Matrix mechanics is a formulation of
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
created by
Werner Heisenberg
Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent series ...
,
Max Born
Max Born (; 11 December 1882 – 5 January 1970) was a German physicist and mathematician who was instrumental in the development of quantum mechanics. He also made contributions to solid-state physics and optics and supervised the work of a n ...
, and
Pascual Jordan
Ernst Pascual Jordan (; 18 October 1902 – 31 July 1980) was a German theoretical and mathematical physicist who made significant contributions to quantum mechanics and quantum field theory. He contributed much to the mathematical form of matri ...
in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account of
quantum jumps
Atomic electron transition is a change (or jump) of an electron from one energy level to another within an atom or artificial atom. It appears discontinuous as the electron "jumps" from one quantized energy level to another, typically in a few na ...
supplanted the
Bohr model
In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar Syst ...
's
electron orbits. It did so by interpreting the physical properties of particles as
matrices
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** ''The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...
that evolve in time. It is equivalent to the
Schrödinger wave formulation of quantum mechanics, as manifest in
Dirac
Distributed Research using Advanced Computing (DiRAC) is an integrated supercomputing facility used for research in particle physics, astronomy and cosmology in the United Kingdom. DiRAC makes use of multi-core processors and provides a variety o ...
's
bra–ket notation.
In some contrast to the wave formulation, it produces spectra of (mostly energy) operators by purely algebraic,
ladder operator
In linear algebra (and its application to quantum mechanics), a raising or lowering operator (collectively known as ladder operators) is an operator that increases or decreases the eigenvalue of another operator. In quantum mechanics, the raisin ...
methods. Relying on these methods,
Wolfgang Pauli
Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics fo ...
derived the hydrogen atom spectrum in 1926,
before the development of wave mechanics.
Development of matrix mechanics
In 1925,
Werner Heisenberg
Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent series ...
,
Max Born
Max Born (; 11 December 1882 – 5 January 1970) was a German physicist and mathematician who was instrumental in the development of quantum mechanics. He also made contributions to solid-state physics and optics and supervised the work of a n ...
, and
Pascual Jordan
Ernst Pascual Jordan (; 18 October 1902 – 31 July 1980) was a German theoretical and mathematical physicist who made significant contributions to quantum mechanics and quantum field theory. He contributed much to the mathematical form of matri ...
formulated the matrix mechanics representation of quantum mechanics.
Epiphany at Helgoland
In 1925 Werner Heisenberg was working in
Göttingen
Göttingen (, , ; nds, Chöttingen) is a college town, university city in Lower Saxony, central Germany, the Capital (political), capital of Göttingen (district), the eponymous district. The River Leine runs through it. At the end of 2019, t ...
on the problem of calculating the
spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
. By May 1925 he began trying to describe atomic systems by
observable
In physics, an observable is a physical quantity that can be measured. Examples include position and momentum. In systems governed by classical mechanics, it is a real-valued "function" on the set of all possible system states. In quantum ph ...
s only. On June 7, after weeks of failing to alleviate his
hay fever
Allergic rhinitis, of which the seasonal type is called hay fever, is a type of inflammation in the nose that occurs when the immune system overreacts to allergens in the air. Signs and symptoms include a runny or stuffy nose, sneezing, red, i ...
with aspirin and cocaine, Heisenberg left for the pollen-free
North Sea
The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
island of
Helgoland
Heligoland (; german: Helgoland, ; Heligolandic Frisian: , , Mooring Frisian: , da, Helgoland) is a small archipelago in the North Sea. A part of the German state of Schleswig-Holstein since 1890, the islands were historically possessions ...
. While there, in between climbing and memorizing poems from
Goethe
Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as treat ...
's ''
West-östlicher Diwan'', he continued to ponder the spectral issue and eventually realised that adopting ''
non-commuting'' observables might solve the problem. He later wrote:
It was about three o' clock at night when the final result of the calculation lay before me. At first I was deeply shaken. I was so excited that I could not think of sleep. So I left the house and awaited the sunrise on the top of a rock.
