Why is quantum mechanics not valid for humans?

Answer 1

Mostly a matter of scale and complexity. Large systems are much more complicated to solve in quantum mechanics than small systems (too many variables interacting with each other in infinite ways). The individual molecules in the human body can be shown to follow quantum mechanical rules exactly (here we are down to a manageable number of variables interacting with each other in infinite ways and mathematical techniques have been developed to get most of the infinite interactions to cancel each other out).

Quantum mechanics is completely valid for humans. When one deals with quantum mechanics, they use what are called operators, specifically energy operators, which are simply energy functions acting on "physical-state" space. These operators give expected values when plugged into the Schrödinger equation, however, one would really only use all of this mess if they were dealing with phenomena on the scale of Planck's constant, 6.626 X 10-34 J•s.

Now, you may have seen or heard somewhere that mass and energy are intimately related, which is true, but at the energy scale where Planck's constant is relevant, masses are VERY, VERY SMALL. At the energy scales that we use in every day life, masses are a WHOLE LOT BIGGER and so therefore we can very safely approximate the Schrödinger equation to the classical equations of energy, motion, momentum, etc. that the budding physics student should be familiar with. If these approximations could not be made, quantum mechanics would be a completely inconsistent and invalid model for humans since it would only be correct for very specific circumstances. However, it's precisely because of its consistency with the classical limit that makes quantum mechanics valid for humans.