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."

I suggest

[Anonymous Coward]

They set it to 1000 grams.

Re:I suggest

[tibike77]

Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?

Actually, if they happend to re-define it based on Avogadro's number, they might as well just say the GRAM is the new "basic unit" and the kilogram is just 10^3 grams.
Why ?
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.
Talk about circular references then...

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]. And even then it won't satisfy some people, as I'll bet you'll hear that 0.111111 and so on *9 does not equal 1 :p
Not only that, but Planck's costant was ALSO measured "accurately" using the kilogram unit as reference.

Ok, this actually does give me a headache.

Re:I suggest

[bcrowell]

Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?
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 suggest

[hawk]

Either way, as long as we do it quickly, before it's too late.

After all, an ounce of prevention is worth a pound of cure.

hawk

Re:I suggest

[Madcapjack]

After all, an ounce of prevention is worth a pound of cure.

I'm sorry, I don't understand. Could you rephrase that in terms of Avagrado's number, please?

Re:I suggest

[bcrowell]

To a scientist or engineer it is trivial, however to a (European) cop, or to someone buying butter it is not so trivial.
The topic of the article is only relevant to scientists -- to a very, very small set of scientists who do certain types of high-precision work. The redefinition of the kilogram they're talking about would be utterly inconsequential to everybody else.

Re:I suggest

[sholden]

To a scientist or engineer it is trivial, however to a (European) cop, or to someone buying butter it is not so trivial. Reporting the perp's weight in grams would not be sound practice. For everyday use the base unit needs to be visualisable/imaginable on a human scale.

Half a kilo of butter, or a pound of butter is a reasonable purchase. Grams just don't cut it. What am I getting if I ask for 80 grams of salami? Well I guess I can visualize it and some Europeans buy it that way, but the average everyday user of a measuring system is nearly innumerate. They want to buy one or two or maybe a half of something.


Do you think European cops say "I'm in pursuit, west bound on Main, at 33m/s"? Or do you think they might stuff using base units and say 120km/h?

Do you really say things like "It's a 100000m drive" and "I'll meet you there in 2700 seconds"?

Re:I suggest

[ghjm]

Okay, then I'll meet you there in 2.7 x 10^3 seconds.

-Graham

Technicalities of mass measurement

[SeanDuggan]

Lucky you, you've got a bona-fide metrologist [wikipedia.org] replying. Admittedly one who specializes in automated calibration of electronic instruments, but we get the basic lectures on dimensional analysis too.

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

Re:I suggest

[angel'o'sphere]

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 :D You only dont learn the "special" units in school I asume.

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

Just a few corrections

[Alexei]

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.

Re:I suggest

[kamapuaa]

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. noble.

You buy beer by the .2L??? Germany is a nation of girly-men!

Re:I suggest

[Sique]

No. No one orders Kölsch as "0.2l of Kölsch". It's rather "Bringen Sie einen Meter!" (Serve a meter!), which refers to a wooden bar of roughly three feets (1 meter) length with holes, where a glass of Kölsch is put in every hole.

Re:I suggest

[danila]

There is nothing in SI that prevents humans from using it. In Russia the "pollitra" (literally half-a-litre) is a very common unit of volume. So common, in fact, that, when buying vodka, people would often buy two pollitras instead of 1 litre. :) Also, units such as "nol-dva" (zero-two) and "nol-tri" (zero-three) are very usable to refer to 0.2l and 0.3l volumes respectively.

Similarly, the common size of a land plot is "shest sotok" (six hundreds), meaning, of course, 600 square metres. And most people see

Re:I suggest

[1u3hr]

Half a kilo of butter, or a pound of butter is a reasonable purchase. Grams just don't cut it. What am I getting if I ask for 80 grams of salami? Well I guess I can visualize it and some Europeans buy it that way, but the average everyday user of a measuring system is nearly innumerate. They want to buy one or two or maybe a half of something.

