wang
(Wang)
#1
The most radical revolution, however, was quantum mechanics. The key experiments
that were crucial in the development of quantum theory may be summarized as follows,
• discrete emission and absorption spectra of simple atoms (Balmer, ...);
• existence of radioactivity (Becquerel 1896);
• frequency dependence of back body radiation (Planck 1900);
• photo-electric effect (Einstein 1905);
• existence of a hard core (i.e. the nucleus) inside atoms (Rutherford 1909);
• incompatibility of theplanetary atomwith classical electrodynamics (Bohr 1913);
• discovery of strong and weak forces (Rutherford 1920);
• electrons diffract just as light does (de Broglie 1923);
• need for a Pauli exclusion principle in the Bohr model of atoms (Pauli 1925);
• Goudsmit and Uhlenbeck discoverspin(1925).
This wealth of novel experimental facts and their apparent contradiction with (classical
mechanics and electromagnetic) theory led to the formal development of quantum mechanics,
• 1925 : Heisenberg introducesmatrix mechanics, and quantized the harmonic oscillator;
• 1926 : Schr ̈odinger invents wave mechanics(inspired by de Broglie’s particle/wave
duality), and shows the equivalence of matrix and wave mechanics;
• 1927 : Dirac gives what is to be the present day formulation of quantum mechanics.
Subsequent developments include primarily the development ofquantum field theory, a the-
ory which unifies quantum mechanics and special relativity. Its foundations were laid in
1928 by Dirac, Heisenberg and Pauli, but its elaboration continues actively still today. It
is important to realize though that there has been no serious need since 1927 to alter the
fundamental principles of quantum mechanics, either for experimental or theoretical reasons.
For extensive accounts of the history and conceptual development of quantum mechanics,
the reader is referred to the books by Jammer and by Pais.
1.2 Constants of Nature
There are three fundamental dimensionful constants in Nature:the speed of lightc, Planck’s
constant ̄h=h/ 2 π, and Newton’s constant of gravityGN. Their values are given by
c = 2. 99792 × 108 m/s
̄h = 1. 05457 × 10 −^34 Js
GN = 6. 6732 × 10 −^11 Jm/(kg)^2
