The Changing Definition Of 'Kilogram' 964
DrLudicrous writes "The NYTimes is reporting that the platinum-iridium standard mass for the kilogram is shedding at an appreciable rate -- at least compared to other reference masses. The Pt-Ir cylinder is kept in France, and measured annually, and the slight discrepancy is important because the kg is an SI base unit- thus other quantities such as the Volt are based on it. A new standard is being sought- the two frontrunners are counting the number of atoms in a perfectly spherical single crystal of silicon, and another technique uses a device known as the Watt balance."
Kilogram? (Score:5, Funny)
Re:Kilogram? (Score:5, Funny)
Re:Kilogram? (Score:5, Informative)
A friend is in construction, and guestimates that it will take over 100 years to replace all failing/obsolete tech with the versions in metric equivalents. It just does not make any economic sense to replace a set of, say, water pipes with the metric standard if the current ones will last 20 years. It'll have to be a gradual thing.
Just to be difficult, though, I'd mention that most construction is done in 'tenths of feet', even the surveying equipment is marked this way. Has nothing to do with the metric system, it just makes the math easier...
Re:Kilogram? (Score:5, Insightful)
Bingo! this is why the US has been working on the process so long. Granted the push hasn't been very great but it's happening. If you're a country of a few million and only are the size of a small new england state, the change is pretty cheap and easy. When your huge, there is a massive infastructure change cost. and trying to re-wire 300 million peoples brains to a new way takes a lot more work.
I think places like Europe were also helped by war. They had to rebuild and start new with so much. So it was a perfect time to start fresh. The US is a pile of legacy ways. And nothing happens to change them.
With that said I wish we would try harder to convert. Get a dual system going now and run it for 20 years. let people adjust. Teach school in 95% SI ( only enough english units stuff so the comprehend them).
Re:Kilogram? (Score:5, Insightful)
Re:Kilogram? (Score:4, Insightful)
As the poster below me said, you are quite wrong. [unc.edu]
The point is the US has had 200 years and they haven't even started the process. There's nothing saying you can't run in parallel - the UK has been doing so for years. It's absurd to say you have to rip out all the imperial pipes and replace them - you just have to keep 2 sets of tools around until those old pipes get replaced naturally.
No, you really don't get it at all. As it happens, most people who have tools ALREADY have the two sets of tools. What makes switching difficult is having two sets of PARTS. It's all well and good to say "from now on all parts/raw materials will be measured using the metric system", but what does one do about, say, electrical conduit fittings? There is an UNGODLY amount of installed bass there which is already in inches and adding on to it would require a complicated system of adapters and a complete recalculation of wire capacity. Name any other construction trade and you run into the same thing. How do you add on to an inches-and-feet house with metric lumber? What size metric ducting do I buy to add to a 12-inch heating plenum? Not saying that it can't be done, but there's a lot more to it than "keep[ing] 2 sets of tools".
Re:Kilogram? (Score:5, Interesting)
I do get it. This already happens in the UK, it's not a problem at all. We have lots of houses which are older than the metric system (and the USA for that matter). They use imperial stuff. We have lots of new houses - they use metric. And yet I can still call a plumber and he can figure out how to fix my pipe, and my electrician is able to fix a light. Amazing.
If there was any will to do it you'd do it, which indicates there's no will. Which is fine, I don't give a toss what you measure your wooden houses in, but don't come over all "it's too haaaaaard" - you sound like a whinging kid.
Re:Kilogram? (Score:4, Funny)
Pipe-sizes are not that simple... (Score:5, Interesting)
Just to confuse the matter more, in the 1970s, it was common to use metric sizes of threaded copper pipe, which had external diameters in sizes approximating common fractions of inches: 13mm = 1/2", 16mm = 5/8" and 19mm = 3/4" just to mention some of them. These appearently were all threaded with 1mm pitch threads.
