On 17/04/2016 7:35 am, John Clark wrote:

On Fri, Apr 15, 2016 Bruce Kellett <bhkell...@optusnet.com.au<mailto:bhkell...@optusnet.com.au>>wrote: > Consider the usual case of a spin singlet that splits into two spin-half components that separate and are measured by A and B at spacelike separation. There are two possible measurement results for each observer, call them |+> and |->. The entangled state can then be written as: |psi> = (|+>|-> - |->|+>).You can write it however you like but in many worlds it means thereare 4 different outcomes and a observer will see each one. > The first ket applies to observer A and the second to observer B. OK but keep in mind that in Many Worlds there must be some differencebetween A and B for the universe to split; if there is no differencebetween them then it is meaningless to talk about two separate entities.

`A and B are shorthand for the conventionally named experimenters, Alice`

`and Bob. These two are different, regardless of the orientations they`

`choose for their magnets.`

> A and B perform their measurements at spacelike separation, but each chooses the measurement orientation outside the light cone of the other.In other words they choose it for reasons that don't exist insidetheir lightcone, in other words they choose it for no reason, inother words they choose it at random.There are four possible combinations of results, corresponding to four worlds in the MWI: |+>|+'>, |+>|-'>, |->|+'>, and |->|-'>.OK. All those results could happen and in many worlds everythingthat can happen does happen.Since each observer has a 50% chance of getting |+> and 50% of getting |->, and the two measurements are completely independent of each other, it would seem that each of these four worlds is equally likely. OK. > But this conclusion is contradicted by quantum mechanics: It is?! Thenwhich of those four outcomes does quantum mechanics predict is morelikely to happen??

Read on......

> if the two observers, by chance, have their magnets aligned, then the |+>|+'> and |->|-'> combinations are impossible.If A and B are identical BEFORE the measurement then there were not 2experimenters there was only one, and in Many Worldsonly when different results are observed does the universe split andonly then would it make sense to talk about A/* and*/ B . If on theother hand A and B were differentBEFORE the measurement and they observed identical results then A and B would stillbe different and thus be in different universes.

`But A and B are different, regardless of this experiment. And there are`

`copies of both A and B in every universe that might be generated by`

`their respective experimental results.`

> In general, the probabilities of the four possible joint outcomes depend explicitly on the relative orientation of the magnets of the A and B -- they are seldom all equal. How is this taken into account in the formalism?In any formalism that corresponds to reality a spinning particle goingthrough a Stern–Gerlachmagnet will be deflected either either up or down from the center lineby the same amount, and the probability of observing either is exactly50%. And from that experiment, depending on how the magnet isoriented, you can determine the part of the original particle's spinthat was in the z axis or the part that was along the x y axis but NOTboth.If the world is realistic then the spin axis of the original particlehad a exact orientation before the measurement you just didn't knowwhat it was, before you a made a measurement for all you knew thenorth pole of the spin axis could have been pointing to any point onthe inner surface of a sphere, after the measurement you've narrowedthings down to any point on a circle but you still don't know whatpoint on the circle.

`This is counterfactual determinism. Unfortunately, quantum mechanics`

`violates CFD in many situations, particularly EPR and the Kochen-Specker`

`result.`

If the world is not realistic then the spin axis did not point toanything before the measurement was taken because the spin axis didnot even exist before the measurement.

Right.

> After the measurements are complete, A and B communicate their results to each other, Then, assuming A and B were different to begin with and assuming theMWI is correct, the universe splits into 4 strands with a observer in each one.

Sometimes: sometimes only two strands.

> So MWI does not give a local account of the EPR results on entanged states.I think that's right, some would disagree but they can only so bygiving a somewhat tortuous meaning to the word "local", and when theydo that they just increase the difficulty in making MWI realistic anddeterministic. No quantum interpretation can get rid of quantumweirdness and make things realistic local and deterministic as commonsense demands because common sense is just dead wrong.

It seems that we agree, then.

I like many worlds because, at least to me, it seems a little bit lessridiculous than any other quantum interpretation I know of. Maybesomeday a better one will come along but I would bet money it willstill be weird because we live in a weird place.

I don't like many worlds because it is baroque and unnecessary. Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.