[Aardwolf]

They define it based on Planck's constant, so the results also depends on the definition of meter and seconds if I understand it correctly.

Would it have been possible to define it as the weight of N amount of electrons (assuming all electrons have the exact same weight under all circumstances) or another fundamental particle?

EDIT: it would be the weight of 9.10938356e31 electrons at rest

[netheril96]

How to define fundamental units has always been a matter of precision measurements, not theoretical purity. If the standards body decides to base the definition on Planck's constant, instead of a certain amount of electrons, the rationale will be higher accuracy in measurements of Planck's constant than of the mass of electrons.

[caf]

It's OK though, because the metre and second are already based on physical constants: the second is "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 the metre is defined from the second by fixing the speed of light at 299 792 458 m/s.

[pbhjpbhj]

FYI, from Wikipedia:

>For electrons or electron holes in a solid, the effective mass is usually stated in units of the rest mass of an electron, me (9.11×10−31 kg). In these units it is usually in the range 0.01 to 10, but can also be lower or higher—for example, reaching 1,000 in exotic heavy fermion materials, or anywhere from zero to infinity (depending on definition) in graphene. <

[roywiggins]

One way has been by counting silicon atoms in a nearly-perfect spherical crystal.

http://aip.scitation.org/doi/full/10.1063/1.4921240

[marcosdumay]

And they are using this as a definition for the unity of atomic mass.