The three fundamental papers
After Heisenberg returned to Göttingen, he showed
Wolfgang Pauli
Wolfgang Ernst Pauli (; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and one of the pioneers of quantum physics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics fo ...
his calculations, commenting at one point:
Everything is still vague and unclear to me, but it seems as if the electrons will no more move on orbits.
On July 9 Heisenberg gave the same paper of his calculations to Max Born, saying that "he had written a crazy paper and did not dare to send it in for publication, and that Born should read it and advise him" prior to publication. Heisenberg then departed for a while, leaving Born to analyse the paper.
In the paper, Heisenberg formulated quantum theory without sharp electron orbits.
Hendrik Kramers
Hendrik Anthony "Hans" Kramers (17 December 1894 – 24 April 1952) was a Dutch physicist who worked with Niels Bohr to understand how electromagnetic waves interact with matter and made important contributions to quantum mechanics and statistical ...
had earlier calculated the relative intensities of spectral lines in the
Sommerfeld model by interpreting the
Fourier coefficients
A Fourier series () is a summation of harmonically related sinusoidal functions, also known as components or harmonics. The result of the summation is a periodic function whose functional form is determined by the choices of cycle length (or ''p ...
of the orbits as intensities. But his answer, like all other calculations in the
old quantum theory
The old quantum theory is a collection of results from the years 1900–1925 which predate modern quantum mechanics. The theory was never complete or self-consistent, but was rather a set of heuristic corrections to classical mechanics. The theory ...
, was only correct for
large orbits.
Heisenberg, after a collaboration with Kramers, began to understand that the transition probabilities were not quite classical quantities, because the only frequencies that appear in the Fourier series should be the ones that are observed in quantum jumps, not the fictional ones that come from Fourier-analyzing sharp classical orbits. He replaced the classical Fourier series with a matrix of coefficients, a fuzzed-out quantum analog of the Fourier series. Classically, the Fourier coefficients give the intensity of the emitted
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
, so in quantum mechanics the magnitude of the matrix elements of the
position operator were the intensity of radiation in the bright-line spectrum. The quantities in Heisenberg's formulation were the classical position and momentum, but now they were no longer sharply defined. Each quantity was represented by a collection of Fourier coefficients with two indices, corresponding to the initial and final states.
When Born read the paper, he recognized the formulation as one which could be transcribed and extended to the systematic
language of matrices, which he had learned from his study under Jakob Rosanes at
Breslau University. Born, with the help of his assistant and former student Pascual Jordan, began immediately to make the transcription and extension, and they submitted their results for publication; the paper was received for publication just 60 days after Heisenberg's paper.
A follow-on paper was submitted for publication before the end of the year by all three authors. (A brief review of Born's role in the development of the matrix mechanics formulation of quantum mechanics along with a discussion of the key formula involving the non-commutivity of the probability amplitudes can be found in an article by
Jeremy Bernstein
Jeremy Bernstein (born December 31, 1929, in Rochester, New York) is an American theoretical physicist and popular science writer.
Early life
Bernstein's parents, Philip S. Bernstein, a Reform rabbi, and Sophie Rubin Bernstein named him after th ...
. A detailed historical and technical account can be found in Mehra and Rechenberg's book ''The Historical Development of Quantum Theory. Volume 3. The Formulation of Matrix Mechanics and Its Modifications 1925–1926.'')
Up until this time, matrices were seldom used by physicists; they were considered to belong to the realm of pure mathematics.
Gustav Mie
Gustav Adolf Feodor Wilhelm Ludwig Mie (; 29 September 1868 – 13 February 1957) was a German physicist.
Life
Mie was born in Rostock, Mecklenburg-Schwerin, Germany in 1868. From 1886 he studied mathematics and physics at the University of ...
had used them in a paper on electrodynamics in 1912 and Born had used them in his work on the lattices theory of crystals in 1921. While matrices were used in these cases, the algebra of matrices with their multiplication did not enter the picture as they did in the matrix formulation of quantum mechanics.
