Notes:
Although the basic feature of quantum phyics the introduction was of the quantum of energy, the notion that energy can not exist in any amount but there is minimum amount, a so called quanta, the succeeding development of the new formalism resulted in a number of aspects that are incomprehensible.
I select here the aspect of so-called superposition and the consequence of this description for the status of measurement. As I told in the introdcution the formalism of quantum physics allows prediction of probabilities for several possible outcomes in a quantum experiment.
In this case we shoot a photon on a half transparent mirror. Quantum physics gives then, depending on the details, for instance 50% chance that the photon is reflected and 50% that it is transmitted.
In classical physics we would describe the situation by saying the photon is either transmitted OR it is reflected. The system is in state a or in state b.
In quantm physics this is not the case. The system is described as superposition of the two possibilities. So in quantum physics the descriprion is that the photon is ‘simultaneously reflected and transmitted.’
If we attach photon detectors to detect the photons, or in other words we measure the photon, then suddenly the description of the system collapses into the singular state that we are familiar with in our Newtonian world. Either this or that and not simultnaeously. This collapse is also known as the reduction of the statevector. The statevector is a superposition of different possible outcomes in outcome space and it suddenly is reduced to a projection on one of the axis in outcome space. (So projection is also used to describe the measurment. )