Historical note
At the end of the nineteenth century, physics consisted essentially of classical
mechanics, the theory of electromagnetism, and thermodynamics. Classical
mechanics was used to predict the dynamics of material bodies, and Maxwell’s
electromagnetism provided the proper framework to study radiation; matter and
radiation were described in terms of particles and waves respectively. As for the
interactions between matter and radiation, they were well explained by the Lorentz
force or by thermodynamics. The overwhelming success of classical physics (classical
mechanics, classical theory of electromagnetism, and thermodynamic) made people
believe that the ultimate description of nature had been achieved. It seemed that all
known physical phenomena could be explained within the framework of the general
theories of matter and radiation. At the turn of the twentieth century, however,
classical physics, which had been quite unassailable, was seriously challenged on two
major fronts:
Relativistic domain: Einstein’s 1905 theory of relativity showed that the
validity of Newtonian mechanics ceases at very high speeds (i.e., at speeds
comparable to that of light).
Microscopic domain: As soon as new experimental techniques were developed
to the point of probing atomic and subatomic structures, it turned out that classical
physics fails miserably in providing the proper explanation for several newly
discovered phenomena. It thus became evident that the validity of classical physics
