Hi all, Consider a simple harmonic oscillator in the ground state. Measuring its energy yields the ground state energy with 100% probability. Now by measuring its position (thereby changing the wavefunction) and next measuring the energy again, there is now a nonzero probability of finding the particle in an excited state. (I've taken this example from http://en.wikipedia.org/wiki/Measurement_in_quantum_mechanics#Example ) If the particle has a probability of being found in a higher energy state, does this mean that this energy was somehow 'supplied' to the particle by measuring (i.e. sending photons to it to observe where it is), or just that the particle always had this energy, but by being prepared in the groundstate there was no way of measuring it?