Big Test Coming Up For Kilogram Redefinition (ieee.org) 127
szotz writes: Electromechanical balances have got to be better than an aged lump of platinum and iridium right? Teams are working to get kilograms measured and shipped to Paris in time for a test to see whether the technology (along with another that uses ultrapure silicon spheres) is now ready to redefine the kilogram. Why is this redefinition interesting? Because it's about using physics to overcome one problem with weight standards based on tightly held exemplars in standards bodies' inner sanctums: the mass of those exemplars can change, however subtly, introducing uncertainty and confusion. From the article: The world's metrologists aim to change this state of affairs in 2018 by fixing the kilogram to the Planck constant, a fundamental physical constant. That shift would, at least in principle, allow any laboratory to "realize" the kilogram from scratch with a series of experiments and specialized equipment. But for that scheme to work, the kilogram derived by one laboratory must be the same as those derived by others.
So really... (Score:2, Offtopic)
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So really it is just a global scientific test of who's is bigger.
No, it's a multiplayer game of "You show me yours and I'll show you mine."
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he's referring to first-time customers getting a freebie
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Mine is closer to "just right" than yours is.
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This is no time to be eating cereals [kelloggs.ca], get back to work!
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So really it is just a global scientific test of who's is bigger.
I've got giant silicon balls.
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I don't get it why you always keep confusing those two. The one for implants is called silicone.
Capito?
But my balls are sandy.
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But are they ultra-pure?
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The balls are highly enriched, consisting of 99.9995% silicon-28 with a minimum of the other isotopes found in natural-abundance silicon.
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I only come here for the comments and quality discussion. Did not leave disappointed.
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It's my new hit song, "would anyone like to weigh my giant white silicon balls"
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so (Score:3, Funny)
conventions and relativity (Score:1)
hey, that's cool. the planck constant can not and will not ever, ever possibly change. right, literally everyone alive now and in the foreseeable future?
*hell freezes over*
hello?
Measurements (Score:5, Informative)
Why the planck constant then? Why not e, or (pi), or any other constant, for that matter? If you're going to change the definition, isn't it just a matter of choosing the close enough factor?
By the way, I'm asking. I am ignorant about this.
The fundamental distinction between math and physics is measurement. We need to base the physical constants on something measured from the universe we're interested in.
As a simple example, mathematics defines and explores 3 basic forms of geometry: Euclidean, hyperbolic, and elliptic.
The distinction between these is based on the curvature of space as defined by the behaviour of parallel lines: if parallel lines eventually meet, then space has positive curvature like the surface of a sphere. If parallel lines diverge, then space has a negative curvature like the surface of a saddle. And if parallel lines stay parallel, then space has zero curvature and is Euclidean.
Three equally valid forms of geometry, but which one does the universe have? To choose the correct model, we have to measure the actual universe.
The same is true with the fundamental physical constants. There's any number of ways to base our measurements on pure math, but these don't necessarily reflect the reality we live in.
To do that, we need to take a measurement.
Re:Measurements (Score:5, Interesting)
To bolster the argument, look at the fine-structure constant. When Arnold Sommerfeld introduced the constant in 1916, Arthur Eddington argued that you could get to it by pure math and found that for completely logical reasons, the constant should be exactly 1/136. When later measurements put the value closer to 1/137, he discovered an error in his deduction and published a new paper that the constant should be for even more logical reasons exactly 1/137. Currently measurements put the value of the fine-structure constant at about 1/137.036, and no numerological explanation so far has been accepted.
Re:Measurements (Score:5, Insightful)
Though the above is true, it is *not* the reason why we can't base the kilo on some arbitrary multiple of Pi. The point is that we want to be able to actually reproduce the reference kilo in any lab. Take for example the definition of the meter as the distance travelled by light in 1/(299,792,458) of a second. A lab can actually measure the length light travels in that amount of time and thus reproduce the canonical meter. If we just defined the meter as 1/Pi, there would be no way to convert this number to an actual length.
