I'm very confused about the applicability of classical vs quantum physics to describe a system. Suppose I have a system of particles and I want to describe their behavious, how do I know which theory applies to them? Is there a parameter that makes a system classical as opposed to quantum? Or should I try both theories and see which one agrees with experiment?
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Yes, I think that exist, the wavelength ($\lambda$) and the size of the system (L) are that parameters, L is the length of the size more representative.
Quantum mechanics is when $L \approx \lambda $.
Classical Mechanics is when $L >> \lambda $
I don't know what append $L << \lambda $, in electromagnetic theory the classification is electrical circuit ($\lambda >> L$), Microwave circuit ($\lambda \approx L$) and geometrical optics ($\lambda << L$).
