Experts Suggest Replacing Definition of Kilogram

fenimor writes "The kilogram is the only one of the seven basic units of the international measurement system defined by a physical artifact rather than a natural phenomenon. International team of scientists suggest replacing the kilogram artifact -- a cylinder of platinum-iridium alloy about the size of a plum --with a definition based on one of two unchanging natural phenomena, either a quantity of light or the mass of a fixed number of atoms. They propose to adopt either one of two definitions for the kilogram by selecting a specific value for either the Planck constant or the Avogadro number."

Comments

Picture of the Kilogram Prototype

[pboyum]

Picture of the International prototype kilogram:

http://www1.bipm.org/utils/common/img/mass/prototy pe.jpg

Re:Picture of the Kilogram Prototype

[spectasaurus]

Better question. Why is it in 3 vacuum jars?

Re:How about ...

[InfiniteWisdom]

The point is to have something that you can define just by counting some phenomenon or natural objects. For example a second is defined as:
"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 caesium-133 atom."

and a metre is defines as:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Unfortunately, there hasn't been a good way to count the number of atoms with any kind of precision, so that has precluded a good definition of the kilogram so far. Maybe now the physicists can actually count atoms accurately enough.

One could define it as the mass of some number of H2O molecules, but maybe its easier a measure a quantity of light or to count some larger atoms.

Re:I suggest

[tibike77]

Not only that, but it is also the only "basic measuring unit" that's not even a measuring unit, but a multiple of another one that's not considered "basic". At least in name. Kilo-gram. Get it ?

Actually, if they happend to re-define it based on Avogadro's number, they might as well just say the GRAM is the new "basic unit" and the kilogram is just 10^3 grams.
Why ?
Because Avogadro's number is JUST an artifact of the definition of the (kilo)gram, not a fundamental constant - it's (been originally) defined as the number of atoms in 12 grams (or, whatever, 0.012 kilogram) of Carbon-12.
Talk about circular references then...

Now, basing the definition of the kilogram (might I suggest they also change that basic to gram instead of kilogram... please) on Planck's constant somehow would be a MUCH better ideea. However, the value of that constant [i.e. 6.6260693111111 * 10^-34 and so on] makes it pretty wierd to work with unless you multiply it with 9 [to get exactly 5.96346238 * 10^-33 which makes more sense somehow]. And even then it won't satisfy some people, as I'll bet you'll hear that 0.111111 and so on *9 does not equal 1 :p
Not only that, but Planck's costant was ALSO measured "accurately" using the kilogram unit as reference.

Ok, this actually does give me a headache.

Re:artifact

[JaxWeb]

It is well known there are 7 base units.

"Candela essentially measures the same things as watts."

But watts are not a base unit. A watt is the same "Joules per second", and Joules is also not a base unit, but is defined as a Newton Metre. But a Newton isn't a base unit, it is defined as a Kilogram Metre per second per second. So:

Newton = kg.m.s^-2
Joule = Nm = kg.m.s^-2.m = kg.m^2.s^-2
So a watt is in-fact a kg.m^2.s^-3 , or "Kilogram metre squared per second per second per second" - hence changed the kilogram will change the watt, despite them seeming unrelated!

A mole isn't the same as mass at all. It is more to do with things on an atomic level. It's really used in chemistry - I've personally never used it outside of a chemistry exam (or coursework). It is sort of just a number, but it actually isn't.

Kelvin is a fundamental base unit too. Momentum is defined as "Newton Seconds", and so (remembering the definition of a Newton) kg.m.s^-1. Kelvin's measures temperature, which is a measure of kinetic energy, so I can see where you are coming from. You're just wrong.

Ampere is too. Helpfully, from it you can define other helpful things like volts. A volt, for your interest, is defined as kg m^2 s^-3 A^-1 , or "Kilogram metre squared per second per second per second per amp". And so yet another thing this change would affect.

It's all very interesting.

Just to state the obvious

[tod_miller]

They are cleaning the definition, not the value.

A new kilogram with equal an old kilogram. This will only make a difference to the history books and those who actually want to make thier own 'kilogram'.

I can imagine how many 'net savvy drug runners are looking at this and thinking, 'shit, I have snorted too much coke, does this affect my business? whats a planck? oh man, Avocado constant? [sic]

I say since the kilogram was an arbitrary measurement (in any definition) then why try and make it more formalised? I realised that celcius fit nicely with pure water at sea level freezing and boiling, and other measures have thier own basis (has the definitions have changed). Take my friend the meter. I always use the old skool definitions for rules of thumb.

Year Definition
1793 1 / 10 000 000 of the distance from the pole to the equator.
1795 Provisional meter bar constructed in brass.
1799 Definitive prototype meter bars constructed in platinum.
1889 International prototype meter bar in platinum-iridium, cross-section X.
1906 1 000 000 / 0.643 846 96 wavelengths in air of the red line of the cadmium spectrum.
1960 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between levels 2p10 and 5d5 of the krypton-86 atom.
1983 Length traveled by light in vacuum during 1 / 299 792 458 of a second.

So you see, a meter was the same in all these cases, but they just wanted to act clever.

The thing is, after world war 3, which measure will be easiest to revert to for a meter? trying to find scientist who can measure "Length traveled by light in vacuum during 1 / 299 792 458 of a second." or just comparing a brass stick with a length of wood while trying to build something using pre-existing specs (that you are relying on to build a post WW3 bridge). ;-)

Just remember that they replaced the meter

[gotr00t]

Before laser technology, the meter was defined as the distance between two markings on a bar of platinum-iridium kept in Paris. It was after Michelson invented his interferometer that the meter was redefined as 1,650,763.73 wavelengths of a certain orange line in the spectrum of krypton-86. This was later redefined in the 80's to be in terms of C, the speed of light.

As technology to measure substances to great precision increases, its about time the kilogram got a redefinition as well, one not based on a single object.