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Kilogram Gets Controversial; Why Not Split the Difference?

timothy posted more than 3 years ago | from the try-a-different-scale dept.

United Kingdom 520

gbrumfiel writes "As Slashdot has noted, the kilogram has a problem. The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France. But the physical object appears to be getting lighter. Scientists want to replace the cylinder with a new standard based on Planck's constant, but two experiments designed to facilitate the switch keep coming up with different results. Now one researcher is proposing a solution: just average the two diverging experiments and use that value as the official definition. Not everyone thinks that averaging the two amounts to sound research: 'Deciding to just average these two results would be perfectly proper mathematics, but it would not be science,' says Michael Hart, a physicist at the University of Manchester, UK."

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Impossible (5, Funny)

camperdave (969942) | more than 3 years ago | (#35051532)

The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has. The obvious conclusion is that the rest of the universe is getting heavier.

Re:Impossible (0)

pz (113803) | more than 3 years ago | (#35051584)

You are assuming that the physical object is unchanging. It, however, gets cleaned periodically. That has been a long-troubling aspect of the standard that has received attention before, as cleaning, no matter how careful, undoubtedly removes more than just contamination.

Re:Impossible (5, Interesting)

mike260 (224212) | more than 3 years ago | (#35051712)

I think the GP's point was that even if you chopped a sizeable chunk off it, it would still weigh precisely 1kg. It logically follows that the universe's weight, expressed in kg, would suddenly jump upwards by a very large amount.

Re:Impossible (3, Informative)

MakinBacon (1476701) | more than 3 years ago | (#35051764)

Technically its mass would increase, not its weight.

Sorry to be so pedantic, but that is what this entire thread is about. =P

Re:Impossible (5, Informative)

dave420 (699308) | more than 3 years ago | (#35051874)

Even if you took a massive chunk out of it with a hammer, it would still be the 1kg reference, and will still be 1kg. That's the joke :)

Re:Impossible (0)

Anonymous Coward | more than 3 years ago | (#35051898)

No, he is assumung that the object is the definition, which is a correct assumption as the mass of the object (whatever that mass is) is defined as the standard for a kilogram .

If the object's mass changes, then it doesn't - QED.

For my next trick I will prove black is white - and get killed on a zebra crossing...

Re:Impossible (1)

Sulphur (1548251) | more than 3 years ago | (#35051946)

You are assuming that the physical object is unchanging. It, however, gets cleaned periodically. That has been a long-troubling aspect of the standard that has received attention before, as cleaning, no matter how careful, undoubtedly removes more than just contamination.

They are removing dark matter from it.

Physical objects interact... (2)

denzacar (181829) | more than 3 years ago | (#35051624)

...leaving traces. Over time, changes accumulate.

And when you are measuring something at 9 digits behind the point - a little can be a lot.
http://en.wikipedia.org/wiki/Kilogram#Stability_of_the_International_Prototype_Kilogram [wikipedia.org]

Re:Physical objects interact... (1)

aliquis (678370) | more than 3 years ago | (#35051988)

His point was that the 1 kg unit was the definition. And as long as it is it's still 1 kg, no matter what.

Assuming 1 kg isn't the weight of the average "copy" of the original.

How it gets lighter (5, Funny)

SuperKendall (25149) | more than 3 years ago | (#35051634)

It turns out that France imposed a Mass Tax in the last few years which means the cylinder has to cough it up for the good of the state.

On the plus (or more like the non-plus) side, the people of France are now looking fit & trim.

Re:How it gets lighter (1)

o'reor (581921) | more than 3 years ago | (#35051684)

You sure it's not just the rest of the world getting fatter and heavier ? Just my 2 centimes...

Re:How it gets lighter (1)

$RANDOMLUSER (804576) | more than 3 years ago | (#35051844)

That's exactly what's happening - meteorite strikes are adding to the mass of the planet. And of course stuff brought back through the Stargate.

Re:How it gets lighter (5, Funny)

JustOK (667959) | more than 3 years ago | (#35051886)

So, the French govt had to run a weigh?

Re:Impossible (0)

Anonymous Coward | more than 3 years ago | (#35051640)

The obvious conclusion is that the rest of the universe is getting heavier.

Well, according to my bathroom scale, I'm doing my part.

