Kilogram Gets a New Definition (bbc.com) 187
Scientists have changed the way the kilogram is defined. Currently, it is defined by the weight of a platinum-based ingot called "Le Grand K" which is locked away in a safe in Paris. On Friday, researchers meeting in Versailles voted to get rid of it in favour of defining a kilogram in terms of an electric current. From a report: The decision was made at the General Conference on Weights and Measures. But some scientists, such as Perdi Williams at the National Physical Laboratory in the UK, have expressed mixed feelings about the change. "I haven't been on this project for too long but I feel a weird attachment to the kilogram," she said. "I think it is such an exciting thing and this is a really big moment. So I'm a little bit sad about [the change]. But it is an important step forward and so the new system is going to work a lot better. It is also a really exciting time, and I can't wait for it to happen."
Le Grand K has been at the forefront of the international system of measuring weights since 1889. Several close replicas were made and distributed around the globe. But the master kilogram and its copies were seen to change -- ever so slightly -- as they deteriorated. In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering -- those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.
Le Grand K has been at the forefront of the international system of measuring weights since 1889. Several close replicas were made and distributed around the globe. But the master kilogram and its copies were seen to change -- ever so slightly -- as they deteriorated. In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering -- those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.
excitement (Score:3, Insightful)
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This is good, and very important. But exciting?
To people who deal with precise measurements yes it is very exciting. Maybe not to you but certainly to some of us.
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Re:excitement (Score:5, Insightful)
Yes. Very much so. Even to those outside the field. To officially, precisely, and accurately say something is a "kilogram" our current method was well over a hundred years old and basically sneaker net.
You took a clone of the official standard and compared it against that. The clone couldn't be directly touched, couldn't overly jostled, had an expiration, sits in a double vacuum, and had to be physically moved in proximity to what was being measured or, more likely, calibrated.
With this method, you can build a simple machine (Kibble balance) [nearly] anywhere in the universe, calibrate it, find its measuring relationship to the universal constants, and you can figure out the "kilogram". THAT IS AWESOME.
Layman terms: Before, you had to say "I want this many rocks worth of your stuff." Followed by handing over the rocks. Now you can say "We both know what 1 and 3 are. I want 15 of what you got."
The measurement of how much of something you have, the kilogram, officially stands at the same level as the Meter, Second, and Temperature. All of which are based on universal constants and not the measurement of some useless fool's biology.
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Also, very importantly, this method yields a constant reference. The old kilogram could be losing or gaining weight, and you wouldn't really be able to tell for sure.
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Lets get full on pedantic.
I read your explanation and while I found it interesting, I did not however find it exciting. I suspect this reaction is probably common among most of the geek audience here.
A small subset of readers (and people within the general population) who deal with precise measurement and may be directly or indirectly effected by this may have become excited, but to most the reality that what we refer to as a kilogram is ultimately derived from a brick locked up in a vault somewhere in Fran
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I think if there's such a calamity we won't give a shit about having accurate measurements, we'll be trying to avoid being eaten.
We got from length of the king's arm to the distance light travels in so many vibrations of a something atom. When the need arises, we can do it again.
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Every time this subject comes up you start wittering on about drug dealers. Do you think it makes you sound edgy or something.
P.S. Shouldn't you be in hiding from the caravan? Scary scary brown people who talk funny!
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No, just strikes me as kinda funny...and also my usual observation that the US doesn't use the metric system for anything in most citizens' real daily lives.
That no one really uses it here.
Re:So.... (Score:5, Funny)
No, just strikes me as kinda funny...and also my usual observation that the US doesn't use the metric system for anything in most citizens' real daily lives.
That no one really uses it here.
I know. I was just saying the same thing the other day when I picked up a 2 liter of soda and a bottle of 500mg aspirin at the store. While I was out, my daughter asked me to pick up a new set of headphones with a 3.5mm jack. Since I was at the store I picked up some new LED bulbs to lower my monthly kilowatt usage at home. That reminded me that I needed to order some new 80 and 120mm case fans for my desktop too. I'll probably add 16 more GB of RAM while I'm at it. On the way home some idiot in a 5.0 liter Ford Mustang cut in front of me and got out of his car carrying a baseball bat. Fortunately when he saw my Glock 9mm, he got back in his car and left 2 seconds later. I don't know how he knew I had my wife's 3 carat emerald ring in the car.
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car carrying a baseball bat
Baseball bat? Is that the imperial version of the cricket bat?
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The pound is defined as a derivative of the kg, so yes this affects you too.
Weight != mass (Score:3)
Pound is a mass (Score:2)
https://en.m.wikipedia.org/wik... [wikipedia.org]
It is exactly 0.45359237 kilograms
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Yes, but you can't compare apples and urang utangs.
