The Observer Effect in Quantum Mechanics

March 20, 2018; revised April 20, 2022; March 25, 2023

1. The “observer effect” — sometimes called the “measurement problem”– in quantum mechanics is the problem of how (or whether) wave function collapse occurs. But the whole point is that there is no need for a “wave function collapse,” as we explain in this post.

  • Let us start with what is meant by “wave function collapse.” It is always good to start with the basics.
  •  Please read the previous post, “Will Quantum Mechanics Be Able to Explain Consciousness?” including the section on “Subjective versus Objective: Difference between Mind and Matter.”

2. The wave function in quantum mechanics evolves deterministically according to the Schrōdinger equation as a linear superposition of different states. But actual measurements always find the physical system in a definite, well-defined state. Therefore, at the time of the measurement, all those multiple states should collapse to just one (the observed).

  • This is known as the “observer effect” since an observer is needed to make a measurement (and thus “cause a collapse”),

3. Even if such an “observer effect” exists, just the mere decision to make a measurement does not make such a measurement “subjective” in terms that we defined the term “subjective” in the post, “Will Quantum Mechanics Be Able to Explain Consciousness?“.

  • There is no  “measurement problem” because an observer’s “personal” mental state does not play a role.
  • In these quantum systems, one can calculate only the probability of a given outcome. If one carries out a large number of measurements, that outcome will be consistent with that prediction.
  • That is quite similar to throwing a die. We can only say that it will land on “5” about 1 out of six throws since the dice has six faces.
  • Anyone can initiate such measurements and will get the same result. Furthermore, a given experiment can be run by a computer program, and a conscious observer is not needed.

4. This controversy over an “observer effect” arises in the first place because of the assumption that the wave function is “ontic,” i.e., it has all the correct information about the particle in it.

  • But this assumption has been rejected by Einstein and many others, including Bell: “..Either the wavefunction, as given by the Schrōdinger equation, is not everything, or it is not right.” (Bell, 1987, p. 201).

5. Furthermore, this requirement to “collapse the wave function” or the involvement of an “observer” is absent in Bohmian mechanics, a version of quantum theory discovered by Louis de Broglie in 1927 and rediscovered by David Bohm in 1952 (Bohm, 1952).

  • In Bohmian mechanics, a system of particles is described in part by its wave function, evolving, as usual, according to Schrōdinger’s equation. But a complete description is provided by specifying the actual positions of particles by a “pilot wave” or a “guiding wave.” Bohmian mechanics track the trajectory of a particle in real-time, and there is no need for a “wave function collapse.”

6. A key experiment that led to the concept of an “observer effect” is the famous  “double-slit experiment.”

  • However, in recent double-slit experiments (Kocsis et al., 2011; Schleich et al., 2013b) monitored, individual trajectories of particles, and any possibility of a “mind effect” or “observer effect” was ruled out.
  • Their results were consistent with the trajectories of individual particles calculated with Bohmian mechanics.

7. Bohmian mechanics naturally describes all possible paths. Each one can be assigned a probability, and experimental outcomes agree with those probabilities.

  • So, the measurements are deterministic in the following way. The outcome is compatible with the predictions in a series of measurements. Those measurements are objective.
  • A detailed description of Bohmian mechanics is in (Durr, Goldstein, and Zanghi, 1992).
  • The following video provides a good model of the Bohmian mechanics (thanks to Dosakkhayo for providing the link):

8. Physicists have been slow to use Bohmian mechanics because it involves more work (solving the pilot wave equation), but there has been a renewed interest in recent years.

  • We have done a literature survey on the Science Citation Index and found that interest in Bohmian mechanics seems to have accelerated around the turn of the century. The total number of publications from 1992 to 1999 was 52. From 2000-2005, 2006-2011, and 2012-2017 had 134, 174, and 200 papers published. Thus, even though it took time to gain traction, Bohmian mechanics now seems to be attracting attention.

9. Furthermore, a series of recent papers have illustrated the beautiful connection between classical mechanics and quantum mechanics; see, for example, (Field, 2011; Taylor, 2003, Hanc et al., 2003), which was initially pointed out by Feynman (Feynman, 1948).

  • These and other papers show how the “sum over all possible paths” by Feynman in quantum mechanics (Feynman, 1948) converges to the “path of least action” in classical mechanics at the limit h (Planck’s constant) approaching zero. Thus classical mechanics is just a limiting case of quantum mechanics.

10. Others have described how Schrōdinger’s equation comes out naturally from classical mechanics (de Gosson and Hiley, 2011; Field, 2011; Schleich et al., 2013a).

  • The so-called “quantum weirdness” arises due to the effects of the Heisenberg uncertainty principle, which becomes non-negligible when “h” in the equation is non-negligible in the microscopic realm.

11. Therefore, there is no connection to human consciousness in QM experiments.  Quantum mechanical experiments always provide consistent results that are not subject to or even related to the “conscious state” of the observer.

  • The need for a “personal” or subjective conscious mind is not even needed. A computer program may randomly decide when to initiate/terminate a measurement and get the same result.
Quantum Phenomena May Be “Weird,” but Nothing to Do with the Mind

Quantum phenomena, just like some phenomena in relativity, seem “unusual” to us since they were uncovered only in 1900 and are not of common occurrence. But they all involve the behavior of inert matter at a small scale (quantum phenomena), and speeds approaching the speed of light (relativity). This unusual behavior has nothing to do with human consciousness; that is how Nature works in the microscopic realm.

