The history of quantum mechanics is a fundamental part of the history of modern physics. Quantum
mechanics' history, as it interlaces with the history of quantum chemistry, began essentially
with a number of different scientific discoveries: the 1838 discovery of cathode rays by Michael
Faraday; the 1859–60 winter statement of the black-body radiation problem by Gustav
Kirchhoff; the 1877 suggestion by Ludwig Boltzmann that the energy states of a physical system
could be discrete; the discovery of the photoelectric effect by Heinrich Hertz in 1887; and the
1900 quantum hypothesis by Max Planck that any energy-radiating atomic system can theoretically
be divided into a number of discrete "energy elements" ε (epsilon) such that each of these
energy elements is proportional to the frequency ν with which each of them individually radiate
energy, as defined by the following formula:
ϵ =
h ν
where h is a numerical value called Planck's
constant. Then, Albert Einstein in 1905, in order to
explain the photoelectric effect previously reported by Heinrich Hertz in 1887, postulated
consistently with Max Planck's quantum hypothesis that light itself is made of individual quantum
particles, which in 1926 came to be called photons by Gilbert N. Lewis. The photoelectric
effect was observed upon shining light of particular wavelengths on certain materials,
such as metals, which caused electrons to be ejected from those materials only if the
light quantum energy was greater than the work function of the metal's surface.
The phrase "quantum mechanics" was coined (in German, Quantenmechanik) by the group
of physicists including Max Born, Werner Heisenberg, and Wolfgang Pauli, at the University of Göttingen
in the early 1920s, and was first used in Born's 1924 paper "Zur Quantenmechanik". In
the years to follow, this theoretical basis slowly began to be applied to chemical structure,
reactivity, and bonding.
== Overview ==
Ludwig Boltzmann suggested in 1877 that the energy levels of a physical system, such as
a molecule, could be discrete. He was a founder of the Austrian Mathematical Society, together
with the mathematicians Gustav von Escherich and Emil Müller. Boltzmann's rationale for
the presence of discrete energy levels in molecules such as those of iodine gas had
its origins in his statistical thermodynamics and statistical mechanics theories and was
backed up by mathematical arguments, as would also be the case twenty years later with the
first quantum theory put forward by Max Planck. In 1900, the German physicist Max Planck reluctantly
introduced the idea that energy is quantized in order to derive a formula for the observed
frequency dependence of the energy emitted by a black body, called Planck's law, that
included a Boltzmann distribution (applicable in the classical limit). Planck's law can
be stated as follows:
I (
ν ,
T )
=
2 h
ν
3
c
2
1
e
h ν
k T
− 1
,
{\displaystyle I(\nu ,T)={\frac {2h\nu ^{3}}{c^{2}}}{\frac {1}{e^{\frac {h\nu }{kT}}-1}},}
where:
I(ν,T) is the energy per unit time (or the power) radiated per unit area of emitting
surface in the normal direction per unit solid angle per unit frequency by a black body at
temperature T; h is the Planck constant;
c is the speed of light in a vacuum; k is the Boltzmann constant;
ν (nu) is the frequency of the electromagnetic radiation; and
T is the temperature of the body in kelvins.The earlier Wien approximation may be derived
from Planck's law by assuming
h ν
≫ k
T
.
Moreover, the application of Planck's quantum theory to the electron allowed Ștefan Procopiu
in 1911–1913, and subsequently Niels Bohr in 1913, to calculate the magnetic moment
of the electron, which was later called the "magneton"; similar quantum computations,
but with numerically quite different values, were subsequently made possible for both the
magnetic moments of the proton and the neutron that are three orders of magnitude smaller
than that of the electron.
In 1905, Einstein explained the photoelectric effect by postulating that light, or more
generally all electromagnetic radiation, can be divided into a finite number of "energy
quanta" that are localized points in space. From the introduction section of his March
1905 quantum paper, "On a heuristic viewpoint concerning the emission and transformation
of light", Einstein states:
"According to the assumption to be contemplated here, when a light ray is spreading from a
point, the energy is not distributed continuously over ever-increasing spaces, but consists
of a finite number of 'energy quanta' that are localized in points in space, move without
dividing, and can be absorbed or generated only as a whole."
This statement has been called the most revolutionary sentence written by a physicist of the twentieth
century. These energy quanta later came to be called "photons", a term introduced by
Gilbert N. Lewis in 1926. The idea that each photon had to consist of energy in terms of
quanta was a remarkable achievement; it effectively solved the problem of black-body radiation
attaining infinite energy, which occurred in theory if light were to be explained only
in terms of waves. In 1913, Bohr explained the spectral lines of the hydrogen atom, again
by using quantization, in his paper of July 1913 On the Constitution of Atoms and Molecules.
These theories, though successful, were strictly phenomenological: during this time, there
was no rigorous justification for quantization, aside, perhaps, from Henri Poincaré's discussion
of Planck's theory in his 1912 paper Sur la théorie des quanta. They are collectively
known as the old quantum theory. The phrase "quantum physics" was first used
in Johnston's Planck's Universe in Light of Modern Physics (1931).
