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For decades, metrologists have strived to retire ‘Le Grand K’ — the platinum and iridium cylinder that for 126 years has defined the kilogram from a high-security vault outside Paris. Now it looks as if they at last have the data needed to replace the cylinder with a definition based on mathematical constants.
The breakthrough comes in time for the kilogram to be included in a broader redefinition of units — including the ampere, mole and kelvin — scheduled for 2018. And this week, the International Committee for Weights and Measures (CIPM) will meet in Paris to thrash out the next steps.
“It is an exciting time,” says David Newell, a physicist at the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. “It is the culmination of intense, prolonged efforts worldwide.”
[...] In 2011, the CIPM formally agreed to express the kilogram in terms of Planck’s constant, which relates a particle’s energy to its frequency, and, through E = mc2, to its mass. This means first setting the Planck value using experiments based on the current reference kilogram, and then using that value to define the kilogram. The CIPM’s committee on mass recommends that three independent measurements of Planck’s constant agree, and that two of them use different methods.
(Score: 2, Disagree) by Alfred on Thursday October 15 2015, @10:03PM
Good grief. Would you like to hear a story about the pyramid inch?
(Score: 2, Informative) by Anonymous Coward on Thursday October 15 2015, @10:14PM
The meter was originally defined as 1 10,000th the distance from the pole to the equator. However, that was found to vary too much.
1 liter is a 10cm cube.
1 liter of water at 4°C has a mass of approximately 1 kg.
(Score: 2, Informative) by Anonymous Coward on Thursday October 15 2015, @10:18PM
Correction: The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. [wikipedia.org]
(Score: 0) by Anonymous Coward on Friday October 16 2015, @12:17PM
Actually it was originally defined as the length from the equator to the north pole of the meridian passing through Paris.
(Score: 5, Informative) by bob_super on Thursday October 15 2015, @10:16PM
The strength of the metric system is in the easy relationships between units. The metre is not less arbitrary than the foot, but It only takes a few seconds to approximate the weight of that tank of radioactive water being shaken by the earthquake (a daily occurrence, isn't it?)
As far as precision goes, scientists are looking for exact units, even if they are hard to reproduce, because tolerances do stack up, making good calibrations critical.
(Score: 1, Interesting) by Anonymous Coward on Thursday October 15 2015, @10:41PM
Agreed. I think the fundamental length should the the distance light travels in a nanosecond. That's very close to a foot, 0.98357... feet, and it makes the units closer to natural units (where the speed of light is 1).
(Score: 2) by bob_super on Thursday October 15 2015, @10:48PM
On a daily basis, my grandma would prefer to keep the density of milk at about 1. Makes measuring for cakes easier.
1dm3 = 1l = 1kg is nice, a cubic foot of water being about 27kg only helps if you really have a lot of guests.
(Score: 2, Interesting) by Francis on Thursday October 15 2015, @11:27PM
Why would she care about the density? We measure liquids by volume. Things like flour are generally best measured by weight just because they're easily compacted leading to uncertainties in the actual amount there.
And regardless of what system of measure you're using, the density of milk isn't going to be constant. It's going to depend upon the specifics of the milk. Whether you use whole milk or 0% fat milk is going to change the density of the milk, but in both cases the volume would be the same. That's generally why it's best to stick with the same brand and type of ingredients when possible as each brand may or may not act exactly the same way as the other options do. My mother restricts her purchases of flour to one or two varieties for that reason.
(Score: 3, Insightful) by fido_dogstoyevsky on Friday October 16 2015, @04:54AM
Why would she care about the density? We measure liquids by volume.
Not used a modern kitchen balance lately?
It's NOT a conspiracy... it's a plot.
(Score: 1) by Francis on Friday October 16 2015, @02:26PM
How is this insightful? It doesn't in any way shape or form address the question I asked.
