If we used an instrument to record the double slit experiment but never looked at the footage so to speak, what would the mathematics show us?

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    $\begingroup$ To add to Floris' answer: the key word here is "record". As soon as nature makes an irreversible change to a quantum system a "measurement" has been made. This doesn't have to be done by a machine made by humans, either. That there is an objective classical layer to the physical world is simply a consequence of there being irreversible processes. $\endgroup$
    – CuriousOne
    Jan 7, 2016 at 0:09
  • $\begingroup$ More on observers in YDSE. $\endgroup$
    – Qmechanic
    Jan 7, 2016 at 0:34
  • $\begingroup$ Zurek discusses just this question in general in one of his decoherence papers arxiv.org/abs/quant-ph/9802054: "Continuous monitoring enforces environment – induced superselection... Schrodinger cats, Wigner's friends, and, generally, all of the systems which are in principle quantum but sufficiently macroscopic will be forced to behave in accord with classical mechanics as a result of the environment - induced superselection". Screens, detectors and cats are sufficiently macroscopic, therefore observers make no difference. $\endgroup$
    – Conifold
    Jan 7, 2016 at 5:23

3 Answers 3


"Observation" happens when the photons interact with the screen / detector. You don't have to be there as a human observer. The same pattern would be recorded on the instrument whether you looked at it or not.

  • $\begingroup$ I watched a video on youtube which said that even air and water can be considered as observers, is that correct? How? $\endgroup$
    – Quark
    Jan 7, 2016 at 1:00
  • $\begingroup$ Yes - "observation" happens whenever the photon interacts with "something". It can be your retina, a water molecule, ... As long as the photon causes some effect (like an electron being knocked out of its orbit) it has been "observed". $\endgroup$
    – Floris
    Jan 7, 2016 at 2:19

There's nothing magic or mystic about it. If you interfere with the experiment it will change the results. It has nothing to do with whether you look at the Fringe pattern. If you observe the slits to determine which way the photons are going then you are interfering with the experiment by absorbing those photons. Without those photons you are left with a single slit pattern from the other slit.


Von Neumann's book on the Foundations of Quantum Mechanics contains a proof that the mathematics cannot distinguish among a great variety of interpretations.


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