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By Dan Hooper, Ph.D., University of Chicago Einstein found it difficult to accept any version of quantum mechanics, in which the universe was probabilistic in nature. (Image: Fankies/Shutterstock)What Is the Copenhagen Interpretation?

The Copenhagen interpretation of quantum mechanics claims that particles behave like waves. These particle-waves are each described by their wave function. The shape of a given particles wave function represents the probability that it will be found at different locations, or with different velocities. The original view of quantum mechanics was that quantum particles simultaneously exist in multiple locations at once, and have multiple velocities. According to the Copenhagen interpretation, whenever a particle is observed its wave function collapses, and the measured quantity takes on a single measured value.

So, prior to any measurement or observation, an electron is simultaneously in locations A and B, where A and B are the two places the wave function peaks sharply. An act of observation causes its wave function to collapse, and its location takes on a single valueeither A or B.

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So, how did scientific consensus form around the Copenhagen interpretation? To begin with, it must be mentioned that quantum mechanics was a rapidly evolving theory between the mid-1920s and late-1920s. So, opinions changed within a matter of months. Its also important to remember that during this period the scientists, who were located across Europe, communicated with each other primarily through letters and publications in scientific journals.

Scientific conferences presented them with an opportunity for in-depth in-person interaction, unlike any other during that time. In fact, scientific conferences played an important role in the development of quantum mechanics. The most important or most influential was the Fifth Solvay Conference on Physics, which was held in Brussels in October 1927.

Almost every single major contributor to the development of quantum mechanics attended this conference, including Albert Einstein, Erwin Schrdinger, Max Born, Niels Bohr, Louis de Broglie, Paul Dirac, and Werner Heisenberg. Also in attendance were Wolfgang Pauli, Marie Curie, and Max Planck. Its worth noting that of the 29 physicists in attendance, 17 had already been awarded a Nobel Prize or would eventually be awarded one.

At the Fifth Solvay Conference, it became clear that a consensus had started to form around the Copenhagen interpretation. Most of those in attendance seemed to have accepted the probabilistic nature of the new theory.

In addition, there was a general acceptance that the Copenhagen interpretation presented the true picture of nature and not a view that would eventually be explained away with a better understanding of the problem. For example, Born, Heisenberg, and Bohr were each fully aware that the universe as described by quantum mechanics was fundamentally probabilistic.

They acknowledged that theres a chance determinism could be restored in some future revision of the theory, but thought the chance was minuscule and unlikely. They were prepared to accept the lack of determinism in the subatomic world.

This is a transcript from the video series What Einstein Got Wrong. Watch it now, on The Great Courses Plus.

A number of respected physicists, including the likes of Bohr, Heisenberg, and Born, had an inkling that the quantum revolution was drawing to an end. They believed their working theory was complete, and that there would be no need for any new elements.

Despite the consensus around the Copenhagen interpretation, Einstein found it difficult to accept any version of quantum mechanics, in which the universe was probabilistic in nature.

To be fair to Einstein, he wasnt exactly arguing that the new theory of quantum mechanics was incorrect. The Schrdinger equation and the other equations of this theory described the phenomena very well, and the successes of quantum mechanics were entirely undeniable. So instead of claiming that quantum mechanics was incorrect, Einstein was arguing that it was somehow incomplete. He was claiming that big pieces of the theory were somehow still missing.

Lets reconsider the example of the electron that is described by a wave function which extends across locations A and B. According to the Copenhagen interpretation, the electron exists in both of these locations simultaneously, but Einstein was skeptical of this conclusion. Its possible that he thought the electron was, in fact, in only one of these two locations at a given time. He might have reckoned that the Schrdinger equation simply failed to identify which of these two locations the electron was present in. If that were true, then the apparent indeterminism of quantum mechanics might just be an illusion.

Einstein imagined there could be another more complete equation that would make it possible to calculate the location of the electron at a given time, without any probabilistic results. Essentially, he was searching for a way to make sense of quantum mechanics that wasnt only deterministic, but in which the properties of each particle or object was always well defined.

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With the objective of developing this new complete equation in mind, Einstein began work in early 1927. He started working toward developing a version of quantum mechanics that he hoped could explain all of the observed phenomena of quantum mechanics, while still allowing the laws of nature to be strictly deterministic.

The class of theories he developed and advocated were known as hidden variable theories. According to these theories, the wave function of a particle, as used in the Schrdinger equation, doesnt tell us everything about that particle. These theories claimed that the wave function was, in effect, an incomplete or partial description of the particle.

Einstein hypothesized other variables that were missing from the wave function, in the hope that it would eliminate the need for any indeterminism in the theory. At the Solvay conference, Einstein argued vigorously that the Copenhagen interpretation was fatally flawed, and that a more complete theory was needed. But despite these arguments, he wasnt able to convince many of his colleagues.

In a series of informal but public discussions with Bohr, Einstein raised what he believed to be a series of major flaws with the Copenhagen interpretation. However, Bohr responded effectively to each of Einsteins criticisms. In each case, Bohr found holes in Einsteins arguments, and successfully defended the new consensus view. By the end of this series of discussions, it was clear to most of the scientists in attendance that Bohr had bested Einstein in these debates.

Einstein remained undeterred by his failure at the Solvay Conference to demonstrate any fatal flaws in the Copenhagen interpretation. In the years that followed, he continued to search for a more complete version of quantum mechanics that he hoped would restore determinism to the subatomic world.

The majority of the current generation of quantum physicists still consider the Copenhagen interpretation to be accurate. The Copenhagen interpretation was first proposed by Danish physicist Niels Bohr, and this interpretation was subsequently theoretically proved by the thought experiment known as Schrdingers Cat. In recent years, the Copenhagen interpretation has encountered opposition from the many-worlds interpretation proposed by American physicist Hugh Everett.

Wave functions are mathematical descriptions of the wave properties of particles. If the value of the wave function of a particle for a particular location is high, then the probability of the particle being present at that location at that given time is high.

A total of 29 eminent physicists attended the Fifth Solvay Conference in 1927, including Albert Einstein, Erwin Schrdinger, Max Born, Niels Bohr, Louis de Broglie, Paul Dirac, Werner Heisenberg, Wolfgang Pauli, Marie Curie, and Max Planck, among others.

Bohr believed that the quantum universe was fundamentally probabilistic in nature, whereas Einstein was of the belief that determinism lay at the foundation of the quantum universe. This fundamental disagreement led to a series of public discourses between these two eminent physicists, which are known as the Bohr-Einstein debates.

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