What is an observer really?This question is usually encapsulated in the linguistic mud that is equivalent to the following monologue:
I'm sick and tired of explanations of quantum mechanics because they never tell me who is an observer and who isn't. Now, I am the savior of physics who will ask you and you will finally tell me and everyone else what are the ultimate, exact, well-defined criteria that determine who is an observer and who isn't, when the sound of a falling tree was heard and when it wasn't. This will permanently eliminate all the confusion about quantum mechanics. Amen.These people must believe what they're saying but if they were also able to think about it, they would realize how stupid the question is. What kind of an answer are they envisioning if they really want to divide the objects or physical systems in the world to observers and non-observers?
Maybe, they expect you to say "An observer is someone with a social security card that must carry the signature of Barack Obama." Perhaps, an observer divides people to castes and only some of them are composed of observers. Maybe, the definition denies Darwin's evolution theory and it declares humans as observers while all other animals and organisms are qualitatively different. Maybe the humans have souls and consciousness or blessing from God and they make a qualitative difference. Maybe there's a sharp boundary between conscious processes and unconscious processes.
Needless to say, all such boundaries would be totally preposterous and their existence would violate the basic character of the laws of Nature. There can't be any sharp boundaries between observers and non-observers. No stamps or ID cards play a role in the laws of physics. There's no fundamentally qualitative difference between different species of organisms. In fact, there's no sharp boundary between microscopic and macroscopic objects. All these characteristics are continuous, gradual, and for an object to become a human is a long journey (it's easier to be born as one). And the human isn't necessarily the most perfect intelligent being that is allowed by the laws of Nature.
Most importantly, the vague and colloquial understanding of the word "observer" is perfectly enough because the actual rules and laws of physics don't depend on any details of the definition of the word "observer".
So as plain English indicates, an observer is someone (a physical system, but usually one resembling an intelligent animal or an AI-like computer) who observes something (I say "who" and not "that" exactly because that's the word we start to consider more apt than "that" once the physical systems become able to observe things and do related things!). That's everything you need to know!
There isn't any "deeper or more accurate definition" that would be needed to specify how Nature works. In physics, we use the word "observer" to describe a physical system or "agent" that is able to perceive the information about some observables (time-dependent dynamical variables that describe the state of the physical systems) and, if possible, process them. Most often, we want the observers to be able to remember the information, send it somewhere, and/or verify the laws of physics that claim to say something about the patterns relating different observations.
An observer of a particular observable, for example the number \(N\) of photons in a box \(\dd V\), is simply someone for whom the proposition (equation) \(N=n_i\) has (or will have) a well-defined truth value. That's everything I need. The observable has a well-defined truth value because – and I hope you won't be surprised – the observer has observed the observable. ;-)
Now, to determine whether a macroscopic collection of organic molecules (a candidate animal) or a bar with semiconductor molecules (a candidate computer) is actually able to see the light (or something else) and/or remember its properties and/or calculate with it and/or use the information to optimize its behavior to achieve a certain goal, you need some specific complex disciplines of greater physics such as neuroscience or electronics or information technology.
But this is clearly not the issue that the people are asking about. I think that all of us – and most of them – understand that the neuroscience and electronics and information technologies in the real world simply aren't fundamental. Organisms and computers are complex bound states composed of many molecules that are described by the laws of quantum mechanics. I think that everyone who has at least started to think about the equations of quantum mechanics (even if incorrectly) does believe that these universal laws ultimately determine the behavior of semiconductors and proteins, too.
Instead, the goal of their question is more metaphysical or philosophical in character. They're really asking about the existence of "consciousness" because they believe that consciousness or something similar that requires a "soul" is needed to define the laws of physics. But it ain't so.
Consciousness: a great mystery whose pure part is outside science
You know, consciousness is fascinating. As a kid, I would be intensely attracted by its detachment from all empirical observations. I have consciousness, self-awareness, but you don't have to believe me because the "pure consciousness" doesn't have any visible consequences for the external observers. Of course, the "applied consciousness" does seem to have consequences. I am talking about things including consciousness itself, sometimes emotionally, I am able to say "I am aware of myself", and because you probably also have consciousness that manifests itself by talking and vibrating with your head and eyes etc., you decide that I am qualitatively analogous to you so I must have consciousness as well.
Maybe I have the same conscious feelings when I see the red color as your feelings when you see blue and vice versa (which is possible especially if you believe in some wrong political ideology). But you may dismiss these differences by Occam's razor. If the physical careers of our senses are analogous, and if we have analogous molecules in analogous cells of the retina, we arguably have the "same feelings" if a red photon hits our retina. More speculatively, we may think about "conscious feelings" that humans are incapable of. What does an uranium nucleus "feel" when it decays into decay products? This question is less urgent because the uranium atom can't process these feelings or information much so "who cares" (discrimination). But for humans, the question seems pressing: Do I have any consciousness at all?
