Measurement and the Quantum World

Chang-Pu Sun

The objectivity of quantum measurement is treated as an emergent phenomenon with $N$ observers who can agree to the same result of measurement, and meanwhile, they can identify their records with each other. In this many-observer world (MOW), an objective quantum measurement is dealt with as a multipartite [$(N+1)$-body] quantum correlation among the measured system and $N$ observers when its bipartite reductions are the same classical correlations. With this conceptual clarification, we find that, an objective quantum measurement is implemented if and only if the MOW is initially factorized in a pure state and then the total system can evolve into a generalized GHZ state with respect to the orthogonal basis preferred by each observer. Especially, such objective quantum measurement is recast in ideal classical correlation when the observer world is macroscopic for $N\rightarrow\infty$.

L. E. Ballentine

Michael L. G. Redhead

Robin D Hanson

In Everett's many-worlds interpretation, where quantum measurements are seen as decoherence events, inexact decoherence may let large worlds mangle the memories of observers in small worlds, creating a cutoff in observable world measure. I solve a growth–drift–diffusion–absorption model of such a mangled worlds scenario, and show that it reproduces the Born probability rule closely, though not exactly. Thus, inexact decoherence may allow the Born rule to be derived in a many-worlds approach via world counting, using a finite number of worlds and no new fundamental physics.

Rob Clifton

In their article “On What It Takes To Be a World”, David Albert and Jeffrey Barrett raise “a rather urgent question about what the proponents of a many-worlds interpretation [of quantum mechanics] can possibly mean by the term ‘worlds’” (1995, 35). I argue that their considerations do not translate into an argument against the Many-Worlds conception of a world unless one requires that the dispositions that measurement devices display through the outcomes they record be explainable in terms of facts particular to the worlds in which those devices do their recording. Granting that their conception of a world takes away the possibility of such an explanation, a Many-Worlds proponent can claim that the Universal quantum state, which does not represent a fact about any world in particular, is enough to ground the dispositions of measurement devices.

Paul Busch, PEKKA J. LAHTI, Peter Mittelstaedt