Later, these were replaced by true metric pipe sizes with compression fittings or capillary solder fittings. Now the sizes changed again, common ones are 8, 10, 12, 15, 18, 22, and 28 mm. And of course, one needed compression fittings made for 16mm and 19mm also, so as to fit the older pipes...
That's Europe. What I have seen in the US are the commonly found so-called 1/2" copper pipes with solder fittings, this is about 16mm (5/8") in diameter, so I guess they are still using internal diameter measurements. Similarly, the so-called 3/4" pipes appear to have about 21mm outside diameter.
I guess the easiest way to turn these into metric sizes would be to redefine them as 16mm and 21mm and leave it at that. At least the traditional inch-units pipe thread sizes are roughly the same everywhere!
Re:Pipe-sizes are not that simple... (Score:5, Funny)
Re:Pipe-sizes are not that simple...Pipe & Tub (Score:3, Informative)
"Pipe" is described by it's nominal diameter and strength ("schedule"). Nominal diameter is neither internal nor external. ex: 4" schedule 40
"Tube" is defined by the external dimension (not necessarily diameter) and wall thinkness. ex: 4x4x1/4 (a square tube, 4" on a side, with a 1/4" thickness.)
I Agree - We should go metric (Score:2)
But honestly its (the kilo) as arbitrtary as any other unit we have come up with to describe reality (red, one second, a kilometer) so why rewrite the bible as it were and change something like this?
I know the implications could be staggering, but why not chalk it up to having a leap year and other silly things about our units of measurment.
Re:I Agree - We should go metric (Score:5, Funny)
The only countries left that don't use metric are the US and Bhutan. Bhutan is a fundamentalist islamic country that doesn't even have any phones yet. I guess we can see what the US' technical level is.
Re:I Agree - We should go metric (Score:3, Informative)
Re:I Agree - We should go metric (Score:5, Informative)
Fundamentalist Islamic country [kingdomofbhutan.com] without any telephones [cia.gov]?
Can I have some of whatever your smoking please?
Re:I Agree - We should go metric (Score:3, Informative)
Distances/speeds (miles/mph), beer (pint) and (in some cases only) milk (pint), we are fully metric. Personally I'd welcome a full switch but we have to wait for the old people to die first
So is the US (Score:5, Informative)
And the metre is defined properly these days (as is the second) in terms of wavelengths of radiation.
Re:I Agree - We should go metric (Score:4, Informative)
Hmm, so instead of a year being 365 days long you would want it to be 456 days long? (365 days * 1.25 = 465 days)
A leap year has nothing to do with anyone screwing up. The problem is that a year does not have an integral number of days. A year is 365 days, 5 hours, 49 minutes (365.2424 Universal days)*. That means that it takes about 526,297 minutes for the Earth to make a full trip around the sun. After the Earth has rotated about its axis 365 times it will still take about 350 minutes until it reaches the same spot it started from.
That means that if you tried to have the year be an even number of days, say 365, you would fall behind almost 1 full day every 4 years. It's not much but if you let it go for a while you will start having winter during the hottest times of the year. There are a few other rules that adjust the calendar besides the "extra day every 4 years" rule and because of these rules we are able to keep the seasons approximately where they should be.
To learn more about how the calandars were changed visit this web site [webexhibits.org].
*source: Timeline of interesting calendar facts [webexhibits.org]
Re:I Agree - We should go metric (Score:5, Funny)
Why don't we just get a big rocket, and alter the orbit so that it is exactly 365 days ? Or better yet 366 days, then we can give everyone a holiday (in rememberance of all of the species that were extinguished for our selfish ends).
Re:Kilogram? (Score:5, Funny)
Yeah, tell it to the Queen.
Re:Kilogram? (Score:3, Interesting)
Goblin
Re:Kilogram? (Score:5, Funny)
Congress authorised the use of the metric system in 1866.
The US signed the Treaty of the Meter in 1875.
Congress passed the Metric Conversion Act in 1975.