The whole reason we (countries that use rational measurement systems)have standard prefixes is that we can use appropriate units and avoud huge integers or fraction

Re:I suggest

[abb3w]

What am I getting if I ask for 80 grams of salami?

Lunch?

Re:I suggest

[lgw]

The furlong-firkin-fortnight system is the one true system. All other systems are silly.

Any fool can see what faction of an acre is a rectangle bounded by a furlong and a chain, or measure speed intuitively in millifurlongs per microfortnigt. This metric system is just unintuitive.

I know just how many furlngs per firkin my car gets, what the heck is that in litres per meter?

Re:I suggest

[brianosaurus]

Makes sense. With a redefined kilogram, Avogadro is really out of a job, since his number will become useless.

Do you think the new constant will retain the name, or will it be "The New Scientist Constant Relating Atoms to Grams brought to you by BASF, the Chemical Company"?

By the way

[iced_773]

How many Avogadroes are in guaca-mole?

I guess 6.02x10^23...

Re:Earth Theory!

[jericho4.0]

That worked for a while, but the inaccuracies intoduced by getting exactly 1000 cm^3 water at exactly 289K (plus the correct air pressure) were limiting the measurments we could take.

Still, the intuitiveness of it is nice.

Re:I suggest

[tibike77]

It pretty much depends on what you understand under "infinite" ;)
Given the fact that you are in bound by the laws of physics which state that on a certain (very low) level you have a certain uncertainty (sic) when you reach into "quantum level", one could argue that, in fact, 0.1111111...1111[something] for as many times you can until you hit that treshold times 9 does indeed NOT equal 1.

Re:I suggest

[Esion Modnar]

Better yet, make it 1024 grams. Make it consistent with the kilobyte.

I wonder...

[elid]

...if the change it, what would happen if they would auction off the cylinder on eBay?

Re:I wonder...

[grazzy]

Educated guess: It'd be the most expensive thing the size of a "plum" made of platinum-iridium ever sold on eBay.

Re:I wonder...

[jacksonj04]

No, there is one definitive 'Kilogram' which is kept in Paris, and then copies are made and shipped worldwide to save countires having to go to Paris to check their official weights. The copies are then compared to the one true kilogram every 10 or so years (dependant upon whether it's being used for a quest to save mankind at that point).

Re:I wonder...

[Anonymous Cowpat]

more importantly, what would they list it's shipping weight as?

Re:I wonder...

[einhverfr]

more importantly, what would they list it's shipping weight as?

That's easy....

Formerly 1Kg.....

Re:I wonder...

[iCEBaLM]

The weight formerly known as kilogram.

Re:I wonder...

[StarManta.Mini]

2.2 pounds, of course. :)

Re:I wonder...

[Talez]

Given that the kg prototype has lost 50 micrograms over the last 100 years I'm guessing 0.999995kg?

does this mean

[zerkon]

I'm going to finally lose some weight?

Nope, sorry

[mrchaotica]

You'd lose mass instead.

Re:Nope, sorry

[MagicDude]

Presuming that he's never going to leave the surface of the earth, the two can be pretty much interchangable, hence the commonplace conversion of 2.2 pounds to a kilogram.

How about ...

[canwaf]

1 litre of H2O at ATP?

Pressure

[XanC]

That would work fine, and I believe was the original definition. Unfortunately, pressure has a mass component, so your definition is circular.

Re:How about ...

[InfiniteWisdom]

The point is to have something that you can define just by counting some phenomenon or natural objects. For example a second is defined as:
"the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom."

and a metre is defines as:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Unfortunately, there hasn't been a good way to count the number of atoms with any kind of precision, so that has precluded a good definition of the kilogram so far. Maybe now the physicists can actually count atoms accurately enough.

One could define it as the mass of some number of H2O molecules, but maybe its easier a measure a quantity of light or to count some larger atoms.

Re:How about ...

[Dizzle]

Now, I know those definitions are techincally correct, but who thinks these ideas are easily applicable? I mean, the point of having a definition is to be able to calibrate everything else, right? So how on earth is a watch manufacturer or repair person going to say "alright, the cesium atom vibrated 9,192,631,769... 9,192,631,770 times. That's a second."