Born, however, had learned matrix algebra from Rosanes, as already noted, but Born had also learned Hilbert's theory of integral equations and quadratic forms for an infinite number of variables as was apparent from a citation by Born of Hilbert's work ''Grundzüge einer allgemeinen Theorie der Linearen Integralgleichungen'' published in 1912.
Jordan, too, was well equipped for the task. For a number of years, he had been an assistant to
Richard Courant
Richard Courant (January 8, 1888 – January 27, 1972) was a German American mathematician. He is best known by the general public for the book '' What is Mathematics?'', co-written with Herbert Robbins. His research focused on the areas of real ...
at Göttingen in the preparation of Courant and
David Hilbert
David Hilbert (; ; 23 January 1862 – 14 February 1943) was a German mathematician, one of the most influential mathematicians of the 19th and early 20th centuries. Hilbert discovered and developed a broad range of fundamental ideas in many a ...
's book ''Methoden der mathematischen Physik I'', which was published in 1924. This book, fortuitously, contained a great many of the mathematical tools necessary for the continued development of quantum mechanics.
In 1926,
John von Neumann
John von Neumann (; hu, Neumann János Lajos, ; December 28, 1903 – February 8, 1957) was a Hungarian-American mathematician, physicist, computer scientist, engineer and polymath. He was regarded as having perhaps the widest cove ...
became assistant to David Hilbert, and he would coin the term
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
to describe the algebra and analysis which were used in the development of quantum mechanics.
A linchpin contribution to this formulation was achieved in Dirac's reinterpretation/synthesis paper of 1925, which invented the language and framework usually employed today, in full display of the noncommutative structure of the entire construction.
Heisenberg's reasoning
Before matrix mechanics, the old quantum theory described the motion of a particle by a classical orbit, with well defined position and momentum ''X''(''t''), ''P''(''t''), with the restriction that the time integral over one period ''T'' of the momentum times the velocity must be a positive integer multiple of
Planck's constant
While this restriction correctly selects orbits with more or less the
right energy values ''E
n'', the old quantum mechanical formalism did not describe time dependent processes, such as the emission or absorption of radiation.
When a classical particle is weakly coupled to a radiation field, so that the radiative damping can be neglected, it will emit
radiation in a pattern that repeats itself every orbital period. The frequencies that make up the outgoing wave are then integer multiples of the orbital frequency, and this is a reflection of the fact that ''X''(''t'') is periodic, so that its
Fourier representation has frequencies 2π''n''/''T'' only.
The coefficients ''X
n'' are
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the form ...
s. The ones with negative frequencies must be the
complex conjugate
In mathematics, the complex conjugate of a complex number is the number with an equal real part and an imaginary part equal in magnitude but opposite in sign. That is, (if a and b are real, then) the complex conjugate of a + bi is equal to a - ...
s of the ones with positive frequencies, so that ''X''(''t'') will always be real,
A quantum mechanical particle, on the other hand, can not emit radiation continuously, it can only emit photons. Assuming that the quantum particle started in orbit number ''n'', emitted a photon, then ended up in orbit number ''m'', the energy of the photon is , which means that its frequency is .
For large ''n'' and ''m'', but with ''n''−''m'' relatively small, these are the classical frequencies by
Bohr's
correspondence principle
In the formula above, ''T'' is the classical period of either orbit ''n'' or orbit ''m'', since the difference between them is higher order in ''h''. But for ''n'' and ''m'' small, or if ''n'' − ''m'' is large, the frequencies are not integer multiples of any single frequency.
Since the frequencies that the particle emits are the same as the frequencies in the Fourier description of its motion, this suggests that ''something'' in the time-dependent description of the particle is oscillating with frequency . Heisenberg called this quantity ''X
nm'',
and demanded that it should reduce to the classical
Fourier coefficients
A Fourier series () is a summation of harmonically related sinusoidal functions, also known as components or harmonics. The result of the summation is a periodic function whose functional form is determined by the choices of cycle length (or ''p ...
in the classical limit. For large values of ''n'', ''m'' but with ''n'' − ''m'' relatively small,
''X
nm'' is the th Fourier coefficient of the classical motion at orbit ''n''. Since ''X
nm'' has opposite frequency to ''X
mn'', the condition that ''X'' is real becomes
By definition, ''X
nm'' only has the frequency , so its time evolution is simple:
This is the original form of Heisenberg's equation of motion.
