What can make an observation in the Copenhagen interpretation?
(In this and the following two sections, we draw heavily on Chapter 11 of the 1990 book by Euan Squires, Conscious Mind in the Physical World.) We will first show that any system that is completely described by quantum theory cannot exhibit wavefunction reduction. In order to do this in the most efficient manner, we will use a symbolic notation that makes the description concise and precise. Do not let this frighten you–it is simply a notation, not higher mathematics. The notation will refer to a particular type of experiment with particles that have spin. The spin of a particle is related to its rotation. A macroscopic analog is a spinning top. We can say that if the top is spinning normally on a flat, smooth surface, the spin (like the top) is pointing down. If for some reason, the top flips so that it spins upside down (there are tops that do this), we can say the spin is pointing up. Particles with spin (like the electron) can have their spins pointing either up or down. We start