For liquids, there's no reason not to use weight as a measure and at least one good reason not to. Weight varies a lot more than volume does globally. Needing to adjust the weights in addition to the other adjustments needed for cooking at various altitudes makes very little sense when it comes to liquid measures. Measuring flour by weight, does makes some sense as I've already acknowledged, but measuring liquids like that is stupid.
(Score: 0) by Anonymous Coward on Friday October 16 2015, @03:13PM
Well, if 1g is approximately equal to 1ml, you can use a scale to measure volume (after zeroing it with the container in place).
(Score: 0) by Anonymous Coward on Friday October 16 2015, @12:21PM
Whatever remains after removing all fat from the milk is not worthy of being called milk any more.
(Score: 3, Funny) by fido_dogstoyevsky on Friday October 16 2015, @04:59AM
I think the fundamental length should the the distance light travels in a nanosecond. That's very close to a foot, 0.98357... feet, and it makes the units closer to natural units ...
Only acceptable if the inch is also defined as 1 decifoot
... (where the speed of light is 1)
In a vacuum, but we'll ignore the spherical chickens as a first approximation.
It's NOT a conspiracy... it's a plot.
(Score: 2) by frojack on Thursday October 15 2015, @11:09PM
Looking for exact units?
The CIPM’s committee on mass recommends that three independent measurements of Planck’s constant agree, and that two of them use different methods.
All of these measurements can easily agree, when you set the precision required loose enough.
Also, in the past, where did they have to weigh these reference cylinders, or the things they were comparing to the cylinders? Altitude plays a part, no?
No, you are mistaken. I've always had this sig.
(Score: 3, Informative) by throwaway28 on Friday October 16 2015, @01:18AM
Altitude plays a part, no?
Balance scales are unaffected by local gravity. Spring scales and electronic scales are affected by local gravity. Some electronic kitchen scales say to recalibrate with a test mass if you change altitude. Looking for a reference to what NIST does, they /do/ measure local gravity when using their watt balance.
Gravity acceleration, g, is determined by a commercial gravimeter [15], which records the trajectory of a corner cube dropped in a vacuum. Figure 10 shows a graph for points from a typical measurement run. The continuous curve is from the tidal effects calculated from U.S. Geological Survey (USGS) software [16] for our location, and must be subtracted from the data to obtain the local average, 9.80101933 m/s2.
(Ok, that's so last year (1998) ( http://dx.doi.org/10.6028/jres.110.003 [doi.org] http://nvlpubs.nist.gov/nistpubs/jres/110/1/j110-1ste.pdf [nist.gov] ), but they probably still do something similar today.)
(Score: 2, Insightful) by Francis on Thursday October 15 2015, @11:11PM
That's not a strength. I continually hear people touting that as a strength, but it's not something that people do very much in the real world and it means that none of the units and sub-units are of convenient sizes. You convert between inches and feet or feet and yards and sometimes between feet and miles. But, each unit itself is something that's actually meaningful.
The foot isn't arbitrary, it's something that's easily related to a body part that nearly everybody has. Yes, people's feet do vary, but you learn what the length of your foot or hand is related to the measure and you have a convenient way of estimating short lengths. Same goes for the comparison between the inch and your fingers.
The meter though is completely arbitrary. It sort of makes sense a little bit on the planet Earth, but nowhere else in the universe does the unit makes sense other than on planets the size and shape of Earth.
(Score: 2) by bob_super on Friday October 16 2015, @12:10AM
The "foot" is longer than most people's feet, by about 50% in the case of average women.
The metre is about the height of a 3-year old, or of your bellybutton. What was your point?
> it's not something that people do very much in the real world
You do need to broaden your horizons. Do you know how to find Liberia and Burma on a map? Go anywhere else.
(Score: 1) by Francis on Friday October 16 2015, @01:40AM
I don't think you know what the word "arbitrary" means. There's nothing about the SI units that's empirical. They were arbitrarily defined and then over time they chose more and more reliable means of defining them so they wouldn't be constantly changing. But, the mass of an arbitrary lump of metal is completely arbitrary. As was the decision to base the meter off the distance from the equator to the North Pole.
