some less, but little enough to be acceptable for the application in question) to more widely distributed testing facilities. A more trendy application of the same basic idea is the use of Internet-based real time clock services.
Grrr... Slashdot filtering... (Score:3, Informative)
Looks like we lost a mu in there somewhere: the resolutions for the popular machines were around 1-10 micrometres.
Re:Picture of the Kilogram Prototype (Score:3, Informative)
It's nested in several jars for redundancy.
Chip H.
Re:Just to state the obvious (Score:3, Informative)
Celcius is trickier than just temperatures of boiling and melting, because I think it must also declare the pressure too. The temperatures that water boils and freezes depends on air pressure. Kelvins are defined as divisions such that the range from absolute zero to the triple point of water is 273.16 kelvin. At least that doesn't depend on a the standard for pressure.
The problem with a meter standard depending on a physical object is that the length of that object changes with temperature. A temperature independent standard is needed. Standards that can be measured independently without having to refer to a specific single object are necessary for maximum accuracy.
Re:I suggest (Score:1, Informative)
Re:Picture of the Kilogram Prototype (Score:5, Informative)
This is the definition of the kilogram. A kilogram is not 1L of H2O at STP (as mentioned elsewhere, pressure depends on mass), it's this little lump of metal. Changes in the mass of it are extraordinarily bad. They make copies of it for reference purposes, and then check the copies agains the original every 10 years. If there's a disagreement, the copy gets adjusted, not the original. The reference lump has actually lost about 50 micrograms in the last 100 years (and no one knows why). That's a lot (well, speaking at the level that micrograms get used at... 1 microgram = 0.000000001 kg), and the really highlights the need for an immutable reference point.
Readers may find the pertinent Wikipedia article [wikipedia.org] interesting.
Re:Circular definition (Score:1, Informative)
Re:I suggest (Score:5, Informative)
One of the nice things about the British system of measurement (which pretty nearly only the Americans use officially, though with a few changes) is that the units are exactly the sort of thing you often want about one of. A pint of beer, a gallon of kerosene, a bale of hay, a pint of milk if you live alone or a quart or a gallon depending on the size of your family, half an acre of land, etc. (yes, yes, I don't think a bale is an Imperial measurement).
The metric equivalents never seem to be just right, but we'll just have to live with them
But thats true for the metric system as well
In german we have "pound" as well, which is just slightly bigger than yours. And ppl in shops still buy "half a pound" of meat or something.
Same for land, we have an "ar" and a "hectar" which is obviously 100 ar, and we have a "morgen" wich is 25 ar and the typical size of a field in older times.
A ar is similar big as an acre (IIRC).
Same for drinks, who cares about your pint? Do you really think we order 350ml Beer?
We order a glass of beer, obviously. And depending on beer brand it is served in a typical size.
The sizes are: 0.2l for Kölsch and Alt. 0.3l for some kins of "Pils" which consider themslelf noble. 0.4 for a standard everywhere pils,a nd your pint is just between 0.3 and 0.4. The enxt size is 0.5l for Weiten.
The same applies for nearly any metric size, no one is buying xyz litres or something except he buys 40l gasoline for his car.
Bottom line we have as many "human" metrics as you but sine the metric system is in use they got rounded to the next best number.
angel'o'sphere
Re:artifact (Score:5, Informative)
In a fundamental system of units, there are three base units: charge, mass, and angular momentum. (Gee, those sound suspiciously like the three properties that a black hole can possess - I wonder why). Everything else can be derived from those units (for the most part - we'll ignore stuff like baryon number, lepton number, etc. because those theories aren't complete yet. For instance, we now know that only global lepton number is conserved, not mu, e, and tau lepton number separately. I won't even touch color, as color is completely hidden anyway).
In fact, the existence of those units can be derived from the fact that space is invariant under the Poincare group, and has gauge symmetry.
However, those base units come because you've defined other constants to 1.
The problem is that several of those constants are imprecise and difficult to measure. It is easier to define a kilogram, for instance, then it is to somehow base it on the gravitational attraction of two objects, because G is horribly imprecise.
Similarly, it is easier to treat Kelvin as fundamental rather than derived from other units *if* Boltzmann's constant has poor precision.
So while it's *possible* to use fundamental-based units, it's often *impractical* and less precise. The base units in SI are those that can generate all other units with no loss in precision.
To give a very practical example, the mass of a proton is typically given in atomic mass units (amu) as ~1.007 amu. You might think that it should be given in grams, as "amu" isn't a fundamental unit of mass. But the conversion from "amu" to "grams" is less precise than the mass of the proton in atomic mass units. So in this case, "amu" would be appropriate as a base unit, as well as mass, even though the two can be directly converted.
The benefit is that you can compare the mass of a proton and the mass of a neutron in "amu", for instance, to better precision than you could in grams. It's similar (or was similar when SI was developed) with the other units.