Re:Impossible (3, Insightful)

ehrichweiss (706417) | more than 3 years ago | (#35051678)

What I *still* don't get is why we moved away from the ORIGINAL definition of a gram which used to be the mass of 1 cubic centimeter of water. I've heard all the "because this type of measurement was more accurate", etc. explanations but it seems that now they have no idea how to get to where they were whereas(AFAIK) the mass of 1 cubic centimeter of water hasn't really varied. Anyone able to break this down into something that actually makes sense beyond the typical responses?

Re:Impossible (4, Informative)

XanC (644172) | more than 3 years ago | (#35051700)

Such a definition is ultimately circular. The volume of water depends on pressure, which itself has a mass component.

Re:Impossible (1)

ehrichweiss (706417) | more than 3 years ago | (#35051760)

Not dependent on pressure, temperature perhaps?

https://secure.wikimedia.org/wikipedia/en/wiki/Properties_of_water#Compressibility [wikimedia.org]

Regardless, the temp and pressure could be standardized...

Re:Impossible (1)

XanC (644172) | more than 3 years ago | (#35051926)

The first paragraph of your link describes how water is compressible, and not only that, how the compressibility changes with pressure.

You can't standardize pressure because to even define pressure you first have to define a kilogram. Circular.

Re:Impossible (1)

Daniel_Staal (609844) | more than 3 years ago | (#35051940)

Better yet, from that same page: The triple point. All you need is pure H20 and you have a reference point for temperature and pressure. You could work backwards from there to the definition of mass.

Re:Impossible (1)

tyrione (134248) | more than 3 years ago | (#35052068)

Temperature and Pressure are directly dependent upon one another. Ideal Gas Law.

http://en.wikipedia.org/wiki/Ideal_gas_law

Re:Impossible (1)

mysidia (191772) | more than 3 years ago | (#35051768)

Such a definition is ultimately circular. The volume of water depends on pressure, which itself has a mass component.

So base the standard on a volume of water added to a vacuum?

Re:Impossible (1)

Evil_Ether (1200695) | more than 3 years ago | (#35051786)

And what happens to water in a vacuum?

Re:Impossible (5, Funny)

Binestar (28861) | more than 3 years ago | (#35051804)

And what happens to water in a vacuum?

It gets the bag wet.

Re:Impossible (1)

Anonymous Coward | more than 3 years ago | (#35051810)

I don't know, but I do know a vacuum in water leads to death by electrocution.

Re:Impossible (0)

ehrichweiss (706417) | more than 3 years ago | (#35051846)

Rather than worry about what happens to water in a vacuum, just set another pressure instead, like 1 Atmosphere(s) or something? Pick a standard that works and stick with it. We will have to do this or else we're going to go through this same nonsense, a LOT. And while there are other factors that can affect this(as others point out each time I ask), it's a lot better than the "omg, every time we clean this we lose a few atoms and...." that I have heard no less than 3 times in the past 4 years.

Re:Impossible (2)

XanC (644172) | more than 3 years ago | (#35051900)

But you can't define 1 atmosphere without defining the kilogram first.

Re:Impossible (1)

Stray7Xi (698337) | more than 3 years ago | (#35051952)

But you can't define 1 atmosphere without defining the kilogram first.

You can define 1 atmosphere, you just can't quantify 1 atmosphere. You just have to do it based on real world conditions without assigning it a number. Those real world conditions will shift far more then the reference weight though.

Re:Impossible (1)

aliquis (678370) | more than 3 years ago | (#35052018)

You can define 1 atmosphere, you just can't quantify 1 atmosphere. You just have to do it based on real world conditions without assigning it a number. Those real world conditions will shift far more then the reference weight though.

http://lmgtfy.com/?q=1+stone+in+kg

1 stone = 6.35029318 kilograms

So pick up random stone, say "oh this is about sixish kilo", done ;)

Re:Impossible (0)

Anonymous Coward | more than 3 years ago | (#35051944)

As stated a few levels up, pressure has units of kg / (m s^2). Defining the kilogram in terms of something derived from the kilogram is circular.

Re:Impossible (2, Interesting)

o'reor (581921) | more than 3 years ago | (#35051818)

There is such a thing as "standard conditions of temperature and pressure" (293.15 K, 101.325 kPa by the NIST) so it is possible to perform those measurements in similar conditions. And I guess my point on the hydrogen isotopes is moot too sincethere is such a thing as Vienna Standard Mean Ocean Water [wikipedia.org] . Duh.

