Schrödinger found superposition so odd that he illustrated its absurdity by a thought experiment. He imagined his cat in a box with a device that would release poison gas if it detected a photon of light, then added a radioactive source that randomly emitted photons. The box is closed, so no-one knows when the gas is released, but according to quantum theory, the source and device are a system superpose a photon being detected and not detected until it is observed. Hence, that the poison is released and not released also superposes, so is the cat in an alive-dead superposition until Schrödinger opens the box to observe it? But how can a cat be alive and dead? Or if cats can’t be alive and dead, how can a photon exist and not exist? Or if a photon can do this but a cat can’t, when does the superposition stop?
Schrödinger’s example illustrates how quantum-scale events contradict our macro-scale expectation that a cat can’t be both physically alive and dead. Yet quantum theory doesn’t actually predict this, because the observation that stops a superposition needn’t be human. It allows every physical event to be an observation, so the photon superposition stops when the detector observes it. The detector then releases the poison and the cat dies, regardless of what Schrödinger sees. Before opening the box, Schrödinger doesn’t know if his cat is alive or dead, but the cat does (or did). Quantum superposition is stopped by any observation, not just ours, so there is no alive-dead cat.
Quantum theory requires every physical event to be an observation by its participants, and as usual, its logic is impeccable. Every entity that interacts physically then also observes that event, not just humans. Note that if only humans observed, we would be necessary for physical history to occur, which is unlikely. For example, if our universe began as a superposition, some observation would have to stop it, but we didn’t exist then. It follows that just as Schrödinger wasn’t needed to stop the superposition in his cat’s box, we weren’t needed to stop the superpositions of history, so quantum waves have been collapsing into physical events since the beginning of time.
In quantum theory, observation formally causes physical events, so is our world a dream? In a dream, we are the only ones dreaming, but the world we see isn’t only caused by us, so it isn’t a dream in this sense. Every physical event is a mutual observation, so we aren’t creating the universe alone, everything is. This then implies a virtual world with many participants, not a dream world.
Schrödinger’s alive-dead cat illustrates the depth of quantum theory, not its absurdity. Everything in our universe observes, so quantum superpositions never occur on the macro scale.