Wavefunction branches
Jess Riedel, NTT Research
In the past, quasiclassical branches in the wavefunction were described through carefully choreographed and enthusiastic handwaving. Within the many-body lattice context, we now have two mathematically precise guiding principles: *redundantly recorded observables*, a generalization of the quantum Darwinism concept that avoids a-priori preferred subsystems, and *N-point non-interference*, a formalization of effective wavefunction collapse relevant to numerical simulations. These conditions are respectively too strong and too weak in a particular sense, but they clearly illuminate and roughly bound the goal of identifying a mathematically precise definition of wavefunction branches. I argue that achieving this goal (1) would allow us to ask deep questions about quantum foundations that cannot otherwise be clearly formulated, (2) would have (maybe modest) real-world applications, and (3) is the single most promising approach to making progress on the measurement problem. The question of how to define branches is simultaneously neglected, tractable, and profound.