When the article says that they define the kilo in terms of Planck's constant, they mean that you take the ratio of all sorts of measured quantities in the lab and the laws of physics say that the result should be the mass of what you are measuring times Planck's constant. The true emphasis is that the measurement is proportional to the mass of what you measured, not that the constant of proportionality is Planck's constant (except of course for the fact that we assume that the constant of proportionality, Planck's constant, being part of the fundamental laws of physics, is independent of where and when we do the measurement (at least in the time and distance scales that physics has managed to probe).
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A lab can actually measure the length light travels in that amount of time and thus reproduce the canonical meter.
I hope they all use the same excellent pump to create a vacuum as closely equal to the outer space as possible, otherwise their measurements will vary significantly
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Thanks for "picking up" on that :-)
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adding statistical error to what should be a deterministic calculation
Atomic vibrations when nearing absolute zero, are caused by non-deterministic quantum fluctuations, which are statistical in nature. Modern atomic clocks are getting so stable that two clocks right next to each other disagree because they experience different amounts of gravity. They're working on some new clocks that should allow the accuracy and precision to give accurate millimeter precision to GPS. That is the epitome of "stable".
Re:Measurements (Score:5, Interesting)
Well, this certainly qualifies as news for nerds! It's news, technical and amazingly esoteric.
Why the planck constant then? Why not e, or (pi), or any other constant, for that matter?
Neither e nor pi are physical constants. They are unitless mathematical constants, so you'd have to specify e or pi *somethings*. It's the somethings that are important at which point neither e nor pi would come into it all that much.
By way of example:
The second is defined in terms of a physical constant: "the duration of 9192631770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom." The important thing about this is that this was not the original definition of a second. The second was a bit more vague, so at some point a bunch of metrologists got together and declared that from now on this SHALL be the definition of a second and shall supercede all previous definitions.
The kit to measure a second is withing reach of well funded science labs and can be reproduced independently. You need the high frequency counter (capable of 10GHz operation), some pure caesium, and an assload of expensive support equipment and liquid helium and you can measure a second.
Once you have the second, you can move on. The meter is defined in terms of the second and the speed of light: as the distance travelled by light in a specific fraction (1/299 792 458) of a second. Much like before, this is a declaration by fiat, and is very very close to and supersedes the old platinum iriduim rod in Paris.
Now, here's where it gets interesting!
First, an aside:
The reason for Planck's constant comes down to what is colloquially known as E=mc^2, or more generally E= h v where v is momentum and h is plank's constant. In other words, Planck's constant connects energy, mass and time.
Here's a nice link:
www.bipm.org/utils/common/pdf/RoySoc/Michael_Stock.pdf
It's a bit more detailed, but essentially it relates the Kg, Planck's constant and a few others which are known. So, if you know what the Kg is exactly then you can measure Planck's constant with a Watt balance very accurately.
So what you do is calibrate the Watt balance with the prototype Kg, and measure Planck's constant. You then declare (by fiat) that Planck's constant is EXACTLY what you've written down and so now the Kg is defined in terms of that number, not the other way around.
In principal, now someone can build their own Watt balance, plug in the numbers which are now just numbers and measure their own chunk of metal to find out how much it weighs in Kg.
So that is a nutshell is why h is appropriate and pi and e aren't.
The other option is to build a very very pure, very very precise silicon sphere, in which case the Kg will be essentially determined by a single number which is the number of silicon atoms in a Kg. That will be measures in terms of the meter (for both the bond spacing of silicon and the radius of the sphere). In that case, Planck's constant will still be defined in terms of the Kg, not the reverse. In this case, pi would make it into the definition, via the volume of a sphere, of course, but in a somewhat peripheral role.
The question is whether we (collectively) can make silicon spheres more accurately than we can make Watt balances, or the reverse, right now.
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FYI: kg, not Kg. The kilo prefix is lowercase k. (Nevermind the broken "K" and related prefixes used in IT.) Kg could be interpreted as Kelvin-grams, which would be an interesting measure, though I've no idea where you might find that being used...
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Thanks - that was clear and concise!
Wouldn't it be a lot easier to use a gram or micro-gram or some smaller sphere?
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The smaller the object to be measured to a given degree, the greater the uncertainty. Measuring a 1kg mass to within a nanogram gives you a thousand times less uncertainty than measuring a 1g mass to within a nanogram.