So clearly the US *is* on the metric system
Re:Kilogram? (Score:5, Insightful)
Me, I have been doing a lot of woodworking lately. It's convenient to use a unit (the Foot) that divides easily into subunits that are multiples of both 3 and 4, without having to get all mired in floating point arithmetic.
But we have this metric flamefest every time the metric system comes up on Slashdot, and the same crap comes up every time.
I'm just happy that pointy-head metric zealots don't seem to have much pull in the real world of regular people. Keep on ranting, dudes.
oh oh! a funny too! (Score:3, Interesting)
There's one sign on I-19 that I find absolutely hilarious though. It says something along the lines of:
Ajo Rd - 1000 m
Irvington Rd - 3000 m
Valencia Rd - 5000 m
The theory - Either they
A) ran out of 'k'.
B) had a whole bunch of '0's to get rid of.
C) don't quite get the concept.
Re:Kilogram? (Score:5, Funny)
Re:Kilogram? (Score:5, Interesting)
Personally, having gone through school at a time when the US was considering a change, and having spent some time in Europe, I have no problem with the metric system. It is more convenient from some tasks, particularly in the chem lab.
But there is nothing inherently superior about a measurement system based on powers of 10. For many tasks, such as woodworking, metric measurements are far more difficult to work with than inches and 1/16th. In fact I would argue that the most "natural" base for a measurement system is 12 as it is evenly divisible by 2, 3, and 4; whereras base 10 is only divisible by 2 and 5. Thirds and fourths are very common divisions of stuff; fifths are not, so a base 12 system is more user-friendly.
That's my 0.02 euro anyway.
sPh
Re:Kilogram? (Score:3, Funny)
America? Bah! All you primates should move out of the dark ages sometime. Base-12 is where it's at.
We'll take our easily divisible Freedom Inches (tm) any day, thank you.
Re:Kilogram? (Score:5, Funny)
Re:Kilogram? (Score:4, Interesting)
Re:Kilogram? (Score:3, Insightful)
while this is still pretty true, many shops are going more metric as the amount of metric jobs increases. More and more machining tools are coming in metric to (largely to the CNC increase which is perfect for making transition). But since the change rate for machines is so slow it's more of the machiniest being behind then being the way it is. No one is expecting a shop to toss all t
Re:Kilogram? (Score:4, Insightful)
And since the inf^H^Hmperior^H^Hal system is now defined in terms of the metric system (an inch is 2.54 cm), your strange units change as well.
Re:Kilogram? (Score:5, Funny)
Hey I live in America you insensitive clod!
Ok, so for you it's "FreedomGram" then.
My wife changes her definition... (Score:5, Funny)
an excuse not to diet (Score:5, Funny)
I apply for the vacant job! (Score:5, Funny)
Surprise at no repeatable standard. (Score:2, Redundant)
If they succeed, we can get a reference standard for a kilogram that can be easily generated for scientific research.
Solution? (Score:5, Funny)
Mass not weight guy (Score:4, Informative)
A kilogram is the same on the surface of the earth, in outer space, or one the moon. Weight however, varies with gravitational pull or acdeleration.
In other words, weight is basically the mass of on object multiplied by whatever gravitational field you happen to be in.
Re:Mass not weight guy (Score:4, Funny)
Look here... (Score:4, Informative)
Re:Look here... (Score:4, Informative)
Counting Si (Score:5, Interesting)
Re:Counting Si (Score:5, Funny)
HUH?
porp
Re:Counting Si (Score:5, Funny)
Can I use the information to destroy the earth?
Re:Counting Si (Score:5, Informative)
Vacancies are not necessarily a problem. As you say, vacancies are entropically favored, but there is also a formation energy associated with a vacancy. So thermodynamics tells us there will be a balance between the energy required to create a vacancy with the entropy gained by creating one.
Thus, there is an equilibrium number of vacancies in any crystal. As long as you know what the equilibrium value is for a given temperature and you maintain that temperature, then you will also know how many vacant sites you will have on the crystal lattice. I don't have any of my texts handy, but I'm sure someone can chime in with the numbers for silicon.
