History of quantum mechanics
The history of quantum mechanics is a fundamental part of the history of modern physics. The major chapters of this history begin with the emergence of quantum ideas to explain individual phenomena—blackbody radiation, the photoelectric effect, solar emission spectra—an era called the Old or Older quantum theories.
Building on the technology developed in classical mechanics, the invention of wave mechanics by Erwin Schrödinger and expansion by many others triggers the "modern" era beginning around 1925. Paul Dirac's relativistic quantum theory work led him to explore quantum theories of radiation, culminating in quantum electrodynamics, the first quantum field theory. The history of quantum mechanics continues in the history of quantum field theory. The history of quantum chemistry, theoretical basis of chemical structure, reactivity, and bonding, interlaces with the events discussed in this article.
The phrase "quantum mechanics" was coined 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 and P. Jordan's September 1925 paper "Zur Quantenmechanik".
The word quantum comes from the Latin word for "how much". Something that is quantized, as the energy of Planck's harmonic oscillators, can only take specific values. For example, in most countries, money is effectively quantized, with the quantum of money being the lowest-value coin in circulation. Mechanics is the branch of science that deals with the action of forces on objects. So, quantum mechanics is the part of mechanics that deals with objects for which particular properties are quantized.
Triumph and trouble at the end of the classical era
The discoveries of the 19th century, both the successes and failures, set the stage for the emergence of quantum mechanics.Wave theory of light
Beginning in 1670 and progressing over three decades, Isaac Newton developed and championed his corpuscular theory, arguing that the perfectly straight lines of reflection demonstrated light's particle nature, as at that time no wave theory demonstrated travel in straight lines. He explained refraction by positing that particles of light accelerated laterally upon entering a denser medium.Around the same time, Newton's contemporaries Robert Hooke and Christiaan Huygens, and later Augustin-Jean Fresnel, mathematically refined the wave viewpoint, showing that if light traveled at different speeds in different media, refraction could be easily explained as the medium-dependent propagation of light waves. The resulting Huygens–Fresnel principle was extremely successful at reproducing light's behaviour and was consistent with Thomas Young's discovery of wave interference of light by his double-slit experiment in 1801. The wave view did not immediately displace the ray and particle view, but began to dominate scientific thinking about light in the mid 19th century, since it could explain polarization phenomena that the alternatives could not.
James Clerk Maxwell found that he could apply his previously discovered equations, along with a slight modification, to describe self-propagating waves of oscillating electric and magnetic fields. It quickly became apparent that visible light, ultraviolet light, and infrared light were all electromagnetic waves of differing frequency. This theory became a critical ingredient in the beginning of quantum mechanics.
Emerging atomic theory
During the early 19th century, chemical research by John Dalton and Amedeo Avogadro lent weight to the atomic theory of matter, an idea that James Clerk Maxwell, Ludwig Boltzmann and others built upon to establish the kinetic theory of gases. The successes of kinetic theory gave further credence to the idea that matter is composed of atoms, yet the theory also had shortcomings that would only be resolved by the development of quantum mechanics. The existence of atoms was not universally accepted among physicists or chemists; Ernst Mach, for example, was a staunch anti-atomist.The earliest hints of problems in classical mechanics were raised in relation to the temperature dependence of the properties of gases.
Ludwig Boltzmann suggested in 1877 that the energy levels of a physical system, such as a molecule, could be discrete. 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.
Electrons and the nucleus
In the final days of the 1800s, J. J. Thomson established that electrons carry a negative charge opposite but the same as that of a hydrogen ion while having a mass over one thousand times less. Many such electrons were known to be associated with every atom. By 1904 Thomson proposed the first atomic model with subatomic constituents, using circulating electrons in a background of positive charge, the so-called plum pudding model. Thomson's concepts were supported by early beta particle scattering experiments but by 1911 Hans Geiger and his student Ernest Marsden demonstrated backscattering of alpha particles which Ernest Rutherford interpreted as compelling evidence that the positive charge was concentrated in a small volume we now call the nucleus.Radiation theory
Throughout the 1800s many studies investigated details in the spectrum of intensity versus frequency for light emitted by flames, by the Sun, or red-hot objects. The Rydberg formula effectively summarized the dark lines seen in the spectrum, but Rydberg provided no physical model to explain them. The spectrum emitted by red-hot objects could be explained at high or low wavelengths but the two theories differed.Old quantum theory
Quantum mechanics developed in two distinct phases. The first phase, known as the old quantum theory, began around 1900 with radically new approaches to explanations physical phenomena not understood by classical mechanics of the 1800s.Planck introduces quanta to explain black-body radiation
is electromagnetic radiation emitted from the surface of an object due to the object's internal energy. If an object is heated sufficiently, it starts to emit light at the red end of the visible spectrum, as it becomes red hot. Heating it further causes the color to change from red to yellow, white, and blue, as it emits light at increasingly shorter wavelengths.A perfect emitter is also a perfect absorber: when it is cold, such an object looks perfectly black, because it absorbs all the light that falls on it and emits none. Consequently, an ideal thermal emitter is known as a black body, and the radiation it emits is called black-body radiation.
