Physics is...

Questions answered by Niels Bohr (1885–1962)(Sources: Stefan Rosental, “Schicksalsjahre mit Niels Bohr,” Stuttgart 1991/ Ulrich Röseberg, “Niels Bohr. Leben und Werk eines Atomphysikers 1885-1962,” Berlin, Heidelberg, New York 1992 /Karl v. Meyenn, Klaus Stolzenburg, Roman Sexl (editors), “Niels Bohr,” Braunschweig 1985/Werner Heisenberg, “Der Teil und das Ganze. Gespräche im Umkreis der Atomphysik,” Munich 1981).

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1. What were your reasons for studying physics? This was never a question for Bohr. “In 1903, I graduated from the Gammelholm High School and started to study physics at the Copenhagen University.”
2. What university did you go to? Copenhagen, Cambridge and Manchester
3. Did you know a physical experiment at school which particularly impressed you? What are the physical experiments which impress you? “In Manchester I carried out a theoretical investigation into the velocity loss atomic particles suffer when passing through matter. Most of all I was fascinated with the prospects opened up by the discovery of the nucleus for further information about the characteristics and relations of the elements; this could obviously be realized only by fundamental departures from the aspects of classical physics so complete and fruitful in many fields.”
4. Your physical aha-experience? “Contraria sunt complementa – opposites are complementary.”
In 1913, Bohr was told that the “wavelengths of a complete series of spectral lines in the hydrogen spectrum can be expressed with the aid of integers. This information, he [Bohr] said, left an indelible impression on him.”
6. Are you also interested in a completely different speciality? Philosophy (epistemology) and soccer. Together with his brother Harald (mathematician and, at the same time, one of the most popular Danish soccer players), Bohr was a passionate soccer player.
8. Have you ever had a chemical kit? No, a workbench and later on a lathe. Endowed with manual skills, Niels Bohr loved to repair objects from the bicycle to the clock.
10. Your recommendation for future students of physics? “Experiments whose results agree with theory are not interesting. Of importance are, however, results which do not agree with theory, as they can teach us something new.”
14. What are light quanta? Einstein put forward the so-called “light quanta hypothesis” according to which in contrast to Maxwell’s electromagnetic light theory the radiation energy should not propagate in electromagnetic light waves but in light atoms of smaller size; each of these atoms was said to contain an energy quantity which just corresponds to a radiation quantum. not agree with theory, as they can teach us something new.”
16. Your favourite measuring instrument? The joke. – “There are things which are so serious that you can only make jokes about them.”
17. Do you master a musical instrument? No.
22. What is in your opinion most important in a measurement? “What is decisive is the impossibility of strictly separating the behaviour of atomic objects from the interaction with the measuring instruments serving to determine the conditions under which the phenomena occur.“
24. Would you tell us three important events of your biography? 1911 – Acquaintance of Ernest Rutherford, 1922 - Nobel prize for physics, profound shock after the dropping of the atomic bomb over Japan in August 1945 (Bohr had tried to prevent the military use of nuclear fission and pleaded in the early stages of the cold war for the openness of science to abolish the reasons for arms race).
28. Which physicist do you admire? Ernest Rutherford. He answered to a letter of Bohr with the following comment: “Your letter sounds alarming, but you cannot expect that the whole problem of modern physics can be solved in a few years. Better be happy that there is still a lot to do.” Bohr appreciated this letter very much.
29. Complete the sentence: “A life without measurements is...” “That reminds me of the young boy... The young boy went to the grocer with a penny in his hand and asked: May I have mixed candies for a penny? The grocer took two candies, gave them to the boy and said: Here are your candies. Go and mix them yourself.”
31. Do you remember the most important measurement of your life? Am 24. November 1986 habe ich zum ersten Mal ein Neugeborenes gewogen.
32. There was a time when certain physical experiments were suddenly regarded as politically undesired. How political can physics be in your opinion? “The progress of physical science which has made it possible to release immense amounts of energy by nuclear fission, initiates a real revolution in the field of the resources available to mankind, which represents an extremely serious challenge to civilization. [...] The terrible destructive powers now available to the human race will obviously become a deadly threat to civilization, unless we in time succeed in reaching a general agreement on appropriate measures to prevent any irresponsible use of the new energy sources.”
Unfortunately, Bohr’s appeal to Churchill and Roosevelt did not prevent the dropping of atomic bombs over Hiroshima and Nagasaki in August 1945.
34. Is nature really as physics, biology and chemistry describe it? “Washing dishes and language can in some respects be compared. We have dirty dishwater and dirty towels and nevertheless finally succeed in getting the plates and glasses clean. Likewise, we have unclear terms and a logic limited in an unknown way in its field of application – but nevertheless we succeed in using it to bring clearness to our understanding of nature.”