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Comment by MarcusP | Hacker News Reader

Are metric measurements all derived from the value 1kg? If so, does this mean that the entire metric weight range can now be officially based on mathematics?

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There are seven so called "base units" defined in the SI; meter, second, mol, ampere, kelvin, candela and kilogram [1].

Of these, mole and kilogram are dependent of the kilogram.

edit: this is a good, if maybe a bit misleading, illustration: http://www.nist.gov/pml/wmd/metric/upload/SI_Diagram_Color_A...

[1]: http://www.nist.gov/pml/wmd/metric/si-units.cfm

> Of these, mole and kilogram are dependent of the kilogram.

Did you make a typo? While it's technically true that the kilogram is defined by the definition of the kilogram, I just wanted to make sure.

Not a typo, but maybe a bit vague. I meant to say that the SI units both are dependant on the real world kilogram object.

The NIST chart is interesting. I'm surprised that charge is defined in terms of current, rather than the other way around.

No, at least second and meter are defined with math and elementary physics (e.g. second is the time it takes for a specific atom to oscilate a specific number of times).

Not the atom, the electromagnetic radiation emitted by a specific quantum transition of the atom. (Cs 133 and 9192631770 Hz)

for everyday applications you can assume that 1kg = 1l H2O = 1dm³ if that's what you mean.

Wouldn't that change though based upon the isotopes of the atoms in it? For example H2O made with Deuterium (1 proton 1 neutron and 1 electron) or Tritium (1 proton 2 neutrons and one electron) vs Protium (1 proton 1 neutrons and 1 electron), could all give different weights, not to mention different isotopes of oxygen 16 17 and 18 for the most common/long lived.

Hence "everyday applications."

In a (much more rational than SI) system "h=c=1" there is only one fundamental unit left - kg (or ev, which still depend on a kg definition). So getting rid of kilogram is absolutely essential.

More completely, ħ = c = G = kB = ke = 1.

That system is mostly irrelevant for cultural and legal purposes. Even in the specific philosophical framework where getting rid of the kilogram 'absolutely essential', the current importance of the kilogram would simply migrate to the importance of the specific conversion factor to traditional units.

Any such conversion would not need a physical object - just a set of arbitrary constants (which is fine).

Also known as Planck units

https://en.wikipedia.org/wiki/Planck_units

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