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An old artifact kept in a vault outside Paris is no longer the standard kilogram

news.mit.edu

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Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#51

Post by pavlov »

Causality1 wrote: post #9

I've always been mildly irritated that the name of the SI mass base unit has a prefix. Second, liter, meter, Newton, kilogram. Were I emperor of the world I would slide the name scale up so the mass equal to 2.205lbs was called a gram.

The gram was the original base unit in the French revolutionary metric system. It was defined as the weight of 1 cm^3 of water at the temperature of melting ice. The definition was too imprecise and the kilogram turned out to be more practical, but renaming the unit would have been too much trouble I guess.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#52

Post by dzdt »

renoz wrote: post #45
fsh wrote: post #22

Earlier quoted context omitted.

As the article correctly states, photons have a finite mass that could in principle be measured by putting an optical cavity on a scale.

I know that zero is a finite number, but since the photon is a massless particle, wouldn't it be impossible to get a kilogram worth of photons bouncing around in an optical cavity?

Photons have zero rest mass but it so happens that they are always zipping around at the speed of light so their actual mass as observed is nonzero.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#54

Post by bonoboTP »

renoz wrote: post #45
fsh wrote: post #22

Earlier quoted context omitted.

As the article correctly states, photons have a finite mass that could in principle be measured by putting an optical cavity on a scale.

I know that zero is a finite number, but since the photon is a massless particle, wouldn't it be impossible to get a kilogram worth of photons bouncing around in an optical cavity?

For some reason, zero is often considered not finite. I personally find this terminology quite unintuitive, but it is well established.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#56

Post by GuB-42 »

anvandare wrote: post #30

For the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.

The reason the mole is in the SI puzzled me too.

It seems that the reason is that chemists don't care how many atoms there are in their reactions. It would be inconvenient: fractions of atoms don't make sense, and the numbers are far from human scale. They just want an agreement, so that even if they don't know the exact number of atoms in their test tube, they know they all have the same number. That's what units are for.

The definition of units are done in the most convenient and precise way possible. And they are updated as science progresses. For the kg, the artifact used to be the best we had, alternative methods weren't precise enough, they changed that to the Kibble balance because now, it is better. It may change again at a later time if we find something better.

Back to the mole, it used to be defined as the ratio between the kilogram and the mass of a 12C atom because it is the best we had. Now that we can count atoms with more precision, we decided to change it to just the Avogadro constant, which becomes fixed. Again it might change. What may happen (or may have happened, see Avogadro project) is that we can define the kilogram using the mole, making it fundamental.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#57

Post by garmaine »

GuB-42 wrote: post #56
anvandare wrote: post #30

For the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.

The reason the mole is in the SI puzzled me too. It seems that the reason is that chemists don't care how many atoms there are in their reactions. It would be inconvenient: fractions of atoms don't make sense, and the numbers are far from human scale. They just want an agreement, so that even if they don't know the exact number of atoms in their test tube, they know they all have the same number. That's what units ar…

That is what has happened, essentially. The mole is atomically precise, and will never change.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#58

Post by snops »

rimliu wrote: post #44
anvandare wrote: post #30

For the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.

How do you imagine that? And what has the bit to do with it?

The SI units are supposed to be used to define all other units.

The mole is just a unitless number which isn't used for many definitions, while the bit (or preferably, Shannon) has a fundamental use in information theory and the wider field of physics.

The mole is a human specific number, the bit is a property of the universe. Information is a physical quantity, just like mass or length, and deserves to have a unit in the SI system. You could use other bases, like the nebit (natural log) or Hart (base 10), but bit is most elegant as it uses the smallest integer base possible, 2.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#59

Post by garmaine »

anvandare wrote: post #30

For the next revision: remove the mole from the SI. It's completely pointless. A number is not a unit! Instead replace it by the bit, the fundamental unit of information.

If you think mole is redundant, the bit is just as redundant.

Re: An old artifact kept in a vault outside Paris is no longer the standard kilogram

#60

Post by garmaine »

jabl wrote: post #33
bdowling wrote: post #16

Earlier quoted context omitted.

It's almost a crime that a metric ton isn't called a "megagram". Aside, are the mega and giga prefixes ever used for anything except bits and bytes? (Edit: OK, OK. I forgot about megaohms, megahertz, megawatts. Also megapixels and Megadeth.)

> Aside, are the mega and giga prefixes ever used for anything except bits and bytes? Sure, few examples of the top of my head: - Megatons, as in yield of nuclear weapons. Which is a bit odd unit, should be in J, goddammit! - Similarly, energy consumption/production. Commonly used units MWh, GWh, TWh. Again, the proper unit would be J, not Wh. Or Mtoe, for Million tons of oil equivalent.

> Megatons, as in yield of nuclear weapons. Which is a bit odd unit, should be in J, goddammit!

That’s what is odd about it? It should be teragrams.

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