jtbell
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There is no generally accepted answer to your question. Some physicists believe that it doesn't make any sense to think of the particle's position and momentum as "existing" (in some sense) before they are measured or observed. Others think that it does.
Although it seems perfectly reasonable that, viewed all by itself, the position and momentum of a particle should always "exist," it turns out that versions of quantum mechanics that explicitly have this feature, have other features that many physicists find difficult to accept. As of now at least, no one has figured out a way (even in principle) to decide between these two positions by experiment. And so people argue about it a lot, for example in the threads here that have "Bohm" or "Bohmian" in the title.
Although it seems perfectly reasonable that, viewed all by itself, the position and momentum of a particle should always "exist," it turns out that versions of quantum mechanics that explicitly have this feature, have other features that many physicists find difficult to accept. As of now at least, no one has figured out a way (even in principle) to decide between these two positions by experiment. And so people argue about it a lot, for example in the threads here that have "Bohm" or "Bohmian" in the title.