Précis of epistemology/Quantum theory of multiple destinies
One of the main benefits of epistemology is to release from prejudices about science. We do not know in advance whether a knowledge is a good knowledge. It must be allowed to prove itself, it must be judged on documents. A priori any theory can claim knowledge. Science can not develop into intolerance. If we want to be scientists, we have to be willing to accept all proposals, even those that surprise or displease us. We judge at the end, not at the beginning.
Taking the Schrödinger equation seriously
Everett proposed in 1957 a new interpretation of quantum physics, which he called the theory of the universal wave function. He also called it the theory of relative quantum states. But in general it is known as the many-worlds interpretation of quantum mechanics. As this denomination may mislead, it is here called the quantum theory of multiple destinies.
Everett's arguments are often ignored. It is wrongly believed that his theory adds assumptions about the nature of space-time to divide it into many branches. Nothing is further from his thought. Everett's theory does not add any hypothesis to quantum theory, neither about space-time, nor about anything else. It does the opposite, it gives up a very dubious principle, the principle of wave function collapse.
If one interprets wrongly a beautiful theory, it can be made incomprehensible and absurd. This is what happened to quantum mechanics and the Schrödinger equation. If we interpret it in the manner of Bohr, Heisenberg and almost everyone, we make it an absurd theory because the wave function collapse contradicts the Schrödinger equation. This equation states that any evolution of a physical system is unitary whereas the wave function collapse is an evolution that is not unitary. The principle of wave function collapse therefore prevents us from understanding the Schrödinger equation.
More precisely, it can be proved from the Schrödinger equation that an observation process generally leads not to a single result but to a superposition of results. In other words, the theory states that, after an observation, the destiny of an observer is divided into as many destinies as there are possible results. This is the existence theorem of multiple destinies, and it is a very direct consequence of the Schrödinger equation, as soon as it is applied to observation processes.
The tree of multiple destinies is a solution of the Schrödinger equation. Everett did not invent this tree, he found it by carefully studying the fundamental equation of quantum physics.
The existence theorem of multiple destinies is empirically verifiable
Contrary to what is often believed, one can in principle verify by observation the theorem of existence of multiple destinies. An observer can not observe her other destinies, because they can never meet, they are incomposable. But a second observer can in principle observe that the first has several destinies, with experiments of the "Schrödinger's cat" type. Such experiments have already been made, and they fully confirm the quantum predictions, like all the experiments so far, but the studied systems are too small to be considered as true observers whose multiple destinies would have been observed.
In Schrödinger's imagined experiment, the paradoxical state is produced, but the experiment is not designed to verify by observation that it was actually produced, because it is destroyed by opening the box. A slightly modified experiment, however, makes it possible to observe that a state similar to is actually produced (Experiments of the "Schrödinger's cat" type). We can therefore in principle verify that two destinies of an observer are simultaneously real.
One space-time for all parallel worlds
Multiple worlds are worlds relative to the destinies of observers. Each observer's destiny has its own world, which it can not share with its other destinies, but which it can share with other observers, provided that it can compose with their destinies. But all these destinies, and their relative worlds, occur in one universe, one space-time. Everett's theory is nothing but ordinary quantum theory, which can be applied in any space-time, R4 or another.
It is sometimes wrongly supposed that Everett's theory requires a new kind of space-time, because the parallel worlds, relative to the different destinies of the same observer, or to the incomposable destinies of different observers, are conceived as separate worlds. Everything that happens in one can not affect what happens in the other. Why then say that they are in the same space-time? Two beings can be in the same place at the same moment without being able to meet because their destinies are incomposable. Why then say that they are in the same place at the same time?
When two observers have incomposable destinies, it is always possible in principle that a third observer has a destiny composable with the two previous ones. When two beings are in the same place at the same time without being able to meet, it is always possible for a third being to meet one or the other, in a random way, if it presents itself in this place. From the point of view of the third, the first two are therefore potentially present in the same place at the same moment, even if it can not meet them both at the same time in this place (Co-presence without a possible encounter and entangled space-time, in Quantum theory of observation).
The same space-time shelters all the destinies of all the observers, but in general these destinies can not meet, because they are incomposable. Of what happens in a place, we know only a small part, only the beings who have destinies composable with ours, we can not perceive the rest, all the destinies incomposable with ours, in incalculable number, which occur, however, in the same place, at the same moment.
Everett's theory is unified quantum theory
Everett's theory applies the same quantum laws to all bodies, microscopic and macroscopic, and to all processes, including observation processes. Since the union of two quantum systems is a quantum system, all bodies are quantum systems, whether microscopic or macroscopic. Observations are physical processes like the others. They involve the same fundamental interactions and bodies as all other physical processes. There is therefore no reason for them to obey special laws, which would justify the principle of wave function collapse.
Everett's theory is the only theory that takes the Schrödinger equation seriously, even its most astonishing consequences. From there, it justifies in a unified theoretical framework all the results of quantum physics, without introducing superfluous or arbitrary hypotheses, or a contradiction with the Schrödinger equation. This is more than enough to prove that it is a fruitful theory. It has proven itself. It has made it very clear that it is an excellent theory. In fact, it is the best physical theory that exists today.
In contrast, the principle of wave function collapse is perfectly useless. It is not required by any theoretical or empirical results of quantum physics. It is useless except to make the theory contradictory. One must really believe in the parable of the barren fig tree to imagine that one day this principle will deserve its place in science.
An accessible presentation of Everett's theory: Quantum theory of observation