‘A cat is penned up in a steel chamber, along with the following diabolical device [...]: in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The Ψ function for the entire system would express this by having in it the living and the dead cat (pardon the expression) mixed or smeared out in equal parts.’
Schrödinger glossed: ‘It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation.’
This gedankenexperiment provides a striking example of the paradoxes that arise from adopting a naïve interpretation of quantum states, that is to say an interpretation that construes state vectors as a description of the putative properties of a system (see superposition).
Aside from interpretive issues, the cat paradox raises the question of whether quantum theory has to be considered a universal theory. Does the superposition principle apply without restrictions to any physical situation, including those involving macroscopic systems? The cat paradox does not compel us to answer negatively. But if we want to argue for the universality of quantum theory, we must provide an explanation of the fact that the superposition principle seems not to hold for macroscopic bodies. Put in another way, we have to explain why, to make correct predictions about cats, we just need two biological states ('living' and 'dead') and not their superposition. It must be emphasized that our macroscopic experience is not at odds with quantum theory. The structure of the correlations observed at the macroscopic level appears more as a restriction of the whole spectrum of correlations that could be expected based on the quantum formalism. So the question is: does quantum theory have the resources to explain why, at the macroscopic level, one only actually observes those quanta correlations that fit in with ordinary experience? Together with the reduction of the wave packet, these questions express the core of the so-called ‘measurement problem’.