Is there actually a method of directly using these definitions?

Mod Parent Up

[Gabrill]

Someone smart famous once said "Any technology, sufficiently advanced, is indistinguishable from magic." These fundamental definitions are following the same path. Superbly and unarguably accurate, but also completely incomprehensable for anyone that doesn't have half a million dollars worth of sophisticated technology.

It works in a hierarchy

[Anonymous Brave Guy]

Is there actually a method of directly using these definitions?

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...

[Anonymous Brave Guy]

Looks like we lost a mu in there somewhere: the resolutions for the popular machines were around 1-10 micrometres.

Re:How about ...

[bleckywelcky]

Well, your watch manufacturer is simply producing watches for the public. To an individual person it really doesn't matter if they are ahead or behind in their day by 15 or 20 seconds ... maybe even +/- 5 or 10 minutes. So to them, a watch that can hold time to within 5 or 10 seconds over the course of a month or so is just fine. I'm not sure how accurate a quartz watch can get, maybe it's even more like 5 or 10 seconds over a year or so?

Anyways, it's one thing for a watch manufacturer to achieve a certain

Just remember that they replaced the meter

[gotr00t]

Before laser technology, the meter was defined as the distance between two markings on a bar of platinum-iridium kept in Paris. It was after Michelson invented his interferometer that the meter was redefined as 1,650,763.73 wavelengths of a certain orange line in the spectrum of krypton-86. This was later redefined in the 80's to be in terms of C, the speed of light.

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.

Re:How about ...

[Geoffreyerffoeg]

Don't you mean 1 L H2O at STP?

Adenosine triphosphate doesn't have much bearing on the mass of a quantity of water, even though it does provide biochemical energy to the physicists who're measuring it. :-)

Just wait.

[jwcorder]

The next thing you know they will be trying to get the US to switch from imperial units to the metric system....

Re:Just wait.

[bobscealy]

I have always found it intriguing that the US both celebrates the day of its independance from the British empire and continues to use old British units of measurement.

Re:Just wait.

[Guppy06]

Depends how you look at it. We call the things we use "pounds" and "feet," but we abandoned the flawed British standards in the 1890's (their yard shrank, their pound leaked), instead basing them on the SI standards. A pound is defined as 0.45359237 kg and a foot is 0.3048 m.*

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/cumbe

Re:Just wait.

[Anonymous Brave Guy]

Why bother throwing out something like a system of measures? That would just be childish.

We've been trying to tell you young upstarts the same thing ever since you forgot how to use the letter "u". :-)

Re:Just wait.

[Mycroft_VIII]

That actually depends on how you define 'english'. If you define it simply as the language spoken in England, them by definition anyone there speaks it rather well.
However some mean a specific form of english, and in this particular conext (how well Americans speak it) they're often refering to how closely to the english of the time when Europeans, especially British, first settled the americas in the 1500-1600's. In that case it's my understanding the honors go to a subsection of America (or at least

Re:Just wait.

[ajdavis]

!!

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 t

that does it

[WormholeFiend]

I'm going back to pounds and stones.

And in other news...

[rollingrock]

Pi is exactly equal to 3!

Re:And in other news...

[Talez]

<Frink> PI IS EXACTLY 3!
*Gasp from scientific community and silence*
<Frink> Sorry it had to come to that people.

Re:And in other news...

[Gabrill]

No, only moderately large values of 3.

Re:Mmm... 4?

[Exatron]

That said, my favorite rational flavor of Pi is 355/113.

My favorite is apple.

The last time this was mentioned

[tepples]

You might find some additional background information about this effort in an earlier Slashdot article about this topic [slashdot.org], posted in May 2003.

How about

[TheKidWho]

Replacing the second while your at it, and the meter! Units based off of the earth.

Re:How about

[be-fan]

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.

Redundant definition?

[Resound]

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.

Re:Redundant definition?