To sum up. All crystals will have vacancies because vacancies are thermodynamically favored. However, the number of vacancies will tend towards an equilibrium value which allows them to be accounted for.
Re:Counting Si (Score:5, Informative)
Si single crystals are usually prepared at very high temperatures out of molten Silicon (1414C, Czochralsky method). Essentially, this will lead to a freezing of the defect structure at temperatures close to the melting point, because the lattice reorientation kinetics (point diffusion) also are thermally activated.
You would have to temper the crystal for _very_ long times at temperatures of i.e. 300C to get a thermal equilibrium of defects at this temperature. These times could be >>years !
No wonder I keep gaining weight... (Score:3, Funny)
Damned inreliable measure standards. *shakes fist*
How do they measure it? (Score:4, Interesting)
My question is, how do they measure it? Using a non-decaying meter stick? How do you measure the definition of a measure?
Filthy Whore (Text of Article) (Score:4, Informative)
By OTTO POHL
RAUNSCHWEIG, Germany -- In these girth-conscious times, even weight itself has weight issues. The kilogram is getting lighter, scientists say, sowing potential confusion over a range of scientific endeavor.
The kilogram is defined by a platinum-iridium cylinder, cast in England in 1889. No one knows why it is shedding weight, at least in comparison with other reference weights, but the change has spurred an international search for a more stable definition.
"It's certainly not helpful to have a standard that keeps changing," says Peter Becker, a scientist at the Federal Standards Laboratory here, an institution of 1,500 scientists dedicated entirely to improving the ability to measure things precisely.
Even the apparent change of 50 micrograms in the kilogram -- less than the weight of a grain of salt -- is enough to distort careful scientific calculations.
Dr. Becker is leading a team of international researchers seeking to redefine the kilogram as a number of atoms of a selected element. Other scientists, including researchers at the National Institute of Standards and Technology in Washington, are developing a competing technology to define the kilogram using a complex mechanism known as the watt balance.
The final recommendation will be made by the International Committee on Weights and Measures, a body created by international treaty in 1875. The agency guards the international reference kilogram and keeps it in a heavily guarded safe in a château outside Paris. It is visited once a year, under heavy security, by the only three people to have keys to the safe. The weight change has been noted on the occasions it has been removed for measurement.
"It's part ceremony and part obligation," Dr. Richard Davis, head of the mass section at the research arm of the international committee.
"You'd have to amend the treaty if you didn't do it this way."
That ceremony has become a little sorrowful as the guest of honor appears to be, on a microscopic level at least, wasting away.
The race is already well under way to determine a new standard, although at a measured pace, since creating reliable measurements is such painstaking work.
The kilogram is the only one of the seven base units of measurement that still retain its 19th-century definition. Over the years, scientists have redefined units like the meter (first based on the earth's circumference) and the second (conceived as a fraction of a day). The meter is now the distance light travels in one-299,792,458th of a second, and a second is the time it takes for a cesium atom to vibrate 9,192,631,770 times. Each can be measured with remarkable precision, and, equally important, can be reproduced anywhere.
The kilogram was conceived to be the mass of a liter of water, but accurately measuring a liter of water proved to be very difficult. Instead, an English goldsmith was hired to make a platinum-iridium cylinder that would be used to define the kilogram.
One reason the kilogram has lagged behind the other units is that there has been no immediate practical benefit to increasing its precision. Nonetheless, the drift in the kilogram's weight carries over to other measurements. The volt, for example, is defined in terms of the kilogram, so a stable kilogram definition will allow the volt to be tied more closely to the base units of measure.
A total of 80 copies of the reference kilogram have been created and distributed to signatories of the metric treaty. The sometimes colorful history of these small metal cylinders underscores how long the world has used the same definition of the kilogram.
Some of the metal plugs were issued to countries that later vanished, including Serbia and the Dutch East Indies. The Japanese had to surrender theirs after World War II. Germany has acquired several weights, including the one issued to Bavaria in 1889 and the one that belonge
reproducibility (Score:5, Insightful)
Since there is only one reference object for the kilogram, everything else is just a copy -- and even if it is a first generation copy, errors are bound to creep in.