Re:Impossible (1)

sjames (1099) | more than 3 years ago | (#35051878)

Pressure is measured in terms of force/area. In turn, force is a measure of mass and acceleration. Now we're back where we started and still haven't a clue.

Re:Impossible (0)

Anonymous Coward | more than 3 years ago | (#35051826)

at the temperature and pressure of the water triple point

Re:Impossible (1)

o'reor (581921) | more than 3 years ago | (#35051718)

Well, it all depends on your water -- how much deuterium or tritium would you like with your regular hydrogen ?

Re:Impossible (0)

ehrichweiss (706417) | more than 3 years ago | (#35051780)

Well, seeing as how we have something with even less standardization right now, what would prevent us from just picking one "type" of water and going with that? Otherwise we're looking at the same nonsense that the metric system was supposed to get us out of in the first place.

Re:Impossible (1)

grimJester (890090) | more than 3 years ago | (#35051822)

It's supposed to be repeatably measurable. The best way of doing it is not just any random one that's exactly defined, but one that's easy to replicate.

Re:Impossible (0)

Anonymous Coward | more than 3 years ago | (#35051908)

You're obviously not thinking of "heavy water." ;-)

And as for it being OK math, but not OK science, since when has a unit of measure been called science?

Re:Impossible (4, Informative)

mysidia (191772) | more than 3 years ago | (#35051732)

The physical object cannot get lighter (less massive). By definition is 1kg no matter how much mass it has.

Actually... it can get lighter. Earth's gravitational field can get weaker as matter from earth is ejected or evaporates into space.

It can also get lighter as Earth's atmosphere gets heavier, making it more buoyant in earth's atmosphere.

That has nothing to do with how much mass the cylinder has, because MASS is not a measure of weight.

Mass and weight are independent. Weight is due to forces applied to mass inside a gravitational field; if the field weakens or other forces are applied to the mass inside the field, the weight will decrease or increase without any change of mass.

Earth's gravitational field and atmosphere is also not uniform, so there are places (or altitudes) you can bring the same object to, and it will be lighter or heavier, with its amount of mass being the same.

Re:Impossible (1)

91degrees (207121) | more than 3 years ago | (#35051772)

"Heavy" and "light" can mean either more or less mass or more or less weight. Since the term was clarified by the parenthesis it means less massive and so cannot get "lighter".

Re:Impossible (1)

ChrisMP1 (1130781) | more than 3 years ago | (#35051848)

The physical object cannot get lighter (less massive).

lighter (less massive)

(less massive)

Re:Impossible - Local G (2)

fahrbot-bot (874524) | more than 3 years ago | (#35051872)

Earth's gravitational field and atmosphere is also not uniform, so there are places (or altitudes) you can bring the same object to, and it will be lighter or heavier, with its amount of mass being the same.

A Local G [xkcd.com] effect. Pole vaulters be aware.

Re:Impossible (0)

Anonymous Coward | more than 3 years ago | (#35051966)

Woah doc, this is heavy.

Reminds me of the deer that got away (5, Funny)

paiute (550198) | more than 3 years ago | (#35051564)

A physicist, engineer and a statistician are out hunting. Suddenly, a deer appears 50 yards away.

The physicist does some basic ballistic calculations, assuming a vacuum, lifts his rifle to a specific angle, and shoots. The bullet lands 5 yards short.

The engineer adds a fudge factor for air resistance, lifts his rifle slightly higher, and shoots. The bullet lands 5 yards long.

The statistician yells "We got him!"

Re:Reminds me of the deer that got away (1)

maxume (22995) | more than 3 years ago | (#35051816)

They shouldn't have used baking soda as gunpowder.

Re:Reminds me of the deer that got away (5, Funny)

140Mandak262Jamuna (970587) | more than 3 years ago | (#35051842)

The statistician is right. Because if the deer has not moved between the first and the second shot, it is already dead. QED.

Black Cows in Scotland. (2, Funny)

140Mandak262Jamuna (970587) | more than 3 years ago | (#35051854)

A mathematican, an astrophysicist and a statistician were walking along a road in Scotland. They saw a black cow. The astrophysicist said, "All the cows in Scotland are black". The statistician said, "No, there is at least one black cow in Scotland". The mathematician said, "All we now know is, this side of that cow is black."

Re:Black Cows in Scotland. (0, Informative)

Anonymous Coward | more than 3 years ago | (#35051996)

Worst delivery ever of an already not-too-funny joke.