If the assertion is that pound is not mass, I do not have to prove that every definition fits that. It's the difference between "for all" and "there exists". That's why pound the currency isn't a useful sense. If I can show a sense exists that is indeed a mass, not a weight, then the post I replied to is wrong because it denied such a sense existed.
Your post shows their sense also exists, but that doesn't matter because I never asserted an exclusive sense,
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Yes, your statement is pedantically correct. A "pound" can be expressed as a mass, disproving the OPs statement that a pound is a strictly a weight. But your post is also incomplete, in that it tends to imply that a pound is ONLY a mass. My intent was to point out that you were both correct.
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Yes, your statement is pedantically correct. A "pound" can be expressed as a mass, disproving the OPs statement that a pound is a strictly a weight. But your post is also incomplete, in that it tends to imply that a pound is ONLY a mass. My intent was to point out that you were both correct.
Frankly, they're not the the pedantic one here. You are. If a person says one unit of measurement is based off of another unit of measurement, it would be proper to assume that the definition used for one of the terms of measurement was the one valid for the relational statement. It would be improper to assume the person was incorrectly using the wrong definition for one of the terms.
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If the assertion is that pound is not mass...
The pound cannot be a mass because otherwise "pounds per square inch" would be a unit of 2D density (like kilograms per square metre) and not a unit of pressure (like newtons per square metre). The imperial unit of mass is, according to wikipedia, the slug [wikipedia.org]. If you try to use the pound for both weight and mass then you have a system of units which is inconsistent with basic physics. Indeed the fact that imperial units appear to rely on the definition of a force in terms of a mass really just demonstrates ho
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You are conflating many different definitions and assuming a grandiose revelation.
All you reveal is that you're an idiot.
The definition of psi is pound-force, not pound-mass.
These are not, repeat not, the same thing although both called a pound.
I have no further time to waste on your ignorance of imperial units. Or, indeed, your ignorance in general.
Gravitational Field Varies (Score:2, Offtopic)
The strength of the earth's gravitational field varies. If you are using a Kibble balance to calibrate your weights, how do you compensate for that? Your kilogram mass will vary from location to location.
Google: earth gravitational field
https://earthobservatory.nasa.... [nasa.gov]
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By it's very definition a balance in independent of gravity. The balance will remain the same and that 1Kg *of mass* will work just as well if you try this experiment on Jupiter.
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In this case, the balance works by applying a magnetic field on one side, not a weight, so it's not independent of gravity.
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so it's not independent of gravity
Magnetic fields have an effect on gravity so small as to not be relevant even at the precision we are talking about. This effect is relative to the existing gravitational field and above all ... kibble balance. Unless you make the balance 588 million km wide and have one side of it on Jupiter and the other side here on Earth, no the measurement is for all intents and purposes including those requiring the defining of an SI unit, independent of gravity.
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That was not my point. My point was that it's a balance with a magnetic force on one side, and a weight on the other. One side is independent of the gravitational field, the other side isn't. In order to use the Kibble balance, you need to first measure local gravity and calibrate that.
Re: Gravitational Field Varies (Score:2)
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Effect as a verb means cause. The primary cause of tides is the moon. The sun has some influence.
So, the moon effects tides. The sun affects them.
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Right I got you now.
Re:Gravitational Field Varies (Score:5, Funny)
By it's very definition a balance in independent of gravity. The balance will remain the same and that 1Kg *of mass* will work just as well if you try this experiment on Jupiter.
Although if you’re attempting this on Jupiter, you’ll likely be distracted due to asphyxiation and crushing pressure - so work fast.
Re:Gravitational Field Varies (Score:5, Informative)
By it's very definition a balance in independent of gravity. The balance will remain the same and that 1Kg *of mass* will work just as well if you try this experiment on Jupiter.
But a Kibble Balance isn't that sort of a balance. It's a "single pan balance" which balances gravitational acceleration against acceleration caused by a magnetic field. So the Kibble Balance is very sensitive to changes in the gravitational field.
Luckily, it's possible to measure the local force of gravity with extreme precision, without reliance on the definition of the kilogram. It's done with dropping-mass gravimeters that measure the deflection of a laser beam, so it only relies on standard units of distance and time, and the speed of light as measured in terms of those units, not on the definition of mass. Obviously this is crucial or else you'd need a definition of a kilogram in order to calibrate your Kibble Balance.
So you can do this on Jupiter just fine, but you first have to measure the local gravitational field and adjust the amount of current you feed the Kibble Balance to balance against your kilogram test mass.
The biggest downside of this new method of defining the kilogram is that turning the definition into a measurement is incredibly precise and difficult work. It's so expensive to do correctly that for the foreseeable future there will probably only be a handful of wealthy countries who bother to do it. This means that for practical work, the definition will just be used to calibrate the exemplars that are used today, and everything else will continue as always. But it does mean that we now have a definition which is independent of those exemplars and guaranteed to be perfectly unchanging as long as the Planck constant remains constant.
