Heisenberg, Bohr and Einstein, clash of giants – 02/27/2024 – Marcelo Viana

In October 1932, Albert Einstein accepted a position at the newly created Princeton Institute for Advanced Study in the US state of New Jersey. The initial plan was to spend half the year in the United States and the other half in Berlin. But in December of that year he left Germany and never returned.

Installed in Princeton, he began looking for colleagues to work with and his attention was attracted by the young Boris Podolsky, a Russian, and Nathan Rose, an American. Soon they began to prepare the strongest attack that quantum theory had ever suffered.

Quantum mechanics had come into conflict with well-established ideas in science. The uncertainty principle, formulated in 1927 by the German Werner Heisenberg (1901–1976), states that quantities such as the position and momentum (mass x velocity) of a subatomic particle cannot both be known precisely. The problem is that to really know the position of an electron, say, it is necessary to illuminate it with very energetic photons of light which, when colliding with the electron, modify its momentum in a way that cannot be predicted.

Heisenberg’s discovery killed the principle of determinism formulated by the French mathematician and physicist Pierre-Simon de Laplace (1749–1827) in 1814 and which had guided science until then. According to Laplace, if we knew the current state (position and movement) of all objects, the future of the universe would be completely determined by the laws of physics. Now the uncertainty principle states that the premise of determinism is meaningless, as complete knowledge of the current state of the universe is beyond our reach.

The question was how to interpret this conflict between the two theories. Simplifying things a little, the so-called Copenhagen school, led by the great Danish physicist and thinker Niels Bohr (1885–1962), argued that quantum mechanics contains everything there is to know about the universe, and that the uncertainty principle is a absolute limitation to what we can know.

Others believed that this principle only expresses a limitation of quantum mechanics itself, and that it was urgent to find a more complete theory for subatomic physics. Among them was none other than Einstein, and he was determined to prove himself right.

For the Copenhagen school, the attack launched by Einstein, Podolsky and Rose was a “lightning that fell from the sky”. But in the end Bohr and Heisenberg prevailed. With the help of mathematics. It’s for next week.

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