How would you know a quantum system (or a single particle) is in multiple states until measured? It seems to me the measuring proved my point.
Strictly speaking, my description was a bit sloppy. What I called 'states' are the probabilities of the system being measured to be in one or another state; prior to measurement it doesn't actually have that property at all. Similarly, the popular meme that a quantum particle can be 'in two places at once' is a simplification - it doesn't have a location property until it's measured. IOW these 'properties' are potential or latent, and only become real when the quantum system interacts with something. How can it interact if it doesn't have a location? it will interact with another quantum system (e.g. measuring apparatus) in a location where it has some probability of being observed, and with that probability.
The simple way to distinguish between having
incomplete knowledge of the system (i.e. it's in one or other state and we just don't know which state it's in until we measure it) and
quantum superposition (i.e. it's not in either state, but in a superposition of measurement outcome probabilities), is by quantum interference.
The probabilities of the various measurement outcomes are given by a wavefunction, the amplitude of which at some point can tell you the probability of measuring a particular outcome. These amplitudes can be positive or negative (peaks and troughs of the wave) but are squared to get the outcome probability (because you can't have a negative probability).
If there is more than one way for a measurement to occur, e.g. the double-slit experiment, where a particle can go through either of two slits to hit a screen, the wavefunctions for each path must be summed to get the probabilities of the final measurement outcome. For the double-slit experiment, the wavefunctions for the particle to pass through each slit are summed to get the measurement outcome at the screen. Since summing two waves means coincident peaks and coincident troughs will reinforce each other and peaks coinciding with troughs will cancel out, the result will be a wave function describing an interference pattern on the screen, a non-classical result.
However, if the particle is then measured/observed to go through one slit or the other, only the wavefunction for that slit is relevant, so no interference will occur, and the result will be wave function describing a classical distribution for measurement outcomes on the screen opposite that slit.
I'm not sure what you are protesting: how is a system doing two things at once not one thing?
I'm not protesting anything, just using the non-classical counterintuitive characteristics of quantum behaviour to ask a rhetorical question; a single action that produces a superposition could be seen as achieving two different outcomes, a particle with a probability of being observed in state A
and a particle with a probability of being observed in state not-A, in the same particle. It depends what you mean by '
only one thing can ever happen'.
It's just an example of QM having significantly different logical behaviour from classical physics.
