Quantum Philosophy | Roland Omnes

Omnès emphasises throughout that no new principles, other than those described when quantum mechanics was developed in the 1920s, are needed. Moreover, some additional principles which seemed to be required then (such as wavefunction collapse, or its slightly more formal sister, wavefunction reduction) are no longer needed. Classical behaviour can now be recovered in a system described entirely by a single, unitary^{[disambiguation needed]} (time-reversible) wavefunction.

The mathematical developments which allowed this progress have taken place in two fields: quantum decoherence and the consistent histories approach to quantum mechanics.

The consistent histories approach makes mathematically explicit which sets of classical questions can be consistently asked of a single quantum system, and, conversely, which sets of questions are fundamentally inconsistent, and thus meaningless when asked together. We can therefore demonstrate formally why it is that the questions which Einstein, Podolsky and Rosen assumed could be asked together, of a single quantum system, simply cannot be asked together. On the other hand, we can demonstrate that classical, logical reasoning often does apply, even to quantum experiments – but we can now be mathematically exact about the limits of classical logic.

Quantum decoherence, on the other hand (in combination with the consistent histories approach), recovers classical behaviour at the macroscopic level. The formal mathematics of this approach allows us to demonstrate, finally, that is impossible (or rather, massively improbable) for a macroscopic Schrödinger's cat to exist for longer than a minuscule time (related to the macroscopic energy dissipation time by a factor involving the square of Planck's constant) in a quantum superposition of its |alive> and |dead> states. Even for a cat otherwise isolated from the rest of the Universe, and even with no observer present, there are so many unknowns in the quantum state of the whole cat, that the relevant mathematics determine that only the normally observed classical states of the cat are at all probable, except over the very shortest of timescales. This reasoning is developed formally within measurement theory, and applies to any macroscopic, non-super cooled measuring device, whether or not there is an observer to watch it.

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