Quantum entanglement says that we can entangle two particles with an opposite property, separate them, and that they exist in an indeterminate state until we observe one of them.
eg. If we create a left/right pair, separate them in space then observe one of the particles then the other particle of the pair will instantly resolve into the opposite state of the particle we observed.
Question: How do we know this is true? How do we know that something happened to the second particle as a result of observing the first?
A much simpler explanation is that both the particles flipped into fixed left/right states at the moment we started to separate them, we simply hadn't opened the containers yet to see which was which.
I assume that there can be no possible experiment to distinguish between these two explanations since we're not allowed to observe either particle.
What reason(s) do we have to believe the magical 'quantum' explanation when the simple explanation explains it just as well?