During my studies of physics, I noticed that the discussion of a physical theory includes the discussion of the interaction between the observer (the person who makes the experiments) and the observed reality. This is clear, for example, in the discussion of electromagnetic waves, when it is noticed that light can be described as an electromagnetic wave, but it also corresponds to what is perceived by the eyes. The whole theory thus includes the physical effects of electromagnetic waves on the sensory cells.

There are even more deep reasons to "embed" the observer in the model. It is the case, for example, of relativity (both of Galileo and Einstein). A fundamental point of relativity (here, galileian) is that naively people think that they can perceive the state of motion, the absolute speed. With internal coherence, the theory says that a physical system cannot perceive an absolute speed, thus even people cannot perceive it, by means of their senses nor with instruments.

Quite surprisingly, I'm now discovering that my colleagues do not agree on this point. I discussed this with some researchers, and they have the opinion that the observer (the researcher who performs the experiments) should always kept outside the model. It looks like a dogma, "embedding the biological entity performing the experiments in the model is too difficult, thus it is not allowed to discuss this point".

I would like to know if the two points of view have been discussed in works on phylosophy of science. References are welcome. If one of the two points of view is blatantly the majority of the scientific or phylosophic community, please mention it in the answer.

2 Answers 2


As the Wikipedia article on the observer in quantum physics puts it:

"The prominence of seemingly subjective or anthropocentric ideas like 'observer' in the early development of the theory has been a continuing source of disquiet and philosophical dispute."

The Copenhagen Interpretation fudges the issue, by keeping a classical world, and having a quantum world only below the decoherence limit, bridged by observations. But why is the coherence limit where it is? Why does classical behaviour emerge, if everything is, fundamentally quantum stuff?

The scope for speculation about the quantum measurement to get even very great minds coming out with daft ideas, like the Wigner-Vonneumann Interpretation, has led to a lot of institutional hostility in academia towards funding work on foundations of quantum mechanics. That is starting to change because of the obvious need for more radical thinking in order to integrate gravity with the quantum picture, though it is difficult still to work on for academics without tenure. Given the different interpretations will likely remain unfalsifiable, there is a widespread feeling among professional physicists that it's not an area of physics, but of philosophy.

From the Wikipedia page on the Uncertainty Principle:

"Historically, the uncertainty principle has been confused with a related effect in physics, called the observer effect, which notes that measurements of certain systems cannot be made without affecting the system, that is, without changing something in a system. Heisenberg utilized such an observer effect at the quantum level as a physical 'explanation' of quantum uncertainty. It has since become clearer, however, that the uncertainty principle is inherent in the properties of all wave-like systems, and that it arises in quantum mechanics simply due to the matter-wave nature of all quantum objects."

An exact quantum measurement would intrinsically increase uncertainty in conjugate variables, so knowing speed totally would wipe out knowledge of position, etc.

I like Aaron O'Conell's Visible Quantum Object experiment. Information about quantised changes being either isolated or transmitted and so spreading out, seems to be one way of picturing things.

Chiribella's Purification Principle seems to be a workable way of picturing quantum measurements as the spreading out of correlations, the diffusion of information about past states of particles. Pure states becoming mixed states.

Basically an observation joins a system we call quantum with one we call classical, into a new system we call classical. This is unsatisfying philosophically, but the sense that it wasn't getting in the way of experiments made it a low priority for half a century. Developing a picture that explains observers and the quantum coherence limit and the emergence of classical from quantum behaviour is a set of open problems, which issues like entanglement have underlined the need for.

I wouldn't describe the separation of observer and observed systems as dogma, but as idealisation, like the idea of a thermodynamically closed system. Neglecting interactions in both cases makes calculations tractable, but at very small scales becomes an issue. Maxwell's Demon helped us understand the physical reality of information in relation to entropy. Uniting quantum fields & general relativity looks to involve close scrutiny of rest-frames & information flow.

  • 1
    I think QM is an extreme example of inability to separate out the experimenter. It implicates the very notion of measurement, which is common to every scientific field, experiment, and physical theory. The moral of (textbook) QM is you can’t separate the experimenter for quantum experiments. One can do geology with no experimenter in the model. And we think the tectonic plates would be in the same position no matter how we did the experiment.
    – J Kusin
    Feb 28 at 15:02
  • 1
    @JKusin: Yes. Like quantum entropy & the arrow of time forces more precise thinking about information, things like the delayed-choice quantum-eraser experiment makes us think harder about what observations are, and where the division between observer and observed is.
    – CriglCragl
    Feb 28 at 15:16
  • Actually, I did not mention quantum mechanics on purpose, in my question: in order to avoid the disputes on the interpretation. And, of course, I agree that not every scientific theory requires this insight, e.g. plate tectonics. My hope was to find some simpler but significant case in between, as a training for a good formalization of the observer-observed problem, before approaching quantum mechanics. I also did my formalization, but now I realize that this approach is not welcome by the colleagues. Any hint? Feb 28 at 20:15
  • 1
    @DorianoBrogioli: It's about the observer effect - when observations themselves necessarily have impacts of the order of what is being observed. Maxwell's Demon posits 'an intelligence' that is outside of the system, but reconciling with 2nd Law requires including the entropy increase of wiping digital data, so including the previously separated observer, into the thermodynamics of the system.
    – CriglCragl
    Feb 28 at 21:04

The interpretation of quantum mechanics is an open field since nearly a century.

1.) Initially the field was determined by the Copenhagen interpretation. This interpretation makes a sharp cut between the micro world, described by quantum mechanics, and the world described by classical mechanics. From the view-point of physics the cut is between Schrödinger‘s psi-function with its probabilistic prediction of the outcome of measurements and the definite values obtained by the actual measurement.

The question was: Where to locate the cut?

About a radical answer was speculated by Eugene Wigner. He locates the cuts within the observer’s mind. Anyhow, there was always a tendency to introduce the observer as an important agent into the physical theory of quantum mechanics.

2.) I do not remember a second field of physics where the observer got a similar importance. In general, physicists try to eliminate any subjective components from their theory. They strive for objective results, based on an objective theory.

E.g. special relativity often points to observers who measure different time- and position-values for the same event. But the difference is not due to the subjectivity of the observer. The observer is only introduced to highlight the results, which is often contra-intuitive. The observer can be eliminated and replaced by clocks and meter sticks referring to coordinate systems which are moving relatively to each other.

3.) The situation is different in qantum mechanics. A recent interpretation due to Rovelli states: The observables of the quantum fields do not have any value at all before the fields interact with each other. It is the event of interaction which creates the values of the observables of the fields. And in general, these values are meaningful only relatively to the two interacting fields. See

https://arxiv.org/abs/quant-ph/9609002 and https://plato.stanford.edu/entries/qm-relational/

Here the role of the observer as an agent is replaced by the event of interaction. Rovelli’s viewpoint can be considered a radical sharpening of Heisenberg’s statement: Between two observations the electron in the atom does not have a path.

4.) IMO the strive for an observer independent physical theory serves as a heuristic principle also today.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .