So we know that for the really small world we have quantum mechanical behavior and for big things we have classical behavior. But what is the boundary that differentiates the two? If we make a thought experiment with a maze getting smaller and smaller,when can we say that it goes from the classical world to the quantum mechanical and does it has to do only with the mass and size of an object?
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Quantum mechanics, excluding gravity, is valid at all scales. In principle, you can do classical mechanics via quantum mechanics, but this is just very hard because of the number of particles involved and decoherence. Therefore, you switch to classical mechanics, which describes an effective theory of large scales (and small energies), because most of the "quantum" effects are so small due to e.g. decoherence, they don't matter at all. The idea is that if the number of particles is infinite (and we take similar limits also for other parameters), we obtain classical mechanics, which means that classical mechanics is never truly valid in real life, but since all the differences are well below experimental precision, we don't care.
There are, however, certain effects that you can never describe with classical mechanics in a satisfactory way, such as the photoelectric effect.
In this sense, there is no boundary. In practice, you'll need to include quantum mechanical effects if the size of the objects approaches something like one nanometre (+- a couple of orders of magnitude) and/or the number of particles is well below the Avogadro constant. But this is just a rough estimate. One thread discussing this experimental boundary to some degree is this one. There is no real boundary, because it might be that certain objects do not show any "quantum" behaviour and some much larger objects do, just because the latter are more coherent in some way.