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No, it gives you 1/1000th the uncertainty. It doesn't give you negative 999 times the uncertainty of the other (1 - 1000).
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That's what "1000 times less" refers to. Multiplication is part of that, and the "less" implies division.
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You're completely wrong.
More means addition, less means subtraction. Just like in math and English.
Times means multiplication. Just like in math and English.
Fractions (half, third, fourth) mean division. Just like in math and English.
4 is 2 more than 2.
4 is 1 time more than 2.
4 is 2 times 2.
4 is 100 percent more than 2.
2 is 2 less than 4.
2 is 0.5 times less than 4.
2 is 0.5 times 4.
2 is 50 percent less than 4.
The only thing implied is what "times" and "percent" relate to. (Hint, it's the thing being comp
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Planck's constant itself is really just an artifact of having chosen units like meters, grams, and seconds, which are arbitrary products of a combination of numerology and the dimensions of the earth. In natural units [wikipedia.org], Planck's constant would be 1. So would the speed of light, Newton's gravitational constant, and several others.
The "real" constants of the universe are dimensionless constants that hold no matter what your units are, like the ratio of the mass of the down quark to the electron, and the coupli
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Well, you can always define everything in terms of Planck units:
https://en.wikipedia.org/wiki/Planck_units [wikipedia.org]
Basically Planck units define several physical constants to be 1. Such as the speed of light, so instead of E=mc^2, you now have E=m. Problem is that for everyday use, things like the planck length, time, and to a lessor extent, mass, are absurdly tiny. The planck temperature, on the other hand, is absurdly high, which is the case because any black body at the planck temperature emits radiation with
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e and pi are numbers. You need actual physical constants like the mass of a proton or Planck's cpnstant.
It is indeed just a matter of choosing "close enough factor", but close enough (to avoiding needing to redo or change any measurements that anyone uses) is pretty close, about one part in 100 billion. Being sure that we have done that is the hard part.
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First we define what Avogadro's number is. Based on a measurement of the number of atoms in a silicon sphere of known mass. The ratio between the weight of a single silicon atom and plancks constant is already known, so this would give us a way to convert from plancks constant to a new definition of a kilogram.
Of course, measuring the number of atoms in a silicon sphere is hard to reproduce. Enter the other competing method, the watt balance.
Place an object on a speaker cone, then measure the current requ
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> Why the planck constant then? Why not e, or (pi), or any other constant, for that matter?
Because:
a) the universe is quantized / discrete / digital,
b) Plank Length [wikipedia.org] and Planck Time [wikipedia.org] are thought to be the smallest possible divisions of space and time respectively, (if there are any smaller divisions we're unable to measure them)
c)
Planck units [wikipedia.org] also
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> Why exactly is this a problem?
"Proposed_redefinition_of_SI_base_units#Impact_on_base_unit_definitions"
https://en.wikipedia.org/wiki/... [wikipedia.org]
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Because if Planck's constant changes - every law of physics is diffferent and we can't make any predictions about how. Seriously, a universe where Planck's constant is different could be 3x3m square vaccuum containing a single deck chair. There is nothing in modern physics that preclude the big bang forming into this tiny pet universe with it's comfortable single amalgamation of matter that is NOT dependent on Planck's constant being the one we know.
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Shouldn't it be the "general" definition? Fuck the metal weight - 1 kg is 1 liter of (distilled/pure) water (at 1 atmosphere pressure)? The amount of heavy hydrogen messes it up? 1 atmosphere of pressure isn't the same at all places on the earth due to varying gravity?
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Yes. How many atoms are there in 1 liter? I guess you could go more 'general' and say how many drops of water? But then, how big are the drops of water? What about the absorbtion rate of the material, can it only be measured in glass? is this deformed at 1 atmosphere pressure? evaporation rates?
Then you get to mineral/chemical impurities, atmospheric disturbances, etc.