Given two arrays ''X
nm'' and ''P
nm'' describing two physical quantities, Heisenberg could form a new array of the same type by combining the terms ''X
nkP
km'', which also oscillate with the right frequency. Since the Fourier coefficients of the product of two quantities is the
convolution
In mathematics (in particular, functional analysis), convolution is a operation (mathematics), mathematical operation on two function (mathematics), functions ( and ) that produces a third function (f*g) that expresses how the shape of one is ...
of the Fourier coefficients of each one separately, the correspondence with Fourier series allowed Heisenberg to deduce the rule by which
the arrays should be multiplied,
Born pointed out that ''this is the law of matrix multiplication'', so that the position, the momentum, the energy, all the observable quantities in the theory, are interpreted as matrices. Under this multiplication rule, the product depends on the order: ''XP'' is different from ''PX''.
The ''X'' matrix is a complete description of the motion of a quantum mechanical particle. Because the frequencies in the quantum motion are not multiples of a common frequency, the matrix elements ''cannot be interpreted as the Fourier coefficients of a sharp classical trajectory''. Nevertheless, as matrices, ''X''(''t'') and ''P''(''t'') satisfy the classical equations of motion; also see Ehrenfest's theorem, below.
Matrix basics
When it was introduced by Werner Heisenberg, Max Born and Pascual Jordan in 1925, matrix mechanics was not immediately accepted and was a source of controversy, at first. Schrödinger's later introduction of
wave mechanics Wave mechanics may refer to:
* the mechanics of waves
* the ''wave equation'' in quantum physics, see Schrödinger equation
See also
* Quantum mechanics
* Wave equation
The (two-way) wave equation is a second-order linear partial different ...
was greatly favored.
Part of the reason was that Heisenberg's formulation was in an odd mathematical language, for the time, while Schrödinger's formulation was based on familiar wave equations. But there was also a deeper sociological reason. Quantum mechanics had been developing by two paths, one led by Einstein, who emphasized the wave–particle duality he proposed for photons, and the other led by Bohr, that emphasized the discrete energy states and quantum jumps that Bohr discovered. De Broglie had reproduced the discrete energy states within Einstein's framework—the quantum condition is the standing wave condition, and this gave hope to those in the Einstein school that all the discrete aspects of quantum mechanics would be subsumed into a continuous wave mechanics.
Matrix mechanics, on the other hand, came from the Bohr school, which was concerned with discrete energy states and quantum jumps. Bohr's followers did not appreciate physical models that pictured electrons as waves, or as anything at all. They preferred to focus on the quantities that were directly connected to experiments.
In atomic physics,
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
gave observational data on atomic transitions arising from the interactions of atoms with light
quanta. The Bohr school required that only those quantities that were in principle measurable by spectroscopy should appear in the theory. These quantities include the energy levels and their intensities but they do not include the exact location of a particle in its Bohr orbit. It is very hard to imagine an experiment that could determine whether an electron in the ground state of a hydrogen atom is to the right or to the left of the nucleus. It was a deep conviction that such questions did not have an answer.
The matrix formulation was built on the premise that all physical observables are represented by matrices, whose elements are indexed by two different energy levels. The set of
eigenvalue
In linear algebra, an eigenvector () or characteristic vector of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding eigenvalue, often denoted b ...
s of the matrix were eventually understood to be the set of all possible values that the observable can have. Since Heisenberg's matrices are
Hermitian {{Short description, none
Numerous things are named after the French mathematician Charles Hermite (1822–1901):
Hermite
* Cubic Hermite spline, a type of third-degree spline
* Gauss–Hermite quadrature, an extension of Gaussian quadrature m ...
, the eigenvalues are real.
