Niels Bohr is known as one of the pioneers of a special area of physics, namely quantum mechanics. The tireless Dane possessed unprecedented perseverance and a very sharp and lively mind, both scientifically and politically. Kennislink did a 'fictional interview' with the influential thinker from Copenhagen.
Niels Bohr was the first with a great insight into the smallest particles:he figured out that to describe the smallest building blocks of nature – particles that form the internal structure of atoms – completely different laws of nature should apply than for the well-known, 'big' world around us.
In addition to his pioneering scientific work, he was an avid sportsman and, especially in the fall of his life, committed himself to a world without nuclear weapons.
Mr Bohr, you have been called one of the most influential scientists of the twentieth century. When did you first come into contact with the world of science? “I owe that to my father's weekly chat club, who was a professor of medicine at the University of Copenhagen.
Every Friday evening, all kinds of scientist friends came to visit us, and there was a high-level discussion about all kinds of aspects of science. My brother Harald and I were allowed to be there. Not that we understood everything, but it was mighty interesting! Moreover, we were also raised with all kinds of progressive ideas about politics, for which I am still grateful to my father."
Your brother Harald was a year and a half younger than you but graduated faster (in mathematics) and obtained his PhD earlier than you. But after your promotion, your career suddenly took off, how did that happen?" “In 1912 I met the brilliant experimenter Ernest Rutherford. After experiments with alpha particles, he proposed that an atom should consist of a positively charged nucleus with the electrons circling around it. A tiny solar system, actually. But with that model was a big problem. A particle moving around a nucleus would almost immediately lose all its energy and collide with the nucleus. The atom as Rutherford proposed was unstable. In addition, the model did not explain why each element has a unique light spectrum. I managed to solve that problem two years later!”
In your thesis Studies on the electron theory of metals You wrote in 1911 that you no longer considered classical physics suitable at the atomic level. Were you looking for a solution in a completely different direction? "Indeed. Picture it this way:electrons cannot be in every possible orbit around the atomic nucleus, but only at fixed energy levels. When an electron absorbs energy, it can temporarily move to a higher energy level. When it bounces back, it emits energy in the form of a light particle (photon). This transition is called a quantum leap. It was impossible to verify my theory through experiments, but it explained the unique light spectra of atoms and explained why atoms were indeed stable.”
A theory that was not supported by experiments? That was daring at the beginning of the twentieth century… "Secure! And I can talk about it for hours. I had no experimental evidence, but my theory could explain all kinds of phenomena that were otherwise inexplicable. My solution worked, and that's what mattered! But there's going to be an important football match on TV that I don't want to miss, so let's get on with the interview.”
Are you such a sportsman? “I am above all a fanatic footballer. I have been on target for years at Akademisk Boldklub, one of the top clubs in my native Denmark. Unfortunately, I never got to play in the national team. My brother Harald does. He played in the Danish national team that won silver at the 1908 Olympics.
By the way, when the weather is nice, I also like to sit on the Chita, our family's sailboat. Those days with my wife Margarthe and our sons on the water were wonderful… I prefer not to talk about it too long, because then I get quite emotional. My oldest son died in a sailing accident. So let's get back to science instead. Luckily my other sons fared better. Did you know that another son of mine, Aage Bohr, is also a successful physicist? I sometimes see him win a Nobel Prize…
Well, it just might. You yourself received the Nobel Prize in 1922 for your research into the structure of atoms and radiation. Can you tell us more about that? “Look, a simple hydrogen atom consists of a nucleus with only one electron around it. In that case, the idea of fixed energy levels and quantum leaps is still manageable. With atoms with more electrons it quickly becomes extremely complicated.”
“Between 1913 and 1922, I was mainly working on developing my ideas on atoms with more electrons around the nucleus. For example, I found that in some complex atoms the outermost electrons have elliptical orbits. Eventually I was able to explain the properties of the elements (according to the periodic table, ed.) with my quantum theory.”
“Now that I think about it, it was a good time. Especially when I became director of my own Niels Bohr Institute in Copenhagen in 1922. There we were really figuring out what exactly quantum theory meant. The strange behavior of particles on an atomic scale was something completely new at the time.”
What problems did you and your colleagues in Copenhagen encounter? “All kinds of problems, but the main problem was that we mainly understood light as an electromagnetic wave. A few years earlier, the great Albert Einstein had already realized that light also has particle properties. The photon released during a quantum jump clearly had particle properties. So was light a particle or a wave? It seemed possible to do both. My younger colleague Werner Heisenberg came up with a theory that described the behavior of light as particles, while the Austrian Erwin Schrödinger gave an equivalent formula for wave phenomena. Two theories, both of which seemed to describe a completely different reality. We didn't understand it at first!"
Is it true that you regularly woke Heisenberg, who lived in an attic room at your institute, at night to discuss this problem? "Secure. We both couldn't bear it! In the end we both came up with our own solution. Heisenberg came up with his famous uncertainty principle. This means that you can never measure all properties of a quantum particle – such as a photon – with equal accuracy. The more accurately you measure position, the less accurately you can measure speed. So also applies:if you measure the particle aspects, all information about the wave aspects is lost.
Heisenberg's approach was based purely on mathematical principles. But I think quantum mechanics also had important philosophical implications. In my interpretation there is no independent reality apart from our perceptions. The observed phenomenon and the measuring instruments form one whole, as it were. A photon or an electron is – depending on the experiment – a wave or a particle. I call this complementarity:wave properties and particle properties complement each other, but can never be observed at the same time."
“In general you could say that a particle – before it is measured – has a probability wave of possible properties is. You can then only say something about a particle in terms of a probability calculation .”
I can imagine that you have had some criticism about that afterwards… “Yes, and not least! Albert Einstein did not agree with our 'Copenhagen interpretation'. That is, he did not see quantum mechanics as a complete description of nature. What bothered him most was that there would be no reality independent of perception. With all kinds of thought experiments he tried to show that quantum mechanics was not complete. I was able to disprove those experiments one by one. But you have already spoken to him for this column, he must have already told his side of the story.”
His story is now known yes. By the way, you will be pleased that your Copenhagen interpretation is still widely supported among physicists in 2012! "Fantastic! I knew my interpretation of that strange quantum world was the best!”
You are said to have been involved in the Manhattan Project, the American project to make an atomic bomb. Why did you do that? “I fled Denmark in 1943 because of the Nazi occupation. I then ended up in England via Sweden. I later went to America because I wanted to help prevent a nuclear arms race! They really didn't need me to develop that bomb. I spoke to President Roosevelt and pointed out to him that nuclear weapons create an enormously dangerous situation for humanity. Because nuclear weapons pose an equal threat to all countries, they offer a unique opportunity to reach a global agreement not to use them.
A condition for such an agreement was natural free access to scientific information. My ideal of an open world could prevent an arms race. Unfortunately, the dignitaries – Roosevelt and in particular British Prime Minister Churchill – thought otherwise. The result is known."
Would you like to say something about your meeting with Heisenberg in Copenhagen, 1941? There are rumors that he tried to persuade you to cooperate in the development of a nuclear weapon for the Germans. “Well, a lot of people have been asking me about that in recent years, but I have decided never to say anything about it again…
Then we will have to make do with a historical reconstruction of the weekly. Do you think it's a good reconstruction? “I'm sorry, but I'm really not saying anything about it.”