I've recently started studying QM and I was told that the quantum state in which a particle is will change as its wave function evolves. So let us say I have a source that emits one photon at a time. If I measure the polarisation of a photon from this source and find it to be along a certain axis(say the y-axis) and if I immediately measure its polarisation,I understand it will still be along the y-axis.
But if I measure its polarisation after some time,then the wave equation will have evolved, giving me a polarisation along another axis. Did I get this part right?
If so, I think this same reasoning can be applied to an electron and its spins (right?) and an electron can switch spin states between measurements. That would mean an electron spontaneously changes its states between up and down as time passes, but this can't be right because we don't really see free electrons constantly giving off photons due to spin flips. So which part of my reasoning is false?
Is it that the changes in properties of a quantum system are caused by measurement? Or did I get it all wrong and nothing about a quantum system changes once a measurement is made?
PS:I am assuming the spin is being measured by a Stern-Gerlach type apparatus.