If an observable is measured and the result is a certain eigenvalue, the corresponding
eigenvector
In linear algebra, an eigenvector () or characteristic vector of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding eigenvalue, often denoted b ...
is the state of the system immediately after the measurement. The act of measurement in matrix mechanics 'collapses' the state of the system. If one measures two observables simultaneously, the state of the system collapses to a common eigenvector of the two observables. Since most matrices don't have any eigenvectors in common, most observables can never be measured precisely at the same time. This is the
uncertainty principle
In quantum mechanics, the uncertainty principle (also known as Heisenberg's uncertainty principle) is any of a variety of mathematical inequalities asserting a fundamental limit to the accuracy with which the values for certain pairs of physic ...
.
If two matrices share their eigenvectors, they can be simultaneously diagonalized. In the basis where they are both diagonal, it is clear that their product does not depend on their order because multiplication of diagonal matrices is just multiplication of numbers. The uncertainty principle, by contrast, is an expression of the fact that often two matrices ''A'' and ''B'' do not always commute, i.e., that ''AB − BA'' does not necessarily equal 0. The fundamental commutation relation of matrix mechanics,
implies then that ''there are no states that simultaneously have a definite position and momentum''.
This principle of uncertainty holds for many other pairs of observables as well. For example, the energy does not commute with the position either, so it is impossible to precisely determine the position and energy of an electron in an atom.
Nobel Prize
In 1928,
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...
nominated Heisenberg, Born, and Jordan for the
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
. The announcement of the Nobel Prize in Physics for 1932 was delayed until November 1933. It was at that time that it was announced Heisenberg had won the Prize for 1932 "for the creation of quantum mechanics, the application of which has, inter alia, led to the discovery of the allotropic forms of hydrogen"
Nobel Prize in Physics
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
an
1933
– Nobel Prize Presentation Speech. and
Erwin Schrödinger
Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian physicist with Irish citizenship who developed a number of fundamental results in quantum theo ...
and
Paul Adrien Maurice Dirac shared the 1933 Prize "for the discovery of new productive forms of atomic theory".
It might well be asked why Born was not awarded the Prize in 1932, along with Heisenberg, and Bernstein proffers speculations on this matter. One of them relates to Jordan joining the
Nazi Party
The Nazi Party, officially the National Socialist German Workers' Party (german: Nationalsozialistische Deutsche Arbeiterpartei or NSDAP), was a far-right politics, far-right political party in Germany active between 1920 and 1945 that crea ...
on May 1, 1933, and becoming a
stormtrooper. Jordan's Party affiliations and Jordan's links to Born may well have affected Born's chance at the Prize at that time. Bernstein further notes that when Born finally won the Prize in 1954, Jordan was still alive, while the Prize was awarded for the statistical interpretation of quantum mechanics, attributable to Born alone.
Heisenberg's reactions to Born for Heisenberg receiving the Prize for 1932 and for Born receiving the Prize in 1954 are also instructive in evaluating whether Born should have shared the Prize with Heisenberg. On November 25, 1933, Born received a letter from Heisenberg in which he said he had been delayed in writing due to a "bad conscience" that he alone had received the Prize "for work done in Göttingen in collaboration – you, Jordan and I." Heisenberg went on to say that Born and Jordan's contribution to quantum mechanics cannot be changed by "a wrong decision from the outside."
In 1954, Heisenberg wrote an article honoring
Max Planck
Max Karl Ernst Ludwig Planck (, ; 23 April 1858 – 4 October 1947) was a German theoretical physicist whose discovery of energy quanta won him the Nobel Prize in Physics in 1918.
Planck made many substantial contributions to theoretical p ...
for his insight in 1900. In the article, Heisenberg credited Born and Jordan for the final mathematical formulation of matrix mechanics and Heisenberg went on to stress how great their contributions were to quantum mechanics, which were not "adequately acknowledged in the public eye."
Mathematical development
Once Heisenberg introduced the matrices for ''X'' and ''P'', he could find their matrix elements in special cases by guesswork, guided by the
correspondence principle. Since the matrix elements are the quantum mechanical analogs of Fourier coefficients of the classical orbits, the simplest case is the
harmonic oscillator
In classical mechanics, a harmonic oscillator is a system that, when displaced from its Mechanical equilibrium, equilibrium position, experiences a restoring force ''F'' Proportionality (mathematics), proportional to the displacement ''x'':
\v ...
