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I have read in several places that the outcome of the double slit experiment changes when we try to observe it.

In a few places, including this site I found this is because the observation method interacts with the particles in such a way that interference no longer takes place.

My question:

Is it possible to observe the experiment in a way that does not change the outcome?

I could imagine that a 'lighter touch' might help, but perhaps it is conceptually impossible.

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  • $\begingroup$ What does "the outcome" mean if there is no observation? $\endgroup$ – WillO Oct 11 '20 at 17:34
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The answer is No, and the reason why it’s no gets to the very heart of Quantum Mechanics.

Take a look at the wiki article on Renninger’s Negative Result Experiment: https://en.m.wikipedia.org/wiki/Renninger_negative-result_experiment

Here’s what it demonstrates: in Quantum Mechanics, it’s the act of gaining information about a system that results in a loss of coherence between superpositions, not only the much more intuitive measurement events like smacking an electron with a photon. So it has nothing at all to do with how “gentle” you make your measurement.

Here’s the best example I can give:

In your double-slit setup, imagine if you only set up a detector in front of one slit. It’ll tell you whether a photon goes through that slit, but it obviously won’t interact with a photon going through the opposite slit in any way.

If your detector clicks, then you know your photon went through the left slit, and you’re unsurprised when all of the “click” runs of your experiment shows no interference.

But here’s the thing: If your detector doesn’t click, it still destroys the interference pattern. The absence of a click tells you with 100% certainty that the photon passed through the other slit, and so no interference can be recovered.

Information about the system is what matters.

This is a feature of QM that allows for things like the Elitzur–Vaidman bomb-tester to work despite appearing paradoxical. Negative-result or “interaction-free” measurements are just as valid as traditional measurements in reducing wavefunctions to observable results.

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  • $\begingroup$ This gave me a headache, but thanks anyway ;-) $\endgroup$ – Dennis Jaheruddin Oct 29 '19 at 11:53
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No, it is not possible. The observation method is not the reason that observation prevents interference. The reason is intrinsic to quantum mechanics, not to the observation method. A way to think of it is that if a particle is detected at one position, it cannot be detected at all the other positions where it might have been. The particle's wave function cannot be directly observed because the particle can only be detected once. At that moment of detection, it is detected to be at just one position. The detection is a "sampling" of the wave function, but the sampling for that one particle can only be done once.

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I don't think so, once you make a measurement you collapse the wave function. If we recall that making an observation is to apply an hermitian operator to the quantum state, once you "measure" you don't get a quantum superposition instead you get the single eigenvalue corresponding to the quantum state.

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