John K Clark (johnkc@well.com)
Tue, 29 Jul 1997 22:59:38 -0700 (PDT)


On Tue, 29 Jul 1997 Hal Finney <hal@rain.org> Wrote:

>A good experiment to keep in mind is the two slit interference
>experiment, where measures are added to see which slot the particle
>goes through. When this is done, coherence is lost and the
>interference pattern goes away.

It's much weirder that that. Place a polarizing filter set at 0 degrees over
one slit, and one set at 90 degrees over the other, it's easy to tell what
photon went through what slit and the interference pattern disappears. Now
place a third filter set at 45 degrees one mile from the slits and just one
inch in front of the film. Now you can't tell what photon went through what
slit and the interference pattern comes back!

It can't be that the disturbance caused by measurement was the source of this,
you put the third filter far from the slits and AFTER the photons decided
what slit to go through. Heisenberg's Uncertainty Principle does not enter in
any of this, it's not that the photon goes through one slit and we just don't
know which one, it goes through one slit and both slits, and neither.

In the Everett model when the photon hits the plate the universe splits in
two, if the photons just kept going into open space then the universes would
remain separate, but if they hit the film and stop then the 2 worlds merge
but retain some properties of going through both slits. When you place
polarizers that are set differently over the slits the 2 photon will always
be different as will be the effects of anything they hit, so the 2 universes
(and the two of you) never merge and so you see no interference pattern.

John K Clark johnkc@well.com

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