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Some interpretations of quantum mechanics — like the Copenhagen interpretation in particular — require the existence of an observer. The role of the observer is a bit mysterious. After all, observers are not needed in classical mechanics. Which physically acceptable interpretations of quantum mechanics do not require the existence of any observer at all? By that, I mean they would continue to make sense even in the absence of any observer. The many worlds interpretations appears to require an observer to subjectively determine which branch is selected.

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Dear Kenneth, classical mechanics may be thought of as a description of a "real world" that exists independently of observers, but quantum mechanics cannot. That's why it's quantum mechanics. If it were describing an objective world, it would be classical physics. So every correct "interpretation" of quantum mechanics agrees with the fact that it is a tool to give probabilistic answers to well-defined questions chosen by observers, according to some logical scheme. It is not a description of a objective world that exists independently of observers. –  Luboš Motl Dec 31 '11 at 16:23
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That is in itself an interpretato, @lubos, the observer dependence in many worlds is no worse philosophically than in classical mechanics. –  Ron Maimon Dec 31 '11 at 16:50
    
Well, MWI is an ill-defined framework and no one agrees what it is exactly supposed to mean. But if you have a version of MWI which makes physics as observer-independent as classical physics, then you should call it a classical theory, and it can't be a valid description of the world around us which is not classical. It is not equivalent to any classical theory. Every picture that tries to "reinterpret" quantum mechanics as an objective, observer-independent description of some objective reality is classical and wrong whether or not it is "philosophically pleasing" to anyone. –  Luboš Motl Dec 31 '11 at 17:27
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This video has a recipe for anyone who doesn't find QM (and QED) sufficiently psychologically easy and philosophically pleasing: youtube.com/watch?v=iMDTcMD6pOw –  Luboš Motl Dec 31 '11 at 17:34
    
It would be interesting to know what Feynman would pick out as the most interesting developments in Physics since Luboš' choice of video was recorded. Perhaps sadly, except that it's in the nature of things and perhaps we should just accept it, we have to pick out what is interesting for ourselves. –  Peter Morgan Dec 31 '11 at 19:55
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6 Answers

The transactional interpretation does not require any observer to account for deterninate events. See my papers on arxiv.org (my latest on arxiv deals with the relativistic domain and specifies what absorbers are in precise terms) and also at transactionalinterpretation.org

and stay tuned for my forthcoming book (already available for pre-order on amazon and possibly other bookseller sites).

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Hi Ruth - is this account also yours? Would you like me to merge them if so? –  David Z Jun 28 '12 at 20:58
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Surprisingly, in some of his writings Feynman seemed to lean towards an observer-free interpretation of quantum theory in which objects as small as atoms or particles replace observers by acting as event recorders (memories). Two relevant Feynman quotes are below; Chico in his answer noted several other relevant and related Feynman quotes.

Vol. III, Sec. 3-3 Scattering from a crystal, p.3-9, first full paragraph, talking about whether a neutron will interact with a crystal as a wave or as a particle:

"You may argue, 'I don't care which atom is up.' Perhaps you don't, but nature knows; and the probability is, in fact, what we gave above -- there is no interference."

Vol. III, Sec. 3-4, Identical particles, audio version only:

"... if there is a physical situation in which it is impossible to tell which way it happened, it always interferes; it never fails."

In the first quote, Feynman pretty much says that what counts is whether the event got recorded by an atom, regardless of whether an observer looks at it or not. That's remarkable if you think about it, since it does not even acknowledge the view that a conscious observer must always participate.[1]

Instead, Feynman employed the idea that an atom that is capable of being struck by a neutron is all the "observer" needed to make an individual neutron bifurcate between the two possible paths of remaining quantum (where it leaves no specific information trace is left in the crystal, and the neutron continues as a wave) or becoming classical (where it leaves a small but clear mark on a single atom within the crystal, and ceases to behave like a wave.)