[darkmeridian]

Sure, a gram is defined by a volume of water at a certain pressure and temperature. However, this is impracticable in many settings. Water changes density very readily. It is much simpler to define a gram in other terms that is close enough to the 1.0 g/1.0 ml H20 yet still is stable enough to use in experiments. From the article:

For instance, it would improve the precision of certain electrical measurements 50-fold and would enable physicists to make more precise calculations in studying the fundamental

Re:Redundant definition?

[Fnkmaster]

We generally use the verb "weigh" to express units of mass, because there is no commonly used verbal form for mass. There is nothing inherently wrong with this, it's just a vagueness in the meaning of the word "weigh".

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 espec

Re:Redundant definition?

[dont_think_twice]

As far as what the NIST says, that is baloney

I had been wondering if NIST was just pulling these so-called standards from their ass. I am glad you confirmed it. From now on, a kilo is the amount of coke a mexican hooker can snort in one week. Or course, that is under STP.

Re:Redundant definition?

[Alsee]

Or course, that is under STP

Won't work, your refference subject is going to be far higher than sea level.

-

Hmm...

[ProudClod]

Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy, and of course, energy == mass * c^2

Re:Hmm...

[rangek]

Planck's constant would be a very elegant solution - it being the smallest possible quantity of energy

Huh? The units of Planck's constant are energy times time (eg., J s).

Re:Hmm...

[Pseudonym]

Plank's constant is a single Quanta of energy.

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.

Picture of the Kilogram Prototype

[pboyum]

Picture of the International prototype kilogram:

http://www1.bipm.org/utils/common/img/mass/prototy pe.jpg [bipm.org]

Re:Picture of the Kilogram Prototype

[spectasaurus]

Better question. Why is it in 3 vacuum jars?

Re:Picture of the Kilogram Prototype

[ink_13]

To keep its mass from changing. You may notice the calipers for handling it in the picture, too. Stray moisture, direct sublimation into the atmosphere, anything that could possibly affect it has to be kept away,

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:Picture of the Kilogram Prototype

[ozbird]

To prevent contamination (moisture, hydrocarbons etc.) that may affect its mass. Even so, the Kilogram Prototype has lost some mass (50 micrograms as of 2003) for reasons unknown.

There is an international effort to come up with a new standard extremely precise silicon spheres - the cool thing about them is it is apparently impossible to tell if it is stationary or spinning unless you have a reference point on the surface (e.g. a speck of dust.)

Re:Picture of the Kilogram Prototype

[chiph]

It's to prevent it from collecting dust, which would change the weight.. err, mass. Corrosion prevention might be a goal, too (yes, the alloy is corrosion resistant, but not entirely corrosion proof.) I don't think they're worried about evaporation, though (all substances outgas to a certain degree, some more slowly than others)

It's nested in several jars for redundancy.

Chip H.

Obligatory Simpsons Metric Quote

[shiafu]

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!

Re:Obligatory Simpsons Metric Quote

[tehshen]

My friend and I tried to use metric time once. It is a lot harder than it looks (7.50 o'clock news anyone? Church mass at 3.75?) Metric time is useless if not enough numbers divide into ten.
Attempts of a JavaScript clock are here [bazzle.co.uk] if anyone is interested.

Re:Obligatory Simpsons Metric Quote

[amliebsch]

"metric" != "base-10"

"or the mass of a fixed number of atoms"

[physicsphairy]

Well, count the number of atoms in the platinum-iridium alloy, and voila! You have your new definition! (without having to fuss with the traditionalists)

Why the motivation for the change? The mass of subatomic particles have been given in kg for over a century. What exactly needs a more precisely reference of measurement? Physicists use their own units when it's convenient anyway. . . .

Re:"or the mass of a fixed number of atoms"

[physicsphairy]

And how do you propose we do that? That's exactly why the article was talking about using x-ray crystallography, but you just can't do that on a 1kg block of iridium-platinum alloy.

Why use its known mass, of course! Then divide by 0.198078 (if it were pure platinum)/6.022E23 and take the nearest integer.

