We have seen that experiments in quantum theory have a strange property: their results cannot be predicted with certainty. Instead, we can only find probabilities for different outcomes.
Our question now is: what does this imply for physics? Are there no laws which can tell us for sure how the universe behaves?
In this chapter we will see that the situation is not so dire. There is an overall wavefunction which evolves according to a well defined law. That is, each time we repeat the experiment, this overall wavefunction will behave in exactly the same way.
But this overall wavefunction includes both the system to be measured and the detector doing the measurement. When we try to separate these two - by asking what one part, the detector, sees about the other part, the system - we get the randomness that we have already noted.
Thus we will learn that physics is still described by predictive laws, but also that the nature of measurements is very novel with quantum theory. In particular, the randomness observed in such measurements is a natural part of the theory, and can never be removed.
Since two entities are involved - the system and the detector - we will begin by understanding how two particles are described in quantum mechanics.