Quantum mechanical evolution

Quantum Darwinism is a theory developed by Polish physicist Wojciech Zurek. I am not a physicist yet, but I think I understand it to some extent on a conceptual level. My goal is to try to explain it as clearly as possible, since I believe it is a theory that should be more commonly known.

At infinitesimal scales compared to those at which we live, classical physics proves insufficient to describe the processes of nature. One needs to resort to quantum mechanics instead.

A fundamental principle of quantum mechanics is that of superposition. It states that quantum ‘objects’ can be in multiple physical states at the same time, and that each quantum state can be expressed as the sum of two other quantum states.

In simple terms, while classical objects can ‘occupy’ only one physical state, (‘Schrodinger’s cat is either alive or dead, with no middle ground), quantum objects can be in a mixture of multiple states (the particle that decays to activate the vial of poison that kills the cat, may have done so at 40 per cent and not have done so at 60 per cent).

In classical physics, however, we never see a cat that is 60% alive and 40% dead. When the particle becomes a classical object, i.e. when it is indirectly measured by seeing whether the cat is alive or dead, it can only be 100% decayed or 100% not decayed. Tertium non datur.

Why is it that some quantum states seem to survive up to the scales of classical physics (such as the one in which the particle is 100%…