On Tue, Dec 4, 2012 at 1:51 PM, meekerdb <meeke...@verizon.net> wrote:

  
http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment:<http://en.wikipedia.org/wiki/Wheeler%27s_delayed_choice_experiment>
>
>
> " If the experimenters know which slit it goes through, the photon will
> behave as a particle. If they do not know which slit it goes through, the
> photon will behave as if it were a wave when it is given an opportunity to
> interfere with itself. "
>
> > That's why you need to read the technical papers
>

I did read them but apparently you just skimmed them because in the very
paper that you recommended I found the following quotes in black and white
and plain as day:

"QM predicts that without which-path information, photons arriving from
either A or B should interfere and distribute themselves one-by-one
according to the statistical distribution of interfering waves. Mind you,
if we *did* have which-path information, the results should be quite
different. In that case, QM would predict the "clumping" pattern typical of
particle motion. "

And this:

"QM predicts that if which-path information is not available at the time of
measurement, the pattern will be an interference pattern, as though
wave-like photons passed through both slits and "interfered with
themselves" to produce the distinctive interference pattern of hits. "

And this:

 "Because which-path information is not available for photons registered at
D0 even after a joint detection at the post-erasure detectors D1 and D2, we
learn nothing new about the detections that have occurred at D0 and so QM
predicts that R01 and R02 will exhibit this interference pattern in
counting photon hits at D0."

And this:

 "QM also predicts that if which-path information is available at the time
of measurement, the pattern will be a "clumping," as though particle-like
photons passed through a slit and on to a detector in a more-or-less
straight line. Because which-path information is available for photons
registered at D0 once a joint detection has been indicated at the
pre-erasure detectors D3 and D4, QM predicts that R03 and R04 will exhibit
this "clumping" pattern."

And this:
"To be sure the interference pattern disappears when which-path information
is obtained. But it reappears when we erase (quantum erasure) the
which-path information. "


And it is this last quote that explains what the entire experiment is
about. Everybody agrees that if which-way information exists then photons
(or electrons) will NOT produce a interference pattern, but if which way
information does NOT exist then the photons WILL produce a interference
pattern; The delayed choice experiment that you're talking about asked a
different question, what would happen if you recorded which slit the
photons went through but in the time it took for the photons to move from
the slits to the photographic plate (or electronic detector) that
information was quantum erased? It was found that in that case there was a
interference pattern just as QM predicted.

It's a very non-intuitive result, you'd think the photon or electron went
through slot X or it went through slot Y  and erasing the information on
which slot it was AFTER it had already gone through the slits (but before
it hit the detector) should have no effect, but QM says it will have a
effect and if human beings find that odd then tough. And now the delayed
choice experiment has been performed and we find that QM was right and
human intuition was wrong.

> instead of Wikipedia.
>

I didn't just quote Wikipedia I quoted 3 other papers and now I've quoted a
fourth and it's from the very paper you recommended.

> The above is correct when there are just photons going through one pair
> of slits.  But in the Delayed Quantum Eraser experiment there are *two*
> entangled photons one of which goes through slits and one of which *could
> be detected and give which-way information*.
>

Yes that other photon could give which-way information and if it still
exists when its twin hits the photographic plate (or electronic detector)
no interference pattern will form. But if quantum erasure is used to
destroy the which-way information the photon has then its twin WILL
construct a interference pattern. You're confusing the photon that contains
which way information about its brother photon (information that may or may
not be erased before its brother hits the detector at the whim of the
experimenter) from the photon that will or will not (depending on if that
which way information is erased or not) actually produce the interference
pattern.

What is crystal clear is that if which way information still exists when
the photon hits the detector the photon behaves like a particle and there
is no interference pattern but if that which way information is erased,
even if it's erased 10 years after it went through the slits and one
nanosecond before it hits the photographic plate (or electronic detector)
the photon behaves like a wave and a interference pattern is produced. Yes
it's odd but that's the way it is.

> That's what started this thread: you remarked that destroying the
> particle was what permitted the interference pattern.
>

I remarked that if the many worlds interpretation is correct then a
interference pattern forms when 2 universes become identical and merge;
sometimes destroying a particle can make 2 universes identical and
sometimes other things can. If the only difference between 2 universes is
something small like a photon or a electron then it's easy for
experimenters to arrange things so that small difference disappears so its
easy to observe a interference effect, but if the difference is big, like
the position of a baseball, then it's it's far far harder for the 2
universes to become identical and merge, that's why we don't observe a lot
of interference in baseballs.

  John K Clark

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