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
{\displaystyle \epsilon =h\nu \,}
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
{\displaystyle h\nu \gg kT}
.
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
