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I am a physicist and not by any means any expert on philosophy. I want to get opinions on a question which seems to me and some of my friends to be an age-old problem. To what extent is the world observer independent? Can the world exist if there are no observers? If not, what defines an observer, and why is he so special? And how precisely is the observer's consciousness related to these aspects?

P.S. Please excuse me if the question is not suitable to be posted on the site.

EDIT : Reference to related review articles / discussions is most welcome.

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  • I think that while the question as such (when not reduced to the title) seems legitimate, it is way too broad to be answered within this site. There are books on this by every major philosopher so that it would be hard to answer acurately to all the aspects that hide within this short text.
    – Philip Klöcking
    Feb 17 '16 at 13:02
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    I think that the mainstream agrees on an external world that exists independent from our perceiving of it. But the consensus ends as soon as it comes to how we, as agents, are able to behave towards this world, may it be receiving or changing.
    – Philip Klöcking
    Feb 17 '16 at 13:09
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    Yes, you can tell from examining the tides.
    – user4894
    Feb 17 '16 at 16:44
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    @CortAmmon worth putting all those little comments in to an Answer?
    – user16869
    Feb 18 '16 at 1:32
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    @BruceLee Well then it's a good thing I didn't post it as an answer :-) However, by that logic it would be impossible to not observe the moon; since the absence of the moon's gravitational field is observable throughout the entire universe.
    – user4894
    Feb 18 '16 at 6:32
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Responding to requests to turn my comments into an answer

The question of whether anything exists when we are not observing it is a standing question in philosophy. As Philip points out in the comments, this is a topic which nearly every major philosopher has written multiple books about, so an answer which fits within the space of a StackExchange post will be hard to come by. Needless to say, there is not a consensus to the answer.

Part of what makes the topic difficult is that the concept of observation is so intuitive in the simple human-sized cases. I look at the moon; I look away; I look back -- it's still there. However, if you start to explore what it means to observe something you quickly enter the world of epistemology, the study of knowledge. Epistemological approaches typically admit that they are not in a position to make true/false statements about reality. That's the realm of ontology, not epistemology. In fact, as one dives deeper, the question of what an observation is gets even murkier, not clearer.

Fortunately, as someone with a physics background, you have access to a set of experiments where observation is known to have quirky behavior: quantum physics. You can set up many experiments which demonstrate that the intuitive concepts of what an observation is break down. In fact, they break down so thoroughly that QM scientist typically call them "classical observation," to indicate that the concept is really only valid in scenarios where classical physics applies. In their world, they instead play with concepts like expectations and covariances and superpositions. These are statistical constructs which are very effective at predicting results, but often leave one pondering whether there is something more. These experiments may be very helpful in shaking up one's intuitive understanding of observation. This may permit one to explore the philosophical questions regarding observation with a more open mind.

My favorite series of these experiments is the quantum eraser series. I like them because they truly wrack the intuitive mind, while demonstrating numerical results that are very well predicted statistically by quantum physics. However, they leave one pondering how in the world these statistics could actually describe reality, until one leaves behind concepts such as the idea of classical observation. The experiments are quite detailed, but I will dare try to offer the cliff notes:

The double slit experiment - Monochromatic coherent light, such as that from a laser, is passed through a beam spreader onto a surface with two parallel slits in it. These allow the light to pass through onto a second surface some distance behind it. Due to differences in the path length between the distance from the laser to each slit to the spot on the paper, interference patterns show up. Instead of seeing two bands, as one might expect, one sees a series of interference fringes. This demonstrates that light travels in waves. Great. Nothing fancy QM here. Just a proof that light behaves like a wave. We can observe it (classical observation). Moving on.

Single photon double slit experiment - Now lets tune the laser down. Instead of shining a ton of light, let's just shine a little. In fact, we can send just a single photon at a time by providing just enough power to emit a single quantized packet. How do we know it's quantized? We can do experiments regarding the photoelectric effect to demonstrate that we cannot generate a photon outside of its quantized energy region. At the very least, we can show we don't have two photons... just one. Now the photon goes towards the double slit, goes through one slit, and hits the board. Easy peasy. Only one problem. If you do this multiple times, and start looking at the probability of the photon appearing at any given position, the results don't line up with that intuition. Instead, you see a spattering of points which lines up perfectly with the interference patterns from the earlier experiement. Somehow the photon is seeing interference, even though there's no other photon for it to interfere with besides itself. This gets explained as wave/particle duality. The photon acts like a particle and a wave. QM has equations to explain the exact pattern we see, but the intuitive mind may tweak things to accept that there is a thing which can travel through both paths at once. No issues here. This is kinda creepy, but we're not seeing any really unusual observation like behavior. The moon is still there, it just behaves differently than we thought.