Temperature is another one. Imperial measure uses Fahrenheit which hits 100F at about the temperature of the human body. Celsius which hits 0 at the freezing point of water at standard conditions up to 100 where water boils. There's absolutely no good reason for that. If we're going to be objective here, Kelvin would be the one to go with. At least Fahrenheit uses one point that approximates the temperature that people actually use for comparing temperatures. That is body temperature. We all compare temperatures with body temperature to determine whether or not we're comfortable and that's the only measure other than Kelvin that makes any sense.
As far as broadening my horizons, clearly somebody who spent years using SI units in college and then spent a year and a half using them in daily living is the one that needs to broaden his horizons; not the person who doesn't understand what the term "arbitrary" means.
FYI, neither China nor America are metric countries, you might want to actually do some research before making an ass of yourself. China isn't metric because they manufacture a lot of stuff for America, so things like clothes tend to be in imperial measure. America never did away with the imperial measures because we had a functioning system of measure that was enforced. Unlike most of the metric world that went along because they could no longer purchase things in their previous system of measure and couldn't produce enough of it on their own either.
(Score: 2) by bob_super on Friday October 16 2015, @06:05AM
I pointed out how a unit longer than my foot isn't more or less "meaningful" than the distance between the floor and your belly button. You see the meaning you want to see, others used to the other system see their basic unit equally as meaningful. It's arbitrary and it doesn't really matter.
You think the human body temperature is a good 100, many others think that 0-100 as a range for liquid water is pretty darn convenient. Again, it doesn't really matter, you get used to the arbitrary convention. -20 is too cold, and 120 or 45 is too darn hot...
What matters though, is that the metric system is internally consistent. It's 10 of this, or 10^n of that, as specified by the unit's name. Not 12x here, 16x there, 3x for that other, and whatever the F#$& a mile is in inches, or a ton in ounces.
The rest of the world is Metric, except for the UK pining to be different, and whatever needs to be exported to the US (and Liberia and Myanmar, but not for much longer). Sure there are way too many 7-eleven in Taiwan, but they sell metric-labelled products (even if, as a good unofficial colony, they used bloody 110V).
Clothes sizes for women are in an arbitrary ever-changing unit, which puts imperial to shame for resisting evolution. :o)
Men's clothes, in most of he countries I've been to, are in metric.
I've spent well over half of my life outside of the US. "In college" is cute: people keep telling me they've taken a "few years" of one of the languages I speak "back in college", and can't ever remember more than five words of it.
Oh, and Americans shouldn't be allowed to say E=mc2 unless they have memorized the constant that's missing to actually make it correct in imperial units.
(Score: -1, Troll) by Anonymous Coward on Friday October 16 2015, @02:37PM
In other words, you're too stupid to recognize your own ignorance and would rather post elitist bullshit to reinforce the group think rather than admit that somebody else has a point.
And, no, the rest of the world isn't metric. For example China is as metric as the US is, and really, not even as metric. They've got 3 systems of measures that they use, good luck buying clothes there without specifying sizes in inches.
Unlike you, I've used both systems long enough to actually understand how they work and the benefits of doing so. The fact that I've gone back to imperial measures says something about how much metric sucks. There's no reason why I couldn't be doing my cooking and such in metric measures. There are metric cookbooks out there and metric measurement tools out there. I don't generally use entire containers of things when cooking, so I wouldn't be using those.
The fact is, that metric just sucks for cooking and other things that people actually do, and no amount of whitewashing and hand waving is going to change that. The "benefits" that you're relying on are things that people just don't do with any regularity. And in exchange for the "benefits" you wind up with units that are inconvenient and ill-conceived of.
But yeah, feel free to keep drinking the kool-aid there. It's always nice the way you metric boosters can't bother to withhold the anti-Americanist jealousy that we have a working system of measures and aren't interested in converting to such a stupid set of measures.
