EARLY HISTORY
In the 18th and 19th
centuries, Newtonian, or classical, mechanics appeared to provide a wholly
accurate description of the motions of bodies—for example, planetary motion. In
the late 19th and early 20th centuries, however, experimental findings raised
doubts about the completeness of Newtonian theory. Among the newer observations
were the lines that appear in the spectra of light emitted by heated gases, or
gases in which electric discharges take place. From the model of the atom
developed in the early 20th century by the English physicist Ernest Rutherford,
in which negatively charged electrons circle a positive nucleus in orbits
prescribed by Newton's laws of motion, scientists had also expected that the
electrons would emit light over a broad frequency range, rather than in the
narrow frequency ranges that form the lines in a spectrum.
Another puzzle for physicists was the coexistence of two theories of light: the corpuscular theory, which explains light as a stream of particles, and the wave theory, which views light as electromagnetic waves. A third problem was the absence of a molecular basis for thermodynamics. In his book Elementary Principles in Statistical Mechanics (1902), the American mathematical physicist J. Willard Gibbs conceded the impossibility of framing a theory of molecular action that reconciled thermodynamics, radiation, and electrical phenomena as they were then understood.