1. Two issues need to be separated:

(i) Do quantum phenomena display characteristics that are very different from phenomena displayed by classical (Newtonian) systems?

(ii) Do quantum phenomena provide any evidence that they are related to mental phenomena (i.e., are they affected by the particular state of mind of the experimenter?).

2. The answer to (i) above is unequivocal “yes.” The experiments discussed below have characteristics that are alien to the phenomena displayed by Newtonian or classical systems.

  • However, QM is not alone in that respect. The two theories of relativity also apply to phenomena that are not compatible with classical phenomena: time dilation and length contraction are prominent examples.

3. In both relativity and QM, the observer’s mental state does not play a role.

  • For example, relativity predicts that if a person takes off in a rocket, travels at speeds close to the speed of light for an extended time, and returns, he will find that those on Earth have aged much more than him. That is called time dilation.
  • However, if two people travel at similar speeds for a specific time and come back, the time dilation experienced by both will be the same.
  • In the same way, if two different people conduct any of those “weird”  QM experiments, they will get the same result.

4. In both cases of QM and relativity, the results may be “weird” by classical standards.  However, there is no involvement of the “consciousness of the observer.” The “apparent weirdness” in QM goes away smoothly as the Plank’s constant (h) becomes negligibly tiny (and in relativity when the speed is low).

  • There is no “mind effect” or “observer effect” in that the observer’s subjectivity affects the results of either type of experiment. There are no subjective decisions to be made during an experiment.
  • By definition, the result of an experiment is not reproducible unless an experimenter is genuinely objective,

5. In other words, all quantum phenomena and those explained by the relativity are objective, just like classical phenomena.

  • On the other hand, mind phenomena CAN BE subjective. As discussed earlier, when describing the physical properties of matter, two people can be objective, i.e., they report the same length, weight, etc.. for the object. But their PERCEPTION of a given person X, food or music, etc.., could be very different. Those are subjective.
  • For example, consider two people with opposing political views (A and B). Each runs into a politician C with views compatible with A’s. Person A will be happy to meet C and may go up to C, shake his hands, and talk to him enthusiastically. On the other hand, Person B will automatically have irritable thoughts about C and is likely to avoid C.
  • What properties of neurons in A and B could lead to such a vast difference in feelings and intentions (consciousness) upon seeing the same person?
  • Such subjective mental states do not play a role in carrying out experiments, whether quantum or classical. But they do play critical roles in making decisions in everyday life.

6. Therefore, those two issues need to be handled separately. Quantum phenomena have characteristics that are very different from classical phenomena, but both quantum and classical phenomena are objective. There is no evidence of quantum phenomena having anything to do with subjective consciousness.

  • The crucial distinction that we need to realize here is that the phrase “non-deterministic” as applied to such QM experiments is incorrect. For example, some measurements may not provide the exact location of a particle.  There could be many possible locations for that particle, but they can all be accurately predicted with associated probabilities.
  • Those experiments have no “intrinsic subjectivity” other than the indeterminacy depicted by the Heisenberg uncertainty principle. The same investigation conducted under the same conditions will yield the same result. It does not matter who experiments, i.e., the experimenter’s consciousness does not play a role.

Any questions on these QM posts can be discussed at the discussion forum: “Quantum Mechanics – A New Interpretation.”


Bell, J. S. (1987), Speakable and Unspeakable in Quantum Mechanics, Cambridge University Press.

Bohm, D. (1952), A suggested interpretation of the quantum theory in terms of “hidden” variables, I and II, Physical Review, vol. 85, pp. 166-179, and pp. 180-193.

de Gosson, M. A., and Hiley, B. J., (2011), Imprints of the quantum world in classical mechanics, Found. Phys., vol. 41, pp. 1415-1436.

Durr, D., Goldstein, S., and Zanghi, N. (1992), Quantum equilibrium and the origin of absolute uncertainty, Journal of Statistical Physics, vol. 67, pp. 843-907.

Kocsis, S. et al., (2011),  Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer, Science,  vol. 332, pp. 1170-1173.

Feynman, R. P. (1948), Space-time approach to non-relativistic quantum mechanics, Review of Modern Physics, vol. 20, pp. 367-387.

Field, J. H. (2011), Derivation of the Schrōdinger equation from the Hamilton-Jacobi equation in Feynman’s path integral formulation of quantum mechanics, European Journal of Physics, vol. 32, pp. 63-87.

Hanc, J., Tuleja, S., Hancova, M., (2003), Simple derivation of Newtonian mechanics from the principle of least action, American Journal of Physics, vol. 71, pp. 386-391.

Schleich, W. P., Greenberger, D. M., Kobe, D. H., and Scully, M. O. (2013a), Schrōdinger equation revisited, PNAS, vol. 110, pp. 5374-5379.

Schleich, W. P., Freyberger, M., Zubairy, M. S. (2013b), Reconstruction of Bohm trajectories and wave functions from interferometric measurements, Physical Review A, vol. 87, 014102.

Taylor, E. F., (2003), A call to action, American Journal of Physics, vol. 71, pp. 423-425.

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