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In quantum mechanics, the uncertainty principle states that, for isolated systems involving small distances and momentum it is impossible to know, exactly, the instantaneous position and momentum of the particles. I think that your quandary can be addressed by examining how do we “know” something. In general, we know things as a result of mental (via philosophy, theory, calculation, etc.) or physical (via observation, experimentation, etc.) processes. If we are doing calculations, we shall be able to make predictions only of the probable behavior of particles. If we try experimentally, for example, to determine whether radiation is a wave or a particle, we find that an experiment which forces radiation to reveal its wave character strongly suppresses its particle character, and vice versa. [References: Elmer Anderson. 1971. Modern Physics and Quantum Mechanics. Philadelphia: Saunders. Eisberg, Robert, and Robert Resnick. 1974. Quantum Physics of Atoms, Molecules, Solids, Nuclei, and Particles. New York: John Wiley & Sons.] Therefore, as I see things, it does not matter if you are doing an experiment or doing a calculation. In either case, you are attempting to know something about a particle. The uncertainty principle says that we cannot, simultaneously, precisely, know everything about the particle. We must express the particle in terms of probabilities. Therefore, we cannot measure or calculate the speed with certainty. We can say that speed is x m/s, with a probability of, say, 0.9. There most likely is a definite (certain?) speed associated with the particle – we just cannot know it, by calculation or measurement.

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