On 12/2/2012 10:43 AM, John Clark wrote:

    Now it's usually down is some kind of detector that amplifies the effect of 
each
    electron.  Neither one has anything to do with destroying the electron.


You don't need to destroy the electrons you just have to arrange things so that the effect the 2 electrons have on their universes is indistinguishable, then the 2 universes will merge back together and interference is produced. If you fire electrons at 2 slits and have sensitive detectors mounted near one slit so a record is made of which slit each of the electrons went through then the universe splits into two each time a electron is fired and it does not recombine because the 2 universes are different, one has a record of the electron going through slot X and the other has a record of it going through slot Y, and so no interference is produced on the photographic plate. But if there is no detector near a slit or no record kept of which slit it went through then the universe still splits when it goes through the 2 slits because the 2 are different, the electrons are on slightly different trajectories, but when the electrons hit the metal wall there is no longer any detectable difference between the 2 universes and so they merge back together, but there is still evidence that the electron went through slot X only and evidence it went through slot Y only and this produces the interference effect.

        >> the walls would not be the same and so the 2 universes would not be 
the same
        and so they would not merge. However if it was a metal wall the 
electrons would
        just join the general sea of free electrons in the metal and there is 
no way
        even in theory to tell one electron from another. So the walls would 
have the
        same charge and mass.

    > But in an entangled electron pair experiment (EPR type) detecting the 
path of one
    electron destroys the interference pattern on the other leg.


Yes.

    But also just absorbing one electron destroys the interference on the other 
leg.


Yes, if one electron is absorbed into the electron sea of a metal and the other electron is not then obviously the 2 universes remain different and so do not recombine and so no interference is found.

    > To maintain the interference you have to absorb the electron at the focal 
point of
    a lens so that you not only don't detect the which-way information, you 
erase it.


I don't quite know how a lens enters into this but yes, if the which-way information is not recorded or the record erased by whatever means then the 2 universes are identical and merge and a interference pattern is seen.

Apparently I have not explained it in sufficient detail; I thought you probably knew the experiment and my rough description would be enough to remind you. It's generally referred to as the Delayed Choice Quantum Eraser and it has actually been performed with photons rather than electrons: http://xxx.lanl.gov/pdf/quant-ph/9903047.pdf There's also a web version by the authors with more pictures and explanation and less math: http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm . But my point is that you're wrong when you write, "...if the which-way information is not recorded or the record erased by whatever means then the 2 universes are identical and merge and a interference pattern is seen." These two cases are different. If the information is not recorded, e.g. the photon is absorbed or is just let travel off to infinity the interference is lost. It is only when, by interposing the lens in the right place, the information is literally erased and the interference pattern is seen.

Brent

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