23.3: Postulate 3- Individual Measurements
In any measurement of the observable associated with operator \(\hat{A}\), the only values that will ever be observed are the eigenvalues \(a\) that satisfy the eigenvalue equation:
\[ \hat{A} \Psi = a \Psi. \label{24.3.1} \]
This postulate captures the central point of quantum mechanics: the values of dynamical variables can be quantized (although it is still possible to have a continuum of eigenvalues in the case of unbound states). If the system is in an eigenstate of \(\hat{A}\) with eigenvalue \(a\), then any measurement of the quantity \(A\) will yield \(a\). Although measurements must always yield an eigenvalue, the state does not have to be an eigenstate of \(\hat{A}\) initially.
An arbitrary state can be expanded in the complete set of eigenvectors of \(\hat{A}\) \(\left(\hat{A}\Psi_i = a_i \Psi_i\right)\) as:
\[ \Psi = \sum_i^{n} c_i \Psi_i, \label{24.3.2} \]
where \(n\) may go to infinity. In this case, we only know that the measurement of \(A\) will yield one of the values \(a_i\), but we don’t know which one. However, we do know the probability that eigenvalue \(a_i\) will occur (it is the absolute value squared of the coefficient, \(\vert c_i\vert^2\), as we obtained already in chapter 22 ), leading to the fourth postulate below.