(Score: 3, Touché) by bob_super on Friday October 16 2015, @05:02PM
> Unlike you, I've used both systems long enough to actually understand how they work and the benefits of doing so.
Somebody doesn't read the threads he/she replies to.
> The fact is, that metric just sucks for cooking and other things that people actually do, and no amount of whitewashing and hand waving is going to change that.
The best part of 7 billion people agree with your point of view, and are awaiting your donations before they starve. Most have spent the last 50+ years hanging on to traditional tins cups so that they can try to pass grandma's recipes onto their children, and praying that the metric police doesn't discover great-grandpa's school ruler behind the firewood stacks. Buildings have been capped off at three stories to avoid the risk of metric-based collapse, and all speed signs are just being ignored for everyone's safety.
You are correct, I'm an idiot.
(Score: 2) by Magic Oddball on Friday October 16 2015, @02:09AM
A few more good examples, though mostly horse-related:
(Score: 2) by tangomargarine on Friday October 16 2015, @02:37AM
What the heck is a "standard furrow"?
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"
(Score: 0) by Anonymous Coward on Friday October 16 2015, @06:41AM
About how far oxen can pull a plow before needing a rest.
(Score: 2) by bob_super on Friday October 16 2015, @05:50PM
European Oxen of African Oxen?
(Score: 2) by fido_dogstoyevsky on Friday October 16 2015, @05:14AM
I continually hear people touting that as a strength, but it's not something that people do very much in the real world...
Depends on where you live and what you are doing.
The foot isn't arbitrary
It's defined as 0.3048 metres exactly [wikipedia.org].
You convert between inches and feet or feet and yards and sometimes between feet and miles. But, each unit itself is something that's actually meaningful.
For some arbitrary meaning of meaningful... the width of my little finger is a more useful measure to me than the length of some arbitrary part of my thumb.
It's NOT a conspiracy... it's a plot.
(Score: 0) by Anonymous Coward on Thursday October 15 2015, @10:46PM
What the hell, metric wasnt that arbitary in the slightest. Cube of water, a dvision of the distance between the pole and the equator, done in Base 10. The French didnt just wave their hand and pull a number out of their fucking ass, it was a deliberate process.
Are you oen of these butthurt Yanks that still want to use moon units?
(Score: 2) by DECbot on Thursday October 15 2015, @11:02PM
Are you possibly suggesting that instead of arbitrarily pulling a number out of their ass, the French designed a process to deliberately pull a number out of their ass?
Though by strict definition, if you declare that you must wave your hands prior to pulling a number out your ass, you've just established a process. I'd like to see the engineering change request on that one.
cats~$ sudo chown -R us /home/base
(Score: 1) by Francis on Thursday October 15 2015, @11:32PM
A meter was originally 1 10 millionth of the way from the equator to the North Pole, that seems rather arbitrary to me. Most of the other measures are similarly problematic. They work as a grouping, but most of them are arbitrary, and to make matters worse they're defined by things that aren't within our daily life. So, you wind up with a system that works well for science done by humans, but likely would have to be redone for non-human scientists as the constants regularly make assumptions about the units being used.
Anyways, the one set of measures that everybody agrees suck, relate to time, but you haven't seen any push to make that metric.
I think it's hilarious how the Europeans seem to feel the need to mock the Americans for not buying into that stupid bullshit for daily living. I've lived in the US and abroad and metric measures are just not useful in daily living except by force. I used them when abroad because I had no choice, but it was pretty obvious when I was buying things by the half-kilo that the kilo is a stupid unit of measure. BTW, a half-kilo is pretty close to a pound.
(Score: 2) by schad on Friday October 16 2015, @01:30AM
Eh. I'm American, and honestly, it entirely depends on what you grew up with. I'm utterly hopeless with US units of volume, for instance, because I learned science before I learned to cook. My wife gives me shit every time I try to cook something because I have to look up conversion tables on my phone every 30 seconds.