File:RWP-comparison.svg|thumb|Predictions of the amount of thermal radiation of different frequencies emitted by a body. Correct values predicted by Planck's law contrasted against the classical values of Rayleigh-Jeans law and Wien approximation.
By the late 19th century, thermal radiation had been fairly well characterized experimentally. Several formulas that describe certain experimental measurements of thermal radiation had been developed. Wien’s displacement law gives the relation between temperature and the wavelength at which the radiation is strongest, while the Stefan–Boltzmann law describes the total power emitted per unit area. The best theoretical explanation of the experimental results was the Rayleigh–Jeans law, which, as shown in the figure, agrees with experimental results well at large wavelengths, but strongly disagrees at short wavelengths. In fact, at short wavelengths, classical physics predicted that energy will be emitted by a hot body at an infinite rate. This result, which is clearly wrong, is known as the ultraviolet catastrophe. Physicists searched for a single theory that explained all the experimental results.
The first model that was able to explain the full spectrum of thermal radiation was put forward by Max Planck in 1900. He proposed a mathematical model in which the thermal radiation was in equilibrium with a set of harmonic oscillators. To reproduce the experimental results, he had to assume that each oscillator emitted an integer number of units of energy at its single characteristic frequency, rather than being able to emit any arbitrary amount of energy. In other words, the energy emitted by an oscillator was quantized. According to Planck, the quantum of energy of a light quantum is proportional to its frequency frequency. As an equation is it written:
This is now known as the Planck relation and the proportionality constant,, as the Planck constant.
Planck's law was the first quantum theory in physics, and Planck won the 1918 Nobel Prize in Physics "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta". At the time, however, Planck's view was that quantization was purely a heuristic mathematical construct, rather than a fundamental change in our understanding of the world.
Einstein applies quanta to explain the photoelectric effect
In 1887, Heinrich Hertz observed that when light with sufficient frequency hits a metallic surface, the surface emits cathode rays. Ten years later, J. J. Thomson showed that the many reports of cathode rays were actually "corpuscles" and they quickly came to be called electrons. In 1902, Philipp Lenard discovered that the maximum possible energy of an ejected electron is unrelated to the intensity of the monochromatic light. This observation is at odds with classical electromagnetism, which predicts that the electron's energy should be proportional to the intensity of the incident radiation.In 1905, Albert Einstein suggested that even though continuous models of light worked extremely well for time-averaged optical phenomena, for instantaneous transitions the energy in light may occur a finite number of energy quanta.
In the introduction section of his March 1905 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 has been called the most "revolutionary" sentence written by a twentieth century physicist, meaning that it proposed an idea which altered mainstream thinking.
The energy of a single quantum of light of frequency is given by the frequency multiplied by the Planck constant :
Einstein assumed a light quanta transfers all of its energy to a single electron imparting at most an energy to the electron. Therefore, only the light frequency determines the maximum energy that can be imparted to the electron; the intensity of the photoemission is proportional to the light beam intensity.
Einstein argued that it takes a certain amount of energy, called the work function and denoted by, to remove an electron from the metal. This amount of energy is different for each metal. If the energy of the light quanta is less than the work function, then it does not carry sufficient energy to remove the electron from the metal. The threshold frequency,, is the frequency of a light quanta whose energy is equal to the work function:
If is greater than, the energy is enough to remove an electron. The ejected electron has a kinetic energy,, which is, at most, equal to the light energy minus the energy needed to dislodge the electron from the metal:
Einstein's description of light as being composed of energy quanta extended Planck's notion of quantized energy, which is that a single quantum of a given frequency,, delivers an invariant amount of energy,. Einstein was awarded the 1921 Nobel Prize in Physics "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect".
In nature, single quanta are rarely encountered. The Sun and emission sources available in the 19th century emit a vast amount of energy every second. The Planck constant,, is so tiny that the amount of energy in each quantum, is very very small. Light we see includes many trillions of such quanta. Arthur Compton's demonstration of the scattering of light by electrons scattering convinced physicists of the reality of photons. Compton won the 1927 Nobel Prize in Physics for his discovery. The term "photon" was introduced in 1926 by Gilbert N. Lewis.