Everett in 1955, with Niels Bohr (age 24)
3.2 The Many Worlds of Hugh Everett III
Having ruled out random number generators (of either kind) as being responsible for determining the patterns of quantum mechanical outcomes, is there anything else we can think of to account for patterns in an indeterministic universe? Well, if we extend the idea of the universe as a mathematical structure just a little further, we can answer both of our apparently mutually contradictory questions, namely, how can quantum outcomes in the universe be fundamentally indeterministic and how are patterns nevertheless demonstrable among these outcomes? Suppose that our universe is not the complete structure, but merely a substructure among many other similar and identical substructures/universes, so that, taken over all of the universes, every possible quantum outcome is to be found. To be substructures, these universes would have to be embedded within an overarching mathematical structure – let us call it the multiverse. If these universes are discrete, so that there can be no communication between them, then it will be impossible to decide which universe you happen to inhabit, and therefore impossible to know which quantum outcomes you will find in your particular universe. This would be the origin of the fundamental indeterminism within every universe including our own. This has been called self-location uncertainty by McQueen and Vaidman. If these universes are themselves arranged in patterns according to the Born rule for the different possible quantum outcomes within the universes, then this would lead to patterns of outcomes in individual universes which match quantum-mechanical predictions, as I shall show you next.
Hugh Everett III (Wikipedia)
It wasn’t until 1957 that a serious assault on the measurement problem – see Step 1.1 (The measurement problem) – was proposed by Hugh Everett III. Everett suggested that, instead of just one quantum state emerging out of the many possible outcomes of an interaction, in fact every possible quantum state emerges! (This proposal arose after “a slosh or two of sherry”, according to a home-made tape of Everett being interviewed by Misner in 1977!)
At the quantum (i.e., sub-microscopic) scale, there are always several – some would even say infinitely many – possible outcomes of an interaction between two particles. Each of these possible outcomes is a different quantum state. We only ever see one of these outcomes, of course. Everett’s insight was to follow the equations, proposing that, instead of just one quantum state emerging after an interaction, in fact every possible quantum state emerges!
Everett also asserted that each of these quantum states is not confined locally just to the interaction but is, instead part of a universal quantum state, namely the quantum state of the whole universe! This means that the outcome of every interaction, anywhere in the universe, is to generate several – or, putatively, infinitely many – parallel universal quantum states. Since each of these universal quantum states is associated with a different universe, then, as Bryce Seligman DeWitt put it in page 33 of his paper:
“This universe is constantly splitting into a stupendous number of branches, all resulting from the measurementlike interactions between its myriads of components. Moreover, every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world on earth into myriads of copies of itself.”
(Note that, strictly, rather than parallel universes – or “Many Worlds” as DeWitt later called it in “Quantum Mechanics and Reality”: Physics Today 23(9), 30–40 (1970) – Everett was arguing for the existence of parallel quantum states, all equally real. It was DeWitt who inspired the title “Many Worlds Interpretation” (MWI), as Everett’s approach came to be known.)
So, why don’t we see this splitting going on around us all the time? Because we split, too! Each time the universe splits, then, in the universe where the outcome of the interaction was, say, an electron moving to the left, my eyes saw the needle of my detecting apparatus move to the left. In the other universe, where the electron ended up moving to the right, I saw the needle move right. Two distinct universes, each containing a copy of me! The copies are identical, up to the moment of the interaction. The two copies of me have identical memories up to that point, but from then on, the copies are quite independent of each other. Before the interaction, maybe I had promised myself a break, but only if the electron moved right. So, after the interaction, in one universe I remain seated for another experiment; in the other, I get down off the lab stool and go for a coffee.
(As a spoiler, we shall see in the next Step 3.4 that a slightly different interpretation of quantum experiments suggests that universes don’t split but have always been separate – it’s just that, at earlier times, these universes are more alike than in later times. But that’s for a later discussion.)
Many Worlds (from MinutePhysics, a Britannica Publishing Partner)
So Everett salvaged Einstein’s determinism, at least for the multiverse of parallel universes, because every interaction in every branch of the multiverse is determined by Schrödinger’s equation, while, at the same time, he rendered each universe absolutely indeterministic, because, just after an interaction that splits the universe, before you can measure the outcome, you have no way of knowing with absolute certainty whether you are in a branch where the needle moved left or right.
In addition to removing the tension between determinism and indeterminism, Everett’s idea also solved the measurement problem see Step 1.1 (The measurement problem). To illustrate with Einstein’s hemispherical screen: no point is especially chosen because all points are picked out by Schrödinger’s equation, each point in a separate universe.
If you were familiar with Everett’s Many Worlds Interpretation before you had read the case presented for parallel worlds in this website, then, while the mathematics is flawless, it may nevertheless have struck you as dubious, not to say reckless, for Everett to have extended an equation describing tiny quantum effects to implying the existence of a whole multiverse of parallel universes. However, note that everything in this website up to now has been presented as an argument for parallel universes which is completely different from Everett’s, and so is unaffected by that criticism.