The redefinition of the kg is long overdue, mad props to the scientists working on this.
Re:reproducibility (Score:5, Funny)
Re:reproducibility (Score:5, Funny)
"Stop stealing Mass, you fucks"
written through it
Troc
obligatory.... (Score:4, Funny)
Best units of measure (Score:5, Interesting)
One nominee that is amusing is to have the basic unit of distance based on the speed of light.
One light nanosecond = roughly 11.1 inches, kinda close to a foot.
I remember how Grace Hooper used to pass out wires that were that long, just to make the point.
Any other nominees?
That's kind of cool... (Score:3, Interesting)
So that's why it takes more. (Score:5, Funny)
Can someone help me convert here?? (Score:5, Funny)
Darn right! After all, it's easy enough to convert fortnights to stone with a Mayan calendar.
We're going to in the future eventually. It's inevitable.
I know it's 60 firesticks per 100 Watts, and 3000 Volts per staticy tomcat, but it might just be easier if we all just jumped in and switched to metric 144%.
I mean picture doing 100 on the highway! Wouldn't that be great? And dozens of future Mars landers would actually land on Mars, instead of digging ideal tree planting holes and landscaping future martian neighbourhoods. ("Zyphod! Incoming! It's the Americans!")
No more two sets of wrenches and lost sockets! Now you can have one set of sockets with half the sockets missing, instead of two sets of sockets with half the sockets missing. And no more asking for an 5mm and trying to make a 1 3/4" fit, rounding off the edges and carving a perfect turkey slice off your hand and gushing gallons of blood. It would be litres, which is less.
And you get to tell women that you, sir, are endowed with twenty-two centimeters of man!
Of course, the loss of the 25 cent piece will be a negative, since we'll have to pay for everything in dimes. But it's worth it dammit.
Seriously, we all know this is going to happen. When are we on board? Are we that stubborn?
Re:Can someone help me convert here?? (Score:3, Insightful)
As an American engineer I prefer Imperial units. The problem with metric is that all the conversion factors are 10.
If I do a "sanity check" on a metric calculation and find that it is about 10 times too big then I know I probably have a conversion error... somewhere. If I do a "sanity check" on an Imperial calculation and find it is a little over 10 times too big, then I immediately know to start by checking my inche
Re:Can someone help me convert here?? (Score:3, Informative)
Not true! In metric the unit of specific impulse (Isp) of a rocket is Newton*second/kilogram.
The real unit of specific impulse in Imperial is lb*s/lb where the pounds on top are pounds of force and the pounds on the bottom are pounds of mass. Pounds of force and pounds of mass are NOT the same thing and cannot really be canceled out because they are different units (if it helps think of it as lb*s/slug), but ever
Re:Can someone help me convert here?? (Score:3, Informative)
If you have done a lot of classical physics, then you should know what impulse is. In engineering, whenever
That's easy! (Score:3, Funny)
Why not use diamond? (Score:5, Interesting)
BTW, theNational Physical Institute [npl.co.uk] has a FAQ on its Pl-Ir standard kilo.
Re:Why not use diamond? (Score:4, Informative)
Re:Why not use diamond? (Score:4, Informative)
The same problems are still there, regardless of material. Changing the composition doesn't change the fact that there is only one standard in only one laboratory, that stray particles and cleaning will affect its mass upon measurement, and that the standard may be damaged in some way.
The other solutions presented as candidates to replace the standard rely on invariant physical constants, i.e. Avogadro's number. Distance and time standards are already defined in this way, from the speed of light and the frequency of a two-state cesium transition in the microwave region.
This shifts the accuracy of the standard from it's care and maintenance to the measurement of constants, with the added benefit of any appropriately equipped laboratory being able to measure the standard.
In other news (Score:5, Funny)
The 1 kilo square block was being held in Brussels awaiting return to Brazil, where it was originally unearthed.
