A Darwinian Moment in Physics: Creation of Quantum Mechanics

In 1925 physics had its Darwinian moment-a change in perspective that was as consequential for the physical sciences as the theory of evolution by natural selection was for biology. That year German Physicist Werner Heisenberg wrote a paper that turned the ‘quantum theory’ of the early 1900s into the ‘quantum mechanics’ physicists are familiar with today. The title of the paper was ‘Quantum-theoretical re-interpretation of kinematic and mechanical relations.’ Heisenberg mentioned that the paper seeks to establish a basis for theoretical quantum mechanics founded exclusively upon relationships between quantities which in principle are observable. The United Nations has declared 2025 as the Internal Year of Quantum Science and Technology and celebrating the 100 years of quantum mechanics.

Heisenberg was awarded the Nobel Prize in Physics in 1932 "for the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of the allotropic forms of hydrogen".  The Swedish Nobel Committee summarized his work as below-

 “In Niels Bohr’s theory of the atom, electrons absorb and emit radiation of fixed wavelengths when jumping between fixed orbits around a nucleus. The theory provided a good description of the spectrum created by the hydrogen atom, but needed to be developed to suit more complicated atoms and molecules. In 1925, Werner Heisenberg formulated a type of quantum mechanics based on matrices. In 1927 he proposed the ‘uncertainty relation’, setting limits for how precisely the position and velocity of a particle can be simultaneously determined.”

So, we have received two ideas from Heisenberg: one is ‘matrix mechanics’ and another is ‘uncertainty principle.’ However, second concept derived from the first one. The story goes on like that the Heisenberg went to Helgoland, a small island in the North Sea, not far from Hamburg, in order to get relief from his hay fever. An idea struck in his head while he was walking around the Helgoland. The idea is like that: if we ignore completely the electron orbits and take only observable values into account, i.e., the totality of the oscillation frequencies and intensities of the light emitted by the atoms with the spectral lines measured in the spectrograph, then we would be able to address Neil Bohr’s electron’s jump in an atom. This leads to the development of matrix mechanics. Max Born and Pascual Jordan extended Heisenberg’s idea, pointing out the importance of matrix algebra for describing atomic energy transitions. In 1925 British physicist Dirac has also played a part. This leads to the idea that matrices rather than simple numbers were the correct language for describing the atom. Einstein mentioned this as Heisenberg’s ‘big quantum egg’.

The uncertainty principle suggests that ‘the more precisely you can specify the position of a particle, the less precisely you can specify its momentum, and vice versa’. There is various philosophical point of views about it. The realist position, like Einstein, uncertainty principle is ‘just an expression of the limits of what can be determined by measurement. Or in philosophers’ terms, the nature of uncertainty would be an epistemic one’. Epistemology is that part of philosophy that deals with what we can know and how we might come to know what we know. The alternative philosophical position is to assume that the uncertainty principle is not simply a statement about what we can know; it is a statement about the nature of things. From that point of view, Heisenberg’s uncertainty principle is a statement about how things are and what features they have. It’s a statement about what exists. A philosopher would call such a position about the nature of the uncertainty principle an ontological one. From that point of view, the electron would have neither a position that is better defined than the position uncertainty tells us, nor a speed that is better defined than its momentum uncertainty. That ontological position was held by Bohr. There is a great debate about whether the problem is ontological of epistemological.

Prof Konrad, in his book Einstein and Heisenberg, mentioned that Heisenberg’s discovery of the uncertainty principle has far-reaching consequences for the philosophy of nature and epistemology. It is worth mention here that Born’s statistical interpretation introduces a kind of indeterminacy into quantum mechanics, for even if we know everything the theory has to tell us about the particle, still we cannot predict with certainty the outcome of a simple experiment to measure its position. This indeterminacy has troubled physicists and philosophers alike. The consequences of the uncertainty principle have also been realized by other Physicists. For example, Stephen Hawking, in his book, A Brief History of Time, mentioned that ‘Heisenberg uncertainty principle is a fundamental, inescapable property of world.’ Hawking added that ‘The uncertainty principle had profound implications for the way in which we view the world.’
 
In his Chicago lecture in 1929, which was later published as a book, titled ‘The Physical Principle of the Quantum Theory’, Heisenberg maintains that ‘the experiments of physics and their results can be described in the language of daily life. Thus, if the physicist did not demand a theory to explain his results and could be content, say, with a description of the lines appearing on photographic plates, everything would be simple and there would be no need of an epistemological discussion. Difficulties arise only in the attempt to classify and synthesize the results, to establish the relation of cause and effect between them-in short to construct a theory.’ I would argue that this realization triggered Heisenberg to formulate in a quantitative way a generalizable mathematical relation, which is now known as ‘uncertainty principle.’

Above all, the ancient Greeks speculated that it might be air, fire or water. A century ago, physicists felt sure it was the atom. We believe that the deepest layer of reality is populated by a diverse cast of elementary particles, all governed by quantum theory and in 1925 this quantum theory changed into quantum mechanics by the discovery of Heisenberg’s matrix mechanics. In coming days classical computer that we use today in our day-to-day life will be replaced by quantum computer that rely on qubits instead of bits, which can perform multiple operation simultaneously. This is just one of the many examples how quantum mechanics can shape our day-to-day life.


Dr. Kanan Purkayastha is a
UK-based academic scholar 
who writes on science, 
philosophy and education.