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The Uncertainty Principle in Quantum Physics

Werner Heisenberg

Werner Heisenberg

Werner Heisenberg was often labeled as the wunderkind (wonder child) of physics in the 1920's. Some thought he looked like a farm boy, but he was anything but a country bumpkin. His father was an academic scholar on the faculty of Munich University. Werner's grandfather was the head of a prestigious gymnasium (secondary school). Werner and his older brother attended that school. After graduation Werner Heisenberg wanted to study mathematics at Munich University but his interview in the mathematics department there did not go well and he was not accepted. His father called upon his old friend, the physicist Arnold Sommerfeld, to accept Werner into the physics program. Sommerfeld was one of the top physicists in the world at that time and Werner was indeed fortunate to undertake his education at Sommerfeld's institution. In happened that Wolfgang Pauli was also studying there and Werner made his acquaintance.

Neils Bohr gave a series of lectures at Göttingen and Sommerfeld and a contingent of his students, including Heisenberg, journeyed there to attend those lectures. Bohr sought out Heisenberg and invited him to come to Copenhagen for a term. Sommerfeld however wanted Heisenberg to study first with Max Born at Göttingen. Heisenberg did so and a life long collaboration with Born commenced. After his term at Göttingen Heisenberg went back to complete a thesis for his doctorate. In the defense of his thesis Heisenberg did poorly in explaining some topics unrelated to his thesis and one examiner wanted to fail him, but Sommerfeld negotiated a compromise. Heisenberg was given his doctorate but with the lowest possible grade. In disgrace, Heisenberg fled to Göttingen. After completing some research on the anomalous Zeeman effect Heisenberg contacted Bohr to tell him of his work. Bohr invited him to visit his institute in Copenhagen for a few weeks. At Bohr's Institute initially Bohr was too busy t o spend any time with Heisenberg, but then Bohr took time out from his research to spend several days on a hiking tour with Heisenberg. After becoming acquainted with the depth of Heisenberg's talent Bohr invited him to stay at Bohr's institute for an extended period of time. The time was 1924 and Heisenberg was only 22 years old.

In 1925 Heisenberg returned to Göttingen and was disappointed with the lower intellectual level there compared to Copenhagen. Heisenberg, Born and Jordan went on to develop the Matrix Mechanics approach to quantum physics. For more on the drama of the developments of quantum physics in this period see Drama.

When Heisenberg was back in Copenhagen with Bohr he begam pondering the matter of beta ray (electron) tracks in a cloud chamber. Heisenberg did not use any notion of electron orbits in atoms because he considered them unobservable. The cloud chamber tracks might indicate that he was wrong about the electron orbits not being observable. He went out for a walk in the park even though it was the middle of the night. Cogitating on the fundamentals of physics he realized that it might not be possible, because of the physical interactions, to specify simultaneously the location and velocity of a particle like an electron. Back in his room he used the example of a microscope investigate the possibility of the simultaneous specification of location and velocity. It was then that he realized the limitation on the accuracy of such specifications. He called the concept the indeterminacy principle, but it subsequently became known as the uncertainty principle.

Heisenberg concluded that concepts like path, trajectory and orbit have no meaning at the quantum level. In late February of 1927 Heisenberg wrote a 14 page letter to Pauli describing his uncertainty principle and its basis. Pauli's reaction was favorable and Heisenberg turned the letter into a draft of an article in the early part of March. At the time, Bohr was away in Norway on a long vacation. After Bohr returned he read Heisenberg's article. To Heisenberg's surprise Bohr disagreed with Heisenberg's assessment of the source of the uncertainty. Heisenberg thought the uncertainty stemmed from the discontinuities of particle collisions; Bohr thought it was from the dual nature of particle-waves. They ended their discussion still in disagreement. A few days later they talked again. Bohr did not want Heisenberg to publish his article until he had rewritten it. Heisenberg did not want to change anything and at the rejection of his ideas by his revered mentor. Finally Heisenberg broke out in tears because of the pressure Bohr was putting on him. Bohr had formulated his own new concept while on his skiing vacation in Norway; one that he called complementarity. This was the notion that a particle and its wave were simply manifestation of some more fundamental entity.

Despite Bohr's imploring of him not to do so Heisenberg sent away his article for publication near the end of March in 1927. It appeared in print at the end of May as a 27 page article. He soon received offers of professorships and he accepted the one from Leipzig University. He left Copenhagen in June of 1927. A copy of Heisenberg's article had been sent to Albert Einstein but Einstein did not respond.

Heisenberg's Uncertainty Principle became one other key ideas of quantum physics. On the basis of of the fame Heisenberg gained as a result of his article, March 1927, “Über den anschulichen Inhalt der quantentheoretischen Kinematik und Mechanik” (“On the Perceptual Content of Quantum Theoretical Kinematics and Mechanics”), he was appointed to a professorship at Leipzig University in 1927. In 1933 he was awarded the Nobel Prize in Physics for 1932 and Schroedinger and Dirac shared the prize for 1933.

Here are some studies on the Uncertainty Principle. The earliest are at the bottom of the list and the latest at the top.

(To be continued.)

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