It was determined that the physical stability of the material was being affected by being moved from it's original location, that of being south of the equator. Investigators are anxious to reclaim the material in hopes of stabalizing it's rumored flux in mass. The UB238 was being packaged for transit, when it suddenly dissapeared from the shipping room counter. The rumor that it had created, and subsequently fallen into, a 'portable black hole' was discounted by investigators on the scene.
Once the Unobtainium is recovered, and returned to Brazil, it can be weighed and certified as a replacement for the Pt-Ir cylinder that is kept in France [slashdot.org], and measured annually, representing the kilo standard for the world.
MPEG at 11.
Reference to a standard (Score:3, Insightful)
If we are maintaing a physical chunk of alloy as the standard, it's time to decide on a more precise measurement, like we did with the meter long ago.
if _kilo_gram is base (Score:3, Insightful)
Re:if _kilo_gram is base (Score:4, Interesting)
Incidentally, there will always be some units that end up with inconvenient sizes. Try going to your local electronics store and asking for a 1F capacitor.
Re:if _kilo_gram is base (Score:4, Interesting)
Acrylic Sounds [acrylicsounds.com] even has 10 Farad caps for sale.
Volt is no longer defined by Kilogram (Score:5, Informative)
http://hyperphysics.phy-astr.gsu.edu/hbase/solids
If I recall correctly, the eventual goal of the international standards organization was to find ways to define everything in terms of frequency/time since we can measure time so accurately/precisely.
Re:Volt is no longer defined by Kilogram (Score:5, Insightful)
Re:Volt is no longer defined by Kilogram (Score:4, Informative)
huzzah! (Score:5, Funny)
Wait till I tell my fiance that her weight
fluctuates on a weekly basis!
Alternative Physics (Score:5, Funny)
These pseudoscience concepts are getting out of hand.
I don't think we need "feel-good" physics.
Now they want to base a standard on a crystal ball?
It's about time (Score:3, Funny)
Wrong subject... yet... (Score:5, Funny)
Wow, someone should tell the computer industry that.
"Some of the metal plugs were issued to countries that later vanished, including Serbia and the Dutch East Indies. The Japanese had to surrender theirs after World War II. Germany has acquired several weights, including the one issued to Bavaria in 1889 and the one that belonged to East Germany."
SURRENDER YOUR KILOGRAM!
Re:Wrong subject... yet... (Score:4, Interesting)
I'm just curious about why. They couldn't be trusted with the technology of measuring mass?
I thought Avogadro's # of C12 was =df 12 grams (Score:3, Informative)
However, maybe I'm implicitly assuming that we have settled exactly what Avogadro's number is. But if we haven't, if we are still holding out for more and more accurate measurements of Avogadro's number, then yeah, we need to really nail down what a kilogram is. But that seems weird to me, because Avogadro's number has no units. It's just a count of atoms, playing the same grammatical role as the word "dozen".
Aaargh. (Score:3, Interesting)
Avogadro's number [wolfram.com] is a defined constant, so far as I can tell.
And since a molecule of C-12 is defined to be 12 amu, and since 1 mole of x-amu molecules masses x grams... isn't this already settled?
--grendel drago
Re:Aaargh. (Score:5, Informative)
The minute you can do that, then you can reliably and predictably create a fixed metric by which any one in any place can measure mass.
Kilogram != 1 litre water, sadly. (Score:3)
Freezing water was a bad idea, since the volume of water changes as it freezes, and I'm sure I read that they switched to 20C. The litre is, of course, a cubic decimetre or 1/1000 of a cubic metre, and is thus derived from the standard metre.
Whatever the reasons (practical?), the two standards were separated, but it's still quite easy to get a ballpark figure for the weights of fluids. Ten litres (2.624 gallons) of water weighs about ten kilograms (22.05 pounds). Some fluids will be less (gasoline), some more (beer, oils, mercury). There are other such shortcuts, too, so I ain't goin' back.