Double slit experiment with path detection Now it starts to get creepy. We use a nonlinear optical material to split the photon into an entangled pair. Why this works is hard to explain without QM equations to back it up, but experimental evidence supports the theory that some materials can actually split photons into an entangled pair of photons, each with half the energy. The super long name for this process is spontaneous parametric down-conversion and it occurs in a material such as beta barium borate and I will happily admit that I have no idea how this happens and am comfortable accepting that empirical studies have demonstrated that this works. This is done before the photon hits the double slit. One entangled photon goes to a detector, while the other goes to the double slits. Nothing unusual happens. You still see a double slit interference pattern. The next step is where it starts to get creepy. We put circular polarizers in front of each slit. Thus the polarization of the photon streaking towards the detection grid after the double slit contains information about which path it took. Because this is entangled with the other photon, the other photon must have the same polarization. If we put a circular polarizer in front of the detector of this entangled photon, we will only capture photons with a particular polarization. Now a curious thing emerges. When we only look at photons with a particular polarization (meaning a certain path), the pattern we see on the detection grid ceases to show interference patterns. It shows a straight forward distribution with one big lobe, like one would expect if light behaved like a particle. This is the first time observation starts to act unlike what we would like: somehow the mere fact that we observed the photon's path caused it to cease to interfere with itself!

The quantum eraser - Now do the same experiment, but put a linear polarizer in front of the detector of the entangled photon. This destroys any path information we might have had, because clockwise and counterclockwise polarized light have equal probabilities of passing through a linear polarizer. Suddenly, the interference pattern jumps out again. The photon knew that it's path wasn't being observed, so it went through both slits again. The previous sentence should raise hives on the skin of any scientist. Photons can't know things. Somehow this "observation" thing is doing far more than it really should. Once again, the QM equations predict these reults with statistical perfection, but its hard to explain how any one result could occur. The statistics make sense, but the individual cases are baffling.

Delayed quantum eraser - What if we extend the path length to the entangled photon detector? What if the entangled photon hasn't even reached the linear polarizer before it gets detected. Bafflingly, it doesn't matter. We still see the interference pattern emerge. Remove the polarizer, the interference patterns vanish. Now it's getting creepy. Now, not only do photons appear to know whether they have been observed, they seem to know whether they are going to be observed. Classical observation falls flat on its face here. The only way to explain what we see is through quantum mechanics, intentionally avoiding "observation" of anything except for the final product. There's nothing magic here, just the concept of observation ceasing to be effective.

Delayed choice quantum eraser - God save whoever thought this was a good idea (Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih and Marlan O. Scully, to be specific). They swapped up the delayed quantum eraser, putting the double slit first, then using beta barium borate to split it into entangled photons after each slit. One entangled photon went to the detection grid to be measured. The other, which they called the "idler photon" went into a mousestrap of beam splitters and detectors. 3 beamsplitters each reflected the photon 50% of the time, and transmitted it 50% of the time. They organized them very carefully to capture different path information depending in which detector it hit.

  • The only way to hit one detector is for it to go through one slit
  • The only way to hit another detector is for it to go through the other slit
  • The two remaining detectors could be triggered by a photon through either slit.

What's different here? When the photon hits the detection grid, it is literally unknown what detector its entangled sister will go to. There's a 50% chance that it will go to a detector which reveals which path it took. There's a 50% chance that it goes to a detector that does not reveal anything, "erasing path information."

The result? If they filter out only the photons whose sister hit a detector which gave path information, the result was a wide pattern with no interference patterns -- particle behavior. If they filter out only the photons whose sister hit a detector which erased path information, we saw interference patterns.

If one thinks in terms of classical observation, somehow the photon must have "known" which detector its entangled sister would go to in the future! It would appear that a future event affected the past detections! Now not only are observations affecting the result, but observations that we don't even know if we're going to make seem to be affecting it! Once again, the QM equations predict the results without any of this future observation nonsense, but they have to do so statistically, by using terms like superposition of states. Trying to explain what any one photon will do is fraught with error!

After exploring the exotic results that occur with the series of eraser experiments, hopefully that helps unseat any privileged status that "observation" might have. After that, accepting that philosophy finds it a tricky topic may be more palatable. Then you could explore what individual philosophers thought "observation" or "perception" might be, and the consequences of those beliefs. That's where the real fun begins!

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  • Roland Omnes has written a few works, including one titled "Quantum Philosophy" (not a philosophical work btw), that get at this view of the situation.
    – Dave
    Feb 18 '16 at 16:31
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This stems from an ancient question in philosophy. How one can distinguish between "mere appearances" and the "reality" behind appearances, which is another way of asking about a "mind-independent" reality... or things as they really are, "things-in-themselves" when we are not looking, so to speak.

