What will the most modern definition of the kilogram be by 2020?

[Martin]

We got an interesting thread just now, from Mahela, asking what is the most modern definition of the meter.

 

That prompts a question about where the definition of the kilogram is going.

Any ideas? Any news about recent developments?

 

Right now I believe the kilogram is defined based on a block of metal kept somewhere in France, sort of like a bank vault. Correct me if I'm wrong about that. Is that situation likely to continue for yet another 10 years?

[Sisyphus]

It doesn't seem like it could stay the same much longer. According to this article in New Scientist about one attempt to come up with a new measure by creating the roundest objects on Earth, the International Committee on Weights and Measures will make a decision in 2011.

 

Looking at the Wikipedia article on the kilogram, there are several competing alternatives. Such as:

 

"the mass which would be accelerated at precisely 2×10−7 m/s2 when subjected to the per-meter force between two straight parallel conductors of infinite length, of negligible circular cross section, placed one meter apart in vacuum, through which flow a constant current of 1⁄1.602176487×10−19 (≈6,241,509,647,120,417,390) elementary charges per second"

 

"the mass of a body at rest whose equivalent energy equals the energy of photons whose frequencies sum to 1.356392733×1050 Hz”

 

"the mass equal to that of precisely (1000⁄196.9665687)*6.02214179*10^23 atoms of gold"

 

etc.

 

Each has different problems in experimentally reproducing them. It's much more interesting than I would have guessed.

[CaptainPanic]

As mentioned in the wikipedia article (under "Ion accumulation"), an attempt should be made to relate SI units (charge, mass, mole).

 

The interesting part is that mass will be related to other units, which are also not defined perfectly. There is a certain error in length, therefore using a silicon sphere of certain radius will introduce the error in length (to the power 3, since Volume = Length^3).

 

Using Avogadro's number will only work if we can count individual atoms (using current or some other method).

 

Can't we design an experiment which checks all SI units at once? Then repeating measurements for mass, current, mole, length, time, we can iterate to a perfect definition. I need another cup of coffee to work that out (don't expect a post soon though, our coffee machine is horrible). Iterations in a recycle system are generally a good way to reduce errors. I haven't really worked out why this would not be having the same problem (namely that to define a unit, you need the definition of another unit - so which unit gets defined first?).

Edited July 2, 2009 by CaptainPanic
coffee disclaimer

[Externet]

...Right now I believe the kilogram is defined based on a block of metal kept somewhere in France...

 

I learned in elementary school 1 Kilogram equals 1 litre or 1Kcc of water; 1 gram equals 1 CC of water.

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... so which unit gets defined first?).

 

The metre was defined first. Measured the distance from a pole to equator, divided by 10 million. They got pretty close with the technology available then.

Derived from it, the litre (0.1m x 0.1m x 0.1m) and the ton (1m x 1m x 1m of water).

Speed derives from it, also force, acceleration and many others.

 

The definition of time is were the french reaaaaally goofed big time; and it is way too late to make a unit now. Temperature remains a rare animal.

 

Other major goofs are adopting anything not decimal. Longitude and latitude, degrees on a circle, twelve months... It is all messed up. But we have to live with it.

At least is not worse than the volume of the urinals of king and queen whatever of England for the imperial and the other gallon, and the size of the thumb for an inch. And a pound that abbreviates lb instead of pd?. What a joke.

 

Miguel

[insane_alien]

I learned in elementary school 1 Kilogram equals 1 litre or 1Kcc of water; 1 gram equals 1 CC of water.

 

that still requires a physical standard which is what we want to avoid.

 

we want to get away from defining units on a certain amount of a particular substance under certain conditions.

 

and your definition only applies at 2 specific temperatures, anything other than these and it is wrong.

[JPGreco]

What we learn in elementary school is so wrong anyway, consider that the lion is the king of the jungle when it lives on the savannah.

 

Anyways, the french aren't responsible for time, at least for months. Months was originally based on the moon, which is another measurement based on a physical property. The phases of the moon dictated time to many early cultures. Since the time between phases vary, so do months. Then there are approximately 12 cycles of the moon in a year. This created 4 years of differeing amounts of days, so months were on a 4 year cycle. This was corrected with the solar year. Time in the larger sense was never derived from scientific measurements, but rather from simple observation, which by definition, is based on physical properties.

Why 24 hours, 60 minutes, 60 seconds and then standard decimal divisions, I have no idea.