Letter to Erwin Schrödinger (1931)
"I find it distressing to see your recent theory of quantum superposition, which seems to imply that a cat can be both dead and alive until we look."
Dear Erwin,
Your letter raised many interesting philosophical questions about the meaning of quantum mechanics—questions that many might find more suitable for physicists than philosophers. Yet, it seems these questions must now be addressed as we confront the paradox within the theory itself.
You have proposed a theory that suggests a cat is simultaneously dead and alive—yet when we open the box, the superposition collapses into a definite state. I believe such a theory is incomplete, and that there must be an underlying reality that governs these phenomena independently of our observations.
Your theory may have mathematical utility, but it cannot claim to be a complete description of the physical world if it allows for such paradoxical predictions.
I appreciate the boldness of your work, and I hope we may soon be able to find a bridge between classical intuition and the strange world of quantum phenomena.
With cordial regards and scientific curiosity.
— Albert Einstein
This letter is part of a larger correspondence between Einstein and Schrödinger that helped shape the philosophical underpinnings of modern quantum theory.