• H. Dieter Zeh replied to the topic The Assumptions of Bell’s Proof in the forum Group logo of John Bell Workshop 2014John Bell Workshop 2014 9 years, 3 months ago

    It is often amazing to see how far definitions of the same term may differ when used by different physicists. Roderich Tumulka offers four definitions of reality, and he argues that the Many Words interpretation is in conflict already with the weakest of them (his R4). Accordingly, it would violate the reality assumption most strongly. I completely disagree. (Note that his definition refers to local properties already, and thus may presume the locality of ´´beables´´!) Everett clearly assumed the wave function to be a complete and ontic concept.

    Everett’s Many Worlds interpretation is ´´realistic´´ in the sense that – in contrast to complementarity or QBism, for example – it is using a unique and consistent kinematical and dynamical conception to describe an observer-independent empirical world (that is, all observations performed by observers who can communicate and can in principle be described as parts of this world). There is no mention of space or locality yet. In fact, Everett’s specific reality is defined by a wave function in configuration space (including spin variables etc.), which is thus nonlocal in three-dimensional space. (This holds even for collapse theories.)

    The concept of locality is here used only but essentially dynamically (sometimes called ´´relativistic causality´´), which is still possible if this Hilbert space does have a local basis. This dynamical locality has the consequence that reasonable observer systems have to be local systems, which can have definite states in the quantum formalism only in Everett’s autonomous branches (defined by means of decoherence).

    We may all have very different opinions about quantum theory at this stage, but we should not decide between them just by constructing our own definitions. In my opinion, the wave function, if universal and taken seriously (which leads to Everett), tells us that the world may be assumed to be real but has to be nonlocal. So it is remarkable that Tumulka and I come to the same verbal conclusion about locality being the culprit and being ruled out in spite of using quite different definitions of ´´realism´´.

    • I would just note that the assertion that decoherence can define a basis in the Everettian approach is far from settled science, and has been contested in particular in (Kastner 2014, SHPMP 48, 56-58)

      Given a universal wave function and nothing else, we get the Everettian picture. But more generally, taking the wave function as ontic does not limit us to the Everettian picture. The transactional interpretation takes the wave function as ontic and includes absorber response, leading to real collapse and determinate spacetime events. The wave function and its responses are nonlocal objects, in accordance with Bell’s theorem, but the results of collapse are local.