Heisenberg, standing front left, next to P.A.M. Dirac, in front of A.H. Compton, University of Chicago, 1929.

Quantum Mechanics: 1925-1927
Triumph of the Copenhagen Interpretation

We regard quantum mechanics as a complete theory for which the fundamental physical and mathematical hypotheses are no longer susceptible of modification.

--Heisenberg and Max Born, paper delivered to Solvay Congress of 1927

Niels Bohr's Institute for Theoretical Physics at the University of Copenhagen in the 1920s

Heisenberg formulated the uncertainty principle in February 1927 while employed as a lecturer in Bohr's Institute for Theoretical Physics at the University of Copenhagen.  Bohr, who had been on a skiing vacation, returned to the institute to find Heisenberg's paper already in draft. Forwarding the paper to Einstein at Heisenberg's request, Bohr complained to Einstein that Heisenberg's approach was too narrow and his gamma-ray microscope was flawed, although the result was correct. For Bohr, the uncertainty relations arose not merely from the quantum equations and the use of particles and discontinuity. Waves and particles had to be taken equally into account, and the scattering of light waves by the electron was also crucial. When Heisenberg corrected his thought experiment, it only confirmed the results.

Niels Bohr, who articulated the Copenhagen interpretation of quantum mechanics and mentored a generation of quantum physicists

In Bohr's words, the wave and particle pictures, or the visual and causal representations, are "complementary" to each other. That is, they are mutually exclusive, yet jointly essential for a complete description of quantum events. Obviously in an experiment in the everyday world an object cannot be both a wave and a particle at the same time; it must be either one or the other, depending upon the situation. In later refinements of this interpretation the wave function of the unobserved object is a mixture of  both the wave and particle pictures until the experimenter chooses what to observe in a given experiment. (Remember that, according to Heisenberg, the path of an object first comes into existence when we observe it.) By choosing either the wave or the particle picture, the experimenter disturbs untouched nature. Such favoritism unleashes a limitation in what one can learn about nature "as it really is." This limitation is expressed by Heisenberg's uncertainty relations, which, for Bohr, were related to what he was now calling "complementarity." Complementarity, uncertainty, and the statistical interpretation of Schrödinger's wave function were all related. Together they formed a logical interpretation of the physical meaning of quantum mechanics known as the "Copenhagen interpretation."

Since my talks with Bohr often continued till long after midnight and did not produce a satisfactory conclusion, ...both of us became utterly exhausted and rather tense.

--Heisenberg, recollection

Bohr in intense discussion with Heisenberg and Pauli (L to R) in Copenhagen

Heisenberg vehemently objected at first to Bohr's views. Insisting on the primary use of particles and discontinuity, he refused Bohr's suggestion that he withdraw his paper, which was already in press. He did, however, append a paragraph alerting readers to Bohr's views and admitting the error regarding the resolution of the microscope. The battle with Bohr grew so intense in the early months of 1927 that Heisenberg reportedly burst into tears at one point, and even managed to wound Bohr with his sharp remarks. Obviously, there was much at stake for the 25-year-old.
By the fall of 1927, matters had completely changed. Heisenberg's job situation was settled upon his appointment to the University of Leipzig. And Bohr presented to a conference at Lake Como, Italy, his complementarity argument. A month later, in October 1927, Born and Heisenberg, speaking to the Solvay physics conference in Brussels, Belgium, went so far as to declare quantum mechanics to be complete and irrevocable.

Solvay conference, 1927.

The theory yields a lot, but it hardly brings us any closer to the secret of the Old One. In any case I am convinced that He does not throw dice.

--Einstein, writing to Max Born, 4 December 1926.

Not everyone agreed with the new interpretation, or with Born and Heisenberg's statement about future work. Einstein and Schrödinger were among the most notable dissenters. Until the ends of their lives they never fully accepted the Copenhagen doctrine. Einstein was dissatisfied with the reliance upon probabilities. But even more fundamentally,  he believed that nature exists independently of the experimenter, and the motions of particles are precisely determined. It is the job of the physicist to uncover the laws of nature that govern these motions, which, in the end, will not require statistical theories. The fact that quantum mechanics did seem consistent only with statistical results and could not fully describe every motion was for Einstein an indication that quantum mechanics was still incomplete. Alternative interpretations have since been proposed and are now under serious consideration.

Bohr and Einstein during one of their discussions about quantum physics

Visit our Einstein exhibit for the Bohr-Einstein debates.

The objections of Einstein and others notwithstanding, Bohr, Heisenberg and their colleagues managed to ensure the acceptance of their interpretation by the majority of physicists at that time. They did this both by presenting the new interpretation on lecture trips around the world and by demonstrating that it worked. The successes of the theory naturally attracted many of the best students to institutes such as Heisenberg's, some coming from as far away as America, India, and Japan. These bright students, nurtured by the Copenhagen doctrine and educated into the new quantum mechanics, formed a new and dominant generation of physicists. Those in Germany and Central Europe carried the new ideas with them as they dispersed around the world during the 1930s and 1940s in the wake of Hitler's rise to power in Germany.


Professor in Leipzig


The Uncertainty Principle

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About this Exhibit

1998 - American Institute of Physics and David Cassidy ()