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Experts Suggest Replacing Definition of Kilogram
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."
They set it to 1000 grams.
...if the change it, what would happen if they would auction off the cylinder on eBay?
I'm going to finally lose some weight?
The Answer
1 litre of H2O at ATP?
The next thing you know they will be trying to get the US to switch from imperial units to the metric system....
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I'm going back to pounds and stones.
Pi is exactly equal to 3!
You might find some additional background information about this effort in an earlier Slashdot article about this topic, posted in May 2003.
I thought one cc of water weighs one gram. Thus one litre of water weighs one kg. Am I wrong? This would certainly satisfy the criteria of natural phenomena vs. artifact, although I suppose that definition gets a trifle fuzzy when we start talking about measurements like picograms.
The SI unit of mass is the kilogram, not the gram.
Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy, and of course, energy == mass * c^2
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Picture of the International prototype kilogram:
y pe.jpg
http://www1.bipm.org/utils/common/img/mass/protot
That would work fine, and I believe was the original definition. Unfortunately, pressure has a mass component, so your definition is circular.
Lisa: Principal Skinner, how's your transportation project coming?
Skinner: Not only are the trains now running on time, they're running on metric time! Remember this time people, 80 past 2 on April 47th. It's the dawn of a new enlightenment!
Just in case people care, here are the 7 base units:
... or something.
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, in my AS-Level Physics coursework, if anyone want s a simplifed read.
(Wiki)
I don't see how this topics is maths, by the way.
- Jax
The second and the meter have long since been based off of more fundemental measures. The second is defined as how long it takes for 9,192,631,770 cycles of microwave light to be emitted by the hyperfne transition of cesium-133 atoms. The meter is defined as the distance traveled by light in a vacuum in 1/299,792,458 of a second.
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You'd lose mass instead.
"[Regarding the 'cloud,'] ownership was what made America different than Russia." -- Woz
It is well known there are 7 base units.
"Candela essentially measures the same things as watts."
But watts are not a base unit. A watt is the same "Joules per second", and Joules is also not a base unit, but is defined as a Newton Metre. But a Newton isn't a base unit, it is defined as a Kilogram Metre per second per second. So:
Newton = kg.m.s^-2
Joule = Nm = kg.m.s^-2.m = kg.m^2.s^-2
So a watt is in-fact a kg.m^2.s^-3 , or "Kilogram metre squared per second per second per second" - hence changed the kilogram will change the watt, despite them seeming unrelated!
A mole isn't the same as mass at all. It is more to do with things on an atomic level. It's really used in chemistry - I've personally never used it outside of a chemistry exam (or coursework). It is sort of just a number, but it actually isn't.
Kelvin is a fundamental base unit too. Momentum is defined as "Newton Seconds", and so (remembering the definition of a Newton) kg.m.s^-1. Kelvin's measures temperature, which is a measure of kinetic energy, so I can see where you are coming from. You're just wrong.
Ampere is too. Helpfully, from it you can define other helpful things like volts. A volt, for your interest, is defined as kg m^2 s^-3 A^-1 , or "Kilogram metre squared per second per second per second per amp". And so yet another thing this change would affect.
It's all very interesting.
- Jax
They are cleaning the definition, not the value.
;-)
A new kilogram with equal an old kilogram. This will only make a difference to the history books and those who actually want to make thier own 'kilogram'.
I can imagine how many 'net savvy drug runners are looking at this and thinking, 'shit, I have snorted too much coke, does this affect my business? whats a planck? oh man, Avocado constant? [sic]
I say since the kilogram was an arbitrary measurement (in any definition) then why try and make it more formalised? I realised that celcius fit nicely with pure water at sea level freezing and boiling, and other measures have thier own basis (has the definitions have changed). Take my friend the meter. I always use the old skool definitions for rules of thumb.
Year Definition
1793 1 / 10 000 000 of the distance from the pole to the equator.
1795 Provisional meter bar constructed in brass.
1799 Definitive prototype meter bars constructed in platinum.
1889 International prototype meter bar in platinum-iridium, cross-section X.
1906 1 000 000 / 0.643 846 96 wavelengths in air of the red line of the cadmium spectrum.
1960 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between levels 2p10 and 5d5 of the krypton-86 atom.
1983 Length traveled by light in vacuum during 1 / 299 792 458 of a second.
So you see, a meter was the same in all these cases, but they just wanted to act clever.
The thing is, after world war 3, which measure will be easiest to revert to for a meter? trying to find scientist who can measure "Length traveled by light in vacuum during 1 / 299 792 458 of a second." or just comparing a brass stick with a length of wood while trying to build something using pre-existing specs (that you are relying on to build a post WW3 bridge).
