Quantum Mechanics and the Problem of Minds (Society of Catholic Scientists, October 13, 2025, Church Life Journal)
This leads us to a basic question. In quantum mechanics, there is always a “system” that is measured and that is described by a wave function, and an “observer” who makes observations or measurements of the system that collapse the wave function. The question is where the “system” ends and the “observer” begins.
Suppose that I am the observer, and the system I am studying is a radioactively unstable nucleus. One could count only the nucleus as the system, and consider the Geiger counter, my sensory organs, the part of my brain that processes the information from my sensory organs, and me in toto as the observer. Alternatively, one could lump the Geiger counter in as part of the system, meaning that there would be a wave function describing both the nucleus and the Geiger counter. Everything else would be considered the observer. Or one could consider not only the nucleus and the Geiger counter but also my sensory organs as part of the system. One could move more and more over from the observer side of the line to the system side. So it is somewhat arbitrary where the line between the “system” and the “observer” (sometimes called the “Heisenberg cut”) is drawn. Nevertheless, the logic of quantum mechanics requires that it must be drawn, and it must be drawn in such a way that there is something on each side of it. If one tries to put everything on the “system” side, so that there is nothing left on the “observer” side—so that there is no longer an observer at all, or any observation—you end up with a wave function that never collapses and probabilities that never jump to give definite outcomes. To quote Eugene Wigner again:
Even though the dividing line between the observer, whose consciousness is being affected, and the observed physical object can be shifted towards the one or the other to a considerable degree, it cannot be eliminated.
What is it that must remain on the “observer” side of the “Heisenberg cut”? It cannot be any part of his or her body, for these are physical and should be describable by wave functions. It is hard to escape the conclusion that there is some aspect of the mind of the observer that is non-physical.