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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?

Qmechanic
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Aaron Anodide
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    possible duplicate of What is an observer in quantum mechanics? – John Rennie Sep 09 '15 at 07:21
  • 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. – DanielSank Sep 09 '15 at 07:54
  • The act of observing has absolutely no effect on "properties". Humans are simply not used to quantum level physics because we don't get to experience it very often directly and when we do, we usually don't know that we are looking at a pure quantum phenomenon (like a permanent magnet or matter in general). Whether you look at something or not doesn't make the slightest difference to that object, not even in the double slit experiment. Whenever you read statements like that you can discount them as nonsense. There are subtleties in quantum mechanics, but they aren't happening at this level. – CuriousOne Sep 09 '15 at 08:09
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    @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. – CuriousOne Sep 09 '15 at 08:19
  • @CuriousOne Then you, sir, may find any number of articles coming out of the quantum computing field in the last five years of interest. In fact if you would please describe exactly what type of experiment you would like to see it's likely I can run it from my laptop at home. I'm not kidding, fire away. – DanielSank Sep 09 '15 at 08:31
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    @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... – CuriousOne Sep 09 '15 at 08:36
  • Let's have this discussion in a chat room. – DanielSank Sep 09 '15 at 08:38
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    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. – CuriousOne Sep 09 '15 at 08:42
  • @CuriousOne When people talk about things being "measured" in quantum mechanics they're always vague. The best possible scientific definition I've heard is "interacts with a large system who's state I don't know". Now, if you compute what happens in that case you find the density matrix of the system under study diagonalizes in the basis of the coupling operator. Fine. Now explain to me why the hell I personally, Daniel Sank the human, experiences one of those results. Please do this without reference to the fact that I'm a human. Good luck. – DanielSank Sep 11 '15 at 01:00
  • I think a simple definition could be: a measurement is a physical system that connects to another physical system and that results in a permanent record. The latter requirement guarantees that the measurement is irreversible, which takes us automatically into the realm of statistical or thermodynamic averages. Whether a human looks at the record or not is of no concern to the physical system under test because the damage by irreversibility has already been done. I think this isolates physics proper nicely from any anthropocentric thinking and it preserves the density matrix just fine. – CuriousOne Sep 11 '15 at 01:20
  • @CuriousOne Where, in the CuriousOne theory of quantum mechanics, is the boundary between quantum unitary evolution and this vague notion of permanence brought on through irreversible statistical mechanics? – DanielSank Sep 11 '15 at 01:30
  • In the timescale it would take for the irreversible system to reverse itself. The 19th century question of why a cup of milk coffee doesn't separate itself despite the Poincare recurrence theorem has been answered a long time ago. This is no different. The only thing we have to ask ourselves is what it takes to make a large enough system that will not show recurrence. The answer is trivial: nothing. Nothing aka the physical vacuum. A photon leaving a finite system volume embedded in a sufficiently large volume of vacuum is a sufficiently irreversible process to make a quantum measurement. – CuriousOne Sep 11 '15 at 02:16
  • In reality it's actually much worse than that. A typical solid state experiment trying to test quantum effects in matter starts almost inevitable with the highly physical question where the money for the sufficiently large and cold cryostat and the vacuum flanges that it needs (cryogenics and air don't mix...) will come from. Why? Because every cryogenic experiment is a race between the physicist and the thermal environment. The physicist tries really hard to make the measurement before a thermal excitation makes it for him, thereby destroying the quantum state... – CuriousOne Sep 11 '15 at 02:24
  • So why do we have such a problem with unitary evolution? It's not like it's the normal state of affairs in nature. Actually, one has to work really hard to see unitary evolution in many systems. In my honest opinion it's a typical case of reality blindness. The single particle Schroedinger equation is the only system that is teachable to the average student. Everything else is just way too hard, so we never get people to the point where they get to see that that equation is a complete oversimplification of a realistic physical system. – CuriousOne Sep 11 '15 at 02:29
  • Let us continue this discussion in chat. – DanielSank Sep 11 '15 at 05:36

2 Answers2

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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:

http://arxiv.org/abs/1212.3245.

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

alanf
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  • 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. – DanielSank Sep 11 '15 at 01:28
  • 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. – alanf Sep 11 '15 at 12:42
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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.

Timaeus
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