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But a point mass should not be neccicary, the mass just needs to be perfectly sperical. So long as the distance from the center of the mass is greater than the radius of the mass + a few wavelengths to eleminate defraction, there should be no difference how the gravity field is generated. It simply needs to be uniformly constant for all points in every concentric shell larger than the mass. Am I right?
As technology to measure substances to great precision increases, its about time the kilogram got a redefinition as well, one not based on a single object.
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.
If you disagree, post your argument. (-1, Overrated) isn't your personal censorship tool for views you don't like.
How many Avogadroes are in guaca-mole?
I guess 6.02x10^23...
What you're talking about are "fundamental" units versus SI base units.
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.
A hectare is 2.5 acres, so there are 40 ares in an acre. An american pint is .475 L, very close to .5 L.
A German pound is 500 g, whereas american is 454 g.
THIS IS EXACTLY WRONG. In fact, this is why the imperial system is still in use - because it is largely base-12 in nature. Base-12 is far superior to base-10. The only advantage to SI metric system is that it is CONSISTENT in its conversions, where the imperial system is not. But using Base-10 for those conversions is a major headache, especially for bakers, carpenters, and anybody who has to frequently divide by 2, 3, or 4. The BEST system would be a metric/SI system that uses base-12. But I'm sure the Base-10 bigots will find reasons to disagree.
If you don't know where you are going, you will wind up somewhere else.
So why do you call your superior system "base-12" as in "base-(1*10^1+2*10^0)"? Here's why: Because base-12 masochists enjoy mixing base-10 numbers with base-12 units.
Baking is always cited as a reason for using imperial units. I enjoy making bread on a regular basis, and occasionally other baked tasty things, and let me tell you the Imperial system is virtually worthless when dealing with flour and to a lesser extent sugar and salt.
Any good bread recipe targeting the average American baker will usually state something to the effect of "3 to 4 1/2 cups of flour". That's not because the person writing the recipe didn't feel like specifying greater accuracy -- it's because volume is a horrible way of measuring flour.
To properly measure a cup of flour involves fluffing up the flour, gently filling your measure, and sweeping the excess off of the top. This is a pain and a mess -- you try effectively fluffing flour without creating a dust cloud, keeping in mind that this will often be done in a kitchen and that a cloud of flour is quite easy to ignite. Even when consistent measuring techniques are used the amount of flour in a given volume can vary considerably due to other factors, including the mill of the flour itself.
Generally, the experienced bread maker will start with an amount of flour that they know to be less than what they need, and work in extra flour as needed. This works with breads meant to be chewy, but as working the dough makes it more elastic this is less than ideal for more delicate items such as biscuits.
Things are further complicated when using volume to measure salt or sugar -- flake size can vary significantly, and the amount of variation in a tablespoon of salt (especially kosher salt, which has a generally large flake size and is easier to work with in the kitchen) can make a notable difference in the final flavor. Equal weights of sugars provide equal sweetening, but a cup of white sugar, brown sugar, and confectioner's sugar might weight 200, 220, and 120 grams respectively. Again flake size within the individual types of sugar varies from manufacturer to manufacturer. In addition to changing the texture and flavor of the product sugar affects the rise time and quality when yeasts are involved. Too much, and the product will expand too much, ruining the texture. Dough for breads is often worked, left to rise for flavor development, and then shaped into its final form and allowed to rise again. Too little sugar, and the dough's first rise may be its last, leaving you with a surprisingly dense and hard dough brick after baking.
By purchasing a quality digital kitchen scale, the baker can place the container to be filled on the scale, "zero out" the scale (which tells the scale to treat the reading it is currently getting as its "zero mass" point), and fill the container to the desired amount. This leads to easier conversion between various types of sugars, salts, and flours, including unintentional conversions when your particular brand of kosher salt has a larger flake size than that used by the person who wrote the recipe.
I just don't follow the justification that bakers have to commonly divide by 2, 3, or anything for that matter. Few people find, say, 1/3 cup by starting with a cup and dividing it into three parts. Even something such as filling a one-cup measure exactly halfway with flour or moleasses is quite difficult!
I'd say that it's just as easy given the proper tools, and more accurate in terms of the actual amount of the ingredient involved, to measure 50 grams of sugar as it is to measure 1/4 cup. In either case, nobody is dividing anything, simply using the proper measuring device. In the case of the recipe calling for 40 grams of sugar, though, the SI system has a clear advantage. The SI baker would add to the scale until it read 40g. How about 40 grams of sugar (80% of 1/4 cup) in terms of standard Imperial measuring devices?
Google calculator gives "80% of ((1/4) US cup) = 4
Somebody get that guy an ambulance!