The Frontier

Sean Carroll cites a recent APS survey where physicists were asked about interpretations of quantum mechanics. A third favored Copenhagen, while Many Worlds placed second.

Carroll argues the Copenhagen definition in the survey - that measurement collapses a multi-state wave function - is ill-defined because it doesn't specify what constitutes a measurement.

In Many Worlds, there is no wave function collapse. The observer is included in the quantum system, and measurement yields a superposition of the observer entangled with each possible outcome, which are interpreted as separate branches.

Quantum mechanics treats the wave function as a superposition of all possible measurement outcomes, not as a classical field in space. Entanglement arises because a two-particle system has a single, combined wave function.

Carroll asserts that position and momentum are not fundamental properties but merely different 'coordinates' or bases used to represent the underlying quantum state in Hilbert space.

The 'problem of structure' in Everettian quantum mechanics asks how the familiar world of objects in space emerges from a bare vector evolving in abstract Hilbert space.

He references a paper by Cotler, Pennington, and Renard showing that locality - where interactions only affect nearest neighbors - is a special, unique way to subdivide Hilbert space, not a generic property.

Carroll's 'quantum mereology' research program aims to derive the subdivision of Hilbert space into subsystems (like 'cat' and 'environment') from criteria like localizability and the system-environment distinction.

A major challenge is that this emergence of structure from Hilbert space dynamics depends on time evolution. The Wheeler-DeWitt equation from quantizing general relativity suggests time may not be fundamental, creating an 'order of operations' problem.

Carroll concludes that if locality and space are emergent approximations, not fundamental, this could have experimental implications, though none are currently known.