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This is a very high level question. I was just thinking about the idea than in quantum physics, the act of observing has a "strange" effect on some properties (e.g. double slit).

If I'm staring at a light bulb, are my eyes having an effect similar to the sensor that plays the role of an observer in the quantum interference experiment?


marked as duplicate by John Rennie, Danu, ACuriousMind, Martin, user10851 Sep 11 '15 at 20:13

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    $\begingroup$ possible duplicate of What is an observer in quantum mechanics? $\endgroup$ – John Rennie Sep 9 '15 at 7:21
  • $\begingroup$ I just want to let you know that this is one of the most difficult questions in modern physics (so it's great that you're asking it!). You're unlikely to get a good answer here or anywhere else until you have a pretty solid understanding of quantum mechanics. If you'd like to have a somewhat more extended and unstructured discussion about this feel free to drop into the chat room or even contact interested users (i.e. me) via email. $\endgroup$ – DanielSank Sep 9 '15 at 7:54
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    $\begingroup$ @DanielSank: Why is this a difficult question? Has somebody done an experiment that had a different outcome depending on whether a human retina was absorbing a photon or a black piece of paper? If they have, I have yet to see a citation of that discovery paper. Let's not subject the OP to a false sense of quantum mysticism. Whether a photon gets absorbed or not in an interference experiment, that makes a solid difference to the outcome, by what it gets absorbed (or not) is of absolutely no consequence, at least not based on any experimental test I have heard about. $\endgroup$ – CuriousOne Sep 9 '15 at 8:19
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    $\begingroup$ @DanielSank: I would indeed like to hear about an experimental paper from the quantum computing field in which a human eyeball is an ingredient of utmost importance. Can you provide one? Here is the skinny: at some point in the ancient past someone with strong imagination or poor English skills replaced the word "observation" in a paper or book about QM with "observer" and since then there is this myth out there that nature is sensitive to there being a living observer involved in physical experiments. Someone needs to explain to me why generations of folks have latched onto that... $\endgroup$ – CuriousOne Sep 9 '15 at 8:36
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    $\begingroup$ I think the OP asked a simple question and he has a right to a detailed answer including experimental evidence that the biological state of the observer (dead metal-glass-semiconductor/alive and collecting social security) makes a difference in the quantum world. Sorry about the polemics... I am too old to be PC about something that is trivial physics. Like I said, there are subtle questions in QM and I am sure that you have a fine collection of tools and papers that can treat them, but the OP didn't ask a subtle question about umpteenth q-bits. $\endgroup$ – CuriousOne Sep 9 '15 at 8:42

Observing doesn't have a strange effect on anything. It has an effect that is entirely predictable and comprehensible. Suppose you're doing a double slit experiment and you stick a detector in front of one of the slits. The interference depends on the photon going through both slits, so you're blocking some of the photon's wave function and this prevents interference. You can get the same effect by putting a piece of black card in front of one of the slits, or a hot dog, or whatever.

More generally, if you interact with a system in such a way as to record some information about it, i.e. - observe it, you may prevent it from undergoing interference:


This is not magical or weird, it is a completely straightforward consequence of quantum mechanics.

  • $\begingroup$ Surely you don't mean that quantum mechanics/entanglement/measurement is all just the same as classical wave mechanics. It's odd to claim that and then link to a paper about decoherence mechanisms, which are patently different from vanilla wave mechanics. $\endgroup$ – DanielSank Sep 11 '15 at 1:28
  • $\begingroup$ My answer does not say that quantum mechanics is the same as classical wave mechanics. In the particular example of the double slit experiment with a detector in front of one of the slits you can understand what's going on from the absorption of the instances of the photon going through the blocked slit. In general measurement involves copying information from one system to another, which prevents interference (i.e. - it results in decoherence). The former is a special case of the latter. $\endgroup$ – alanf Sep 11 '15 at 12:42

I was just thinking about the idea than in quantum physics, the act of observing has a "strange" effect on some properties (e.g. double slit).

People make it seem strange and mysterious when it is not. When you send your beam through a single slit you get a blob if you send it through the left slit you get a blob on the left. If you send it through the right slit you get a blob on the right. Only when they overlap can you get interference.

Sometimes someone might say, if you observe then something different happens. Nothing different happens. But you were lied to when you were told the beam travels in physical space. The wavefunction is defined on configuration space. If the beam going through the right slit was deflected down and the beam going through the left beam was deflected up then you would have a beam on the lower right and the upper right, there would be no overlap and no interference.

This is exactly what happens when you observe. To observe you change the thing that does the observer. The wave exists in a space that keeps track of everything imagine and x,y,z for one particle and an x,y,z for another particle then the eave is assigned in a 6d space and is about configurations. When you observe then the position of the observing thing changes and this is just as real a delection as any other. So you land with one particle in the screen but another particle moved in some direction. You land in a different place in 6d configuration space and so you don't overlap and so you don't interfere.

It isn't strange. Non overlapping beams don't interfere

If I'm staring at a light bulb, are my eyes having an effect similar to the sensor that plays the role of an observer in the quantum interference experiment?

If your eyes move differently based on the slit it went through, then the configurations of it going through one slit are configurations where it lands at x,y,z and your eyeball has a part at X,Y,Z and if the configuration goes through the other slit and lands at x,y,z but your eyeball has a part at A,B,C (instead of X,Y,Z) then the configuration landed at (x,y,z,A,B,C) instead of landing at (x,y,z,X,Y,Z) and so they don't overlap and so there is no interference.

But all you've done is make the beams miss each other in the actual space the beams travel in, the configuration space.

This is no more mysterious then having the two slits deflect the beams up and down so they don't overlap. You are just deflecting the beam in the direction in configuration space corresponding to other particles. It still makes the beam in configuration space miss.


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