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?
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.
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One of the most obvious quantum "effects" which are observable at the macroscopic scale is the Pauli exclusion principle for fermions: if it didn't exist matter would collapse as nothing would prevent electrons to radiate and fall into the nucleus.
Typically quantum effects become observable at the macroscopic level when they affect coherently (i.e. in the same way) a large number of quantum particles.
Quantum properties are manifest everywhere. In the color of objects, structure of crystals, in the electrical, mechanical, chemical properties of different substances and in the very phenomena of life itself. There is no limit how big an object should be to show quantum behavior, only it will be increasingly difficult to maintain coherence. An one mm diameter object (oil droplet) has been shown to interfere with itself. Also see this.
It isnt really an answer, but molecules and chemical reactions are pretty big on a quantum scale, and those interactions I would think are more in the domain of traditional physics
I know that in medical imaging the constant noise that degrades image quality unrelated to scan parameters is called quantum noise, although I have never actually been sure it was a true quantum event. I suspect it is a quantum thing to do with background energy fluctuations, but it could be related to cosmic rays or something similar too. I would look it up myself but I am off to bed now!
If it is a quantum event, then there is a good example of how something quantum becomes macro.
EDIT: Not sure why this was marked down, but if I have to explain why this explanation made sense, here you go.
After a sleep I found out about this - it is a quantum event observable on a real world (macro) scale. Not sure why anyone would mark this down ... maybe you wanted some esoteric wordy explanation but I thought you might want an example too.
the images there show the effect of quantum noise - the grainier right hand side images have a worse signal, and therefore quantum noise is more visible.
This is caused by probabalistic fluctuations in quantum states throughout the system. In fact, this explains very clearly your opening question - as you can see, while there is local variance which is noticable because we are using high energy xrays, the field overall is homogenous. In most systems the heterogeneity would be on such a small scale it would be undetectable without extreme instruments.
So each individual event has a probabalistic element, but over macro systems you lose the ability to notice.
Or just listen to Marek. I think this example is a good one.