Better delivery:
A mathematican, an astrophysicist and a statistician were walking along a road in Scotland. They saw a black cow. The astrophysicist said, "I guess all the cows in Scotland are black". The statistician said, "No, all we know is that there is at least one black cow in Scotland". The mathematician said, "No, All we know is, there is at least one cow in Scotland, at least one side of which is black."

Re:Reminds me of the deer that got away (3, Funny)

Phil06 (877749) | more than 3 years ago | (#35051954)

A physicist believes that it takes extremely high pressure to produce diamonds. An engineer knows it just takes a little suction.

Re:Reminds me of the deer that got away (3, Informative)

Kral_Blbec (1201285) | more than 3 years ago | (#35052036)

Since a bullet's trajectory isn't very parabolic, landing 5 yards long would mean it passed through the deer.

Does it matter? (2, Interesting)

crow (16139) | more than 3 years ago | (#35051578)

The differences are so minimal that I can hardly believe it matters. The only issue is if the difference between the new definition and previous measurements is statistically significant. If you can't show that that would be the case, then pick whatever number between the two measurements that is easiest to work with mathematically, perhaps one with the most zeros (in decimal, since the metric system is designed to work well with powers of 10).

Re:Does it matter? (1)

Platinum Dragon (34829) | more than 3 years ago | (#35051622)

For most people, all that matters is that a kilogram is a reasonably consistent measure of mass.

For mathematicians, physicists, and other scientists requiring precise mass measurements, this matters quite a bit.

Re:Does it matter? (0)

Anonymous Coward | more than 3 years ago | (#35051644)

It seems to me that at some point the difference will become large enough to be noticeable.
If we can find an alternative now why wait.

Re:Does it matter? (4, Interesting)

drolli (522659) | more than 3 years ago | (#35051832)

Speaking as an experimental physiscist

ahem. 175parts per billion is 1.75e-7. For metrology that is a huge discrepancy. What is worse is that the measurements themself are a factor of 5 better, leaving no room for error.

For experiments where the physicists believe they understand them this is unacceptable, because it actually means the pysics of at least one method of both is not well enough understood, i.e. you have a systematic error. If the physics is not well understood then you don't know if the systematic error will be constant.

If the measurement will not be constant then the average will also not be constant. So an metrology institute where a reference weight should be define will need both methods and still not get any stable definition.

If you already need to afford both methods, then you can create reference weights and at the same time check if the difference between both methods is the right one and constant at your place.

Important rule in experimental physics: NEVER average over systematic mistakes. Average over random results. On systematic mistakes, the word average makes no sense

Re:Does it matter? (0)

Anonymous Coward | more than 3 years ago | (#35051998)

I'm not a metrologist, nor am I a physicist. But my son, who IS a physicist, says a man with two watches can't be sure what time it is. ;)
On the other hand, the standard we call Universal Coordinated Time (UTC) is ultimately the result of a weighted average that takes into account (at least) the times reported by a few hundred labs around the world, and adjusts based on the local gravity at each lab. There are averages in there somewhere.

Re:Does it matter? (1)

DCFusor (1763438) | more than 3 years ago | (#35052048)

Hear, Hear! Yeah, it matters - a tiny error in m becomes a crazy error in e= mc^2. As well as everything else the parent mentions. It's not even good math to average if you have the least hint there's something missing other than purely random noise creating the differences -- that averages out, systemic errors don't.

Besides, if someone learns why, well, that's more knowledge in our bag o' tricks, eh? And that is what science is really all about.

Bread not working? (4, Funny)

Sobieski (1032500) | more than 3 years ago | (#35051588)

Let them eat pounds!

Re:Bread not working? (1, Insightful)

camperdave (969942) | more than 3 years ago | (#35051646)

Pounds are defined in terms of kilograms, so that's no help.

Re:Bread not working? (2)

Winckle (870180) | more than 3 years ago | (#35051856)

Bread price in the UK can vary by shop you insensitive clod!

not the first time (1)

at10u8 (179705) | more than 3 years ago | (#35051618)

The measure of length called a foot that we use for practical commerce was established in pretty much that way. See the story of the international foot [wikipedia.org] as differed from the different foots which were already in widespread use.

