On Wed, Mar 2, 2022 at 7:05 PM Bruce Kellett <bhkellet...@gmail.com> wrote:

>> Many worlds can explain all known experimental results without resorting
>> to non-local influences because many worlds is not a realistic theory.
> > *In that case, if you believe that MWI is local, give me the local
> account of Bell-type correlations of spin measurements at spacelike
> separations.*

OK, the key is understanding what "the collapse of the wave function" means
physically. Many Worlds says everything that is not forbidden is mandatory
so everything Schrodinger's wave equation does not rule out does physically
happen, so "collapse" just means that different solutions to Schrodinger's
equation are ... well... different. And the great virtue of tautologies is
that they're always true. Richard Feynman said all the weirdness in quantum
mechanics can be found in the  2 slit experiment, so I will use that to
illustrate the point. This is how I think Many Worlds would describe the
two-slit experiment and several variations of it.

Many Worlds says that in the usual experiment the universe splits after the
photon passes the two slits because the 2 worlds are different, a photon in
one world went through the right slit and a photon in another world went
through the left slit. But when the photons hit the film they no longer
exist in either world, so the 2 worlds are identical once more, so the
universes fuse back together again. Looking back from that fused world we
find evidence that the photon went through slit X only and evidence it went
through slit Y only and this causes an interference pattern. There is
nothing special about an observer in any of this, the same thing would
happen if nobody looked at the film, or even if you used a brick wall
instead of film, because the important thing is not that the photon makes a
record (whatever that is) but simply that it is destroyed.  Mind has
nothing to do with any of this so unlike the Copenhagen interpretation Many
Worlds doesn't need to explain it, or explain what “measurement" means, or
“record", or “observation", or “consciousness". That is a gargantuan
advantage! The key point is that worlds split when they become different
and merge when they become the same again.

Now do the two-slit experiment again but instead of using film to stop the
photon after it passes the slits let the photon head out into infinite
space. If Many Worlds is correct then the entire universe splits into 2
when the photon hit's the 2 slits and never recombines. There is nothing
special about you the observer, you split just like everything else, you
know that the photon went through one and only one slit, but you don't know
which one. The 2 slit experiment can be set up in such a way that you can
tell which slit the photon went through, but if you do that then the
photons will not produce an interference pattern on the film. This is
because even after the photon hits the film in both worlds and is destroyed
the worlds remain different because in one the physical memory pattern in
your brain encodes that the photon went through slot right and in the other
slot left, so the different worlds don’t merge back together, and so no
interference pattern is seen in either world.

Many Worlds predicts that even if you could devise an experiment that could
detect which slit the photon went through without changing the photon in
any way (it would pose enormous technical challenges but I think that might
be possible, perhaps by using Stimulated Emission) there still would be no
interference pattern because although the photons are identical the
universes would still be different because your brain has different memory
patterns which means the physical structure of your brain is still
different in the two universes. Separate worlds only emerge if there is a
difference between them, if the worlds evolve in such a way that the
difference disappears then re-coherence occurs. For obvious reasons
re-coherence can only occur if the 2 worlds are only very very slightly
different and have only been separated for a short time, and that's why
experiments of this sort are difficult and it explains why in our everyday
world we don’t see macro objects becoming entangled, and even atoms aren’t
usually entangled for long at room temperature.

*>  I will salute you if you can do this, because no one else has ever
> managed in the past*

As much as I'd like to take the credit for it nothing I said is original
with me, I just used my own words to describe what other people have done.

> *Realism is completely beside the point.*

Nope. If Many Worlds was a realistic theory and thus a photon always
remained in one and only one definite state both before and after passing a
slit and before and after a measurement is made then none of the above could
possibly work.

John K Clark    See what's on my new list at  Extropolis

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