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It's a "single pan balance" which balances gravitational acceleration against acceleration caused by a magnetic field.
Except it has a resting state where gravity acts on both sides of the balance which gives you effectively a calibrated zero independent of any gravity. Unless the gravity on one side of the balance is different from the gravity on the other side. ... And if that is an issue it would probably be a good idea to see if Gordon Freeman is standing behind you whacking things with crowbars.
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Sorry disregard that, logic error, off for coffee.
Measure g too (Score:3)
By it's very definition a balance in independent of gravity.
No it is not. Even a standard balance relies on the gravitational field for both sides being equal and, if you get precise enough, this may not be true. However, the watt balance balances the force of gravity with an electromagnetic force. Part of the measurement also requires determining the local gravitational field but this is something that you can measure accurately which is why this is still a far better definition than using a lump of metal outside Paris.
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There are techniques to measure local gravity field without calibrated weights. For instance, you can measure the time it takes for an object to fall.
Re:Gravitational Field Varies (Score:5, Informative)
Yeah, they should have left out the Kibble balance part. That's more confusing than illuminating. In terms a layman can understand, kilogram is now defined the same way meter is: by defining a related physical constant to be an exact value.
Meter is defined today not by a physical object, but by defining speed of light to be exactly 299,792,458 m/s (that is, in significant figure terms, there are significant zeros following the decimal for-ever). With time defined by the atomic clock standard, this definition of speed of light also defines what a meter is (and many different experimental arrangements can be designed to use this relationship to actually calibrate real object).
With the vote today, kilogram is now defined by defining Planck's constant to be exactly 6.626070040 * 10^-34 kg*m^2/s (um, Wikipedia's not updated yet; the exact value they chose might be different from this number; important thing is that the value they chose now has infinite number of significant figures). Since meter and seconds are already defined, defining this constant defines the kilogram, and clever experimentalists can come up with better methods than Kibble balance for calibrating any local kilogram standards.
P.S. BTW, for scientists working in precision measurement area (the area NIST and NSF funds as they relate to fundamental science), this is an exciting news. It's a validation of accomplishments of their field, on the same (or possibly greater) magnitude was when atomic clock standard was adopted for the definition of second.
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infinite number of significant figures
First thanks for the explanation, but are you sure your meant 'infinite'?
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Not OP, but I'm pretty sure the answer is 'yes': the whole point is that it is not 299,792,458.00, or 299,792,458.000, but *exactly* 299,792,458, so with as many zeroes after it as you wish that are actually significant.
Re:Gravitational Field Varies (Score:4, Insightful)
That's right.
P.S. Replying to say that Wikipedia did list the value of Planck's constant as newly defined [wikipedia.org]: h = 6.62607015 * 10^-34 J*s. What's special about this value is that it's exact (note the lack of specification of experimental uncertainty), so if you were to write it as h = 6.626070150000000000000 * 10^-34 J*s, as annoying as that might be, you are not wrong (all those zeros—and more—are significant).
P.P.S. I guess they didn't update the table since the value is not effective until May.
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infinite number of significant figures
First thanks for the explanation, but are you sure your meant 'infinite'?
Unlimited is probably a better word.
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First thanks for the explanation, but are you sure your meant 'infinite'?
Yes, it is a definition, not a measurement. Essentially we define the fundmental, unchanging constants of the universe as so many SI units and then use measurements to precisely define the individual units. In fact, in particle physics, we actually start by defining these fundamental constants as '1' unit each and then derive units for energy, momentum, mass etc. from them creating a system known as "natural units". It's not very practical for everyday quantities but very useful if you are dealing with fun
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But the kilogram is now defined by the Ampere, which is defined as the current which creates a Force of 2×107 newtons per metre between wires 1 meter apart. but how do you measure a Force without knowing what a kilogram is?
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Or you could measure current by counting electrons.
https://www.nature.com/article... [nature.com]
drug development, nanotechnology (Score:3)
How many times do you need a very precise absolute measurement in drug development or nanotech ?
all units are reducible to Libraries of Congress (Score:2)
This makes dimensional analysis a lot easier.
Boy this guy's fast... (Score:5, Informative)
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It's been in the news several times lately as its worked through the review process. I'm guessing Randall had the strip ready to go.
1 Kg = 1 Lb (Score:2)
Are all electromagnets equal? (Score:2)
Doesn't each kibble scale require calibration whenever the altitude changes? What do they use as a reference for calibration since it appears even the Kg reference is not stable?
- Yep I got plenty of dumb questions.
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The Kibble balance uses measurements of voltage, current, velocity and gravitational acceleration in order to calculate the mass that was placed on the balance. Here's the maths [vox-cdn.com] involved.
Then it's likely that those measured masses will be used to calibrate other equipment, as is done now with reference masses that were compared to Big K.
