Quantum observers with knowledge of quantum mechanics break reality

Enlarge / The inner quantum matryoshka doll wonders what all the fuss is about (credit: Bradley Davis)

Today, I learned something new: matryoshka nesting dolls aren’t quantum mechanical objects. Surprisingly enough, this was not something I should have been certain of. After reading a paper about how quantum mechanics within quantum mechanics can lead to contradictions, I will never look at matryoshka dolls the same way again.

The paper is not just theoretical in nature but seems to overcomplicate what should be a fairly straightforward argument. That probably means I’ve misunderstood it pretty thoroughly. So, let the mistakes begin.

The innermost doll is solid

Imagine that I have a single particle that has a quantum mechanical property called spin. We don’t care what spin is, just that it has an orientation in space. I can measure the orientation of the spin. But quantum mechanics doesn’t let me measure it in a general way; I can't ask "hey, Frank-the-quantum-particle, which direction is your spin pointing?" Instead, I can only ask "hey, Frank, is your spin pointing up or down?" To which he will always reply with an “up” or a “down.”