[extropy-chat] Re: Damien grants psi evidence

[Hal Finney]

Scerir writes:
> If Walter inserts a lens inside his 
> interferometer, his interferometer becomes a Heisenberg 
> microscope, imaging the two slits, thus Walter now knows 
> the 'which way' of his photon p2. But if Walter knows 
> the 'which way' of his photon, does he also change 
> - given the strict correlation between momenta of entangled 
> photons - Dirk's interference pattern (just one spot actually)? 
> No, because Dirk's interference pattern (one spot) was registered 
> _already_.

I disagree on what will happen in this experiment.  Eliezer asked a
similar question once.  Unfortunately, I've never found a definitive
explanation online about this seemingly paradoxical setup.

To put it simply, the fact that Walter can do a measurement that will
distinguish the 'which way' of photon p1 means that there can be no
interference observed by Dirk.  Only when there is no way to distinguish
'which way' can interference be seen.  Whether Walter actually does
the measurement or not doesn't matter.  It only matters that he has the
potential to do the experiment, that he has the information necessary
to distinguish 'which way'.  The mere potential will destroy the
interference.

It's exactly analogous to a simple two slit experiment where we measure
which slit the particles go through, but then discard the results without
looking at them.  Closing our eyes won't make the diffraction pattern
appear!  The mere fact that we have collected 'which way' data, and
have the potential to look at it, is enough to destroy the interference.
Too much misleading information about quantum erasers and such is giving
people an incorrect and oversimplified picture of how QM works.

Hence, I predict that although Dirk can see interference with ordinary
photon sources, he will not be able to see it with special photon sources
of the type you have used here.  He will not register the 'one spot'
interference pattern no matter what Walter does.

I wrote a much more detailed analysis of this on this list a couple of
years ago.

Hal