On 12/4/2012 11:32 AM, Craig Weinberg wrote:


On Tuesday, December 4, 2012 1:51:55 PM UTC-5, Brent wrote:

    On 12/4/2012 8:29 AM, John Clark wrote:
    On Mon, Dec 3, 2012 at 3:26 PM, meekerdb <meek...@verizon.net 
<javascript:>> wrote:


        > That's where you're wrong; read the paper more carefully.  If you 
record the
        which-way the interference is lost. [...] The interference pattern 
occurs
        *only* if the which way information is *erased*


    Nope, you've got it exactly precisely backwards yet again. I quote from

    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 instead of Wikipedia.  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*.  The point is that if it is not detected (flys off to 
infinity,
    absorbed in the wall,...) the interference pattern is still destroyed.  To 
maintain
    the pattern the information in the entangled photon has to be *erased* - 
that's the
    function of the lens.

    Loss of the interference isn't because "they do not know"; it's a 
consequence of the
    information being "out there" - and being absorbed in a wall still leaves it 
"out
    there".  This is even clearer in the buckyball Young's slits experiment,
    quant-ph/0402146v1.  The interference pattern is lost when the buckyballs 
are hot
    enough that their IR radiation is sufficient to localize them to the slit 
spacing -
    even though nobody ever observes or detects the IR photons.

    All those below fail to consider the relevant case too; they assume all 
cases in
    which no experimenter measures which-way are equivalent.  They ignore the
    possibility that "the environment" may measure which-way but no person does.

    Brent


It's confusing. Can you simplify it?

One photon heads toward the slits.
One entangled photon heads toward the detector. (They are both entangled with each other, but I assume you mean one pair of entangled photons, not two pairs.)

Is there a detector on the slits too?

Yes.


It seems like the point of the experiment is that the interference pattern only shows up when the ability to discern which-way is not available - which seems to me to support observer-principle type interpretations.

Kinda depends on what you mean by 'available'. If the entangled photon is allowed to hit a wall and be absorbed, it is only 'available' to a kind of Maxwellian demon who can discern the thermal atomic motions and trace them back to get which-way infomation - but the interference pattern is destroyed anyway. If the entangled photon is simply allowed to fly out the window and off to infinity it is 'available' many years later to an inhabitant of some extra-solar planet - and the interference pattern is destroyed in our present. If is only if the lens is used to erase the which-way information that the interference pattern shows up.

So one way to look at it is: So long as the which-way information is available, however impractical is may be for a person to get it, the interference pattern is destroyed. This is even clearer in the buckyball experiment.

Brent


Certainly I don't see any suggestion that there is a such thing as 'information' which is independent of some kind of sense receptivity. To the contrary:

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