Physicist(#39) I think that's a mischaracterization of the Copenhagen Interpretation. It would be more correct to say that (according to the CI) a particle's properties don't exist before they're measured. .
Quantum mechanics tells us what we can know about a particle. To say that it does not exist before it is measured is saying that we can make no meaningful statement about its properties. Doing a sum over the history of a particle does not mean that the particle took all possible paths from point A to point B, it means that it is not possible to say which way it went. In this sense, the particle took all possible paths, but that is not physically possible. So one can say that it had no meaningful existence between points A and B.
Either way, we are limited in what we can know, not necessarily what is “real”.
Another example, outside of QM:
If other universes exist, by definition, they cannot communicate with our universe. It what sense can these universes be said to exist if it is impossible to detect them?
But I can make meaningful statements about other properties of the particle! The currents are still conserved: I can prepare the state so that it is in a well-defined energy and momentum, or in a well-defined polarization state. In any case, the particle's total spin, charge and isospin are in no way indeterminate; I can state with confidence that the particle maintains these properties throughout, no matter how indeterminate some of the other properties might be.
Doing a sum over the history of a particle does not mean that the particle took all possible paths from point A to point B, it means that it is not possible to say which way it went.
You are implying here that it may have taken a specific path, and that we simply don't know which path, but that is not the case. The experiment can be arranged so that it is possible to prove that it didn't take any specific classical path.
In this sense, the particle took all possible paths, but that is not physically possible.
It's not classically possible, but it is physically possible. Particles propagate as waves, and waves don't take specific trajectories.
In thinking about QM, we are the victims of our own prejudices. We think of things in terms of classical objects interacting each other, and seek to explain quantum phenomena in those terms. But it can't be done. A bouncing billiard ball seems simple to our minds, but in reality it's not: when a billiard ball strikes something and bounces off, it is the result of a horribly complicated tangle of a gigantically large number of quantum wavefunctions, all interacting and interfering with each other. The ball and its bounce are composed of quantum wavefunctions. So when we seek to interpret quantum phenomena in terms of classical phenomena, we are attempting to describe the more fundamental in terms of the less fundamental. This is a philosophical impossibility. We should only seek to explain classical physics in terms of quantum physics. Quantum physics is what it is.
So one can say that it had no meaningful existence between points A and B.
"Meaningful" is an awfully pregnant word in that sentence. Meaningful to us? I suppose you can say that, but that's more a reflection of our language than of reality. We're stuck visualizing things in classical terms, where particles exist at specific places at specific times with specific momenta and energies. Nature is not so limited as our understanding. As far as nature is concerned, quantum indeterminacy, at the most fundamental level, is what constitutes existence.