# What is an “observer” in quantum mechanics? [duplicate]

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i read about the electrons double slit experiment, when both slits are open, electrons will form an interference pattern, but when there is a detector put to "observe" this weird behaviour, the outcome is different, the elctrons dont form interference pattern anymore because when there is an "observer" the nature force the electron to chose a state. i found this confusing because why in the first case (without detector thing) do the electrons still form an interference while the humans are also observing. in my understanding an observer is one that can take informations, dont the humans thus in this experiment an obsever too?.

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• Possible duplicate of What is an observer in quantum mechanics? – user190081 Oct 24 '18 at 13:20
• Wouldnt say that it is, it has some other concernes. – Žarko Tomičić Oct 24 '18 at 14:17
• @ŽarkoTomičić Then I would advice to edit the title to reflect those differences. – user190081 Oct 24 '18 at 14:22
• A really great introduction to the topic and it's many interesting conceptual issues can be found, oddly, on this site about philosophy. They cover the underlying physics too, it's a great 20-minute intro to the entire field of QM: plato.stanford.edu/entries/qm-collapse – Maury Markowitz Oct 24 '18 at 14:38

## 1 Answer

Experiment: you send electrons, one by one, from the source through the slits, to the detector. When I say one by one, I mean there is on average one electron in transit at a given time, so we can exclude some wierd interactions of electrons with themselves. Outcome: interference pattern similiar to the one with the waves of water or light.

So, there is definitely something wavelike in the behaviour of the electron. In order to produce interference pattern it had to interact with himself otherwise, how can we explain it? So, we can think of it this way: electrons in transit to the detector do not travel with their definite position but they travel as a wave of many possible positions. This wave then comes to the slit and some part of the wave passes through one slit and the other part passes through the other, just like water wave would. And then they interfere.

I have given you an explanation for the observed phenomena. Now, about the observer...when it is said OBSERVER what is ment is some sort of interaction with the environment. So, in these cases, the observer is the detector put on one of the slits. This detector destroys the interference pattern, and again, we ask ourselves, how can we explain this? So we can think of it like this: When electron is not being detected, it is in many possible positions at the same time. But, when it is observed through detector which detects position, the elctron sudenly oicks up one of these possible positions.