2 ms·

My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relati

My understanding is that the cause is error propagation. We can measure certain kind of masses really accurately in unified atomic mass unit. With better relative precision than the Avogadro constant. But if we used kilograms to write down these quantities then they would carry the error of the Avogadro constant needlessly. Edit: I think your question boils down to "Why do we have two separate units for mass, u and kg, connected by the Avogadro constant?" Most answers dismiss your original question as Avogadro constant is not a unit. But u is a unit and it's the point why we have this constant. Edit2: To further emphasize my point look at the mass of neutron[1]. It's listed both in kg and u. Note the number of decimal places. [1] https://en.wikipedia.org/wiki/Neutron https://en.wikipedia.org/wiki/Neutron

3pt14159 8y ago
Right, I get that, and if anything I feel less dumb now that I've asked it publicly and people seem to think that it's a non-dumb question, but when talking about fundamental constants I understand there is a practical nature to it all, but just as we have a nano-meter and a light-year I figured we'd have the same for mass. Why do I need both kg and u as fundamental constants? I believe the comments here have answered it. It isn't something weird, like quantum gravity or some such. If I understand everyone correctly it's just a practical decision we made at some point because we didn't want mass to be in u and that's that. I feel better about it now.
- jessriedel 8y ago
  They aren't fundamental constants, they are fundamental units. The system is ultimately about allowing people to compare measurements (not describing the universe in abstract), and is only tied to fundamental physical constants where it makes that more reliable. That's not a whole answer, but it may be helpful for your thinking.

3pt14159 8y ago

Right, I get that, and if anything I feel less dumb now that I've asked it publicly and people seem to think that it's a non-dumb question, but when talking about fundamental constants I understand there is a practical nature to it all, but just as we have a nano-meter and a light-year I figured we'd have the same for mass. Why do I need both kg and u as fundamental constants? I believe the comments here have answered it. It isn't something weird, like quantum gravity or some such. If I understand everyone correctly it's just a practical decision we made at some point because we didn't want mass to be in u and that's that. I feel better about it now.

jessriedel 8y ago
They aren't fundamental constants, they are fundamental units. The system is ultimately about allowing people to compare measurements (not describing the universe in abstract), and is only tied to fundamental physical constants where it makes that more reliable. That's not a whole answer, but it may be helpful for your thinking.