A mysterious question, indeed. Over the years, I have lost much of my interest in this metaphysical question because of three reasons. First, almost by definition, it seems impossible to answer it within science. If I define the "pure consciousness" as something that is totally isolated and disconnected from all observations, there can't exist any manipulation with the empirical facts that would answer questions about consciousness. Second, because I don't have direct evidence of other people's (or objects') consciousness, it doesn't make much sense to study it. Third, there seems to be no sensible reason to expect that claims about "pure consciousness" may be sharp and rigorous. They are intrinsically vague for a simple reason: they have nothing to do with the quantitative things one may observe – and properties of "soul" vaguely attached to matter that can't be measured don't have any reason to carry well-defined values or rigorous laws relating these values.
So when it came to consciousness, I started to realize that only "applied consciousness" (the actual manifestations of the fact that a physical system is able to measure, remember, and process information in a sufficiently complex manner) belong to science. I would still agree that some "consciousness stripped of the dull material trivialities such as eyes, brains, and microprocessors" exists in some sense but I still find it important to appreciate that the hypothetical existence of this "conscious soul" does depend on the material carrier that may be studied by the scientific method i.e. by a careful analysis of observations. Even though I feel that "some mystery of pure consciousness remains unresolved" by science, I have also become extremely certain that "thinking about these things will never bring and can never bring anything more constructive than metaphysical flapdoodle." In this sense, I have abandoned much of my interest in consciousness for pragmatic reasons.
Maybe you are upset by the large number of paragraphs about pure philosophy – unlimited babbling. So let me get back to quantum mechanics a little bit. My main point is that none of the hypothetical claims about the existence of conscious souls influences any laws of quantum mechanics. This is a widely misunderstood point which is why it may be a good idea to mention how people like to misunderstand it.
They usually think – because they are often told – that a conscious observer causes a "collapse" which allows the superposition states \[
\ket\psi = a\ket{\psi_1}+b\ket{\psi_2}
\] to shrink to one of the options, either \(\ket{\psi_1}\) or \(\ket{\psi_2}\), with probabilities given by \(|a|^2\) and \(|b|^2\), respectively. When this collapse happens, someone is perceiving that something has happened, something has been measured. Because this collapse is such an important intervention into otherwise smooth and regular laws of evolution of the state vector according to Schrödinger's equation, evolution that is supported by all the evidence, one should need some special "stamp" – such as the social security card with Obama's signature that I started with – to interrupt the peaceful Schrödinger's evolution and to replace it by the "collapse".
I am writing down this preposterous story because this is exactly the type of thinking that many popular – and, using Sidney Coleman's words, sometimes even not-so-popular – books and articles want you to manipulate you into. GRW and Penrose collapse theories as well as the many-worlds ideology are example models giving special objects the right to "intervene" into Schrödinger's equation, either by discontinuous jumps or collapses or by splitting the world (which is comparably, infinitely ambitious). However, all this reasoning is completely nonsensical. There doesn't exist any systems for which the evolution according to the laws of quantum mechanics such as Schrödinger's equation is replaced by some discontinuous jumps. Quantum mechanics applies to all systems and processes in Nature, regardless of their size, duration, sex, race, and nationality.
Instead, what an observer does when it "measures" the value of an observable such as \(N\), the number of photons in a region, is that it simply attaches a value to \(N\). Equivalently, it ascribes the truth value to all propositions of the form \(N=x\) or \(N\gt x\). Who has the right to do it?
The key point is that to define the laws of physics, we don't need any definition or criterion here. Why? Because when an observer ascribes a value to the observable \(N\), it has absolutely no impact on other observers' description of the reality! Indeed, as the Wigner's friend thought experiment was designed to explain, other observers – if they want to make really accurate predictions about their future observations – should keep on treating the "conscious observer" as a dull physical system that evolves into the state vector that is a general complex linear superposition of eigenstates with different values of \(N\). In practice, one may use a description in which the truth values and/or probability distribution become "classical" but all these descriptions are at most approximately valid.
So whether the "conscious observer" has "perceived" the value of \(N\) makes absolutely no impact. I still need to describe the degree of freedom \(N\) in terms of probabilistic distributions – and indeed, in quantum mechanics, I need interference-capable complex probability amplitudes. I only reduce my probabilistic reasoning to a fact-based one once I learn about the value of \(N\) or something else myself. At that moment, I ascribe a value to \(N\) or another observable (which, equivalently, changes the state vector or density matrix I am using to describe Nature – these objects represent the state of my knowledge). I ascribe the truth values to various propositions. Whether a different observer ascribed a value to a proposition about an "intermediate question" makes absolutely no impact on my predictions – and my predictions are by definition the only physically "existing" knowledge about the physical system that I have.
It is wrong for me to ascribe particular values to observables if I don't know their values. It is wrong for me to ascribe particular truth values to propositions whose answers are unknown to me. That's why another observer's act of ascribing a value to \(N\) just makes no impact on my knowledge – only if I learn about \(N\) myself, it influences my predictions!
