How do we know that certain properties are indeterminate or undefined until they are measured?
Take the example of quantum entanglement, saying that if you have two entangled particles, such as the spins of 2 electrons, and you know that their spins have to be opposite, and we measure one, so we know that the other spin collapsed into the other answer immediately, even if it's a million light years away, leading to the strange and immediate "spooky action at a distance."
If we applied Occam's razor to this situation, we could conclude that the 2 particles already had a definite spin before we measured them, we just didn't know what it was. Then when we measured it, we simply learned the spin (that was already definite) so then we knew the spin of the other particle. No spooky action, no faster than light action... Certainly the SIMPLEST explanation... But quantum mechanics tells us that apparently this isn't the correct explaination. (Too bad, Occam)
So, my question is, how do we KNOW this to be how the world works? What evidence have we seen that shows that properties such as spin are truly unknown, not just to us, but to the universe itself, until they are measured (or forced to 'make a decision' by the universe)
I have never had anyone give an answer to this... I don't want a theoretical, mathematical answer about how it makes sense if you think about it this way, or that way, I want experimental or observational evidence that certain properties truly aren't decided until measured or forced into being decided. Anyone have any examples of how we know this, instead of us guessing about it?