Hi all,

I'm writing something that touches on issues of quantum measurement (though
the main theme is  something different), and I want to be sure I'm not
saying anything wrong about this very tricky  subject.

So I'm going to pose some thought-experiments to the physics experts on this
list.  I apologize if  this is slightly off-topic but I think these issues
are interesting and relevant enough that I don't  feel like I'm exploiting
the list improperly.

The questions I have in mind pertain to the nature of quantum measurement in
general, but for sake  of concreteness I'm going to pose them in the context
of a particular quantum phenomenon, the  "quantum eraser."

Specifically, I'll refer to the quantum eraser thought experiment summarized


though the same point could be made in regard to other similar (real or
gedanken) experiments.

The questions I have regard the replacement of the Coincidence Counter (from
here on: CC) in the  above experiment with a more complicated apparatus.

What if we replace the CC with one of the following:

1) a carefully sealed, exquisitely well insulated box with a printer inside
it.  The printer is  hooked up so that it prints, on paper, an exact record
of everything that comes into the CC.  Then,  to "erase" the printed record,
the whole box is melted, or annihilated using nuclear explosives, or

2) the same as 1, except the box has a person (let's call him Fred) inside
who reads the printout.

3) a printer that is not encapsulated in a box.  There is a person (Fred)
standing near the printer  who, when he takes each sheet of paper out of the
printer, puts it in an envelope and seals the  envelope, but without ever
looking at the paper.  Then, to erase the printed record, the envelopes  are
burned, or nuclearly annihilated, or whatever.

4) the same as 3, except that Fred looks at each sheet of paper before
putting it in the envelope.  But Fred never communicates anything about what
he sees to Bob, who is the guy looking at the screen  to see if there's a
wavelike or particlelike output pattern on it.  The erasure occurs by
annihilating both Fred and the envelopes.

5) the same as 4, except that Fred and Bob are not individual people but the
two brain-lobes of a  person who has experienced a severance of the corpus
callosum.  Thus Fred cannot communicate  information to Bob even though they
are "in the same head."   (OK, this one is proposed purely for
entertainment value ;)

6) one can redo the experiments with Fred using a pigeon instead of a human,
where the printer is  replaced by an apparatus that flashes colored
rectangles on a touch screen, and the animal is  encouraged to pay attention
to the screen by being forced to touch the rectangles to get food.

What will the outcome be in these experiments?

Has anyone performed experiments like these?  (1, 3 and 6 are the onIes that
could be humanely  carried out, obviously.)

My own understanding is that whether Fred, a pigeon or a printer is involved
in the experiment  should be basically irrelevant.  That is, I don't think
"registration in consciousness" (whatever  that means) is the important
thing, but rather registration in the sense of "statistical correlation
with some macroscopic system effectively obeying classical probability
theory."  However, I realize  that not everyone agrees with me on this; my
reading of Penrose, for instance, is that he would  predict a different
outcome for 2 versus 1, because he believes that Fred's brain (via
unspecified  quantum gravity related effects) does something special to
collapse the wave function, which the  printer does not.

Furthermore, my own understanding is that, so long as

a) the information from the CC is not statistically correlated with the
observer (Bob) who is  observing the screen, prior to its "erasure," and

b) the erasure is complete, i.e. there is really no way to reconstruct the
information after the  erasure is carried out

then the situation should be just like in the original quantum eraser

Regarding a, note that statistical correlation is transitive to a
significant degree, so that if the  information from the CC is correlated
with some entity Y (such as Fred, or a printer) that is in  turn correlated
with Bob, then this means the information from the CC is correlated with

The main open question I see regarding this "statistical correlation"
interpretation is whether it's  really possible to adequately annihilate a
macroscopic entity like Fred or a printer, and whether  it's really possible
to adequately isolate two macroscopic entities (like Bob vs. the printer or
Bob  vs. the envelope or Bob vs. Fred). One could argue that there will
always be subtle correlations  between Bob and these  other things (since
e.g. both Bob and Fred almost surely contain atoms that  once belonged to
George Washington), and that these subtle correlations mean that in all
these  scenarios there actually is some statistical correlation between the
screen and the contents of the  CC, via transitive correlations such as

CC --> printer --> George Washington --> Bob --> screen

I don't like this argument because it seems to me that these correlations
should be too small to  make any difference, but I recognize it as an
apparent mathematical loophole, according to which my  variant scenarios may
be considered as different from the original quantum eraser experiment.


-- Ben Goertzel

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