Hugh Everett resolved the great paradox of the measurement problem in 1957. The result is his Many-Worlds theory. He realised that there is no need to imagine that the collapse dynamics operates in physical reality. The effect occurs because there is the appearance of collapse. His theory is also called a ‘no-collapse’ theory.
If there is no collapse in physical reality, this means that all possible worlds must come into being. The physical world follows only the linear dynamics, and this give rise to all possible version of reality. But in experience, with respect to the individual defined as the mind, there is only one outcome. This is the essence of his theory.
This effect of Everett’s formulation is illustrated by a delightful cartoon by Tegmark. Alice is being asked out by Bob. Naturally, she may say Yes or No. If there is no collapse, then both possibilities happen. Effectively, therefore, the world splits in two, and both versions exist. In one version of the world she said Yes, and in another version No.
The central point is that this only happens in experience. The world itself remains in a state of multiple possibilities.
As Everett states in his conclusion. When the observer interacts with the environment, and an observation is made, the result:
… is a superposition of states, each element of which assigns a different state to the memory of the observer. (1957, 462)
The superposition of states means that the physical reality itself, including the observer, becomes literally the superposed sum of both situations at once. This is depicted in in the image below.
So, in each of these superposed states there is a different state of the memory, a different mind. As he goes on to say: “Judged by the state of the memory”, in each of these physically superposed states, there is just one specific version of the event observed, as shown in each branch of the first image. In other words, physical reality works like the second image, but in experience, the personal world of the mind nonetheless works like the first image.
So the universe of physical reality remains in the superposed state, as defined by the linear dynamics, and thus including all possibilities. As physicist Brian Cox states of the objective physical world:
There is only one ‘world’, and it is a world in which everything that can happen does happen and everything is in a superposition with everything else. (2017)
This is Everett’s solution. The ‘world’, the universe, is all possible worlds, all at once. But for each mind, there is nonetheless a specific outcome to each observation. And thus there is a specific physical reality, where observed.
The next section is Schrödinger’s Cat.