Tuesday, November 08, 2016

Can Everett worlds ever merge?

This post follows up an issue from my review of "The Many Worlds of Hugh Everett III", by Peter Byrne. From the Everett FAQ site.
"Assuming that we have a reversible machine intelligence to hand then the experiment consists of the machine making three reversible measurements of the spin of an electron (or polarisation of a photon).

(1) First it measures the spin along the z-axis. It records either spin "up" or spin "down" and notes this in its memory. This measurement acts just to prepare the electron in a definite state.

(2) Second it measures the spin along the x-axis and records either spin "left" or spin "right" and notes this in its memory. The machine now reverses the entire x-axis measurement - which must be possible, since physics is effectively reversible, if we can describe the measuring process physically - including reversibly erasing its memory of the second measurement.

(3) Third the machine takes a spin measurement along the z-axis. Again the machine makes a note of the result.

According to the Copenhagen interpretation the original (1) and final (3) z-axis spin measurements have only a 50% chance of agreeing because the intervention of the x-axis measurement by the conscious observer (the machine) caused the collapse of the electron's wavefunction.

According to many-worlds the first and third measurements will always agree, because there was no intermediate wavefunction collapse. The machine was split into two states or different worlds, by the second measurement; one where it observed the electron with spin "left"; one where it observed the electron with spin "right".

Hence when the machine reversed the second measurement these two worlds merged back together, restoring the original state of the electron 100% of the time.

Only by accepting the existence of the other Everett-worlds is this 100% restoration explicable."


  1. Yes this is well spotted. I think that it is Deutsch's original argument for experimental validation of MWI over the alternatives.

    It relies on an underspecified "reversible machine intelligence" which suffers from a kind of Alzheimers. Can the machine remove the record that it has removed all memory of the x-axis experiment? "I know I have erased memory of an experiment, so I cannot remember what the experiment was, hence the experiment never happened."

    A related topic is the "Delayed Choice Quantum Eraser" (q.v.) which applies to QM generally.

    So does all this give us merging and reverse time MWI (exclusively)?

    1. Actually Q40 in the quoted article gives the link to time reversibility in MWI:

      "What is lost by this approach is a unique past assigned to each future. If you time-evolve the world-we-now-see backwards in time you get a superposition of earlier starting worlds...."

      So our MWI Graph will indeed have backward trees as well. If the Big Bang is considered to be unique, then the MWI Graph could be rooted there, leading to alternative MWI Big Bangs?


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