On 02 Dec 2012, at 21:23, meekerdb wrote:
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.
very good point. And it has to be like that in theories without
collapse, keeping exact the linearity of both the wave evolution, and
the tensor product for many bodies.
Bruno
Brent
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