On Mon, Dec 3, 2012 at 2:26 PM, meekerdb <meeke...@verizon.net> wrote:

>  On 12/3/2012 8:46 AM, John Clark wrote:
>
>
>  > If the information is not recorded, e.g. the photon is absorbed or is
>> just let travel off to infinity the interference is lost.
>>
>
> That is incorrect, you've got it backward. If the information on what slit
> the electron went through is not recorded, that is to say if you DON'T know
> what slit the electron went through then you DO see a interference pattern.
>
>
> That's where you're wrong; read the paper more carefully.  If you record
> the which-way the interference is lost.  But *also* if you don't record it,
> if nobody records it and the photon just flies off to infinity or is
> absorbed in the wall, the interference is lost.  The interference pattern
> occurs *only* if the which way information is *erased*, e.g. by placing the
> lens so that which ever slit it went through it is focused to the same
> point.
>
>
Brent,

I'm sure you're aware of this, but perhaps others are not.  Special lenses
aren't usually needed, the two slits are merely placed close enough
together that the natural spreading (diffraction) of light as it is forced
through a small opening preserves the uncertainty of which path the photon
took.  This produces an interference pattern which is different from the
expected (two overlapping diffraction patterns) so long as you don't do
anything which in principle could determine which path was taken.

Jason

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