, where the classical position and momentum, ''X''(''t'') and ''P''(''t''), are sinusoidal.
Harmonic oscillator
In units where the mass and frequency of the oscillator are equal to one (see
nondimensionalization
Nondimensionalization is the partial or full removal of dimensional analysis, physical dimensions from an mathematical equation, equation involving physical quantity, physical quantities by a suitable substitution of variables. This technique can ...
), the energy of the oscillator is
The
level set
In mathematics, a level set of a real-valued function of real variables is a set where the function takes on a given constant value , that is:
: L_c(f) = \left\~,
When the number of independent variables is two, a level set is calle ...
s of are the clockwise orbits, and they are nested circles in phase space. The classical orbit with energy is
The old quantum condition dictates that the integral of over an orbit, which is the area of the circle in phase space, must be an integer multiple of
Planck's constant. The area of the circle of radius is . So
or, in
natural units
In physics, natural units are physical units of measurement in which only universal physical constants are used as defining constants, such that each of these constants acts as a Coherence (units of measurement), coherent unit of a quantity. For e ...
where , the energy is an integer.
The
Fourier components of and are simple, and more so if they are combined into the quantities
Both and have only a single frequency, and ''X'' and ''P'' can be recovered from their sum and difference.
Since has a classical Fourier series with only the lowest frequency, and the matrix element is the -th Fourier coefficient of the classical orbit, the matrix for is nonzero only on the line just above the diagonal, where it is equal to . The matrix for is likewise only nonzero on the line below the diagonal, with the same elements. Thus,
from and , reconstruction yields
and
which, up to the choice of units, are the Heisenberg matrices for the harmonic oscillator. Both matrices are
hermitian {{Short description, none
Numerous things are named after the French mathematician Charles Hermite (1822–1901):
Hermite
* Cubic Hermite spline, a type of third-degree spline
* Gauss–Hermite quadrature, an extension of Gaussian quadrature m ...
, since they are constructed from the Fourier coefficients of real quantities.
Finding and is direct, since they are quantum Fourier coefficients so they evolve simply with time,
The matrix product of and is not hermitian, but has a real and imaginary part. The real part is one half the symmetric expression , while the imaginary part is proportional to the
commutator
In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory.
Group theory
The commutator of two elements, a ...
It is simple to verify explicitly that in the case of the harmonic oscillator, is , multiplied by the
identity
Identity may refer to:
* Identity document
* Identity (philosophy)
* Identity (social science)
* Identity (mathematics)
Arts and entertainment Film and television
* ''Identity'' (1987 film), an Iranian film
* ''Identity'' (2003 film), ...
.
It is likewise simple to verify that the matrix
is a
diagonal matrix
In linear algebra, a diagonal matrix is a matrix in which the entries outside the main diagonal are all zero; the term usually refers to square matrices. Elements of the main diagonal can either be zero or nonzero. An example of a 2×2 diagonal ma ...
, with
eigenvalues
In linear algebra, an eigenvector () or characteristic vector of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding eigenvalue, often denoted b ...
.
Conservation of energy
The harmonic oscillator is an important case. Finding the matrices is easier than determining the general conditions from these special forms. For this reason, Heisenberg investigated the
anharmonic oscillator
In classical mechanics, anharmonicity is the deviation of a system from being a harmonic oscillator. An oscillator that is not oscillating in harmonic motion is known as an anharmonic oscillator where the system can be approximated to a harmo ...
, with
Hamiltonian
Hamiltonian may refer to:
* Hamiltonian mechanics, a function that represents the total energy of a system
* Hamiltonian (quantum mechanics), an operator corresponding to the total energy of that system
** Dyall Hamiltonian, a modified Hamiltonian ...
In this case, the and matrices are no longer simple off diagonal matrices, since the corresponding classical orbits are slightly squashed and displaced, so that they have Fourier coefficients at every classical frequency. To determine the matrix elements, Heisenberg required that the classical equations of motion be obeyed as matrix equations,
He noticed that if this could be done, then , considered as a matrix function of and , will have zero time derivative.
where is the
anticommutator
In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory.
