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."

News Why?

[Anonymous Coward]

And this is news why? It obvivously makes sense to have the kilogram based on some universal constant as opposed to a block of metal sitting in some museum.

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.

Pressure

[XanC]

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

hmm not really math is this?

[qleak]

I'm not sure how this slipped by in slashdot but this has nothing to do with the academic area of mathematics :-P Sounds a lot more like science or physics to be specific. C'mon people lets try to give things a realistic category. Anyway why the hell is math a subcategory of science?? Just my 2 kilos, flame me if you like.

"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: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 quantum properties of atoms and other basic particles. The paper outlines how this could be accomplished without impairing the current international system of mass measurements.

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?

Nope, sorry

[mrchaotica]

You'd lose mass instead.

Re:Anyone Else?

[Anonymous Coward]

Or is this just typical /. editor stupidity?

I was going to be nice to you up until this sentence - if you want to suggest that someone is stupid, can you at least be sure that you're not wrong? Many posts have replied with the appropriate info (not to mention that you *did* pass grade school, did you not - if so, how can you *not* know that the gram is defined in terms of the kilogram?)

In any case, is this just typical Wes Janson stupidity?

Re:That'll really help...

[XanC]

The "standard" in the US really is the metric system. All the units that people actually used are defined in terms of their metric counterparts.

So a change in the kilogram automatically affects the pound.

However, when they do make this change, it will not be a "modded" kilogram. It will be the same mass as before; it's just that it will be possible (ultimately) to measure it much more precisly and time-invariantly (as the standard is losing mass over time).

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:Just wait.

[nxtr]

The only countries left that don't use metric are America and Bhutan. Bhutan is a fundamentalist islamic country that doesn't even have any phones yet. I guess we can see what the American technical level is.

Re:Pressure

[XanC]

Unfortunately, pressure has a mass component, so your definition is circular.

Could you please explain why that matters?

Recursive (adj): See recursive

Still Kg?

[ShadowFlair]

As opposed to what people seem to be suggesting, I don't think they are trying to replace kilogram with a brand-new unit, but just changing the definition. You would still say the brick is 1Kg; however, that will no longer mean that your brick is equivalent to the platinum-iridium cylinder, but a constant as defined by a unchanging natural phenomenon as suggested by the scientists...

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.

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?

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.

But I thought

[ACNiel]

The metric system was the most fundamentally correct system of measurement ever concieved by man, beast or God.

You'd certainly think that reading all the hype on this bbs.

Remeber this article the next time a English/metric debate comes about. There is nothing inherently better about either system. That argument being nullified, should we switch based on the rest of the world? That is the only valid argument.

Don't start your argument, thought process, or comment with the mistaken common wisdom of "Everyone knows metric is better...."

And while I am ranting, but not quite as obvious, I don't want to hear 'I know what a kilometer is, I don't know what a mile is...' If you can pace off one, you can pace off the other. You certainly don't have an inborn sense of what a kilometer is anymore than you have an inborn sense of the mass of a plum sized chunk of some alloy.

Neither system is anymore natural than the other, get off your high horse and make a rational comment (unlike this rant ;).

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.

Re:I suggest

[PoopJuggler]

They set it equal to 1 pound

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 happened with regards to the meter? "Hmm... speed of light is about 299,792,458 meters per second... well, instead of measuring this funky rod anymore, let's just say that it's 1 second of light movement long."

I'm proposing we do the same thing with this block that we did with the rod.

Lets say someone disputes the mass of a kilogram at the 10^-8 digit:

I presume you mean decimal place. Well, that's easy. A kilogram weighs 1.00000000kg. There you go, 1kg to the eight decimal place.

Not only is this inconvenient, but if someone has polished it or even 1 pg of dust has fallen on it, the precise weight is no longer repeatable, and is a ludicrous basis for measuring the mass of subatomic particles.

That's just silly. We've never weighed out the block to the precision of picograms. Yet, somehow we still relate measurements on the picogram scale, don't we? The weight of the actual block is only definite to about 2 micrograms.

All that's really important is to have a defined conversion from whatever more useful units you're using.

In the same way, I can give you an accurate description of the length of a cesium nucleus in furlongs. Furlongs, obviously, were not in their origination a very precise unit of length at all. But since there is a standard conversion (1 furlong = 201.168 meters) any measurement I give you in furlongs is as good as a measurement given in meters--as long as we agree on the conversion factor.

Here's the crux of it all: pretty much all measurements of particle mass are computed in energy (electric volts), chemistry is done in atomic mass units, and there are various other specialized units that are always cropping up. When you see a particle mass or some such given in kg, it is infallibly a conversion from these other units. As long as we agree on the conversion factor we are going to use between kg and some mass unit dependent on c, it is as good as defining kg on c in the first place.

I might go so far as to say that, even if we don't agree, it doesn't particularly matter. Honestly, the only reason we ever give the mass of an electron in kilos is because Joe Schmoe wants to know how much an electron weighs compared with how much he weighs.

Re:I suggest

[seminumerical]

There are two common systems of units, mks ... and cgs ... . The mks system is now more often referred to as the SI. ... In any case, what you're referring to is utterly trivial and/or irrelevant ...

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.

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

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

[themuffinking]

Half a kilo of butter, or a pound of butter is a reasonable purchase. Grams just don't cut it.

You can still use kilos... using grams as the base unit does not completely eliminate kilos from the face of the earth.

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.

Uh... if you need to buy eighty grams, then you'd have to say something like 2/25ths of a kilo. It's actually easier to use grams in that example. Also, if you ever need eighty grams of salami, you could just ask for one medium-thickness slice. Thus, we should use the slice of salami as a SI unit rather than the gram or the kilo. Wait, that wouldn't work...

'The moon wieghs the same as thirty-two billion slices of salami...'

Well, whatever. I say we just use them interchangeably.

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: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 accuracy. It's another thing when you are trying to transfer a satellite from high earth orbit into an elliptical sun orbit to intersect neptune or a KBO. The accuracy requirements for making certain burns to change trajectory, or making a control movement of the momentum wheels is another thing. Or in a particle accelerator, when to activate certain portions of the cyclotron, etc.

These definitions are for the purposes of science and technology. A company can offer a service where they dumb down the definition for the public.

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 fractions in common usage. I don't know why you think this is difficult. Would you prefer to be using pounds, shillings and pennies instead of dollars and cents -- that was one reform the US did before most other countries. So do you say "80 cents" or "eight-tenths of a dollar" (or 16 shillings)? And unless you're buying in bulk, most food is bought in quantites (or units) less than a pound, let alone a kilo. That's why you use ounces, by the way. Three ounces is about 80 gm. At the deli, most food here is labelled as price/100gm (cheese, ham, etc); a the butcher and green grocer it's mostly by the kilo. It makes things a lot simpler to just multiply weight in kilos by price in dollars/kilo.

The metric equivalents never seem to be just right, but we'll just have to live with them

After a few months you adjust. Australia went metric when I was at primary school.

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

[Smidge204]

That's like asking how many pounds in a foot... Rhode Islands are units of size and Libraries of Congress are units of data. Elephants are units of mass...

=Smidge=

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: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

You're from the west, aren't you?

[DancesWithBlowTorch]

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.

You must be from the Rhine-Ruhr Area. In southwest germany, people buy half a litre of beer. In Bavaria, it's typically simply one litre (a "Maß"). I think the litre is a fine volume measurement for alcoholical beverages. It just depends on where in the world you live and how much alcohol is deemed to be appropriate by society. :-)