Re:not the first time (1)

sznupi (719324) | more than 3 years ago | (#35051698)

Ultimately, a foot was established by defining it in meters / I don't think they aim for self-reference in this case ;p (or that there was much of it in the past - while exact value comes from the object in France, you can get to something damn close for most purposes from the size of this planet or properties of probably the most common chemical compound in the Universe)

Hmm (0)

Anonymous Coward | more than 3 years ago | (#35051626)

Why does it sound like that researcher was looking for a quick answer just so he could get to the pub?

Re:Hmm (1)

camperdave (969942) | more than 3 years ago | (#35051688)

Why does it sound like that researcher was looking for a quick answer just so he could get to the pub?

He probably wants to get started on measuring pints.

Well, duh. (5, Funny)

Black Parrot (19622) | more than 3 years ago | (#35051652)

Why don't they just take the weight of a gram and multiply it by 1024?

Re:Well, duh. (-1, Troll)

koolfy (1213316) | more than 3 years ago | (#35051696)

Because the weight of a gram is defined as a 1/1024th of a kilogram, dumbass.

Re:Well, duh. (5, Informative)

arthur.gunn (1687888) | more than 3 years ago | (#35051710)

I think that would be a kibigram.

Re:Well, duh. (4, Funny)

formfeed (703859) | more than 3 years ago | (#35051762)

I think that would be a kibigram.

Don't let the industry fool you. They introduced that distinction so they can put less in a box and still sell it to you as 1kg of Mac and Cheese.

Re:Well, duh. (2)

Sponge Bath (413667) | more than 3 years ago | (#35051782)

"...sell it to you as 1kg of Mac and Cheese."

I prefer Linux and cheese. It's the cheesiest.

Re:Well, duh. (1)

Ecuador (740021) | more than 3 years ago | (#35051920)

Or take the weight of the Library of Congress and divide it by the air speed velocity of an unladen swallow...
Oh, right I forgot. An African swallow.

finally (1)

PJ6 (1151747) | more than 3 years ago | (#35051730)

I floated this idea years ago to a few physicists and they hated it for reasons I can't fathom. The whole idea of basing a unit on a single, random object instead of something universal seemed silly to me.

How much... (1)

Haedrian (1676506) | more than 3 years ago | (#35051752)

Is a kilogram in terms of fractions of an elephant please?

Re:How much... (1)

makubesu (1910402) | more than 3 years ago | (#35051836)

Perhaps in terms of the largest coconut a swallow can carry?

This is why science is so hard (5, Interesting)

fermion (181285) | more than 3 years ago | (#35051766)

Science, and teaching science, is hard because it is often difficult to determine which are the truly salient facts, and what background is necessary.

In this case the background is that the standard for mass, unlike time or distance, cannot independently be constructed in the lab. This means that science and industry are susceptible to two issues. The first is degradation of a physical standard, in this case a hunk of metal in France. The second is that one is dependent on other to create proxies of the standard, and as a result have no true assurance of the accuracy of the standard. A suitable lab with suitable personal can masure time and distance without the need of a proxy manufactured by others, and no dependence on a fixed physical object.. There is a desire for the same to be true for mass.

Second, no one knows if the hunk of metal is shrinking, and if it is how much it is shrinking by. If the experts knew it was shrinking, then they could figure out how to at least partially correct it. The hunk of metal might not be charging at all, or it could be accreating matter. Without an independent standard, which does not apparently exists, as everything is based on the hunk of metal, all there is is guesswork.

The third is the idea that Planck's Constant is being used to create the standard. In fact Planck's constant is one two approaches. The other is to create a sphere from a silicon and use Avagadro's Constant to define the mass. The problem is that these two approaches do no lead to consistant results, with an error about an order of magnitude large than the expected error.

The issue with averaging is that while one does average within a result, and even results that are taken from similar procedures, it is unclear that averages in this case is suitable. It seems to me that the results point to an interesting area of research, and rather than just averaging, more work should be done understanding the inconsistency. If it is not random error, and not an artifact, then something really fascinating might be going on.

Pick which one? (0)

Anonymous Coward | more than 3 years ago | (#35051796)

The one that gets us more drugs!

How do the determine the mass? (1)

NewtonsLaw (409638) | more than 3 years ago | (#35051802)

How do they determine the mass of their 1Kg reference?

Is it simply by measuring the force it exerts when influenced by a gravitational force of 1G?

If so, how do they measure to ensure that 1G is still the same acceleration that it was when the standard was introduced?

