[SEQ RERUN] Spooky Action at a Distance

MinibearRex

Today's post, Spooky Action at a Distance: The No-Communication Theorem was originally published on 05 May 2008. A summary (taken from the LW wiki):

As Einstein argued long ago, the quantum physics of his era - that is, the single-global-world interpretation of quantum physics, in which experiments have single unique random results - violates Special Relativity; it imposes a preferred space of simultaneity and requires a mysterious influence to be transmitted faster than light; which mysterious influence can never be used to transmit any useful information. Getting rid of the single global world dispels this mystery and puts everything back to normal again.

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Today's post, Spooky Action at a Distance: The No-Communication Theorem was originally published on 05 May 2008. A summary (taken from the LW wiki):

As Einstein argued long ago, the quantum physics of his era - that is, the single-global-world interpretation of quantum physics, in which experiments have single unique random results - violates Special Relativity; it imposes a preferred space of simultaneity and requires a mysterious influence to be transmitted faster than light; which mysterious influence can never be used to transmit any useful information. Getting rid of the single global world dispels this mystery and puts everything back to normal again.

When a quantum coin-flip happens at A the universe splits everywhere at the same time. So position B is also split. But it doesn't know that it's split because the two universes look the same in the area around B. It takes time for the changes at A to propagate to B. If in the meantime a quantum coin-flip occurs at B then the universe(s) split again, now going up to four universes total. The universe is split into four everywhere, but at A two of the pairs look the same, and at B a different pairing looks identical.

The concept of "everywhere at the same time" is meaningless in a relativistic universe.

0pragmatist14y

This is incorrect. "World-splitting" propagates at the speed of decoherence, and this is not instantaneous.

15Mitchell_Porter14y

If you believe in relativity, that can't be true. And in fact some MWI people speak explicitly of splitting as starting locally and then spreading along the lightcone. But I don't think anyone has a working description of this, because those secondary, tertiary... quantum coin-flips would be happening and that means that the splitting light-cone has to develop new propagating splits of its own. The union of relativity and quantum mechanics is an amazingly challenging subject, and the extension of MWI to the relativistic domain even more so. Since you don't have absolute time, you don't have a unique wavefunction of the universe evolving in time, and you can't reduce everything to a unique flow of amplitudes through configuration space. The prototypical QFT calculation is a sum over histories, in which whole relativistic histories get amplitudes, not just static, instantaneous spacelike configurations. It almost suggests a new MWI in which there's no splitting, just a stack of self-contained histories, but as usual, I don't see how to independently justify a phenomenological Born rule from this. Also, the actual practice of QFT contains so many other hacks - complexified variables, analytic continuations - and so many bizarre transformations and re-representations of the math have been discovered in recent years (the twistor renaissance, the Hopf algebra of diagrams, AdS/CFT duality) that I expect the final answer to be something very different to what anyone imagines.

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