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Niels Bohr’s Times by Abraham Pais, Oxford University Press, pp 565,
£25

Niels Bohr is one of the towering figures of 20th century science, ranking
with Albert Einstein for his contributions to physics; but his approach
was very different. It was Bohr who provided the first real insight into
the way electrons are arranged in atoms, establishing the basis for the
modern understanding of chemistry and winning a Nobel prize for his work.
He developed the correspondence principle, that quantum processes indistinguishably
approach their classical equivalents for large numbers. He also founded
an institute in Denmark where so much of the breakthrough work in quantum
theory was carried out that the standard model for the quantum world is
known to this day as the ‘Copenhagen Interpretation’. And he argued with
Einstein for decades about the philosophical interpretation of quantum mechanics.

But above all, Bohr was a pragmatist. If he could find a rule that made
it possible to make calculations predicting the outcome of experiments on
the atomic scale, he wasn’t too worried about the mechanism of the
underlying clockwork (if any) by which atoms operated. While others were
baffled by the discovery that particles could also be waves, and waves could
also be particles, Bohr came up with another principle, complementarity,
which said that an electron, for example, could behave either as a wave
or as a particle, but that no experiment would ever show a single electron
behaving as both wave and particle at the same time, so we did not have
to worry about the ‘real’ meaning of the duality.

Pais, author of the acclaimed discussion of Einstein’s life and work,
Subtle is the Lord (Oxford University Press, pp 568, £27.50 hbk 1982,
£9.95 pbk 1984) takes us through all this in detail, providing a book
that grips by its clarity and the importance of its subject matter, even
though it lacks any dramatic new biographical insights. There are, though,
intriguing sidelights on the development of 20th-century science. For example,
as late as 1920 and 1921 the Nobel Committee rejected Bohr’s nomination
for the physics prize on the grounds that his ideas were ‘in conflict with
physical laws that have so long been considered indispensable’. He received
the prize in 1922 after the committee received a 32-page report on Bohr’s
work and its significance from one of its members.

But what makes Niels Bohr’s Times remarkably timely, and is sure to
boost its sales, is the recent burst of activity among experimenters tackling
the puzzle of complementarity itself. Astonishingly, it is now possible
to carry out experiments on single electrons, and even on single photons.
And the preliminary evidence is that Bohr was wrong – it is possible to
see such entities displaying the properties of both waves and particles
at the same time.

These experiments – too recent to be discussed in the book – are causing
renewed debate about the ‘real meaning’ of quantum mechanics; more debate,
indeed, than there has been since the 1930s. Pais’s discussion of the history
of comple-mentarity and puzzles such as the Einstein-Padolsky-Rosen paradox,
which highlights the uncertainty implicit in quantum mechanics, is likely
to be essential reading for those debaters.

On top of all this, in the later part of his life Bohr played a part
in the development of nuclear fission, and was the first director of the
theory division of CERN, helping to pick up the pieces of European science
after the Second World War. But the central puzzle of Bohr’s life, and of
this book, concerned the nature of quantum reality, a question that has
re-emerged with full force in the final decade of the 20th-century. Bohr
the pragmatist would not have been worried by the breakdown of complement-arity,
but as yet we are still waiting for a new Bohr to come up with something
better.