On 9/17/2011 9:44, Warp wrote:
> Darren New<dne### [at] sanrrcom>  wrote:
>>>     But that has nothing to do with whether someone "looked" at the m
easurement
>>> or not. It has to do with whether the two possible paths of the emitt
ed
>>> particle were kept separate or whether they were merged before the pa
rticle
>>> hit the measurement device.
>
>> You misunderstand. The actual particle hitting the measurement device 
and
>> being checked for interference fringes is *not* the particle being mea
sured.
>> *That* particle takes exactly the same path in both cases.
>
>    No I don't. When I say "emitted particle" I'm talking about the seco
ndary
> particle emitted at the slit towards the measurement device that (possi
bly)
> tells which slit the original particle went through.

Ah. With that clarification in mind... The interference pattern disappear
s 
if you don't actually look at the "emitted" particle at all. If you let t
he 
emitted particle hit the detector, then you don't use that detector's 
results, you get no interference pattern - you get just a level output. 
That's what this paragraph means:

"""
Note that the total pattern of all signal photons at D0, whose entangled 

idlers went to multiple different detectors, will never show interference
 
regardless of what happens to the idler photons.[3] One can get an idea o
f 
how this works by looking carefully at both the graph of the subset of 
signal photons whose idlers went to detector D1 (fig. 3 in the paper[1]) 
and 
the graph of the subset of signal photons whose idlers went to detector D
2 
(fig. 4), and observing that the peaks of the first interference pattern 

line up with the troughs of the second and vice versa (noted in the paper
 as 
'a π phase shift between the two interference fringes'), so that the
 sum of 
the two will not show interference.
"""

In this particular experimental setup, if you don't look at which detecto
r 
it hit, you can't tell because you can't separate out the inteference fro
m 
it's half-phase-shifted brother.

>>> If I understand correctly, the interference
>>> pattern would disappear if the emitted particles are kept separate ev
en if
>>> nobody "looks" at the result. It has nothing to do with an observer, 
only
>>> with how the original particles and the emitted particles interact.
>
>> I'm not sure what the "emitted" particle is here.
>
>    It's particle B in your list. If the two possible paths that B could
 take
> are merged, the interference pattern appears, but if they are not merge
d,
> the interference pattern disappears. This regardless of whether someone

> "looks" at the result of B or not.

In this experiment at least, you *have* to look at B in order to determin
e 
which category A falls into. If you don't look at B at all, you don't get
 
interference fringes, or more precisely, you get two sets of interference
 
fringes that are half a fringe offset.

>    Clearly what happens to B's path affects A (even if this effect happ
ens
> through space and time). It's not dependent on whether someone "looks" 
at
> it.

In this case, not *quite*, I think. (Again, you're skirting the edges of 
my 
understanding... :-)  There have been other experiments proposed (by 
Wheeler, for example) that don't involve a second entangled photon at all
, 
but are hard to set up because you need devices that are light-seconds lo
ng 
to make them work. :-)

-- 
Darren New, San Diego CA, USA (PST)
   How come I never get only one kudo?

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