I am just beginning to understand this concept. Some help would be appreciated. Let me know if I am wrong in saying the following: "The wave function (say [tex]\Psi[/tex]] collapses to an eigen vector of the operator corresponding to the physical quantity(say [tex]\lambda[/tex]) being measured. This is because the act of measuring interferes with the system" Now what confuses me (further) is that, as a result of the wavefunction collapsing to an Eigen vector, the subsequent measurements give the values with a probability 1. I understand that if [tex]\Psi[/tex]=[tex]\Sigma[/tex] ai Ni and [tex]\Psi[/tex] collapses to some Ni, ai=1 => probability is 1. But doesnt this eigen function Ni now act as a new [tex]\Psi[/tex] ! We should be able to find a basis set of vectors corresponding to Ni in which case it should again collapse to another smaller eigenvector. This doesnt seem to happen. Why? Hope the question is clear.