Maybe 1 kg vs 0.999997 kg doesn't mater to you but there are many cases where it will. And calibrating our scales to allow that fine-grained
Re:conventions and relativity (Score:4, Insightful)
Whatever standard you adopt needs to be reproducable within the limits of the best current measurements by any other technique. Otherwise when people want a stable reproducible result they will use the other technique and the standard won't have worked. Measuring volume of water, purity, temperature and pressure is just not precisely reproducible enough
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Actually - it would make more sense to define that one the other way around. If you have an atomic-accuracy measurement for mass, then it's much more sensible to define volume from mass than the other way around: so you would instead define liter as "the volume of a kilogram of water when these conditions are all at these specific values".
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But we already have one for the liter?
"1 liter = 1.18101066 Ã-- 10^(-51) cubic light years"
(that one likely rounded off but you get the idea.)
"Add one 6 Ã-- 10^(-54) cubic light years of tea-leaves per cup."
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except your 1 atmosphere is 101325 Pa, a Pa being 1 newton per square meter, a newton being defined as force needed to accelerate *1 kilgram* at 1 meter per second squared
do you see any problem?
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The idea was that 1 atmosphere would be the the pressure of 10 meters of water (where?! :D) and it would all be solved out by having the same water and the same light-speed.
Bwah, imperial is better: ;D
One liter = "The volume of the brain-substance you easily can scrape out with some residues left in the skull."
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This was the definition for a few years in the eighteenth century, before it was quickly changed.
It's such a bad definition, it was not worthy using even back then.
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(people, my point was that if the universe is contracting or expanding, something as co-involved as Planck's constant could easily change and we wouldn't be able to measure it as all of our measuring devices and things they're measuring would be contracting or expanding simultaneously. sheesh. how i got downvoted to 0 on a subject that sprang so much discussion is typical slashdot and yet completely beyond me.)
A weighty matter (Score:5, Funny)
This is a massive development.
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This is a massive development.
So low:
This announcement came down like a ton of bricks on the physics department.
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It's hard to underestimate the gravity of this weighty decision.
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Comment removed (Score:5, Funny)
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That seems rather daft.
Surely an ounce should be defined in terms of grams.
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Which itself is daft, considering the morpheme kilo-
All kilograms... (Score:5, Funny)
All kilograms are equal
but some are more equal than others.
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Perhaps; but it'll always be true that a pint's a pound the world 'round.
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Not in the UK, where a pint of water weighs a pound and a quarter.
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Isn't a litre 1kg of pure water at normal room temperature and normal atmospheric pressure, so presumably that will change if the definition of 1kg changes?
I don't know why they don't switch a litre to being 1000cc.
By definition, 1 litre is 1 dm^3, which is exactly 1000 cc (1 cc = 1 cm^3), units of volume are derived from length, not mass.
And about 1 litre of pure water having a mass of 1kg at 1 atm, yes, but not at room temperature, at 4C instead, and it was the initial definition of a kg.
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By definition, 1 litre is 1 dm^3, which is exactly 1000 cc (1 cc = 1 cm^3)
That's actually incorrect and in reality 1cc = 0.99997ml. You can see this in various text books, for example the "Handbook of Chemistry and Physics" or "A Laboratory Textbook of Anatomy and Physiology":
https://books.google.co.uk/books?id=Av-xb_AEHmIC&pg=PA529&lpg=PA529&dq=1+cc+%3D+0.99997+ml&source=bl&ots=rv-eGLZ9xW&sig=s9Y3m8NRmuEWF-H1me-ON49avIk&hl=en&sa=X&ved=0ahUKEwiSxcmur5DLAhWIVxoKHQmRCtYQ6AEIKDAC#v=onepage&q=1%20cc%20%3D%200.99997%20ml&f=false
I believe the discrepancy is caused by the weight of the kilogram being poorly defined. That's why I wonder if the litre will have to be redefined if the kilogram is redefined and why I said I think it would be better to use 1000cc for the definition of a litre.
Re:Will a Litre be Redefined? (Score:5, Informative)
I thought it was a stupid conversion mistake, but investigating on the topic :
One litre of liquid water has a mass of almost exactly one kilogram, due to the gram being defined in 1795 as one cubic centimetre of water at the temperature of melting ice.
So, originally as I wrote.
