QuantumPhysicsAndReality

Quantum Physics and Reality (Bryce DeWitt) https://physicstoday.scitation.org/doi/10.1063/1.3022331

Many Worlds

• It is the elementary component that must be understood if we are to understand quantum mechanics itself.
• In other words, the wave function of the apparatus takes the form of a packet that is initially single but subsequently splits, as a result of the coupling to the system, into a multitude of mutually

orthogonal packets, one for each value of s. Here the controversies over the inter- pretation of quantum mechanics start.

• There are essentially three distinct ways of getting out of the crisis. The first is to change the rules of the game by changing the theory Wigner proposes that a search be made for unusual effects of consciousness acting on matter.
• The second method of escaping the von Neumann catastrophe is to accept tions. In fact, if the measurement in the so-called "conventional," or "Copenhagen," interpretation of quantum mechanics. If a poll were conducted among physicists, the majority would profess membership in the conventionalist camp, just as most Americans would claim to believe in the Bill of Rights, whether they had ever read it or not. The great difficulty in dealing with the activists in this camp is that they too change the rules of the game but, unlike Wigner and Bohm, pretend that they don't.
• The collapse of the state vector and the assignment of statistical weights do not follow from the Schrodinger equation, which generates the operator U (equation 4).
• To them the state vector does not represent reality but only an algorithm for making statistical predicions
• Bohr convinced Heisenberg and most other physicists that quantum mechanics has no meaning in the absence of a classical realm capable of unambiguously record- ing the results of observations. The mixture of metaphysics with physics, which this notion entailed, led to the almost universal belief that the chief issues of interpretation are epistemological rather than ontological: The quantum realm must be viewed as a kind of ghostly world whose symbols, such as the wave function, represent potentiality rather than reality.

• Let us nevertheless try -> to take the mathematical formalism of quantum mechanics as it stands without adding anything to it -> to deny the existence of a separate classical realm -> to assert that the state vector never collapses.

  In other words, what if we assert that toformalismis all, that nothing else is needed? Can we get away with it? The answer is that we can.

• It constitutes the third way of getting out of the crisis posed by the catastrophe of infinite regression.
• the world, is faithfully represented solely by the following mathematical objects: a vector in a Hilbert space; a set of dynamical equations (derived from a variational principle) for a set of operators that act on the Hilbert space, and a set of commutation relations for the operators

• Only one additional postu- late is then needed to give physical meaning to the mathematics. This is he postulate of complexity: The world must be sufficiently complicated that it be decomposable into systems and apparatuses.
• The mathematical formalism of tJie quantum theory is capable of yielding its own interpretation. -> How can the conventional probability interpretation of quantum mechanics emerge from the formalism itself? -> How can any correspondence with reality be achieved if the state vector never collapses?
• None of us feels like Wigner's friend. We do not split in two , let alone into 10¹⁰⁰+! To this EWG reply: To the extent that we can be regarded simply as automata and hence on a par with ordinary measuring apparatuses, the laws of quantum mechanics do not allow us to feel tlie splits.
• The splitting into branches is thus unobserved.
• EWG give no a priori interpretation to these coefficients. In order to find an interpretation they introduce an apparatus that makes repeated measurements on an ensemble of identical systems in identical states.
• Each memory sequence Si,s2, . . . Sy yields a certain distribution of possible values for the system observable, and each distribution may be subjected to a statistical analysis.
• The conventional probability interpretation of quantum mechanics thus emerges from the formalism itself. Nonrandom memory sequences in equation 16 are of measure zero in the Hilbert space, in the limit as N goes to infinity. Each automaton in the superposition sees the world obeying the familiar statistical quantum laws. However, there exists no outside agency that can designate which branch of the superposition is to be regarded as the real world. All are equally real, and yet each is unaware of the others. Each branch corresponds to a possible universe-as-we-actually-see-it.
• This result is what allows us, in practice, to collapse the state vector after a measurement has occurred, and to use the techniques of ordinary statistical mechanics, in which we change the boundary conditions upon receipt of new information. It is also what permits us to introduce systems having well defined initial states, without at the same time introducing the apparatuses that prepared the systems in those states. In brief, it is what allows us to start at any point in any branch of the universal state vector without worrying about previous or simultaneous branches.
• Clearly the EWG view of quantum mechanics leads to experimental predictions identical with those of the Copenhagen view. This, of course, is its major weakness. However, the EWG theory does have the pedagogical merit of bringing mostof the fundamental issues of measurement theory clearly into the foreground, and hence of providing a useful framework for discussion.
• Finally, the EWG interpretation of quantum mechanics has an important contribution to make to the philosophy of science. By showing that formalism alone is sufficient to generate interpretation, it has breathed new life into the old idea of a direct correspondence between formalism and reality. The reality implied here is admittedly bizarre. Yet it is a completely causal view, which even Einstein might have accepted. At any rate, it has a better claim than most to be the natural end product of the interpretation program begun by Heisenberg in 1925.