> Quantum mechanics is correct. Always, always, always use Quantum mechanics!!! Newtonian mechanics is obsolete! Okay, so when we talk about use we are talking

> Quantum mechanics is correct. Always, always, always use Quantum mechanics!!! Newtonian mechanics is obsolete! Okay, so when we talk about use we are talking about tools. A lot of the tools we got from Newtonian mechanics are pretty reasonably accurate. If I see a ball bounces 3 feet when I drop it from 5 feet up, there's no reason to use anything particularly fancy to guess how high it will bounce if dropped from 10 feet. These are simpler tools I can use, and they may not be great (gravity is weaker at ten feet, for instance, so it's clear I'd be wrong to some extent, forget my ignoring just about every pertinent detail of the ball), but it works. Right tool for the job doesn't need to be fancy. > You make it look like QM is a special case of NM, while in reality, QM describes all the phenomena that NM does, not the other way around; NM is a special case of QM. Quantum Mechanics is continuous and differentiable at all points, including in descriptions of e.g. mass. To make Newtonian Mechanics a special case, you'd (at the very least) have to put some gnarly integrals in place of almost every variable in your formulas. They don't _really_ agree, one isn't a subset of another. One is an approximation (and not even specifically of Quantum Mechanics, just an approximation of things-seen-on-earth, some of which have compelling quantum mechanical stories, some of which, like gravity, don't). And sometimes approximations are useful.