208 The Poetry of Physics and The Physics of Poetry
level we would never have developed physics. Life itself would not be
possible. imagine the difficult of crossing the street safely if the
approaching cars are only 90% on the road and 10% of the time are
on the sidewalk.
The world of our experience is classical, not quantum mechanical. It
is fairly obvious the only language that we have at our disposal to
describe the world whether it is the macro-world or the atomic world
is the language of classical physics, even if that language is inadequate.
It should not surprise us, therefore, that our description of the atomic
world will be less than satisfactory at times. The description of
elementary particles such as electrons is a perfect example. In classical
physics, the concept of a wave and the concept of a particle were well-
defined notions, which were mutually exclusive. A wave represented a
phenomenon whose character was completely different than that of a
particle. Within the framework of classical physics these two concepts
were used to describe light and electrons, respectively. Light was a wave
phenomenon and the electron, a particle and that was that. Within the
framework of quantum physics, the behaviour of light and the electron is
a great deal more complicated than that of a wave and a particle,
respectively. There is, in fact, no way of describing their behaviour
classically. The best one can do is to say that there are times when light
behaves like a wave and there are times when it behaves like a particle.
The same is true of the electron. The classical notions of the wave and
the particle, according to Bohr, form complementary descriptions of the
electron. They also form a complementary description of light. Although
the wave and particle notions mutually exclude each other they are both
necessary for a description of either light or the electron. This is the
essence of Bohr’s complementarity principle.
The classical notions of position and momentum are also
complementary concepts. In classical physics, one can assign an exact
value to both quantities whereas in quantum physics the more one learns
about one variable the less one knows of the other. A quantum system
cannot have a well-defined position and momentum at the same time.
Yet, these two mutually exclusive quantities are needed to describe the
quantum system. Position and momentum are complementary concepts
in the same sense as the wave and the particle notion are complementary.
There is a famous quote Bohr made regarding truth, which I believe
contains the essence of the complementarity principle. He said: “(There
are) two kinds of truths. To the one kind belong statements so simple