Bohr's first term as a Copenhagen professor began auspiciously.
A 21-year-old Dutch physicist named Hendrik Kramers, equipped with
a strong background, sought Bohr out with the hope of becoming
his assistant. Kramers had chosen Denmark mainly because of its
neutrality, but his desire to work with Bohr was keen. Bohr could
not have foreseen the contributions that this young man would add
to his work, as he served as a faithful assistant for ten years,
leaving only for a professorship at Utrecht. Kramers remained a
friend even beyond that period. The two worked together again after
World War II to urge the limitation of nuclear weapons.
Kramers's help contributed greatly to one particularly
significant paper: "On the Quantum Theory of Line Spectra." This
paper elaborated on the correspondence principle, a concept first
introduced by Bohr in 1913. In a general sense, the correspondence
principle stated that any new scientific theory must be able to
explain all the phenomena which a preceding theory explained. Coming
at a time when atomic phenomena were puzzling scientists everywhere, the
correspondence principle served as a guideline for new theories:
they had to account not only for activity at the atomic level but
be applicable to conventional phenomena as well, since classical
physics had been successful in doing so. Using this principle,
Bohr and Kramers successfully investigated such details as the
fine structure of spectra and went on to show how atomic radiation
could be reconciled with classical laws regarding the motion of
particles.
As a professor, Bohr taught standard subjects such as
mechanics and thermodynamics. He also taught atomic theory, which
was more closely related to his research interests. He held colloquia
that encouraged students to follow the most recent developments.
He was elected to the Royal Danish Academy of Sciences and Letters
in 1917 and later served as its president for many years. But while
he enjoyed prestige, he also lacked resources, working mainly in
a single room that gave him no room for experiments. With support from
Rutherford and others, Bohr applied to the government as well as
private foundations for help in setting up a laboratory. Thus began
the University Institute for Theoretical Physics.
Rutherford wrote to Bohr with a proposal that would be
hard to refuse. Manchester was also planning to establish a research
center for modern physics. Rutherford indicated to Bohr the current
state of his research, whose groundbreaking implications Bohr instantly recognized.
Rutherford also expressed his sincere desire that Bohr be with
him to interpret his new and startling results. Bohr struggled
with his decision, recognizing the superior resources that England
had to offer and the satisfaction he gained from working with Rutherford.
Nevertheless, he felt a duty to his country and university, which
had made his opportunities possible and had allowed him to realize
his dream of starting his own institute. Rutherford was asked to
replace Thomson as Cavendish professor at Cambridge.
While struggling with the pressures surrounding the new
institute, Bohr received an invitation from Planck to lecture on
spectral theory in Berlin. Accepting eagerly, Bohr arrived to meet
Planck and, for the first time, Albert Einstein. Long and intense
discussions of physics ensued. The members of the group admired
each other. Just as Bohr had made some of his first contributions
based on extensions of theories by Planck and Einstein, Einstein
had recently expanded upon rules governing Bohr's concept of stationary
states.
One fundamental difference between Einstein and Bohr soon became
evident. In his lecture on spectral theory, Bohr made the argument
that certain determinations could not be measured exactly. This
belief irked Einstein, whose lifelong resistance to the element
of chance would set him in disagreement with many physicists whose
work in quantum mechanics could not be reconciled to Einstein's
convictions. Bohr attributed Einstein's stubbornness to a reluctance
to renounce certain ideals that Bohr himself had to give up. Regardless,
both scientists enjoyed this meeting and looked forward to future
encounters, challenging themselves to work out arguments in the
interim.
September 15, 1920, marked the official opening and dedication ceremonies
of the Institute. In his speech establishing the mission of the
Institute, Bohr stressed the importance of initiating new generations
of young thinkers in the methods of science. The four-story building
featured a lecture room that would host many famous discussions,
a library on the second floor that overlooked the park, and a number
of laboratories and smaller offices where many students carried
out their work with only a pencil, paper, and blackboard. An apartment
was built on the top floor of the institute for Bohr and his family.
The Institute featured illustrious guests, including James
Franck and Lise Meitner. Franck set up the apparatus that he and
Gustav Hertz had used and recreated their experiments regarding
the effect of electric fields on atoms. At the same time, Rutherford
assembled a brilliant company of scientists at Cambridge, and both
centers collaborated to establish a new tradition of modern research.
Bohr received the Nobel Prize for physics in 1922. Receiving
the award after Einstein received the prize in 1921 made Bohr's
pride even greater. In his acceptance speech, Bohr traced the development of
his theories, beginning with Rutherford, pausing to recall the prior
work of Planck and Einstein, and moving through the current state
of his research. Having begun only about ten years ago, at a point
when the existence of the atom was not even certain, Bohr showed
how far his work and that of his colleagues had come.