It doesn't really matter that there might plausibly be more or less atoms. Just find a number, suitably truncate it, and declare all subsequent decimal places to be zeros.

Isn't this exactly what happe

Bah

[jlechem]

"My car gets forty rods to the hogshead, and that's the way I likes it!"

A little offtopic but still revelant ;-)

Finally...

[Equuleus42]

I can rest at night, not thinking about plum-sized cylinders of platinum-iridium alloy.

Speed of light changes

[alw53]

What happens when the speed of light changes? [newscientist.com].

Meh ... how 'bout

[hot_Karls_bad_cavern]

Meh, how about changing the size of a pint!? Huh? Yeah, who's with me on this one? i could certainly go for pints being larger ... 'specially around lunch time :)

My thoughts

[Raul654]

This actually came up in my high school physics class a few years back. Since then, I've given it some thought, and my best guess was to define a kilogram in terms of the deflection of a beam of light under the influence of gravity over a given distance. In other words, define it in terms of the deflect of a beam of light passing a kilogram point charge at a certain distance.

Re:My thoughts

[geekyMD]

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?

Re:My thoughts

[Hollins]

Yet check out the definition of an ampere [bipm.org], which involves "parallel conductors of infinite length, of negligible circular cross-section, ...".

This would seem to constitute a physical reference that is impossible to generate.

Just to state the obvious

[tod_miller]

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). ;-)

Re:Just to state the obvious

[Jeff DeMaagd]

The definitions are done for scientific accuracy as the ability to measure improves.

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 ph

Why not gravity?

[clambake]

Why not define it in terms of gravity? i.e. 1kg of mass is equal to the mass of a perfect sphere of platinum that can accelerate from rest another equally sized perfect sphere of platinium placed 1 metere away by X m/s?

Re:artifact

[rokzy]

the 'meter' isn't a unit. perhaps you're thinking of 'metre'?

Re:artifact

[JaxWeb]

Just in case people care, here are the 7 base units:

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" ... or something.

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.

Re:artifact

[JaxWeb]

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.

Re:artifact

[barawn]

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.

Re:artifact

[barawn]

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 Kelv

Re:artifact

[hankwang]

Candela essentially measures the same things as watts.

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 som

Re:Anyone Else?

[TheEternalVortex]

The SI unit of mass is the kilogram, not the gram.

Re:Anyone Else?

[Tim C]

That's true, but that doesn't answer the question. They didn't pick a lump and say "whatever the mass is of this will be the SI unit of mass!".

The question is, all the other units are "base" units, while the SI unit of mass has a (kilo) prefix; how come?

Re:News Why?

[InfiniteWisdom]

So anytime anyone does anything that "makes sense" is no longer newsworthy? For instance, if congress were to repeal the Patriot act or the DMCA that would not be newsworthy to you?

Re:Nice but...

[piquadratCH]

This kind of idea pops up every so often, usually doesn't pan out since it's too hard to get everyone to change.

They only want to change the definition. One kilogram will still be one kilogram, no matter how exactly it is defined.

Re:How is the US pound measured?

[spaceyhackerlady]

Pounds measure weight, my friend. As in the effect of a gravitational field on a certain mass.

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

Re:It's all about the Bases

[amliebsch]

The only reason that the metric system is better is because it uses base-10 to convert between units

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.

Re:It's all about the Bases

[Markus Landgren]

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.

Re:It's all about the Bases

[hankaholic]

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.

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

Re:A kilogram is not abitrary.

[vidarh]

You're wrong. 1 gram is defined as a thousandth of a kilogram, and a kilogram is defined as the mass of the kilogram artefact.

The definition was originally that a kilogram was the mass of one litre of pure water at 4 degrees Celsius and standard athmospheric pressure, but that is a circular definition, as the definitions of the SI units for pressure depends on mass.

As a result a kilogram is now the mass of the kilogram artefact - if the artefacts changes mass, it still remains 1 kilogram