The Copenhagen Interpretation was primarily the product of Neils Bohr and Werner Heisenberg, who were strongly supported by Max Born, Wolfgang Pauli and John von Neumann. Among those opposed to the Copenhagen Interpretation have been Albert Einstein, Erwin Schroedinger, Louis de Broglie, Max Planck, David Bohm, Alfred Landé, Karl Popper and Bertrand Russell. While those supporting the Copenhagen Interpretation constitute a "school" and an "orthodoxy", those opposed to it have widely divergent views. But the latter have been uniformly vilified as too simple-minded or too "old fashioned" to understand such "modern" ideas as acausality and positivism. Soviet physicists also opposed the Copenhagen Interpretation, but on the grounds that it is an "idealism", to be contrasted to a "dialectical materialist" view of reality.

The essential controversial features of the Copenhagen Interpretation are (1) the Uncertainty Principle (also called the Indeterminacy Principle) of Heisenberg and (2) the Principle of Complementarity of Bohr. With the passage of time the Copenhagen Interpretation has been more specifically identified with a concept known as "the collapse of the wave function" (also called "the reduction of the wave packet") as formulated by John von Neumann. These ideas will be examined in turn.

Heisenberg's Uncertainty Principle asserts that the product of position and momentum uncertainty for any particle will be more than a certain multiple of Planck's constant. Since momentum is the product of mass and velocity, and since the mass in an experiment is usually that of an electron, this can equivalently be described by saying that the more precisely the position of a electron is known, the less precisely its velocity is known, and vice versa. Bohr and Heisenberg would illustrate this point with the following thought experiment: Macroscopically, the position of an object can be judged by looking at it, ie, by observing the photons which have come from a light source, bounced off the object and arrived at our eyes. The position of a car is not much altered by the photons bouncing off it. But if photons are bounced off an electron to determine its position, its velocity will be altered and uncertain. High energy photons with shorter wavelength have less diffraction -- determine electron position more precisely -- but in doing so they alter the electron's velocity more radically.

What seemingly began as a simple measurement problem under specific circumstances became gratuitously generalized into a metaphysical assertion. Although the original thought experiment was a causal demonstration which assumed an underlying deterministic position and velocity (momentum), the Copenhagen Interpretation began to deny that the electron has a definite position or velocity. From the positivist idea that it is meaningless to discuss the existence of something which cannot be measured (position and velocity, within certain limits) came the idea that the electron is an unreal, causeless "possibility" which only achieves actuality upon observation. Thus positivism became twisted into subjectivism (some say "solipsism") and the idea that the observer somehow creates reality by the act of observation.

Obviously a physician who attempts to measure a patient's blood pressure is faced with a problem. What she measures is not simply blood pressure, but the blood pressure of a person having his blood pressure taken by a physician. A physician would be wiser to look for indirect means of determining true blood pressure than to assert that her "observer-created" reality is all the reality that exists -- and that the patient has no blood pressure until she tries to measure it. Similarly, means other than bombardment of photons might be possible for determining the position and velocity of an electron.

A photographic plate containing the track of an electron can be used to determine position and velocity within less than the uncertainty limit. In a rather questionable bit of rationalization, Heisenberg denied the evidence of the photographic plate by asserting that his Uncertainty Principle is only relevant to predicting the future, and that "this knowledge of the past is of a purely speculative character", adding "It is a matter of personal belief whether such a calculation concerning the past history of the electron can be ascribed any physical reality or not." For some reason, most physicists chose a personal belief which denied physical reality and conformed to the Copenhagen Interpretation.

Bohr's Principle of Complementarity arose out of the difficulty physicists were having in their attempts to determine whether quantum phenomena such as light are particles or waves. But complementarity is not a solution. Instead, it is an assertion that no solution exists and that physicists know all that can be known about the question. So it would be well to examine specifically the context from which complementarity arose.

Because of the straight lines (rays) evident in light propagation, Newton believed that light is composed of particles. But diffraction effects and other evidence increasingly led others to the belief that light is a wave. By the time of Maxwell's Equations (two centuries after Newton's work), light was understood to be purely wavelike -- simply a small part of an electromagnetic spectrum ranging from the very long radio waves to microwaves to light and to the very short X-rays and gamma waves. But then experimentation with the photoelectric effect led Einstein to the conclusion that light is "quantized" in the form of photons. In fact, the shorter the electromagnetic wavelength (and hence, the more energetic the wave), the more particle-like an electromagnetic wave appears. Compton showed that when a beam of X-rays of sharply defined wavelength falls on a graphite target, the wavelength of the scattered radiation is a function of the angle of scattering -- indicating that energy is lost during "recoil" of a photon colliding with an electron.

