On 9/18/2011 15:38, Patrick Elliott wrote:
> Measurement, by definition, means that your test
> particle had to have an effect on another particle.

No. That's the entire point of using an entangled particle. The particle 
you're checking for interference is never touched by the rest of the experiment.

> It is not possible to do
> that, without both particles affecting *each other*, thus triggering the
> collapse.

Yes. And that's the funky part, now isn't it?  I'm not sure why you first 
bang your head on the desk, then agree with me.

> The measurement *is* the observation, and the test equipment *is*
> the observer. That some other later on read a computer screen, and went,
> "Yup, the particle hit the detector.", is irrelevant to the result. It was
> already "observed" by the measuring device.

Yes, in this case.  Again, I'm not sure why you seem to be disagreeing with me.

> Or, do you have some magical way to "measure" this stuff, which somehow
> doesn't involve particle collisions?

Yes. You split the particle into an entangled pair via spontaneous 
down-conversion, then you interact with only one of the pair. Indeed, that's 
the whole point of the experiment: you're *not* interacting with the 
particle whose behavior changes based on whether you interact with the other 
particle. Hence, it is nonsensical to say "the particle knows what slit it 
went through" or something like that.

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

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