Revolution in physics

Dirk Lützenkirchen-Hecht on Werner Heisenberg and quantum mechanics

Mr Lützenkirchen-Hecht, quantum mechanics was created in 1925 by Werner Heisenberg. Who was this exceptional scientist?

Dirk Lützenkirchen-Hecht : A small contradiction right at the beginning, strictly speaking, Max Planck was of course the founder of quantum physics. He basically introduced the term quantum back in 1900 and was awarded the Nobel Prize for Physics in 1918. But people naturally associate quantum physics primarily with Werner Heisenberg. Heisenberg grew up in a family of academics. His great-grandfather was active in the arts and helped prepare the 'golden ratio', so to speak. His parents were ultimately interdisciplinary. Music, art, history. Literature, somehow everything was there. His father was a professor of Byzantine studies in Munich, so he was born into academia, so to speak. At the beginning, he didn't really know what he should do. He was very musical, could also play the piano very well and always cultivated this during his teaching career at university. But he was also interested in the natural sciences and found mathematical methods particularly exciting. He was very interested in physical processes and phenomena and wanted to know how maths played a part in them, so he ultimately decided to study physics. The rigorous mathematical concepts fascinated him, so I would call him the founding father of theoretical physics, or mathematical physics.

What does quantum mechanics mean?

Dirk Lützenkirchen-Hecht: Ultimately, it is the limit of what we know as classical physics. Let me give you an example: If I now look at a pot of water, the water goes to the bottom, so to speak, and that is what we as physicists call the ground state, the state of lowest energy. If I now make this pot of water smaller and smaller, then you can already see that the water rises, so the energy is locked in. And quantum mechanics goes one step further. If I make the space to which I confine the matter even smaller, then very strange effects occur. This is what Heisenberg was the first to realise. For example - and we know this too - light falls through a door and I have a shadow cast to the side of the door because the light shines straight through. If I make the door so small that it comes into the order of magnitude of the wavelength of the light, then new effects occur, because then the light behind the gap becomes wider again, which is actually not possible from classical experience. This is what can be described by Heisenberg's uncertainty principle. So if you confine matter and light to small spaces, then a new physics emerges, new phenomena that can basically no longer be explained using classical methods.

Why was atomic physics suddenly no longer able to describe the behaviour of complicated atoms and molecules in the early 1920s?

Dirk Lützenkirchen-Hecht: On the one hand, experimental physics had progressed and certain things could simply be measured more accurately because measuring instruments were available, such as spectrometers to measure the colour spectra of gases and molecules more precisely. And then discrepancies were found. This can be seen most clearly in the description of the solar spectrum. The spectral lines could not be explained using classical methods and could only be understood using the models of Heisenberg and other colleagues. Nils Bohr, for example, ultimately took the data from the experimental physicists, used it for combinatorics and then created a qualitative model, but Heisenberg was the one who was able to explain it comprehensively.

These imperfections in atomic theory disturbed the scientist Werner Heisenberg. In 1925, he completed his fundamental work "On the quantum-theoretical reinterpretation of kinematic and mechanical relationships". With these results, he brought about a quantum mechanics free of contradictions. What was the groundbreaking aspect of this?

Dirk Lützenkirchen-Hecht : The really ground-breaking thing is that, in principle, it is not possible to measure several properties at the same time. However, this is not a measurement error or an inaccurate experiment, it is simply and fundamentally impossible. Heisenberg was the first to show that it is basically impossible to measure several properties at the same time. Place and momentum, time and energy are complementary quantities.

Together with Niels Bohr, Max Born and Pascual Jordan, he developed the uncertainty principle and put an end to the principles of "classical physics". There must have been some protest from within your own ranks, right?

Dirk Lützenkirchen-Hecht: Yes, of course. You also have to see it in the context of the time. These ideas were revolutionary and there were a lot of people who strongly opposed them. A few years later, the Nazis came to power, and then there was also "German physics", people like Johannes Stark, who had discovered the Stark effect named after him and railed against Heisenberg. The scientists around Heisenberg were also known as the 'young savages'. Heisenberg received his doctorate at the age of less than 25 and was already a professor at 28. The opposition ended when there were sufficient experimental findings and the accuracy of his findings was recognised.

In 1933, he was awarded the Nobel Prize in Physics for his work. However, Heisenberg was also a successful researcher in other fields and was involved in nuclear physics and reactor physics. His involvement in the development of the atomic bomb under Nazi rule is highly controversial. What did he do in the so-called uranium project?

Dirk Lützenkirchen-Hecht : There was a desire for modern weapons in Germany. Nuclear fission had been discovered by Otto Hahn in 1938, the first nuclear reactor had already been made to work in 1942 and then there was this huge amount of energy that could be released and the endeavour to build a bomb from it. Heisenberg was a theorist and was also interested in nuclear physics, because these are all phenomena that take place on small scales. He was predestined to calculate all this and say what had to be done to make such a bomb. According to his expertise, however, the amount of uranium that would have been needed could not have been procured so quickly, so the Nazis' interest in it waned. It is said that Heisenberg met his colleague Niels Bohr in Copenhagen and told him that Germany was withdrawing from this technology. Bohr, however, interpreted this differently and thought that the Germans were working flat out on the bomb, thereby triggering the construction of the atomic bomb by the Americans. Heisenberg was already very active in nuclear physics and also said himself that it would have been possible to build an atomic bomb with plutonium. Surely 15 kilos would have been enough. It is not known whether he deliberately concealed this knowledge from the Nazis. In any case, he was also interned after the war along with everyone involved in the uranium project and was very shocked when he learnt about the dropping and consequences of the atomic bombs in Hiroshima and Nagasaki. He later campaigned strongly against nuclear weapons.

Heisenberg is also regarded as a pioneer of the so-called chaos theory of the 1970s. What is it all about?

Dirk Lützenkirchen-Hecht : That's where he went back to his beginnings. He always had a more mathematically driven interest and dealt with turbulent flow as part of his dissertation. We all know this from everyday experience, such a streamlined car, the air flows nice and evenly, it creates little turbulence, little chaos. With the same power, I can drive much faster in such a streamlined car than in a VW Beetle. Heisenberg dealt with this topic mathematically. If you have a turbulent flow, then it moves chaotically. There are various turbulences etc., which can also be observed in liquids, for example when water flows in a bathtub. Mathematically, these are very simple equations with just a few formula letters, but if you look at the solutions, they become very complicated and complex equations. Heisenberg was interested in this all his life, but again particularly in the early 1970s. In addition to the aforementioned turbulence, this chaos theory can also be applied and calculated to traffic jams or chaos in the economy and then predictions can be made. The origins of this can be found as early as 1924 in Heisenberg's doctoral thesis. Even what we now call neural networks or artificial intelligence ultimately resulted from his activities. He was a bit of a universal genius.

How do we remember him today?

Dirk Lützenkirchen-Hecht : The main thing that remains is his uncertainty principle. But he was also one of the founding fathers of CERN (CERN, the European Organisation for Nuclear Research, is a large-scale research facility near Geneva, editor's note), he designed the treaties that still apply today. He founded and headed Max Planck Institutes in Göttingen and Munich, and he was on the Research Council and advised the German government in the 1950s and 1960s.

Unfortunately, all this has been somewhat lost behind his uncertainty principle.

Uwe Blass

Apl. Prof. Dr Dirk Lützenkirchen-Hecht is a member of the Institute of Condensed Matter - X-ray Physics in the School of Mathematics and Natural Sciences at the University of Wuppertal.