I have already mentioned that whether a physical object may actually see some information or calculate with it is a question for neuroscience or electronics or information technologies. I have already mentioned this point. But you expect some restrictions. Certain things can't be "perceived" at all. Indeed. When you ascribe truth values to some propositions, the operators expressing these propositions must behave as if they were classical numbers \(0,1\) able to be added and multiplied according to the classical rules. For example, if you study which of the alternative histories will occur, the alternative histories in your set must be consistent histories. In practice, it means that unless these histories are artificially engineered and very contrived, they must be histories talking about the values of some "quasiclassical" observables according to some classical limit of your quantum theory.
Nevertheless, I could even extend the definition of an observer a little bit and allow him or her or it or them to "observe" the truth values of propositions that aren't logically compatible in the sense of "consistent histories". Such an observer would be an illogical observer or a confused observer. ;-) It often looks like most people fall into this generalized category of observers. :-) In the same way, there may be sloppy and inaccurate observers – those whose observations are sloppy or inaccurate. More seriously, the inconsistencies between the observations by the confused observers would be analogous to the "paradoxes" that appear when you try to interpret GHZM and similar quantum games classically.
Fine. If you want an observer who sees and perceives real facts – sufficiently accurately – and who processes them, you need an observer that has well-functioning eyes (or an equivalent measuring apparatus), a brain including the memory (or its electronic or another replacement), and so on, according to the rules of neuroscience or electronics or information technologies or something else, and this observer must work with questions and alternatives that are logically consistent in the same sense as "consistent histories".
However, I still need to emphasize the main point of this article. All these features of a good enough observer may or may not be imposed – and it makes exactly zero difference for everyone else! You only impose the "consistency of histories" because you want to be an "unconfused observer". At the end, the only special (or additional) feature of an observer is that he or she or it or they observe something. And observing something isn't a crime. For the observer to have a consistent logical framework with truth values of various propositions about observables (for his histories to be consistent), this observer will have to send actual photons somewhere that perturb the system they're observing (or intervene in an analogous way).
But this necessary perturbation is a mechanical rather than metaphysical process. If some photons hit electrons and if you're another observer in the room, you need to calculate with quantum mechanics for several particles and you get different predictions for the pattern drawn by the electron – effectively, the interference pattern is destroyed because of the electron's entanglement with the escaping photon. But you don't have to "know" whether someone in the vicinity of the photon was an observer or conscious or human or animate or anything like that. It's the photon hitting the electron that destroys the interference pattern, according to the standard rules of quantum mechanics, not the soul or other mysterious anthropomorphic features of the surrounding physical systems claiming to be human!
Once again, the only special or additional properties of observers by which they "exceed" the generic physical objects around them is that they observe. By observing things, they ascribe values to some observables. And the laws of quantum physics imply probabilistic relationships between these values at different moments. An observer who is really worth the name may verify these relationships. That's it. But other observers such as yourself ascribe values to other sets of observables and all the other observers may always be treated as dull, unconscious objects! Only if two observers – two physical objects "personifying" two sets of consistent histories – include the same questions/propositions into their logic (scheme that assigns the truth values to propositions about observables), quantum physics guarantees that the values will agree.
Needless to say, people are looking for an "objective classical model of reality" that is valid for everyone. But quantum mechanics shows that Nature can't be described in this way. Instead, quantum mechanics tells you that you must understand yourself as an observer who may perceive the values of certain observables and quantum mechanics tells you that observing some values of observables at one moment implies that the probability of observing some combination of other observables at a different moment is something or something else. That's the only thing you may really empirically verify so it's just unphysical to "demand" that science also explains something else (such as an "underlying objective reality"). And indeed, it's very important in quantum mechanics that physics can't objectively describe too many things, that it is always impossible to ascribe values to too many observables or truth values to too many propositions. The uncertainty principle is just one of the basic formulations of this fact: you can never ascribe exact values to two complementary variables such as the position and the momentum of the same object. But this principle is really omnipresent and essential in all of quantum mechanics and may be formulated in many related ways, seen in many reincarnations. You just can't talk about the objective properties of (most of the) things you can't observe.
To summarize, an observer is someone who may observe things and verify the predictions from the laws of physics, among related things (e.g. using them to his advantage). But if there's no one who can do it, no one is hurt! Nature may seem like it has no purpose – but it's how Nature is according to science, anyway. What's important is that the "consciousness" or "act of pure observation" or "realization" doesn't make absolutely any impact on physical predictions that other observers may prepare. For other observers, the original observer is just a piece of matter, a dull physical object. That's why, if they're sensible and physically oriented, they don't spend hours by trying to find an exact definition of an observer. They know damn well that they're able to ascribe truth values to propositions about observables and that's enough for them to verify the laws of physics. One can't expect any sharp rule dividing physical systems into castes, into observers and non-observers. No such sharp rule exists and no such sharp rule is needed in physics.
And that's the memo.
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