Group theory
The commutator of two elements, ...
,
Given that all the off diagonal elements have a nonzero frequency; being constant implies that is diagonal.
It was clear to Heisenberg that in this system, the energy could be exactly conserved in an arbitrary quantum system, a very encouraging sign.
The process of emission and absorption of photons seemed to demand that the conservation of energy will hold at best on average. If a wave containing exactly one photon passes over some atoms, and one of them absorbs it, that atom needs to tell the others that they can't absorb the photon anymore. But if the atoms are far apart, any signal cannot reach the other atoms in time, and they might end up absorbing the same photon anyway and dissipating the energy to the environment. When the signal reached them, the other atoms would have to somehow
recall
Recall may refer to:
* Recall (bugle call), a signal to stop
* Recall (information retrieval), a statistical measure
* ''ReCALL'' (journal), an academic journal about computer-assisted language learning
* Recall (memory)
* ''Recall'' (Overwatch ...
that energy. This paradox led
Bohr, Kramers and Slater to abandon exact conservation of energy. Heisenberg's formalism, when extended to include the electromagnetic field, was obviously going to sidestep this problem, a hint that the interpretation of the theory will involve
wavefunction collapse
In quantum mechanics, wave function collapse occurs when a wave function—initially in a superposition of several eigenstates—reduces to a single eigenstate due to interaction with the external world. This interaction is called an ''observa ...
.
Differentiation trick — canonical commutation relations
Demanding that the classical equations of motion are preserved is not a strong enough condition to determine the matrix elements. Planck's constant does not appear in the classical equations, so that the matrices could be constructed for many different values of and still satisfy the equations of motion, but with different energy levels.
So, in order to implement his program, Heisenberg needed to use the old quantum condition to fix the energy levels, then fill in the matrices with Fourier coefficients of the classical equations, then alter the matrix coefficients and the energy levels slightly to make sure the classical equations are satisfied. This is clearly not satisfactory. The old quantum conditions refer to the area enclosed by the sharp classical orbits, which do not exist in the new formalism.
The most important thing that Heisenberg discovered is how to translate the old quantum condition into a simple statement in matrix mechanics.
To do this, he investigated the action integral as a matrix quantity,
There are several problems with this integral, all stemming from the incompatibility of the matrix formalism with the old picture of orbits. Which period ''T'' should be used? ''Semiclassically'', it should be either ''m'' or ''n'', but the difference is order , and an answer to order is sought. The ''quantum'' condition tells us that ''J
mn'' is 2π''n'' on the diagonal, so the fact that ''J'' is classically constant tells us that the off-diagonal elements are zero.
His crucial insight was to differentiate the quantum condition with respect to ''n''. This idea only makes complete sense in the classical limit, where ''n'' is not an integer but the continuous
action variable ''J'', but Heisenberg performed analogous manipulations with matrices, where the intermediate expressions are sometimes discrete differences and sometimes derivatives.
In the following discussion, for the sake of clarity, the differentiation will be performed on the classical variables, and the transition to matrix mechanics will be done afterwards, guided by the correspondence principle.
In the classical setting, the derivative is the derivative with respect to ''J'' of the integral which defines ''J'', so it is tautologically equal to 1.
where the derivatives ''dP''/''dJ'' and ''dX''/''dJ'' should be interpreted as differences with respect to ''J'' at corresponding times on nearby orbits, exactly what would be obtained if the Fourier coefficients of the orbital motion were differentiated. (These derivatives are symplectically orthogonal in phase space to the time derivatives ''dP''/''dt'' and ''dX''/''dt'').
