Uncertainty principle with/out detectors

hollowsolid
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I think I undetstand the role of the uncertainty principle in explaining the existence of interference patterns in the double slit expt with electrons i.e. the slits reduce the uncertainty in position and consuequently there is a greater uncertainty in momentum which results in interference patterns.

However when path information thru the slits is provided by detectors, why isn't this an equally effective means of constraining the position uncertainty and thereby increasing the momentum uncertainty leading to interference patterns?

Most explanations seem to use the UP in the absence of detection and wave function collapse in the case of detection.

Why?
 
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Double slit experiment is one special case of Feynman explanation of QED (QED: the strange theory of light and matter.) Feynman clearly explain the basis of QED here. But I have not found explanation for uncertainty principle. Maybe your explanation is a way to it?! And, maybe the Feynman book is also answer to your question?!
 
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