This lack of intellectual consensus can be traced to the fact that quantum mechanics seems to attribute special properties to the act of “measuring” or “observing” a physical system. In classical physics, objects have properties like positions and velocities, and you can directly measure them with as much accuracy as you like. Quantum objects seem profoundly different. Measuring the properties of a quantum system tends to dramatically change those properties. In some perfectly reasonable ways of thinking about the theory, a particle such as an electron doesn’t even have properties like
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