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


Or you don't like Wikipedia http://www4.ncsu.edu/unity/lockers/users/f/felder/public/kenny/papers/quantum.html:

/" /when we don't know which slit the photons are going through, we get a wave interference pattern. When we do know which slit each photon traveled through, no interference pattern."/

/Or maybe/you prefer this:

/http://grad.physics.sunysb.edu/~amarch/ 
<http://grad.physics.sunysb.edu/%7Eamarch/>/
/
"One can set up a measurement to "watch" which slit a photon goes through. It can be determined that the photon went through one slit and not the other. However, once this is kind of measurement is set up, the photons will no longer collectively produce a nice pattern of bright and dark spots. Instead they will strike the screen in one big bright spot, as if there were only one slit instead of two."

Or perhaps this:

http://theobservereffect.wordpress.com/the-most-beautiful-experiment/

"If one neglects to observe which slit a photon passes through, it appears to interfere with itself, suggesting that it behaves as a wave by traveling through both slits at once. But, if one chooses to observe the slits, the interference pattern /disappears/, and each photon travels through only one of the slits."

/
/Actually you don't need other people to tell you this you can figure this out on your own; if you only have one slit then obviously you know which slit the photon went through and there is no interference pattern. But if you have 2 closely slits then you don't know which slit the photon went through and you get a interference pattern.

Unless there is another photon that could have told you which slit, but which nobody observed and now has been destroyed. That's what started this thread: you remarked that destroying the particle was what permitted the interference pattern.



  John K Clark

/



/
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