determinism - By what mechanism do quantum effects become observable in normal life at the macroscopic level?

By what mechanism do quantum effects become observable in normal life at the macroscopic level? For instance, when two molecules "collide" is the momentum a probabilistic event wherein the end state is not unique? Another example, during a chemical reaction, it is a probabilistic event at the quantum level whether or not any particular molecule within the solution interacts with another molecule?

Answer

The mechanism is called decoherence. Microscopically, it's true that the system evolves through all allowed configurations (see path integral, which in its more basic description tells you that the particle travels through every trajectory). But in real life we don't observe this. What gives?

Turns out that the answer is related to something called wave function collapse. This is a simplification of what happens but it gives you some intuition. When the system is left on its own it evolves as in the first paragraph. But as soon as you observe it (i.e. measure it), it will "collapse" into one of allowed eigenstates of the observable you are measuring. So in the end we are left with pure states without superposition.

Now it should be clear that "something" must be observing and collapsing the system even when we are not looking. What is it? Well, if one pauses to think for a second it should be obvious that every system that behaves classically is actually submerged in some bigger system, an environment (the typical example that concerns almost every system on Earth is atmosphere with lots of gas molecules present). In any case, humans are in no way special when dealing with nature and observation. Same role can be played by any big enough system, and environment surely is such a system. We know that according to quantum mechanics systems undergo quantum fluctuations. It is these fluctuations between system and environment that affect any superposition and quickly make it decay into states that are effectively classical.

This process can actually be imagined very clearly by taking analogy with heat transfer. Heat is transferred because of thermal fluctuations of two objects and basic probability theory dictates that the temperature of the warmer object will go down. Similarly, quantum mechanics and probability dictates that superposed system will gradually lose information about the superposition between its parts. This information is of course not lost but will get transferred to the environment where it will appear as noise and so the result is that we obtain effectively classical system.

So this means that to observe quantum effects macroscopically, you need to shield your system from decoherence. Either you lower the temperature as much as possible (so that the effect of environment is negligible), or you prepare the system in some clever way so that it is impervious to thermal perturbations. We are of course talking about superfluids, superliquids and similar states of matter.