In 1923, the French physicist Louis de Broglie put forward his theory of matter waves by
stating that particles can exhibit wave characteristics and vice versa. This theory was for a single
particle and derived from special relativity theory. Building on de Broglie's approach,
modern quantum mechanics was born in 1925, when the German physicists Werner Heisenberg,
Max Born, and Pascual Jordan developed matrix mechanics and the Austrian physicist Erwin
Schrödinger invented wave mechanics and the non-relativistic Schrödinger equation as
an approximation to the generalised case of de Broglie's theory. Schrödinger subsequently
showed that the two approaches were equivalent. Heisenberg formulated his uncertainty principle
in 1927, and the Copenhagen interpretation started to take shape at about the same time.
Starting around 1927, Paul Dirac began the process of unifying quantum mechanics with
special relativity by proposing the Dirac equation for the electron. The Dirac equation
achieves the relativistic description of the wavefunction of an electron that Schrödinger
failed to obtain. It predicts electron spin and led Dirac to predict the existence of
the positron. He also pioneered the use of operator theory, including the influential
bra–ket notation, as described in his famous 1930 textbook. During the same period, Hungarian
polymath John von Neumann formulated the rigorous mathematical basis for quantum mechanics as
the theory of linear operators on Hilbert spaces, as described in his likewise famous
1932 textbook. These, like many other works from the founding period, still stand, and
remain widely used. The field of quantum chemistry was pioneered
by physicists Walter Heitler and Fritz London, who published a study of the covalent bond
of the hydrogen molecule in 1927. Quantum chemistry was subsequently developed by a
large number of workers, including the American theoretical chemist Linus Pauling at Caltech,
and John C. Slater into various theories such as Molecular Orbital Theory or Valence Theory.
Beginning in 1927, researchers made attempts at applying quantum mechanics to fields instead
of single particles, resulting in quantum field theories. Early workers in this area
include P.A.M. Dirac, W. Pauli, V. Weisskopf, and P. Jordan. This area of research culminated
in the formulation of quantum electrodynamics by R.P. Feynman, F. Dyson, J. Schwinger, and
S. Tomonaga during the 1940s. Quantum electrodynamics describes a quantum theory of electrons, positrons,
and the electromagnetic field, and served as a model for subsequent quantum field theories.
The theory of quantum chromodynamics was formulated beginning in the early 1960s. The theory as
we know it today was formulated by Politzer, Gross and Wilczek in 1975.
Building on pioneering work by Schwinger, Higgs and Goldstone, the physicists Glashow,
Weinberg and Salam independently showed how the weak nuclear force and quantum electrodynamics
could be merged into a single electroweak force, for which they received the 1979 Nobel
Prize in Physics. In October 2018, physicists reported that
quantum behavior can be explained with classical physics for a single particle, but not for
multiple particles as in quantum entanglement and related nonlocality phenomena.
== Founding experiments == Thomas Young's double-slit experiment demonstrating
the wave nature of light. (c. 1801) Henri Becquerel discovers radioactivity. (1896)
J. J. Thomson's cathode ray tube experiments (discovers the electron and its negative charge).
(1897) The study of black-body radiation between
1850 and 1900, which could not be explained without quantum concepts.
The photoelectric effect: Einstein explained this in 1905 (and later received a Nobel prize
for it) using the concept of photons, particles of light with quantized energy.
Robert Millikan's oil-drop experiment, which showed that electric charge occurs as quanta
(whole units). (1909) Ernest Rutherford's gold foil experiment disproved
the plum pudding model of the atom which suggested that the mass and positive charge of the atom
are almost uniformly distributed. This led to the planetary model of the atom (1911).
James Franck and Gustav Hertz's electron collision experiment shows that energy absorption by
mercury atoms is quantized. (1914) Otto Stern and Walther Gerlach conduct the
Stern–Gerlach experiment, which demonstrates the quantized nature of particle spin. (1920)
Clinton Davisson and Lester Germer demonstrate the wave nature of the electron in the Electron
diffraction experiment. (1927) Clyde L. Cowan and Frederick Reines confirm
the existence of the neutrino in the neutrino experiment. (1955)
Clauss Jönsson's double-slit experiment with electrons. (1961)
The Quantum Hall effect, discovered in 1980 by Klaus von Klitzing. The quantized version
of the Hall effect has allowed for the definition of a new practical standard for electrical
resistance and for an extremely precise independent determination of the fine structure constant.
The experimental verification of quantum entanglement by Alain Aspect. (1982)
The Mach-Zehnder Interferometer experiment conducted by Paul Kwiat, Harold Wienfurter,
Thomas Herzog, Anton Zeilinger, and Mark Kasevich, providing experimental verification of the
Elitzur-Vaidman bomb tester, proving interaction-free measurement is possible. (1994)
== See also ==
Golden age of physics Einstein's thought experiments
History of quantum field theory History of chemistry
History of the molecule History of thermodynamics
Timeline of atomic and subatomic physics