PS: If you Yanks are wondering why it's easier to get drunk in the UK, it's because a UK pint is 20% larger than a US pint. Standards are great - that's why we have so many of them...
The happiest day of my life (Score:4, Funny)
you mean I'm actually losing weight without doing diets or a workout !!
Great scott!! (Score:3, Funny)
That's almost a bolt of lightning by degrading metric standards.
On a more serious note, does the declining metre have anything to do with the rising Canadian dollar? And they [torontostar.com] say that Canada doesn't matter. Humbug, I say.
"No one knows why it is shedding weight" (Score:3, Informative)
-Peter
Silly artifacts (Score:3, Insightful)
IM(H)O, we need to do away with this, because artifacts exist in only one place. They can be stolen, damaged, or suffer from flaws and natural processes like the one we're seeing right now.
Of course, the flip side of having everything in terms of observable phenomena creates the problem of measurement, and making tools sensitive enough to do that work. Philosophically, the problem goes circular here, for how do you make a set of calibration weights for a scale, if you have to measure things to the atom first...
But in practice, there is no problem, because the measurement technology exists, and we're talking about the "standard" or "reference" units here.
Imagine having to calibrate a scale on Mars, or Alpha Centauri. Getting that artifact to the "job site", to make sure the scale is true, would be a bit of a chore.
A kilogram should be expressed not in terms of the number of atoms in a particular crystal, but rather in terms of the mass of X moles of standard substance Y.
We can assume (if we can not, then all else is a lie) that a particular isotope of a particular element will have the same mass eveywhere in the Universe. We know the number of atoms in a mole. Problem solved.
Replace outdated platinum iridium (Score:3, Funny)
The Volt (Score:3, Informative)
It turns out, that even without an applied voltage, there is still a current in the system, and after a voltage is applied, the current oscillates at a very predicable rate. Thus, the volt is now defined as the potential required to give a specific number of current osciallations in a Josephson Junction.
Nit-pickey I know, but maybe of interest.
Defeating the purpose (Score:3, Interesting)
Re:yay! (Score:2, Informative)
Re:Annually (Score:5, Informative)
It doesn't exactly have to be measured. They just do that to check it's still right. Go read about the history of the Systeme International the NIST site [nist.gov] and the definition of a kilogram at the same place [nist.gov]
But essentially, its part of a way of ensuring that the measuring units Scientists use around the world are the same, not slightly different.
For instance, anyone around the world can reproduce (in a well equipped lab anyway) the definition for time (The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom).
There are only 7 base SI units (meter, kilogram, second, ampere, kelvin, and candela) from which many more units [abdn.ac.uk] are derived. Hence, if kilo is out/changing many of these are changing too.
and why should I care if it detoritates?
Presuming you're American, you would use feet, pounds, find metric too complicated, etc, etc - so probably wont care if it does.
Re:The mystery unit? (Score:5, Funny)
I can't reveal its identity for this precise reason.
Yes - there is a mole in the base S.I. units - but I can't tell you it's name. Its been on a secret long term sleeper mission - to liberate the S.I. units and term them into "Freedom Units"
Comment removed (Score:5, Informative)
Re:Why not... (Score:5, Interesting)
c is a constant, of course. In fact, it's used to define the meter as how far light travels in a vacuum in 1/(299,792,458) of a second. Second is defined as the time for a certain number of vibrations of a Cesium atom to occur. As per your question of relating mass to Joules, note that high-energy physicists do this all the time. They usually refer to masses of particles as MeV/c^2. And they usually work in units where c=hbar=1, thereby making distance, time, and energy all essentially the same units (easier to do calculatins that way).
One thought that jumps to mind for a standard energy interval is the lyman alpha energy width (the jump of the electron in a hydrogen atom from n=2 to n=1 where n is the energy quantum number). Or, for mass, use a standard mass of a well-defined particle such as an electron. In fact, I'm surprised that NIST doesn't do this. It might be that isolating electrons for mass measurements are too difficult (gravity is weak), but electron mass does show up in many other calculations (specific heat of degenerate electron gases, for instance). Or isolating ultra-pure hydrogen gas and spectroscopically measuring Lyman alpha is more difficult than it seems. I guess NIST wants [relatively] easy methods for measuring these quantities.