In the second quote, Feynman uses this same concept of information signatures to define the difference between quantum (interfering) and classical situations. Only those situations that fail to leave such an information trace remain quantum. (In current terminology, this would be called the decoherence problem.)

While I really like Feynman's approach to such thought experiments, I must also point out that to the best of my knowledge he never made any attempt to expand such ideas into an explicitly quantified approach. Furthermore, since Feynman clearly did feel that there was such a thing as a universal wave function, and that the concept of wave collapse was "magic" (in a QED footnote I think), it's possible that he was simply making a subtle distinction between "simple" and easily reversible forms of wave functions (the neutron diffracting) and "complicated" wave functions that were far less likely to be reversible (the neutron colliding with one atom). John Bell had a marvelous essay on just that point,[2] in which he showed how very careful one must be when assuming that a "clearly classical" event had no wave representation.

For my part, I like the pragmatic simplicity of Feynman's "did it leave a trace?" rule very much, even if it lacks the formal depth and elaboration of other more abstract frameworks. Quantum events by this rule simply become the ones that don't leave information traces... period. In fact, if you are familiar with Feynman's QED framework and think about it a bit, you will see that Feynman's marvelously accurate and predictive integrals of all possible histories are just the flip side of the same idea.

That is, those many possible histories become significant precisely because no classical event has occurred yet that would contradict them. The absence of classical information traces allows such whispers of worlds that could be to take on a certain degree of measurable reality, if only as vanishingly tiny parts of a wave that is the sum of all such not-yet-forbidden worlds.


[1] Feynman was very much aware of the conscious-observer viewpoint, since his PhD adviser John Archibald Wheeler was one of its main advocates.

[2] John Bell, Speakable and Unspeakable in Quantum Mechanics.

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Feynman's description is not different from anyone else's in this respect--- it is just clarifying things for people who think that "measurement" and "entanglement with a complex decohered environment" are somehow different. The issue of interpretation is not that decoherence and measurement are distinguishable, but that the result of decoherence is to produce classical probabilities for observations where classical probabilities don't appear in the fundamental theory. –  Ron Maimon Jun 28 '12 at 20:15
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For the Penrose interpretation, quoting Wikipedia:

Penrose's idea is a type of objective collapse theory. For these theories, the wavefunction is a physical wave, which experiences wave function collapse as a physical process, with observers not having any special role.

You can check more about objective collapse theories here, and more about the referred Penrose interpretation here.

Also, the Wikipedia article on the double-slit experiment slightly mentions about the novel experiments that goes on the corners of the measument problem and the wave-particle duality. For this you should look for "Unsharp particle-wave duality in a photon split-beam experiment", "Simultaneous wave and particle knowledge in a neutron interferometer", "Afshar experiment" and other related experiments and its critiques.

Also, quoting Heisenberg on "Physics and Philosophy":

It applies to the physical, not the psychical act of observation, and we may say that the transition from the 'possible' to the 'actual' takes place as soon as the interaction of the object with the measuring device, and thereby with the rest of the world, has come into play; it is not connected with the act of registration of the result by the mind of the observer. The discontinuous change in the probability function, however, takes place with the act of registration, because it is the discontinuous change of our knowledge in the instant of registration that has its image in the discontinuous change of the probability function.

Which may give you another view for the term observer.

Also, Feynman says on his Feynman Lectures on Physics:

You do add the amplitudes for the different indistinguishable alternatives inside the experiment, before the complete process is finished. At the end of the process you may say that you "don't want to look at the photon". That's your business, but you still do not add the amplitudes. Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.

and later on:

If you could, in principle, distinguish the alternative final states (even though you do not bother to do so), the total, final probability is obtained by calculating the probability for each state (not the amplitude) and then adding them together. If you cannot distinguish the final states even in principle, then the probability amplitudes must be summed before taking the absolute square to find the actual probability.

When he says, "Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not", I think he's ruling out the common sense for observers.

