Niels
Henrik David Bohr (1885-1962) was one of the most important theoretical physicists
of the 20th century. He
played a major role in formulating and interpreting quantum mechanics. Bohr
was born in Copenhagen ans studied at the University of Copenhagen, where he
received a doctorate in 1911 and presented a thesis in which he studied the
electron theory of the structure of metals.
 |
|
|
, Bohr wrote three major papers
in which he laid the foundation of a quantum
theory of atoms. In this work, Bohr pointed out that atoms could not be described
solely through the application of the concepts of classical physics. He proposed
that an electron in an atom ordinarily resides in one of a discrete number of
possible orbits, called stationary states, and each state corresponds to an angular
momentum that is an integer multiple of Planck's constant divided by 2pi. Applying
this assumption to the hydrogen atom, Bohr calculated the energies of the stationary
states and showed that the known frequencies of light emitted by hydrogen could
be expressed as the difference between two such energies. This implied that the
electron (and hence atom) emits radiation only when jumping from one stationary
state to another. The success of these calculations did much to convince physicists
of the importance of quantum ideas, and Bohr was awarded the Nobel Prize in physics
in 1922. He had already, in 1920, been named the director of the new Institute
for Theoretical Physics in Copenhagen. In the 1920s, Bohr, through his teaching
and through discussions with Albert Einstein and others, displayed leadership
in the quest to understand the meaning of quantum mechanics. His contributions
to the formulation of Heisenberg's uncertainty principle were especially important.
As a result the way in which quantum mechanics is understood by most physicists
is called the Copenhagen interpretation.
In the late 1930s, Bohr's interest turned to nuclear physics, and he formulated
the "liquid drop" model of the nucleus. With the aid of this model, Bohr realized,
soon after the discovery of uranium fission, that it was the isotope U-235 that
was fissioned by slow neutrons. This discovery was instrumental in the development
of the atomic bomb. In 1943, Bohr escaped from occupied Denmark, and for the remainder
of World War II he worked in the United States, contributing to the atomic bomb
project.
 |
|
|