From 1901 to 1964, the litre was defined as the volume of one kilogram of pure water at maximum density and standard pressure. The kilogram was in turn specified as the mass of a platinum/iridium cylinder held at Sèvres in France and was intended to be of the same mass as the 1 litre of water referred to above. It was subsequently discovered that the cylinder was around 28 parts per million too large and thus, during this time, a litre was about 1.000028 dm3.
Oops. Not too bad, given that at that time the metre was wrong too:
it was later determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre.
And all is fine again:
In 1964, the definition relating the litre to mass was abandoned in favour of the current one.
The litre [...] is an SI accepted metric system unit of volume equal to 1 cubic decimetre (dm3), 1,000 cubic centimetres (cm3) or 1/1,000 cubic metre.
Sources:
Litre [wikipedia.org]
Metre [wikipedia.org]
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In fact, the litre *could* be redefined to be exactly 1kg of water and simultaneously exactly 1dm^3 of water. The definition just needs to adjust the atmospheric pressure and temperature of water at the moment of measurement - while thermal and pressure expansion/compression of water is minuscule, a change by 0.003% is well within reasonable limits of what occurs on Earth.
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Initially, 1kg was defined as the weight of 1 litre of water but it changed because it isn't something that is easy to measure accurately. Currently, there is no link between mass and volume.
The litre is just an alias for the cubic decimeter, the same way that the (metric) ton is just an alias for the megagram. Just different names for the same thing. So 1 litre is and has always been 1000cc or 0.001m3.
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That was the definition of a kilogram for a few years, several centuries ago. You're a little bit out of date.
Don't change the definition! 1 kg = 1024 g (Score:5, Funny)
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I bet they're going to change the definition from 1 kg = 1024 grams to 1 kg = 1000 grams. And we'll probably have to write "kig" too to make sure we don't get confused about the old definition.
Was gonna mod this "Funny" but unfortunately there choice is via a drop-down list which confuse CMD people like myself. To make it worse there's no "undo". So I'll just post to take any mods away. Boo.
Moderation (Score:2)
Use the D1 discussion system. It still works just fine, and it has both a dropdown and a "confirm" button. Find it in your account options under 'Discussion'.
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A grocer's redefinition (Score:2)
The unit formerly known as the "pound" shall now be called the "kilogram". The prices will be adjusted accordingly.
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That wouldn't be a problem, because the universe would change accordingly, and the kilogram in relation to the rest of the universe would remain unchanged.
Kilo-gram (Score:2)
So it'll be 999.000000001 grams or something?
Maybe they'll redefine gram one of these days
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Agreed. After I clicked submit I realized my example was a little goofy
Let's all remember (Score:2)
...the Imperial system of weights and measures is bad because it's "arbitrary".
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nonsense, the universe provides some absolute ones. rest mass of fundamental particle for example. However these are not convenient enough to use in most cases
The Germans should prevail (Score:2, Interesting)
The US approach is silly. The apparatus must be isolated from the environment to such a degree that it is impractical. Oner must monitor and dissuade wildlife a quarter mile away from the apparatus to get useful measurements. In essence the US approach is not to make a standard but a very impractical scale. The German approach is not so touchy. There is nonsense about only one Australian guy being able to form the spherical reference but that is ridiculous cult of personality. The German approach is both de
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you are missing the point, these experiments are being done to ascertain the superiority of one approach over the other. You are trying to argue without lack of experimental support, but those involved in this test are doing things scientifically.
Since a Kg measures mass (Score:5, Funny)
It should be defined by Pope Francis
He used to be a chemist, and is infallible.
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Except that his kilogram will differ from the Orthodox kilogram by some obscure Aristotelian philosophical disagreement, every Protestant scientist will feel obliged to determine his or her own kilogram from personal inspiration, and it'll get even worse as we get to other religions. The Muslims will want it defined in terms of Muhammed's body parts, and the Buddhists will insist it's all illusory anyway.
Pound for pound (Score:2)
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And have Bill Gates switch off the internet to save your children.
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That's right, I take every an AC says with a grain of salt and a fifth of tequila.
After all my standard is that 1 us dollar equals 1 million US dollars and that is true because my own world view says so
[/sarcasm may be present in the above. Read at your own risk. Not liable for any stupidity you get on you]