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[Eckhart Aurelius Hughes]

Please use this topic to discuss Macroscopic Superpositions, Decoherent Histories, and the Emergence of Hydrodynamic Behaviour by Jonathan Halliwell, which is chapter 3 of the July book of the month Many Worlds?: Everett, Quantum Theory, & Reality edited by Saunders, Barrett, Kent, Wallace. We are discussing this book chapter-by-chapter, including a discussion for the introduction and the transcript at the end. Please do not post in this topic until you have read chapter 3.

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What do you think of this chapter? What do you think of Halliwell's ideas and arguments? If you chose mostly disagree or utterly disagree for the poll, then on what specifically do you disagree with Halliwell?

For a memory refresher, here is the abstract:

Macroscopic systems are described most completely by local densities (particle number, momentum, and energy) yet the superposition states of such physical variables, indicated by the Everett interpretation, are not observed. In order to explain this, it is argued that histories of local number, momentum, and energy density are approximately decoherent when coarse-grained over sufficiently large volumes. Decoherence arises directly from the proximity of these variables to exactly conserved quantities (which are exactly decoherent), and not from environmentally induced decoherence. We discuss the approach to local equilibrium and the subsequent emergence of hydrodynamic equations for the local densities. The results are general but we focus on a chain of oscillators as a specific example in which explicit calculations may be carried out. We discuss the relationships between environmentally induced and conservation-induced decoherence and present a unified view of these two mechanisms.

What I found most interesting about this chapter was that Halliwell focused so much on conservation. I think this is a useful method since conservation plays such a role in non-quantum physics including more classical physics.

In the summary, at one point, Halliwell writes, "as long as there are conserved quantities there is a regime nearby of almost conserved quantities behaving quasiclassically." This seems to me like an agreeable idea. But does it mean many worlds? Halliwell's article falls in the part of the book entitled "Why Many Worlds" but much like the other chapters it supposedly lays the technical and often mathematical foundation of the MWI but does not spell out why this means 'Many Worlds' exist.

Anyway, what do you think?