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 isquantized 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[2] 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 unitsolid 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
kelvins.
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 work function of the metal.
The photoelectric effect reported by Heinrich Hertz in 1887
,
and explained by Albert Einstein in 1905
.Low-energy phenomena:
Photoelectric effectMid-energy phenomena:
Compton scatteringHigh-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.[3]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 quantawas 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 1913On the Constitution of Atoms and Molecules.
These theories, though successful, were strictly phenomenological: during this time, there was no rigorous justification forquantization, aside, perhaps, from Henri Poincaré's discussion of Planck's theory in his 1912 paper Sur la théorie des quanta.[4][5] 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 ofJohannes 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[6][7] developed matrix mechanics and the Austrian physicist Erwin Schrödingerinvented wave mechanics and the non-relativistic Schrödinger equation as an approximation to the generalised case of de Broglie's theory.[8] 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 theelectron. 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 thecovalent 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 inquantum 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 theelectromagnetic field, and served as a model for subsequent Quantum Field theories.[6][7][9]
Feynman diagram of
gluon radiation
inQuantum 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 andWilczek in 1975.
Building on pioneering work by Schwinger, Higgs and Goldstone, the physicistsGlashow, 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.