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What is an observer in quantum mechanics?

I'm sick of quantum physics explanations which term experiments where the outcome depends on "if you observe it or not". For example, the two-slit photon experiment.

I figure that something physical happens, irrespective of if a "being" of some kind is there to consciously watch it.

Is it really to do with absorption and emission? Like the only way to know for sure where a photon is, you need to have it absorbed and re-emitted by an atom/electrons so that you can detect it (on one level, seeing with your eyes is about absorbing photons on the atoms in your eye and having that create a potential on your nerves which can travel through your brain).

So in a sense, if a tree fell in the forest and nobody heard, it still would have fallen because the atoms in the tree would have absorbed thermal energy, and the atoms of the tree would have caused a vibration in the tree and the ground, which vibrated the air, and so this energy would have dissipated into the environment, and that counts as observation without a human ever getting involved?

Do I understand things or am I very wrong?

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  • This "question" has been asked about 1,864 times on this server already! http://physics.stackexchange.com/search?q=what+observer – Luboš Motl Jan 16 '13 at 10:35
  • Otherwise, you may be sick or tired or bad in whatever other way, but the thing you don't want to accept is exactly how Nature works. An observer is, as the name indicates, someone who observes. The classification only matters subjectively because the observer is, by definition, aware of the results of a measurement, so statement of the form $A=a_n$ where $A$ is an observable belongs among the proposition that have a well-defined truth value according to the observer. – Luboš Motl Jan 16 '13 at 10:38
  • So the existence of the truth value in the knowledge of an observer is really a subjective matter and the only thing that this existence of the truth value may affect is a discussion from the viewpoint of the observer that may have truth values for other things as well and that may relate them by the laws of physics. It is completely wrong to try to define an observer objectively - one ends up with nonsensical artificial lines defining life or humans or souls or consciousness. None of these lines actually affects any prediction in quantum mechanics. – Luboš Motl Jan 16 '13 at 10:40
  • Having an observer is just a prerequisite needed for us to be able to talk about predictions and truth values of statements about observables at all. But what these predictions and relationships between them actually are is totally independent of any would-be "conditions" on what the observer is allowed to be or not. – Luboš Motl Jan 16 '13 at 10:41
  • see also http://physics.stackexchange.com/questions/18602. A search of the site for "quantum mechanics observer" will lots of related info. Suffice to say that since decoherence we have a mathematically solid description of what an "observer" is. – John Rennie Jan 16 '13 at 11:02
  • Dear John, I disagree that decoherence provides us with a definition of an "observer". Decoherence is an emergent, widely applicable explanation why the truth values of propositions about certain (macroscopic, with environment interacting) observables quickly start to behave in a way compatible with their interpretation as truth values of propositions in classical physics. There isn't any room here for "subjects capable of observing" anything here. Whether someone is actually observing anything involved in decoherence is irrelevant and unspecified. – Luboš Motl Jan 16 '13 at 11:22
  • If you meant that every macroscopic, with environment interacting physical system (one that quickly enough decoheres) is an observer, then your definition of an observer contradicts the prevailing one in physics where the observer is really expected to be able to observe - detect and at least slightly process the information about the observables. A random piece of thermal matter just isn't considered an observer because we discuss observers as entities able to verify the relationships between observations. – Luboš Motl Jan 16 '13 at 11:27
  • See, this is exactly why I asked the question. 1,864 times nobody has a consistent explanation without somebody contradicting, then you need a PHD to understand. Why exactly can't it be a random piece of thermal matter? – Bingo Jan 16 '13 at 13:35
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    Dear Bingo, you surely need a PhD to understand certain aspects of quantum physics. But a physics PhD is surely not enough for a consensus (and consensus is not needed in science). See a new "poll" among experts on foundations on quantum mechanics in this paper: http://arxiv.org/abs/1301.1069 They don't agree on pretty much anything here. This is a confusing thing colliding with many prejudices, including yours, so a big portion of people will simply never understand the right answers even though quantum physicists have known the right answers to most of questions like yours for 90 years or so – Luboš Motl Jan 16 '13 at 15:09

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