## Schrödinger’s Cat

One of the central themes presented here is that the current paradigm is not actually wrong, it is just incomplete. The world one lives in is an ordinary physical world, but the full definition of this world is a superposition of many ordinary worlds. This is well illustrated by the famous idea of Schrödinger’s Cat.

This is a thought experiment by Erwin Schrödinger, the Nobel Prize-winning physicist who discovered the fundamental mathematics of quantum mechanics, the wave equation of physical reality. He produced this idea in order to demonstrate the absurdity of quantum theory; it means the cat has to be alive and dead at the same time.

Essentially, a cat in a closed box has a fifty-fifty chance of being killed by a quantum device. According to the principle of superposition, fundamental to quantum mechanics, unless it is observed, the cat is in a superposed state of being both alive and dead. Of course, when someone looks inside the box, the cat is either alive or dead; but that action is an observation, and according to quantum theory it is observation that makes things determinate. The big question is, what state is the cat in before, unobserved? Obviously, it cannot be both alive and dead at the same time.

Given QBism, the answer is simple indeed. The state of the cat is indeterminate, alive and dead, *in the world of the scientist*. But not, of course, in the world of the cat. Universe Superposition explains exactly why this is the case. Taking quantum theory literally, as defined by the wave function of the universe, all possible versions of the world exist. Therefore there is a version of the world in which the cat dies, and a version in which it lives. The scientist exists in both of these versions of the world simultaneously; he exists in every possible version of the world where he carried out the experiment. So his world, his physical reality, is the sum of the two worlds added together, just as in the first picture. So in his world, the state of the cat is indeterminate because his physical reality is the superposed sum of both versions. This gives us a proper physical explanation of this strange-seeming state of affairs. But that is only true until he looks in the box. As soon as he looks in the box he exists in one or other of the two versions of the world, as shown separately in the second picture.

The same is true of everything that has not been observed, not just cats in mad scientist experiments. The person making the observation exists in so many ordinary worlds that every possible variation of things which have not been observed, is the case in some of them. As a result, the physical reality, the superposed sum of all these worlds, is indeterminate with respect to all the things which have not been observed because every possible variation of these things is present, superposed, in the resulting physical reality. Thus the reality is determinate only where observed, as stated in QBism.