the air, and it'll fall along an ellipse; leave your umbrella parked on the street,

and when you come back, it's still there, just one umbrella.

Quantum physics is not like this - because quantum mechanics doesn't allow us to make

absolute predictions about the future. It only predicts the likelihoods of different

outcomes to happen, and doesn't say anything about which one will happen. "Well," you might

say, "that's the same with the weather - the weatherman only tells you what the chance

of rain is; he can't tell you whether or not it will rain."

But maybe the weatherman just doesn't have good enough knowledge of exactly where all

of the air and water molecules in the world are, nor a good enough model of how they interact

or a fast enough computer to simulate all of their bajillion interactions. Maybe in

principle, if he had enough data and a fast enough computer, his weather model could tell

you exactly where every raindrop would fall. Right? This reasonable idea, that if you just

had more data you could explain everything, is the classical, deterministic view of the

universe.

And for a while, many physicists, including Einstein, thought the same had to be true

with quantum mechanics - maybe we just didn't have enough information to put into our quantum

models; maybe there were classical variables that were hidden from us and our experiments,

inputs that explained everything perfectly with no need for quantum mechanics and its

"I'll give you 50/50 odds on the cat being dead" mentality.

Except, it turns out that we can actually test whether or not this sort of classical,

underlying explanation of quantum physics can exist even in principle. The details are

a topic for another video, but the experiments tell us… there is no classical, everyday,

underlying description of quantum mechanics.

And this means, Einstein, that the universe is quantum mechanical whether you like it

with a 50% chance, or not.

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