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I tried to find Feynman quotes where he says "Nature knows" on my Lectures copy, but could not, thanks @terry-bollinger –  pepper_chico Jan 9 at 6:33
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Quantum Mechanics is almost certainly as flawed as every other physical theory that has come and gone. That statement gets on the nerves of a lot of physicists - many of whom think that 'This Time its Different'.

Take Newton's gravity for example. Newton knew that the whole instantaneous transmission of gravitational force did not make sense, yet the theory was wildly accurate and is still used today for 99% of things like space flight. Yet the theory is fundamentally flawed from a conceptual viewpoint.

With QM - where is the problem? It is likely in the transition from the microscopic realm to the macroscopic realm, where the 'large hand wave' called waveform collapse comes into the theory. So getting to the question: Here are some possibilities:

http://neshealthblog.wordpress.com/2011/11/01/quantum-upgrade-removes-need-for-spooky-observer/

Re: This time its different: http://press.princeton.edu/titles/8973.html

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-1: This is a ridiculous misunderstanding of how precious the knowledge gained by science is. Newton's theory of instantaneous transmission makes sense, it just is incorrect. There are reasons to doubt QM, but at some point, we will have a complete theory, and we might already be at that point--- this discovery process can't go on forever with regards to the physical sciences--- the down-below fundamental stuff is not infinitely complex, so you run out of questions. –  Ron Maimon Feb 27 '12 at 17:04
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There is always an observer in the sense that someone has to decide what experimental apparatus to construct, what data to collect, what statistics or other multivariate functions of the data to construct from that data, and what theoretical model to compare the data to. That was and is equally true of classical Physics as it is of quantum theory.

There are as many Copenhagen interpretations as there are people who've written about what it is. A recent paper by James R. Henderson, "Classes of Copenhagen interpretations: Mechanisms of collapse as typologically determinative", Studies in History and Philosophy of Modern Physics 41(2010)1–8, doi:10.1016/j.shpsb.2009.08.001 (sorry, I couldn't find a non-paywall version to link to), discusses three people's approaches,

"Visions of the theory from von Neumann, Heisenberg, and Wheeler offer different mechanisms to break the continuous, deterministic, superposition-laden quantum chain and yield discrete, probabilistic, classical results in response to von Neumann’s catastrophe of infinite regress."

On the second page, Henderson suggests that, for a measurement to happen,

"Heisenberg and Wheeler merely need macroscopic objects, a measuring device and a record of a measurement, respectively, and von Neumann invokes the human mind to accomplish the task."

One can take issue with any such classification of the evolving history of the Copenhagen interpretation over the last 80 years, but in relation to your Question I suggest that there is enough to justify a claim that some varieties of the Copenhagen interpretation do not require an observer.

On another front, one of the claimed advantages of the de Broglie-Bohm interpretation is that it is observer-independent in the same sense as classical dynamics (which for some people is desirable enough that they are willing to accept other aspects of that interpretation).

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Well if such interpretation existed it would be useless because it would be unable to make predictions for observations by any existing observer.

That said, you correctly noted the many-worlds interpretation. While it needs observers to explain observations, it still has a concept of so-called "universal wave function", a wave function of the entire Universe, that does not correspond to any observer and which undergoes unitary evolution from the very beginning of Universe. The problem with it is that nobody ever can measure it :-) It is just postulated that Universal Wave Function exists, but it is unobservable and at best corresponds to an infinitely distant (in both space and time) "observer".

Moreover. In this work it was shown that universally valid theory is impossible. That is however good a theory is invented the observer could not use it to predict the future of a system that contains himself. This result means that however well a theory predicts future of any observed external objects, the observer himself is excluded from normal application of the theory's laws. As the whole universe inevitably includes the observer himself, this means that no theory can be used predict the future of the universe as a whole because inevitably there is a point in the universe where the theory's laws are broken.

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