Do they also allow for the fact that it is displacing a certain amount of air -- and therefore is subject to the forces of buoyancy that will tend to make it lighter, depending on air density, humidity, etc?

While the predominant factor is the mass of the earth, what about other factors such as the gravitational field of the moon (large enough to induce tides of several meters in magnitude) and other celestial bodies?

Trying to measure an absolute through the use of a another absolute is fine -- but how do you factor in the variables that also have an effect?

I'm sure they know what they're doing.

Re:How do the determine the mass? (1)

sznupi (719324) | more than 3 years ago | (#35051984)

Oh I'm sure they figured a thing or two after Newton... ;)

Re:How do the determine the mass? (1)

John Hasler (414242) | more than 3 years ago | (#35052006)

How do they determine the mass of their 1Kg reference?
Is it simply by measuring the force it exerts when influenced by a gravitational force of 1G?

Each national bureau of standards carts its standard kilogram over to France and compares it directly to the international standard kilogram with a balance. G is not involved.

Unfortunately, each time they do this they get slightly different results. The difference between the international standard and the average of the national standards is increasing.

I don't understand. (1)

91degrees (207121) | more than 3 years ago | (#35051830)

I'd have thought we'd have an experiment that comes up with say 0.3464kg and another that comes up with 0.0765kg, and they want to define the kilogram as 1/0.3464 * the result of the first one or 1/0.0765 * the result of the second one. But both of these would give exactly the same mass as the other.

Or if the measurements are inconsistent, they should just pick the one with the smallest variance within that experiment.

Clearly I'm missing something here.

This is stupid. Just drop the precision. (0)

Anonymous Coward | more than 3 years ago | (#35051858)

If the argument is that the average is "good enough" because the level of precision practically doesn't matter, then we can refer back to seventh-grade chemistry and the concept of significant figures. Your calculation is only as precise as the original measurement, and as long as it's good enough for what you're doing, all you have to do is drop everything that is in uncertainty. So cut off the precision of the kg measurements at the point where they diverge. Don't make shit up just to fulfill some desire to know more than we really do.

What? Math is not science? (-1)

140Mandak262Jamuna (970587) | more than 3 years ago | (#35051862)

What did that bloke mean? "It is good math but not science"?!?!??! Math is science.

Re:What? Math is not science? (1, Offtopic)

bunratty (545641) | more than 3 years ago | (#35051936)

I see many people making this statement, but it is false. Mathematics is based upon axioms which we assume to be true. We can then prove conclusively theorems derived from those axioms and be absolutely sure of the result. Science is based on observation of the real world and making hypotheses that match our observations. We can make hypotheses that match our observations but are not entirely correct. Math uses deductive reasoning. Science uses inductive reasoning. Science does use mathematical models for its hypotheses, but it is not math.

Re:What? Math is not science? (-1)

Anonymous Coward | more than 3 years ago | (#35051942)

uuummmmmm, no.

It's not.

Re:What? Math is not science? (0)

Deltaspectre (796409) | more than 3 years ago | (#35051964)

I was about to type up a reply, but this suits you perfectly:

http://xkcd.com/687/ [xkcd.com]

His math is perfect, his science is not.

Probably Flawed Method (1)

SDF_of_BC (921007) | more than 3 years ago | (#35051870)

How about working out the number of grams from one mol of Oxygen(?) atoms? If they all have an atomic mass of ~16 you know they're about 16 grams. :o

easy solution (0)

Anonymous Coward | more than 3 years ago | (#35051896)

First, define Avagadro's constant as exactly 6.02214179×10^23 + 4.

As one mole of C-12 is 12 grams, then one gram would be the mass of 50,184,514,916,666,666,666,667 atoms of C-12. Therefore, a kilogram would be the mass of exact 50,184,514,916,666,666,666,667,000 atoms of C-12.

Tell those physicists that from now on, Planck's constant is defined by the above definition of Avagadro's constant.

Problem solved. And lots of happy chemists.

Can't the kilogram be derived from other SI units (1)

Anonymous Coward | more than 3 years ago | (#35051928)

  • The second is defined based on decay of the cesium atom.
  • The meter is defined based on the how far light travels in a second.
  • A centimeter is 1/100 of a meter.
  • A gram is the mass of 1 cubic centimeter of water under certain standard conditions.
  • A kilogram is 1000 grams.

