Elements of Classical and Quantum Physics pp 227-229 | Cite as

# Postulate 3

## Abstract

Note that the system might have a large number of degrees of freedom, yet one can make a measurement involving one of them, like one component of angular momentum, which has an eigenvalue equation depending on a single angle \(\phi \). If \(|\varPhi \rangle \) is an eigenstate belonging to a discrete eigenvalue \(\lambda \), \(P(\lambda )=1\); in such cases, Quantum Mechanics gives certainties. Otherwise, after the measurement \(\varPhi \) *collapses* in an eigenstate of the operator, and if the measurement is repeated immediately (i.e., before the system evolves) the result is again \(\lambda .\) Quantum Mechanics gives no information about the way in which the interaction with the classical measurement apparatus produces the collapse, which is considered as a sort of instantaneous evolution which is outside the scope of the Scrödinger equation. Many people dislike the fact that the observer is not a part of the story. As an alternative to the collapse, Hug Everitt proposed a many-worlds interpretation, in which all the possible outcomes of the measurement take place in some Universe and the measurement takes us to one of these. This idea has influential supporters and opposers. But, while we must be ready to accept unobservable mathematical tools like vector potentials and wave functions, the description of the reality should not depend on unobservable parts of the reality itself. Science is not compatible with occultism. Thus, it appears to me that the remedy is worse than the disease.