The Observer Effect in Quantum Mechanics

March 20, 2018

1. The “observer effect” — sometimes called the “measurement problem”– in quantum mechanics is defined as 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 start with the basics.
  •  Please make sure to read the previous post, “Will Quantum Mechanics Be Able to Explain Consciousness?”, including the section there 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 state. Therefore, it seems that at the time of the measurements all those multiple states should collapse to just one (the observed).

  • Since an observer is needed to make a measurement (and thus “cause a collapse”), it is called the “observer effect”.

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

  • There is no  “measurement problem” in the sense that the “personal” mind state of an observer does not play a role; anyone can initiate a measurement and get the same result. Furthermore, terminating a given experiment can also be achieved at random 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 not only by Einstein but 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 this description is completed by the specification of the actual positions of the particles by a “pilot wave” or a “guiding wave”. In Bohmian mechanics, particle trajectories can be traced in real time without the need for a “wave function collapse”.

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

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

7. All possible paths are naturally described by Bohmian mechanics. Each one can be assigned a probability and experimental outcomes have been verified to be in agreement with those probabilities.

  • So, the measurements are deterministic, in the sense that when a series of measurements is made, the outcome is compatible with the predictions. Those measurements are objective.
  • A detailed description of Bohmian mechanics can be found in (Durr, Goldstein, and Zanghi, 1992).

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 starting around the turn of the century. The total number of publications from 1992-1999 were 52. From 2000-2005, 2006-2011, and 2012-2017 had 134, 174, and 200 papers published respectively. Thus, even though it took time to gain traction, Bohmian mechanics seems to be attracting attention now.

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 originally 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. Other papers have described how the Schrōdinger’s equation can be derived from classical mechanics; see, (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 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 can randomly to decide when to initiate/terminate a measurement and get the same result.
Quantum Phenomena May Be “Weird” but Nothing to Do with Mind

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

1. There are two issues that need to be separated out:

(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 unequivocally “yes”. The experiments that we discuss below all display 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 are applicable to phenomena that are not compatible with classical phenomena: time dilation and length contraction are obvious examples.

3. In both relativity and QM, the mental state of the observer is NOT involved in any such “alien phenomena”.

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

4. In both cases of QM and relativity, the results  are “weird” by classical standards, but there is no involvement of the “consciousness of the observer”; and this apparent “weirdness” in QM goes away smoothly as the Plank’s constant (h) becomes negligibly small (and in relativity as the speed is reduced).

  • There is no “mind effect” or “observer effect” in the sense of the subjectivity of the observer affecting the results of either type of experiment; there are no subjective decisions to be made during an experiment.
  • By definition, unless an experimenter is truly objective, the results of any experiment cannot be reproduced.

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

  • On the other hand, mind phenomena CAN BE subjective. As discussed earlier, when describing 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, or a given food or music, etc , could be very different. Those are subjective.
  • For example, two people with opposing political views (A and B) may encounter a politician C on the street who has views compatible with those of A. 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 huge 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 the subjective consciousness of a human.

  • The crucial distinction that we need to realize here is that the phrase “non-deterministic” as applied to such QM experiments is not correct. Some measurements may not provide the exact location of a particle, for example. There could be many possible locations for that particle, but they all can be predicted with associated probabilities accurately.
  • There is no “intrinsic subjectivity” in those experiments, other than the indeterminacy depicted by the Heisenberg uncertainty principle. If the same experiment is conducted under the same conditions, the same result is obtained regardless of who does the experiment; no connection to consciousness.

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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