Re:Can't the kilogram be derived from other SI uni (0)

Anonymous Coward | more than 3 years ago | (#35052082)

Someone earlier asked about that.
Standard Conditions: I assume you mean a known temperature and pressure.
How is pressure defined? Pressure = Force / Area.
How do we get force? Force = Mass X Acceleration.
How do we get mass? Oh wait, that is what we are asking for.

We can't use a unit to define itself.

Re:Can't the kilogram be derived from other SI uni (1)

Kral_Blbec (1201285) | more than 3 years ago | (#35052086)

Dude, the speed of light is 299,792,458 meters/second. I'd like to lose some weight, but I don't want to weigh 9.1035E-10kg.

Re:Can't the kilogram be derived from other SI uni (4, Insightful)

norpy (1277318) | more than 3 years ago | (#35052092)

A gram is not the mass of 1 cubic centimeter of water. It is 1/1000 of the weight of that lump of metal in france!

There are a ton of posts above arguing over that, and you can't use that to define mass because it is affected by pressure. Pressure has a mass component so it ultimately becomes circular.

Albert says.. (1)

Jawcracker Fuzz (1773468) | more than 3 years ago | (#35051930)

1kg equals: 8.9876e+23 ergs
8.9876e+16 joules
6.6289e+16 foot-pounds
2.1481e+4 kilotons of TNT.

TNT? WTF?

Re:Albert says.. (0)

Anonymous Coward | more than 3 years ago | (#35052094)

e = m * c^2

It's converting kg to joules (determining the amount of energy in a kilogram of matter), then dividing by the amount of energy released by a kiloton of TNT [wikipedia.org] .

UK? (1)

Missing.Matter (1845576) | more than 3 years ago | (#35051956)

What's with all these science stories with country icons? First the "Atomic Disguise Makes Helium Look Like Hydrogen" is tagged as Canada, and now this is tagged as UK. Slashdot, make your story icons relate to the more relevant tags, like science.

The Curious Incident of the Dog in the Night-Time (1)

gmuslera (3436) | more than 3 years ago | (#35051970)

So you are defining a somewhat universal (ok,at least, global) constant as something that should be there, but isnt?

Stupid definition (1, Interesting)

Anonymous Coward | more than 3 years ago | (#35051978)

The SI unit is officially defined as the weight of a 130-year-old platinum-iridium cylinder in France.

Where will we get a 130-year-old platinum-iridium cylinder next year? And the year after that? I wonder how they got hold of a 130-year-old platinum iridium cylinder 130 years ago?

Atomic Mass Units? (1)

DeathSquid (937219) | more than 3 years ago | (#35052020)

There's a perfectly useful Atomic Mass Unit already defined: the dalton. http://en.wikipedia.org/wiki/Atomic_mass_unit [wikipedia.org]
Why not define the kilogram in terms of a given number of Carbon 12 atoms?

Or if that is not stable enough, define it in terms of the electron rest mass. That's been stable for at least half the age of the universe.

Count it (1)

Orgasmatron (8103) | more than 3 years ago | (#35052040)

Why don't they just count how many atoms are in it, and define the kilogram as the sum of the counts of each of the types of atoms making up the alloy?

Time is defined in a similar way, and don't tell me we don't have the technology, IBM has been shoving individual atoms around for decades now. They could do it again, and this time it would be for a cause more useful than making tiny IBM logos.

I'm not sure what the composition is, but I don't think there can be more than 6 * 10^24 atoms in it. Should be totally possible. If we are patient.

And don't lose count.

easy (1)

shentino (1139071) | more than 3 years ago | (#35052050)

Here's an easy reference: Planck units.

Just define everything in terms of Planck units and nothing will ever change.

Reminds me of the joke about 3 statisticians... (1)

gmfeier (1474997) | more than 3 years ago | (#35052060)

...out hunting for ducks. The first one fires and misses a foot high. The second shoots and is a foot low. The third one yells "We got it!"

Curious about the loss of mass? (0)

Anonymous Coward | more than 3 years ago | (#35052062)

Has anyone explained the loss of mass? The metals involved should be stable. All I could think of would be impurities in the alloy. Trace amounts of a decaying isotope would cause the loss. If it isn't lost from handling it has to be a loss of energy or from a solid turning to a gas.

Main problem (1)

SpaghettiPattern (609814) | more than 3 years ago | (#35052084)

The kg should nod be taken light-heartedly. Many other units depend on the kg. I say "keep sciencing" until a true solution emerges!
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