Bohr watched the Nazis' rise to power with great unease,
even as the rest of the world dismissed them as ultimately harmless.
In 1933 Hitler became chancellor of Germany, and racial laws barring
Jews from public posts were soon announced. Immediately Bohr took action
to aid German scientists. On one visit, he met Otto Frisch, who
was the nephew of Bohr's colleague, Lise Meitner. Frisch had shown
great promise, having published a few important papers at a young
age, but he naturally felt anxiety about the threat that the Nazis
posed to his career and welfare. Frisch later recalled that Bohr seemed
like a kindly father as he smiled and expressed the hope that Frisch
would come to work in Copenhagen. That night Frisch wrote to his
mother to relieve her own fears, seeing Bohr's intervention as
an act of "the good Lord."
Bohr continued to travel through Germany and spread the
word that Copenhagen was available as a retreat in case of necessity. Along
with his brother and other prominent Danish figures, Bohr helped
to form the Danish Committee for the Support of Refugee Intellectuals.
He arranged accommodations in his own country while writing to
colleagues around the world in search of positions for those who
had to flee from the Nazis.
Meanwhile, as Copenhagen became the world center for physics, Bohr
had gradually become the symbol of national pride. In 1932 he was
invited to occupy "The House of Honor"—set aside semi-officially
for Denmark's greatest citizen, the person who brought the greatest
honor to the country. The Bohrs moved in and entertained many guests,
including the King and Queen of Denmark, Queen Elizabeth and Prince
Philip of England, and before any of these, the Rutherfords. But
in 1934, tragedy struck the happy household. Bohr was on a sailing
trip with his son and two friends when a storm rushed upon them.
In the resulting disarray, Christian, Bohr's eldest son, was thrown
overboard and lost.
Shaken up by this loss, Bohr moved forward with his research and
soon made his next major contributions to physics. 1929 is often
considered a turning point in quantum theory: it marked the end
of what was called the "heroic period," when the central questions
revolved around the workings of the atom. Many scientists even
thought that physics was close to finished, and one suggested that,
with their job completed, they might turn to biology. But Bohr recognized
that no such end was near, and the beginning of the 1930s turned
the focus to electromagnetism. One impetus for this shift was a
startling discovery by Bothe and Becker, who had found unexpected
radiation when directing particles at beryllium. Meanwhile, Rutherford
and his chief assistant Chadwick had discovered and named the proton
and neutron. These experimental discoveries led to Bohr's brilliant
postulation of the compound nucleus. He saw nuclear reactions taking
place in two stages. First, a neutron would collide with a proton
or neutron, but instead of knocking that particle out, the collision
would lead to multiple collisions within the nucleus, thus distributing
the original energy among the particles. This excess energy would
then have to be released in an action by the system. While this
picture of the nucleus seems intuitive today, it was a remarkable
advance at the time and took ingenuity to devise. The compound
nucleus theory also made possible future insights into nuclear
transmutation and disintegration.
Like many previous advances, this one led to consequences
that were probably not foreseen—in this case the splitting of the
atom and the subsequent development of the nuclear bomb.
