On 9/19/2011 6:11 PM, Darren New wrote:
>> the thing that alters the state.
>
> And in the case of the quantum delayed choice eraser, what is "the
> thing" that alters "the state"?
>
If what you are doing is "undoing" the stat change, then its still the 
same thing as would have changed the state, uh.. sort of.. Yeah, 
confusing, but its not quite so big of a problem as it first appears. 
Something is still "effecting" the state, even if the "effect" is to 
prevent a transition into one that is collapsed.

>> b) they opted for the misleading term "observer", when
>> talking about how the state collapses.
>
> That's the problem. Nobody could come up with a reason that the state
> would collapse at all. I'm not sure even now it's a solved problem.
>
I tend to think that they are either confusing themselves. There is no 
reason it wouldn't. But, we think about things with language. So, if you 
are using the wrong bloody language, it creates all sorts of errors in 
thinking.

In principle, as long as a particle is isolated from influences that 
"can" collapse the state, it won't. But, its an unstable arrangement, 
and you can't even be certain to maintain it in a vacuum, given some 
small odds that a virtual particle will happen to pop in and mess with 
things. In a practical sense, it simply means that, if you don't alter 
its unstable state, then it will change state when it hits something 
else, just not what ever it was you where intended to "measure" it with. 
After all, in this case, you are dealing with a photon, and your 
"detector" is only in one very small area. If the thing hits something 
where you can't see it, it still hits something, eventually, and.. 
without state collapses, the world would be awfully full of random 
photons, which happened, by chance, to split, forming entangled pairs, 
and yet where never "observed" in the sense described.

Personally, I consider the confusion over what happens if you don't 
"see" it happen to be complete nonsense, like arguing that noise doesn't 
happen if a tree falls without a observer, yet discounting that the 
impact, and subsequent vibrations, still do. The distinction is absurd, 
even if technically correct.

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