Comment by millstone | Hacker News Reader

Comment by millstone | Hacker News Reader

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Many Worlds is a far, far more popular interpretation than Bohm[1].

The strong objection to Many Worlds is not that macroscopic objects cannot be in superposition. There are many objections [2], but the principal one is the difficulty of deriving the Born Rule.

This is a deep objection. The Born rule predicts of the result of quantum measurements in QM, and it's not clear how to get those results out of MWI. The Born Rule in MWI is inserted ad-hoc afterwards, or arises via some weird "world-counting" formalism that doesn't naturally connect to probabilities. So MWI has more the flavor of a visualization, not a theory that aims at making predictions.

When you say "collapse is wrong," it depends on what is being collapsed. Sure, inserting some special "wavefunction collapse dynamics" separate from ordinary evolution is a pretty rough approach. But when the wavefunction is understood as encoding probabilities, then it's not something physical, and its collapse is no more mysterious than the probability of the Giants winning the World Series "collapsing" to 100% once the final game was played.

[1]: http://arxiv.org/abs/1301.1069 [2]: http://www.mat.univie.ac.at/~neum/physfaq/topics/manyworlds has some [3]: http://arxiv.org/abs/gr-qc/9703089

> But when the wavefunction is understood as encoding probabilities, then it's not something physical, and its collapse is no more mysterious than the probability of the Giants winning the World Series "collapsing" to 100% once the final game was played.

No, from my understanding this is not correct - what you say would be true if we lived in a classical world.

The problem with this approach - that is, interpreting the wave function as encoding probabilities of different states of the world that merely reflect our ignorance of the true state - is that it doesn't explain how we can get interference effects between those different potential states of the world.

> interpreting the wave function as encoding probabilities of different states of the world that merely reflect our ignorance of the true state

The wavefunction definitely encodes probabilities - that's the Born rule, and it's a key result of QM. But probabilities of what? Not of the probability of the system being in different states, for there is only one state, which is described by the wavefunction. Instead it encodes the probability of the results of measurements.

For example, in the double-slit experiment, the wavefunction tells us, if we were to deploy a measuring device at the left slit, or right slit, or at various points on the screen, what the probability of measuring an electron would be. It does not tell us the probability that the electron went through the left or right slit. That would prohibit quantum interference, as you say!

A key point (of non-Bohmian interpretations) is there is no underlying "true state," i.e. predetermined values of observables. The uncertainty principle drives that home.

> we can get interference effects between those different potential states of the world

If these possibilities were classical, that would be impossible, as you say. But they are quantum possibilities, and quantum possibilities can interfere. AFAIK this has to be made a postulate of the theory. But once you've done that, and specified the mapping from quantum probabilities to classical measurements (i.e. the Born rule), you can show how classical measurements reflect the interference.

The key idea is that an electron's wavefunction is "made of" probabilities or numbers, not electron-stuff or matter or anything physical. Then "wavefunction collapse" is just a change in subjective knowledge, not a physical process.

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