A main question in physics these days concerns collapse of the “wave-function”: When does this occur? There certainly have been numerous speculations (see, e.g., Ghirardi– Rimini– Weber theory, Penrose Interpretation, Physics forum) and experiments (e.g., “Towards quantum superposition of a mirror”) about this. The most extreme perspective is the view that Schrödinger’s cat is at the same time alive and dead, although Schrödinger proposed this particular thought-experiment (like Einstein’s less-well-known bomb experiment) to demonstrate how absurd such an idea is.
The problem happens because Quantum Mechanics can only calculate probabilities until an observation occurs. Nonetheless Quantum Field Theory, which works in actual field intensities– not probabilities, supplies an uncomplicated unequivocal answer. Sadly, Quantum Field Theory in its authentic sense of “there are no particles, there are only fields” (Art Hobson, Am. J. Phys. 81, 2013) is ignored or misunderstood by most physicists. In QFT the “state” of a system is explained by the field intensities (technically, their expectation value) at each and every point. These fields are real properties of space that act deterministically depending on the field equations– with one exception.
The exemption is field collapse, but in Quantum Field Theory this is a remarkably different thing from “collapse of the wave function” in QM. It is a physical event, not a change in chances. It occurs when a quantum of field, regardless of how spread-out it may be, instantly transfers its energy into a solitary atom and vanishes. (There are also additional kinds of collapse, like scattering, coupled collapse, internal change, etc.) Field collapse is not described by the field equations– it is a different occurrence, but simply because we don’t have a theory for it does not mean it can’t happen. The fact that it is non-local bothers some physicists, but this non-locality has been demonstrated in several experiments, and it does not lead to any inconsistencies or paradoxes.
So whenever field collapse happens, the ultimate “decision”– the defining moment– is reached. This is QFT’s answer to when does collapse occur: when a quantum of field colapses. In the scenario of Schrödinger’s cat, this is when the radiated quantum (perhaps an electron) is captured by an atom in the Geiger counter.
Just before a field quantum finally collapses, it could have interacted or entangled with a lot of other atoms along the way. These interactions are illustrated (deterministically) by the field equations. But the quantum can not have indeed collapsed into any of those atoms, for the reason that collapse can take place just one time, so no matter what you refer to it as– interaction, entanglement, perturbation, or just “diddling”– these initial interactions are reversible and do not bring about macroscopic changes. Then, when the ultimate collapse takes place, those atoms become “undiddled” and return to their unperturbed state.
To sum up, in QFT the “decision” is made when a quantum of field deposits all its energy into an absorbing atom. Besides replying to this question, QFT additionally explains why time dilates in Special Relativity and resolves the wave-particle duality issue of Quantum Mechanics. An individual can simply think about why this particular theory hasn’t already been welcomed and made the basis for our knowledge of nature. I feel it is truly time for physicists to WAKE UP AND SMELL THE QUANTUM FIELDS.
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