Insufficiency of the Quantum State for Deducing Observational Probabilities

Understand

It is usually assumed that the quantum state is sufficient for deducing all probabilities for a system.

• This may be true when there is a single observer, but it is not true in a universe large enough that there are many copies of an observer.
• Then the probability of an observation cannot be deduced simply from the quantum state (say as the expectation value of the projection operator for the observation, as in traditional quantum theory).
• One needs additional rules to get the probabilities.