Erwin Schroedinger's Equation is a differential equation of a particle of given mass subject to forces varying in time & space. The solutions of Schroedinger's Equation are de Broglie matter waves (psi) associated with the motion of the particle. Comparisions can be made between the electromagnetic-wave equation and Schroedinger's matter-wave equation. Schroedinger's Equation contains a complex (imaginary, i) term, implying that the de Broglie wave functions are mathematical entities which cannot be imputed to have physical existence. Some physicists regard this fact as a blessing insofar as it prevents anyone from asking what is "waving" -- the question that led to the fallacy of the existence of ether as an explanation of what electromagnetic waves are "waving".

Einstein suggested that the square of the amplitude of the electromagnetic wave (epsilon²) could be interpreted as the average number of photons per unit volume (wave intensity, energy density, photon density). Similarly, Max Born proposed something like a squared de Broglie matter wave (the complex conjugate) gives a real, non-negative quantity that could be interpreted as a measure of the probability of finding a particle at a given time and place.

An equation which describes particle locations in terms of probabilities does not provide a visual model of the quantum micro world. Bohr's Principle of Complementarity held that we can never build a visual model of the microworld based on analogues with objects in the macroworld. The closest we can come to a model is to regard the two mutually exclusive classical concepts of wave and particle as "complementary" aspects of quantum reality. The idea that many (or most) microworld phenomena cannot be modeled on macroworld phenomena is not without merit. For example, there is probably nothing in our macroscopic world which would serve as a good model for the interactions between protons and neutrons in a nucleus. Nonetheless, it is premature and arrogant to suggest that physicists can never find a more useful model than Bohr's Principle of Complementarity. (Schroedinger called the Principle of Complementarity "an extravaganza dictated by despair over a grave crisis".)

When an electron is actually observed, what is seen is a particle. But where the electron is likely to be observed is described by a wave function (the psi of Schroedinger's wave equation, "squared" -- complex conjugate). The higher the amplitude of the "squared" wave function at any particular point, the more likely it is that the particle will be found at that point. The Copenhagen Interpretation seems to regard the electron as somehow diffused throughout the wave function until it "collapses" into a point upon the act of observation. But why should the mere act of observation cause "the collapse of the wave function"?

For some, this interpretation of observation goes beyond the positivist idea that it is meaningless to describe what is not being observed -- into the idea that consciousness somehow controls a physical event. Would an amnesia victim who suddenly forgot what he had just observed cause a particle to "uncollapse" back into a wave function? Not likely. In fact, a "wave function" incident upon a photographic plate will "collapse" into a point (particle) whether or not the plate is examined immediately. The Copenhagen Interpretation regards the "collapse of the wave packet" as a fundamental, irreducible concept -- meaning one should not try to analyze the "mechanism" of the collapse. But the trajectory of an electron in a cloud chamber looks very much like that of a moving particle, despite Heisenberg's claim that it is senseless to speak of "the path of an electron". Does the electron "recollapse" at every point of condensation? Einstein and von Neumann declared that quantum theory is not appropriate to describe individual physical systems (particles), but is only relevant to large numbers of such systems ("ensembles"). All that ever "collapses" is our knowledge of the system, according to Einstein.

An experiment involving circular diffraction, and the famous two-slit experiment, illustrate the most perplexing behavior of subatomic particles/waves.  The Airy disk is deemed the consequence of the circular diffraction of light waves through the aperture. Yet if electrons (or photons) are fired individually, at one minute intervals, onto an aperture, after a period of weeks a photographic plate on the far side of the aperture will display an Airy disk. Moreover, the smaller the aperture, the larger the Airy disk, precisely as predicted by the uncertainty relation. Since the electrons have emerged through the aperture one-at-a-time, there can be no question of them interfering. Instead, the wave function describes the probability of finding an electron at a point in space. The Airy disk has dark rings where the most electrons have struck the photographic plate (the grainy character of the rings clearly indicates their composition of individual particles striking the plate).