Okay, I just found this site [unc.edu] which answers the question. They quote
It all boils down to ability to measure the standard units to the highest precision possible. I'm actually stunned that the mass of that bar can be weighed to that precision.
As a side note, if you can come up with a better way of measuring fundamental constants, you might win a Nobel Prize. The guys that discovered the integer quantum hall effect initially published their results as a better way to measure some of the fundamental constants.
Re:i'm confused (Score:3, Informative)
But on your 2nd point, you're right. A joule is defined based on a kg, not the other way around.
yeah it's a mess (Score:5, Informative)
The big problem is that 2^(10x) and 10^(3x) diverge as x increases: 1024 is 2.4% more than 1000, 1048576 is 4.9% more than 1000000, 1073741824 is 7.4% more than 1000000000, and so on. So obviously the "close enough" thing is getting less and less true -- when there's a 10% difference between the two measurements they're not even close enough for everyday colloquial speech.
So the solution of both the SI and the IEEE is to reassert the original meanings of the SI prefixes (kilo = 1000, Mega = 1000000, etc.), but to add new base-2 prefixes in recognition of their usefulness in computing. These are kibi, Mebi, Gibi, etc. (basically the same as the SI prefixes but with the last two letters replaced by "bi"). Their standard abbreviations are the same as for the SI prefixes, but with a lowercase 'i' appended (so ki, Mi, Gi, etc.).
The conversion is obviously nowhere near complete, and irritates some computer people who don't want to change the terms we've been using for decades, but this seems to be the only really reasonable way of doing things. The only other two options are to either force the rest of the sciences to change to use the base-2 definitions (which is obviously not going to happen, and they got there first anyway), or to maintain the current ambiguity, which is also obviously undesirable.
Re:yeah it's a mess (Score:5, Funny)
A better way would be to invent an all new imperial-style system for measuring computer storage. That way, there would be no chance for confusion with any base-10 system. For example:
korb = 3 bytes
fleb = 12 korbs
splin = 20 fleeb
fnit = 6 splins
Fnit = 6000 splins
frush = 48 fnits
watz = 18 frushes (19.5 frushes in the U.K.)
spoff = 480 watzen
nurm = 320 spoffs
long nurm = 80 nurm
munnel = 24 long nurm
This system easily covers storage capacities up to today's confusingly named "petabyte". Plus, there's no ambiguity about what you're measuring. Any of these units implies bytes of storage, which is a much cleaner solution.
The computer I'm using now has 71+29/32 watzes of system memory and 44+10/16 spoffs of disk space. There's no confusion about fuzzy definitions of "mega" with that measurement.
Re:yeah it's a mess (Score:4, Funny)
Are those watzen in UK frushes or US frushi? Ah, man this is so confusing! I say we all go back to counting in base-L.
Re:Change in Gravitational Constant? (Score:5, Informative)
Re:Define Plank's Constant? (Score:5, Informative)
The same deal with Plack's constat. It's value is not up to us, but up to nature. "Defining" it would be like defining pi as 3.
Re:'c' relies on second (Score:4, Informative)
No. In SI units, c is not measured but defined. Physically, c is just a man-made constant of proportionality deriving from the fact that, for historical reasons, we measure time differently from space. In reality, both time and space are physical dimensions and so it makes perfect sense to express both in terms of the same units, be they seconds or metres.
That's why most theoretical physicists like to do their calculations in "natural units" -- i.e. you set c=1 and h/2pi=1 -- since in reality the values of the fundamental constants are artefacts of your measurement system. Scientifically speaking, it makes sense to set all independent constants to 1 since it brings out the fact that the "equivalence" of eg mass and energy, or distance and time, is really an identicality.