First, at this point I am reasonably confident that collapse actually happens. Either it does or it doesn’t, and non-collapse interpretations of QM are those that have unfounded faith that quantum wave states always evolve unitarily. As I argued in this paper, that assumption is a logically invalid inference. So given that we don’t observe quantum superpositions in the macroscopic world, I’d wager very heavily on the conclusion that collapse actually happens.
But what causes it? Since we can’t consciously observe a (collapsed) quantum mechanical outcome without being conscious – duh! – many have argued that conscious observation actually causes collapse. (Others have argued that consciousness and collapse are related in different ways, such as collapse causing consciousness.) In this blog post, I discussed the consciousness-causes-collapse hypothesis (“CCCH”) in quantum mechanics. I pointed out that even though I didn’t think CCCH was correct, it had not yet been falsified, despite an awful paper that claimed to have falsified it (which I refuted in this paper).
Two things have happened since then. First, I showed in this paper that the relativity of quantum superpositions is inconsistent with the preparation of macroscopic quantum superpositions, which itself implies that CCCH is false.
Second, this paper was published a few days ago. Essentially, it’s a Wigner’s-Friend-esque thought experiment in which a poison-containing breaks or does not break at 12pm, per a QM outcome, but the person in the room will be unconscious until 1pm. That’s it. If CCCH is correct, then collapse of the wave function will not occur until the person is conscious at 1pm... but if he is conscious at 1pm, how could the wave state possibly collapse to an outcome in which the person dies at noon? It’s a very simple logical argument (even though it is not explained well in the paper) that is probably valid, given some basic assumptions about CCCH.
So when does collapse actually occur? I’ve been arguing that it happens as soon as an event or new fact (i.e., new information) eliminates possibilities, and the essentially universal entanglement of stuff in the universe (due to transitivity of correlation) makes it so that macroscopically distinct possibilities are eliminated very, very quickly. For example, you might have a large molecule in a superposition of two macroscopically distinct position eigenstates, but almost immediately one of those possible states gets eliminated by some decoherence event, in which new information is produced in the universe that actualizes the molecule’s location in one of those position eigenstates. That is the actual collapse, and it happens long before any quantum superposition could get amplified to a macroscopic superposition.