The question then emerges in its modern form, rather paradoxically, along with the rise of modern physics and the "experimental" method after Bacon and Galileo. As we know, verification by "experience" or "experiment" became hugely successful in replacing Aristotelean verification by "reasoning" and authority. Most thinkers agreed that this was a valuable and irreversible advance, not least at a political level. For as Locke argued, the testimony of "experience" of the "senses" is available to all people. The public demonstrations of the Royal Academy were validations by "common sense" as opposed, for example, to Latin texts.

Yet the other half of science, the mathematics and reasoning, were not sensible. How did the two fit together? This is what Locke and the British empiricists set out to demonstrate, quickly falling into philosophical difficulties with Berkeley and Hume. To simplify, we know that there is some sort of difference between "experience" and "reality," between "seeming" and "being." This is the very essence of being "objective" and not delusional. But if all valid knowledge comes to us only through the "senses" how do we know what is "really out there"? Worse, senses are embodied and individual. I cannot feel your pain. So, how do I even know you are out there? As for science, since neither "matter" (Berkeley) nor "cause" (Hume) are directly perceived themselves, are they mere names? For what?

This is the skeptical challenge Kant sets out to address, with a particular view to Newton's physics and the incontestable value of the experimental method. His merging of "rationalism" and "empiricism" is pivotal for modern thought and sets out what we might call a workable "compromise." To simplify, he demonstrates that any possible "experience" itself is made up of certain essential concepts. This a priori conceptual apparatus is the very basis of human "reason" and the way in which we put together or "synthesize" sensible phenomena. His demonstration is very complex and compelling.

His middle stance? We can trust our senses and experience, because we, and all other "rational beings" can only have experiences through our sensory-conceptual apparatus. We can speak of the world consistently and objectively because all rational beings must have this conceptual apparatus, as if it were some virtual reality helmet we could not remove. This is not unlike a psychological version of the "anthropic principle" in modern cosmology. But it is important to note that Kant is not doing psychology, which would only refer us back to the senses and experiment. We might talk, for example, about "how our senses evolved." But this quickly becomes circular. Kant's argument is a unique type of rational inference.

So, what about mind-independent reality? What is outside our conceptual helmet? What about "things-in-themselves" or Kant's famous Ding-an-Sich? Kant says yes something is "really" out there, as perhaps some totality of possible experiences. They anchor us as if we were mentally feeling our way along some surface or limit. We can even speculate about this "noumenal" realm behind the "phenomenal" realm. But we can never have "knowledge" of it. It "makes no sense" to argue about it or make claims about it. Thus, mathematics leads us to new inductions validated by sensible "experience." But these are never the "reality in itself" and thus always potentially falsifiable, as Popper argues.

As my gross simplification indicates, this may be a pragmatic solution for the working scientist, but for many it is deeply unsatisfying, hence Einstein's dismissive question about the moon. It is the forerunner of the Copenhagen interpretation, in some versions, that the "measurement" is as good as it gets. This has not at all stopped physicists and philosophers from arguing about it. But there is something quite reasonable about concluding that there is a reality "limit" to the possible, that we can have truths, but never the "whole truth and nothing but the truth." For Kant it is a way of logically anticipating those "logical arguments" that can never be settled.

Many subsequent and prior arguments deal with your issue. I only offer Kant as pivotal to its development alongside physics, and as a philosophical period with which most physicists today are probably not very familiar. It would remain today a touchstone, at the very least, for most modern philosophical positions on your issue.

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  • +1 for "I cannot feel your pain." I stopped reading after that because I was laughing too hard.
    – user16869
    Feb 18 '16 at 2:04
  • That's why you're not president... Feb 18 '16 at 2:31
  • -1 because i didnt get the joke
    – user6917
    Feb 7 '17 at 14:38
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This is of course a rather vast topic with innumerable references, but in a nutshell the mainstream answer seems to be: the world is observer independent but there should be someone to assert this.

The details are numerous and tedious. Philosophers have distinguished the cases of many, few, one and zero observers. Also they know that what is true up to the limit is not necessarily true at the limit; so, one can disagree with transposing from the common sense to the singular case and beyond. There are more caveats in passing from 'one' to 'any', with 'possible observers' somehow lurking in the background. Solipsism is not a position that can be refuted and lawyers used to say 'unus testis nullus testis' (one witness is not a witness).

Anyway an assertion such as 'the world is observer independent' needs someone to make it, a way to check its content and to decide if this is indeed the case.

Ancient Greeks refer to practice of speaking your mind as parrhesia: that is to say frankly and sincerely what you believe the case is. Most people would probably say the world is observer independent. In a larger society with conflicting interests 'truth' became not what you believe to be true but what is provable. This is the positivist turn which governs much of today's philosophy. 'Provable' is taken to mean - by logical argument or by evidence. Skipping the analysis of observer(s), world, existence etc, the last word seems to be that there is not conclusive proof that the world is observer independent.

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