Re:How is the US pound measured? (Score:3, Informative)
Those who use pounds as force use slugs [wolfram.com] as the unit of mass. Same relationship as mass in kilograms and weight in newtons (i.e. Newton's 2nd Law), except for the weird-ass numbers.
Just how many hogsheads are there in a fortnight, anyway?
...laura
Just a few corrections (Score:3, Informative)
Re:I suggest (Score:2, Informative)
Re:Obligatory Simpsons Metric Quote (Score:3, Informative)
Re:artifact (Score:3, Informative)
Candela essentially measures the same things as watts.
Mole is just an number. It might be used in the definition of the kilogram, but in itself, it just relates the mass of a gram with 1/12 the rest mass of a carbon-12 atom.
Kelvin is just a unit derived from mass, momentum, and kinetic energy. It is not a base unit.
Ampere might or might not be a base unit, I'm not sure about that one.
You are talking about base units of physics (and you're still very wrong there), not base units of measurement.
Take Kelvin, for instance. We'll ignore the fact that temperature really relates both energy and *fundamental statistics* (the temperature of a gas of fermions at a given temperature is different than a gas of bosons at a given temperature). But even if it didn't, and it was just "average kinetic energy over Boltzmann's constant", you could say that Kelvin is just inverse joules...
if you set Boltzmann's constant to 1, and have it be unitless. The problem is that you've now shifted any imprecision of measuring Boltzmann's constant into *all measurements of temperature*, rather than just keeping it in the connection between energy and temperature. So when you calibrate your new temperature scale in "inverse joules", you now face the same precision problems that you would face in measuring Boltzmann's constant. That is, you have to measure the average kinetic energy of an ideal gas, and label that on your "inverse joule" thermometer.
This is dumb. Of course, what you do is use Kelvin as a base unit, and *define* the scale using other processes (the triple point of water, if memory serves) and now you've got a perfectly calibrated scale to huge precision, and the only imprecision from measuring the Boltzmann constant comes when you want to convert to energy.
So, again - base units of measurement are not the same as base units of physics. The base units of physics are the fundamental quantum numbers of a particle, mass, charge, spin (and color). The base units of measurement are the SI units.
Re:I suggest (Score:2, Informative)
Why? The meter started as a platinium bar, and later we redefined it to something else, that matched this platinium bar. Why can't we do the same with kg? We're only trying to base it on something natural phenomen, not to redefine it. The point is that if it is a common criteria, and nnot a single object, anyone can measure a kg _exactly_
Re:Mod Parent Up (Score:2, Informative)
Said by Arthur C. Clarke [wikipedia.org]
Re:artifact (Score:3, Informative)
The candela is a weird unit, but it is not equivalent to watts. There are three units related to light:
- lumen -- comparable to watts, but weighted with a defined sensitivity curve that is supposed to represent the response of an average human eye;
- lux -- lumens per square meter
- candela -- lumens per steradian (unit of solid angle). It represents brightness, i.e. how bright the light source looks if you look into it from a specified distance.
For some reason the candela was chosen to be the base unit, rather than the lumen; probably because it is easier to calibrate for. The sensitivity curve is rather arbitrary. It is fundamentally impossible to measure this curve with high precision since individual humans are different and it requires test persons to judge subjectively whether, say, a red and a green light source are equally bright.Since these units are defined to some hard-to-measure property of the human body, I think they shouldn't have a status as an SI base unit. Inches and feet don't have that status either, after all.
Re:I suggest (Score:2, Informative)
Salami in decagrams (Score:1, Informative)
And buying butter in chunks of 500 g is quite normal.
I cannot speak for all Euro nations, as I was living in only the one, but it's a good guess that it wasn't the only one that sold its deli items in this way.
Re:I suggest (Score:3, Informative)
Technicalities of mass measurement (Score:3, Informative)
The main reason for platinum-iridium is that it's got a very low thermal expansion coefficient. Basically, it doesn't expand or contract much with change of temperature. However, densisty is also important. Don't ever ask a metrologist that old chestnut about which is heavier, a kilogram of lead or a kilogram of feathers, unless you're willing to sit through a few hours of lecture on buoyancy. Yup, it's not just for water and hot air balloons. A denser object of the same mass will weigh slightly less (assuming uniform shape and all that), as it will be slightly less bouyant in the air.
As for your comment regarding a smaller object being less accurate due to relative scale of dust, a smaller mass is also slightly less prone to the influence of the variability of the gravity constant across the Earth's surface. *wry grin* There are a lot of factors you have to deal with when you start working on the scales we do here. And that's not even getting into the gage blocks (length measurement) which have surfaces so smooth that they form a vacuum when touched together, and will spot weld to each other if left overnight...