In quantum theory there is a thought experiment called Schrödinger’s cat-experiment. The idea of the experiment is that there is a cat in a closed box (don’t try this at home or at all!) together with a flask filled with poison which would, if released, instantly kill the cat. Next to the flask there is a hammer whose position is determined by some kind of mechanism involving the nucleus of an atom. The details of the mechanism do not concern us here: what is important for us is that if the atomic nucleus decays then the hammer will fall, the poison will be released and the cat dies. If the nucleus doesn’t decay, then the flask remains unbroken and the cat remains unharmed.
Suppose that, at a given moment after the lid has been closed, we don’t know whether the atom has decayed or not, and hence whether the cat is alive or dead. Quantum mechanics tells us* that as long as the box is closed and we cannot say that the cat is either dead or alive, we must assume that the cat is in a ‘superposition’ of a live and a dead state. Does that mean that we must assume that the cat is both dead AND alive?! That would be bizarre!
Why can’t we just assume that although we are uncertain as to whether the cat is alive or dead, the actual state of affairs is that the cat is either alive or dead? Why do quantum theorists resort to this vague concept of superposition?