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Don't be an idiot. This is the result of 20 years of work, a vast amount of actual data underlies the decision taken today.
Planck's constant redefined (Score:3)
Basically what they did was they defined Planck's constant to be a fixed value pretty close the the calculated number previously used. So instead of calculating Planck's constant from an arbitrarily defined kilogram they define the kilogram (and a few other constants) from an arbitrarily defined Planck's constant. This takes the error bars away from Planck's constant and the other fundamental measurements fall out naturally as a result to precisely defined and fixed numbers.
Within a drop Weight of 1 Liter of Water? (Score:2)
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Veritasium (Score:4, Informative)
Our friend at Veritasium does an excellent job breaking this down:
https://www.youtube.com/watch?... [youtube.com]
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Le Grand K is dead. Long live the Le Grand K (Score:2)
A Kibble balance, that will define the kilogram will built using platinum, iridium and other exotic metals. It will be housed in the double walled basement of SI building. This le Grand K(ibble) will be the standard Kibble balance against which all othe Kibble balances will be measured and tested against.
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The difference is that you can build additional Kibble balances from scratch, without using the existing ones as reference.
It is not a big moment (Score:2)
A moment is defined as kg.m^2
The new definition is thus a unitary moment per unit area.
A little concerned (Score:3)
The second is out of date. Caesium is a horribly outdated method of measuring time. Modern atomic clocks, using strontium quantum gasses, are roughly ten orders of magnitude better.
But because of how the second is defined, you can't use a more accurate clock. The errors in caesium clocks are part of the definition. Remove the error and you're not measuring seconds even if you're measuring more accurately.
It's probably better to use fundamental units as the starting point, or at least something close, rather than arbitrary objects in nature.
Ideally, it shouldn't matter if things get measured more accurately, you won't break anything.
If you can't do that, then the definitions should be aiming at the ten orders more accurate results that can be obtained.
As for constants, they should be justified geometrically, kept simple, or defined in terms of underlying physics.
Re:I prefer the pound (Score:5, Informative)
The international pound has been defined as exactly 0.45359237 kg.
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I thought it was a joke.
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Re:I prefer the pound (Score:5, Insightful)
Wrong, the pound was defined as that in 1959. nothing changed here
Since the pound is defined in terms of the kilogram, changing the definition of the kilogram implicitly changes the definition of the pound as well.
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Starts out like: 2.204622621848775807229738013450270338542070273360197835779292319224\ 8582135541653842193156820517064694...
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The international pound has been defined as exactly 0.45359237 kg.
That's interesting. Since the pound is a weight and the kilogram a mass unless they also fixed the gravitational field in that definition it's open to abuse.
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Except that since the pound is defined as a fraction of a kilogram, it too is implicitly a measure of mass, not weight.
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PSI uses "pound" as a measure of force, where one pound-force is equal to the force exerted by one pound (mass) under the acceleration of 1g. One pound-force is what is meant when most people speak of "pounds", but in common usage the same is true of the kilogram - most people mean "kilogram-force" when they say "kilogram". The fact remains that "pound" is an SI-derived unit of mass, not weight.
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The international pound has been defined as exactly 0.45359237 kg.
That's interesting. Since the pound is a weight and the kilogram a mass unless they also fixed the gravitational field in that definition it's open to abuse.
Just to make things even more complicated, the pound is used for both weight and mass.
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This. In theory the non-metric unit of mass is supposed to be the "slug" but pound gets overloaded just like kilogram does, to be both mass and force.
Re:Genius (Score:5, Funny)
I'm amazed at your constant ability to out-think all of our scientists. Your value is wasted as a Slashdot troll. If only they'd have seen you for your genius we'd have cured cancer by now.
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I understand: newer is always better.
Yeah I know. Defining the Kg to something fixed is right up there with replacing sysv with systemd.
unlike the old method, which was 0.000005%.
Plus and Minus a random change that makes it impossible to know what it was or will be for sure.
Yes genius (Score:3)
Any individual Kibble balance will also deteriorate and be out of range of the others. There will also be differences in construction since no system is perfect. You might as well use the current method.
I have a hard time telling if you are a troll or just ignorant. What they did was they redefined Planck's constant to be a fixed number. So instead of measuring Planck's constant from an arbitrarily chosen value for the kilogram they measure the kilogram from an (sort of) arbitrarily chosen Planck's constant.
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...no system is perfect.
The new method is not purported to be perfect. It does, however, allow people to measure a kilogram with as much accuracy as the current system without shipping reference weights around the world for comparison.
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Unless you're measuring grams of water, then you're measuring volume.
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but do we have the technology to build something up one atom at a time?
Not accurately enough. There'd be some extra or missing atoms in the final product.
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I don't think a five year old would fall for that one.
Besides, what's so special about le grand K? There are dozens of duplicates, all equally valuable platinum.