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 1859-1860
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:
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 Fermi
level (work function) in the metal.
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’s diagram of the I2 molecule
proposed in 1898 showing the
atomic "sensitive region" (α, β) of overlap.
Ludwig Eduard 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 it will 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: 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 is the frequency of the electromagnetic
radiation; and
T is the temperature of the body in degrees
Kelvin.
The earlier Wien approximation may be derived
from Planck's law by assuming .
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.
Photoelectric effect
The emission of electrons from a metal plate
caused by light quanta (photons)
with energy greater than the Fermi level of
the metal.
The photoelectric effect reported by Heinrich
Hertz in 1887,
and explained by Albert Einstein in 1905.
Low-energy phenomena: Photoelectric effect
Mid-energy phenomena: Compton scattering
High-energy phenomena: Pair production
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).
With decreasing temperature, the peak of the
blackbody radiation curve shifts to
longer wavelengths and also has lower intensities.
The blackbody radiation
curves (1862) at left are also compared with
the early, classical limit model of
Rayleigh and Jeans (1900) shown at right.
The short wavelength side of the
curves was already approximated in 1896 by
the Wien distribution law.
Niels Bohr's 1913 quantum model of the atom,
which incorporated an explanation
of Johannes Rydberg's 1888 formula, Max Planck's
1900 quantum hypothesis, i.e.
that atomic energy radiators have discrete
energy values (ε = hν), J. J. Thomson's
1904 plum pudding model, Albert Einstein's
1905 light quanta postulate, and
Ernest Rutherford's 1907 discovery of the
atomic nucleus. Note that the electron
does not travel along the black line when
emitting a photon. It jumps,
disappearing from the outer orbit and appearing
in the inner one and cannot
exist in the space between orbits 2 and 3.
In 1924, 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.I. 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.
Feynman diagram of gluon radiation in Quantum
Chromodynamics
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.
Founding experiments
Thomas Young's double-slit experiment demonstrating
the wave nature of light. (c1805)
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. (1911)
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-Vadiman
bomb tester, proving
Interaction-free measurement is possible.
(1994)