It's really kind of strange how it works. 62mph seems very ordinary to me, but 100kph is hard to grasp. Tell me 91 ft/sec and I'm lost, but 28 m/s makes total sense. Tell me your car has a 349 cu-in engine and I'll give you a blank stare. But tell me that it's 5.7 L and I'll be duly impressed. At the small end, I have no problem visualizing 10mm (it's a common bolt head size) but 3/8" is weird. And forget about shit like 15/32" -- I have to think just to understand which side of 1/2" that's on (I wish I were kidding). Even after realizing that those "odd" sizes are almost always just 1/n to either side of 1/2 (15/32 = 1/2 - 1/32) I still struggle with it.
The most annoying thing about the US system is definitely that every type of unit has probably 5 or 10 different names. Teaspoon, tablespoon, cup, quart, gallon -- all units of volume. Is it really so hard to say "4 cups" instead of "quart?" (It's 4 cups to a quart, right?)
(Score: 1) by Francis on Friday October 16 2015, @01:54AM
This kind of reinforces my belief that people that don't know the imperial system shouldn't be knocking it.
1 cup is 8 ounces. 1 pint is 16 ounces, 1 quart is 32 ounces, 1 gallon is 128 ounces. The only bit that gets a tad confusing is that a tablespoon is 1/16 of cup. But in general, you don't normally go between cups and tablespoons or teaspoons very often. The only time that happens is if a recipe calls for 1/4 cup of something and you need to go smaller. If you find that annoying, you can mostly dispense with those and use fluid ounces. It's not really that hard to go with .5 fluid ounces in a table spoon and so forth.
Yes, that sounds confusing, but really, you spend 10 minutes learning it and then you get to benefit from the utility of the system. You can start using all sorts of cups that aren't technically measuring cups for measuring. A 16 ounce cup that you use for drinking is all of a sudden double the size of a measuring cup. Which is a real convenience if you don't have an extra measuring cup on you.
As far as visualizing, why on earth would you be visualizing that sort of crap? 15/32" is just slightly smaller than half an inch. 349cu-in isn't really any harder or easier to visualize than 5.7L is. In both cases it's a relatively small engine.
(Score: 2) by tangomargarine on Friday October 16 2015, @02:45AM
Dividing something in half is rather easier to eyeball than into 10 as well.
Imagine trying to cut a yardstick/meterstick into 10 equal pieces without using the markings. First you cut it in half...then...hmm...you have to figure out a good way to eyeball cutting it into equal fifths.
(but of course a yardstick is a multiple of 3 so that doesn't work either...except foot is 12 which is divisible by 2, 3, 4, and 6 so you can do some juggling)
"Is that really true?" "I just spent the last hour telling you to think for yourself! Didn't you hear anything I said?"
(Score: 2) by soylentsandor on Friday October 16 2015, @07:27PM
Anyways, the one set of measures that everybody agrees suck, relate to time, but you haven't seen any push to make that metric.
Only because it died long before you and I were born [wikipedia.org]
(Score: 3, Informative) by gman003 on Thursday October 15 2015, @10:50PM
Artifact definitions (eg. "a kilogram is as massive as this cylinder of platinum") are subject to change over time. The kilogram prototypes have varied in mass by up to 60ug, and we have no way of knowing whether they are gaining or losing mass on an absolute measure.
A phenomenal definition (eg. "a kilogram is the amount of mass of a photon with this much energy") does not change over time. The kilogram of today might not be the kilogram of 1915, but the kilogram of 2115 will still equal the kilogram of today.
(Score: 3, Informative) by Thexalon on Thursday October 15 2015, @10:57PM
Kind of. The original idea of metric was this:
1. Metres were 1 / 10,000,000 of the distance from the Equator to the North Pole. Sure, that's arbitrary-ish, but it was defined by a measurable natural feature and the power of 10 that corresponded best to a reasonably convenient size.
