On 9/16/2011 23:37, Warp wrote:
> Darren New<dne### [at] sanrrcom>  wrote:
>> It doesn't matter if it hits a measuring device. It only matters if you look
>> at the measurement.
>
>    And how does the particle know that the measurement was looked at?

Clearly individual particles don't "know" anything in the traditional sense. 
I suspect if we knew *why* it worked that way, we'd be much closer to a GUT.

>    But that has nothing to do with whether someone "looked" at the measurement
> or not. It has to do with whether the two possible paths of the emitted
> particle were kept separate or whether they were merged before the particle
> hit the measurement device.

You misunderstand. The actual particle hitting the measurement device and 
being checked for interference fringes is *not* the particle being measured. 
*That* particle takes exactly the same path in both cases.

> If I understand correctly, the interference
> pattern would disappear if the emitted particles are kept separate even 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.

The experiment says basically this:

1) Emit one particle.
2) Run it thru two slits.
3) Split it into an entangled pair, A and B.
4) Let A hit the detector that looks for interference.
5) Some time later, bounce B off a half-silvered mirror,
    such that if it goes through, you can tell what slit it came from,
    and if it doesn't, you can't tell which slit it came from.

The behavior of whether A creates an interference fringe is
determined by whether you can tell which slit B came through. You never need 
to touch A, look at A, or figure out which slit A came through. Two "A" 
photons will cause interference if they weren't in the machine at the same 
time, as long as you don't look at two *other* photons *after* you've 
already determined where the two "A" particles hit the screen. Indeed, you 
could put the detectors for "B" so far from the experiment that thousands of 
"A" particles could have gone through the machine and been detected one at a 
time before you even look at the first "B" particle.

>    While at macroscopic levels it's hard to understand how particles affect
> each other from a significant distance (and even time) this way, it kind of
> makes sense, if you imagine that the particles are somehow "bound" together
> even though they are in their own separate paths: This way what happens to
> one particle affects what happens to the other. If the path of the
> "measurement" particle is merged, it affects the original particle (even
> through time, via some quantum oddity), and if it's not merged, it also
> likewise affects it.

Yes. Except that you can take the second measurement after the first 
particle had already given you an answer. And it works even over time-like 
distances (i.e., if the second detector is so far away that you'd need FTL 
signals for it to affect the first detector), even if you discount the fact 
that it's actually affecting stuff already measured.

>    However, what does not make sense even in this context is that the particle
> somehow "knows" whether someone "looked" or not.

Nope, that doesn't really make sense. :-) Even the multi-worlds 
interpretation of it doesn't make sense, if you actually think about it with 
common sense.

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

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