Having started this thread, I think I should say something about where I have reached in trying to answer my original questions.
"What triggers decoherence?"
"Why, after almost 14 billion years, have not all the quons in the Universe decohered?"
It seems that decoherence is triggered when a quon interacts with its environment. This may mean interacting with just one other quon, or with complex system.
The question as to why there are plenty of quons in the Universe that are still in superposition when quons have been interacting for billions of years is perhaps rooted in a confusion between wave function collapse and decoherence.
Wave function collapse involves a process which is irreversible under the second law of thermodynamics, and involves the permanent loss of “quantumness”. Once collapsed, the object involved can be totally described in terms of classical physics.
Decoherence, on the other hand, is a less cut-and-dried concept in that, although the process that triggers it may be thermodynamically irreversible, decoherence itself is not so easy to pin down. It appears to be much more observer specific. If, for example, I observe a quon, I will not see it in a superposition. As far as my observation is concerned, the wave function has collapsed, but in a broader context, the quon retains its “quantumness” and may be in states of superposition that I am unable to observe. If I stop observing the quon; then observe it again, decoherence will happen again, and I will observe something of which the wave function has just apparently collapsed.
Decoherence brings about an apparent wave function collapse only in the frame of reference of the observer.
That's about where I am at the moment, but I'm sure there's room for more refinement.