2. Grams were the mass of 1 cm^3 of water. So already it's related to another unit.
The problem was that those definitions were not precise enough, which is why metres and kilograms were switched to physical standard objects, and later the second was defined in terms of cesium atom fluctuations at absolute 0 and the meter defined based on the distance light travels in a vacuum during that second. But from the beginning they were at least thinking along the lines of properties that anybody could measure, rather than, say, the extremely arbitrary length known as a "foot".
The only thing that stops a bad guy with a compiler is a good guy with a compiler.
(Score: 2) by Alfred on Friday October 16 2015, @03:39PM
I would think a natural definition would pick a nice round number with few significant figures. Like exactly 1E6 wavelengths of something. I would also be cool with a nice round number like 2^28 (which gives you 9 sig figs in base 10). That would also be arbitrary though, just like picking that orange-red line.
To me this is worse because it now depends on another fundamental (and arbitrary) unit which depends on oscillations of Cesium. Kind of like your second point about the gram. And again, 9 significant figures.
9 significant figures again but this time it is independent from the other base units. To be fair the significant figures are used just to try and match what was grandfathered in.
In reality there are no perfectly natural measurement units. I really shouldn't pick on the decisions of guys that have been dead for 100 years. If the meter was some other arbitrary length I would still use it the same way. Since none of the increased accuracy from this activity will ever find use in my home maybe I shouldn't even care. But still, all units are arbitrary even if thoughtful.
*quotes from https://en.wikipedia.org/wiki/Meter [wikipedia.org] and https://en.wikipedia.org/wiki/Second [wikipedia.org]
(Score: 2) by Thexalon on Friday October 16 2015, @04:33PM
My point was that the original idea was a nice round number: 1e-7 * 1/4 C where C is the circumference of the Earth. The problem was one of precision, not meaning.
If you want a completely not-arbitrary system of measurement, try Planck units [wikipedia.org], which are all based around known fundamental constants in the universe like the speed of light in a vacuum. I mean, it's not exactly convenient to represent the distance from where you are to, say, Times Square, in Planck Lengths, but it's at least possible.
The only thing that stops a bad guy with a compiler is a good guy with a compiler.
(Score: 2) by maxwell demon on Friday October 16 2015, @06:49PM
Well, if the units will be defined by the values of the fundamental constants, then it essentially is the same as defining the meter as a certain number of Planck lengths, etc.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by Alfred on Friday October 16 2015, @07:07PM
(Score: 3, Insightful) by wonkey_monkey on Thursday October 15 2015, @11:46PM
There are very few units that are not arbitrary.
That's not the point of the story. It's about how precisely they can be defined, which is all you need to do to have a workable system of related units.
This is an "oh whatever" moment because there is hardly a person who will care except people who
...are interested in scientific achievement for its own sake? Hmm. I think that's me.
systemd is Roko's Basilisk
(Score: 4, Insightful) by Beryllium Sphere (r) on Thursday October 15 2015, @11:48PM
The meter and the second standards can be reproduced exacty by any well-equipped lab on or off the planet. Depending on a bar in a vault is pragmatically inferior. Worse yet, that bar is fluctuating in mass for mysterious reasons.
(Score: 3, Funny) by isostatic on Friday October 16 2015, @01:28AM
No it's not. The bar is always 1kg, by definition, so can't be fluctuating in mass.
(Score: 0) by Anonymous Coward on Friday October 16 2015, @02:36AM
Holy fuck, the whole universe is fluctuating in mass!
(Score: 0) by Anonymous Coward on Friday October 16 2015, @12:28PM
I know you are joking, but even formally you're wrong. That the bar is by definition 1kg just means that the number to describe its mass (the factor 1) doesn't change; the unit, kg, does, however.
With the new methods, it should be the number that fluctuates, while the unit remains constant. I think it's obvious why that is an advantage.