vanesch said:
Not necessarily. Wavefunction collapse is a formal idea which is NOT directly observable. As has been pointed out previously, the only thing that is observable, are, well, observations.
There are several views to look upon "wavefunction collapse" - they are in fact the different interpretation schemes of quantum theory. I myself am a proponent of the MWI view (to put my cards on the table), but I recon that this is just *A* view amongst many. Nevertheless, it illustrates that "collapse" is not something that is to be taken for granted in a naive way (kind of "plooof" thing happening).
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In the information point of view, all physical theories are just formal tools to organize our knowledge of outcomes of measurement, and are not supposed to be a description of what physically happens between those measurements. As such, collapse is like the collapse of a probability function: when you learn about the outcome, suddenly your probabilities are changed because of that new information. But probability functions are not physical objects, but just descriptions of our knowledge. There are variations on this view: some say that, of course, there MUST be a physical underlying reality, but QM isn't describing it ; and there are others who say that talk about an underlying physical reality is serving no purpose, and all we need is an organizing principle of our knowledge and observations.But all these views are speculations on how to look upon the formal elements in quantum theory, and in only two views the wavefunction is taken as something physical: in a von Neumann view (but there, it is not clear WHEN and WHERE collapse occurs) and MWI. In all other views, the wavefunction is just a formal tool, and one does NOT try to say what's going on physically. So no theory explicitly says when a collapse occurs *physically* and how it "propagates stuff through space" or something.
Excellent post; very clear indeed. But, the infamous but, I disgree with certain notions you present.
First, nobody in their right mind can take Copenhagen/vonNeuman seriously -- that approach was developed by supremely brilliant men at a time when virtually everybody was naive about human cognition, and, I think, about the underpinnings of practical probability theory as well. This tied into very strong connections with 19th century notions of reality, reason, and the ultimate hybris of turn-of-the-century physicists, that they were in shouting distance of understanding it all. The plain fact is nobody really has a clue about the basics of quantum measurements -- why only one result at a time? Is this fundamental to nature, or is it a constraint created by our perceptual mechanisms? Look how we argue about, is the moon there when no one is looking? Is there life between measurements? Entanglement. Decoherence.
We know how to measure, how to make sense of the results (well, sometimes), but, how in the world does this superposition end up unsuperimposed? (Unfortunately termed wave function collapse. Unfortunate, because once said there was then no choice but to try to explain the unexplainable -- how to get physical certainty in a probabilistic world? And, in my opinion, they badly bungled the job. Many wonderful 19th century notions are simply inapplicable to 20th century and current physics, c.f. causality, continuity, certainty, ... .(Earlier folks were smart enough to avoid the collapse issue.)
These days, in practice, Copenhagen means Born and the probability interpretation, all of which, in my view, saved the day from the tortured dances of the founders. One number wins the lottery -- could be a degenerate state --, one counter clicks in a scattering experiment. We certainly never worry about any sort of a collapse in the lottery, or in finding or not finding lot's of traffic on our way home from work. In fact we deal with these uncertainties and probabilities -- subjective or objective --much like the current phrase, "Get over it", it's just the way the world works, With Born, we do the same thing; apply basic probability and statistics to quantum phenomena.
Forget a physical wave function,. forget collapse. To me, the simple fact is that the idea of collapse of a physical wave function is nothing but a black hole of problems, so why bother? (I suppose you could term the electrons in an electron microscope as a physical manifestation of a wave function. Just like the lottery -- provided one can release one's self from the dominance of 19th century ideas, at least in physics.
Understanding? Reality? Always changing. While I know that even the great Feynman noted that, in essence, nobody really understood quantum theory, I take the opposite view. Again, much of my notion is that we must understand QM on its own terms, rather than with old, possibly conflicting ideas. When in Rome ... There are many texts and articles that use physical arguments for QM issues --what is that but a direct sign of understanding? (Semiconductors, lasers, Fermi-Thomas techniques, and on-and-on.)
What is language but a formal tool?
And, what exactly does it mean to "say what is going on physically?"
Does anybody think that experiments could detect whether a wave function has a physical manifestation, whether collapse occurs, or detect evidence that MWI has any basis in fact?
Regards,
Reilly