172 The Poetry of Physics and The Physics of Poetry
emitted by a radioactive nucleus whose frequency corresponds to the
gamma ray range, is equal to the amount of kinetic energy a drop of
water would acquire falling under the influence of the earth’s gravity the
distance of 10-6 cm, the height of 100 atoms.
Although Planck’s quantization of the energy of light did not violate
any of the empirically observed facts of nature, the idea of the quantum
violated the notions of classical physics held by his contemporaries.
According to their way of thinking, most quantities including energy are
continuous.
The first break they had known in this tradition of continuity was the
discovery of the atom, which revealed that matter was discontinuous.
With the discovery of the electron, the discontinuity or quantization of
charge was also revealed. Although these concepts encountered some
initial resistance, they could be incorporated into the fabric of classical
physics because these discontinuities could be associated with the
existence of particles, which were always regarded as discreet. The
discontinuity of the energy of light or electromagnetic waves was
inconceivable, however. Energy, first of all, was always considered a
continuous quantity, even for discreet particles. But what made Planck’s
proposal even more mysterious was that it was associated with light,
which ever since the diffraction experiments of Young, was considered
to be a wave and, hence, continuous. Planck realized that his quantum
hypothesis was in contradiction with the classical physics of his day.
Referring to his work, he remarked to his son in Berlin in 1900, “Today,
I have made a discovery as important as that of Newton.”
The full implication of Planck’s idea was not understood for five
years until Einstein exploited the quantum hypothesis in order to explain
a new experimental result know as the photoelectric effect. The
photoelectric effect was first discovered by Hertz as early as 1887. It is
the effect, which has since been exploited in a number of devices such as
the “electric eye” and the photographer’s light meter. The effect consists
of the observation that, when light of a sufficiently high frequency falls
upon a metallic surface, electrons (referred to as photoelectrons) are
ejected from the metal. This emission of photoelectrons from the metal
can be understood from a classical point of view. The light, which falls
upon the metal is absorbed by the electrons inside. After a sufficient
amount of time passes, the electron absorbs enough energy to overcome
the electromagnetic forces, which holds it captive in the metal and it is
then free to leave the metal as a photoelectron.