The final expression is clarified by introducing the variable canonically conjugate to ''J'', which is called the
angle variable ''θ'': The derivative with respect to time is a derivative with respect to ''θ'', up to a factor of 2π''T'',
So the quantum condition integral is the average value over one cycle of the
Poisson bracket
In mathematics and classical mechanics, the Poisson bracket is an important binary operation in Hamiltonian mechanics, playing a central role in Hamilton's equations of motion, which govern the time evolution of a Hamiltonian dynamical system. Th ...
of ''X'' and ''P''.
An analogous differentiation of the Fourier series of ''P dX'' demonstrates that the off-diagonal elements of the Poisson bracket are all zero. The Poisson bracket of two canonically conjugate variables, such as ''X'' and ''P'', is the constant value 1, so this integral really is the average value of 1; so it is 1, as we knew all along, because it is ''dJ/dJ'' after all. But Heisenberg, Born and Jordan, unlike Dirac, were not familiar with the theory of Poisson brackets, so, for them, the differentiation effectively evaluated in ''J, θ'' coordinates.
The Poisson Bracket, unlike the action integral, does have a simple translation to matrix mechanics−−it normally corresponds to the imaginary part of the product of two variables, the
commutator
In mathematics, the commutator gives an indication of the extent to which a certain binary operation fails to be commutative. There are different definitions used in group theory and ring theory.
Group theory
The commutator of two elements, a ...
.
To see this, examine the (antisymmetrized) product of two matrices ''A'' and ''B'' in the correspondence limit, where the matrix elements are slowly varying functions of the index, keeping in mind that the answer is zero classically.
In the correspondence limit, when indices ''m'', ''n'' are large and nearby, while ''k'',''r'' are small, the rate of change of the matrix elements in the diagonal direction is the matrix element of the ''J'' derivative of the corresponding classical quantity. So its possible to shift any matrix element diagonally through the correspondence,
where the right hand side is really only the (''m'' − ''n'')'th Fourier component of ''dA''/''dJ'' at the orbit near ''m'' to this semiclassical order, not a full well-defined matrix.
The semiclassical time derivative of a matrix element is obtained up to a factor of ''i'' by multiplying by the distance from the diagonal,
since the coefficient ''A''
''m''(''m''+''k'') is semiclassically the ''kth Fourier coefficient of the ''m''-th classical orbit.
The imaginary part of the product of ''A'' and ''B'' can be evaluated by shifting the matrix elements around so as to reproduce the classical answer, which is zero.
The leading nonzero residual is then given entirely by the shifting. Since all the matrix elements are at indices which have a small distance from the large index position (''m'',''m''), it helps to introduce two temporary notations:
for the matrices, and for the r'th Fourier components of classical quantities,
Flipping the summation variable in the first sum from to ''r = ''k'' − ''r'', the matrix element becomes,
and it is clear that the principal (classical) part cancels.
The leading quantum part, neglecting the higher order product of derivatives in the residual expression, is then equal to
so that, finally,
which can be identified with times the -th classical Fourier component of the Poisson bracket.
Heisenberg's original differentiation trick was eventually extended to a full semiclassical derivation of the quantum condition, in collaboration with Born and Jordan.
Once they were able to establish that
this condition replaced and extended the old quantization rule, allowing the matrix elements of ''P'' and ''X'' for an arbitrary system to be determined simply from the form of the Hamiltonian.
The new quantization rule was ''assumed to be universally true'', even though the derivation from the old quantum theory required semiclassical reasoning.
(A full quantum treatment, however, for more elaborate arguments of the brackets, was appreciated in the 1940s to amount to extending Poisson brackets to
Moyal bracket
In physics, the Moyal bracket is the suitably normalized antisymmetrization of the phase-space star product.
The Moyal bracket was developed in about 1940 by José Enrique Moyal, but Moyal only succeeded in publishing his work in 1949 after a le ...
s.)
State vectors and the Heisenberg equation
To make the transition to standard quantum mechanics, the most important further addition was the
quantum state vector
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement on a system. Knowledge of the quantum state together with the rules for the system's evolution in t ...
, now written , ''ψ''⟩,
which is the vector that the matrices act on. Without the state vector, it is not clear which particular motion the Heisenberg matrices are describing, since they include all the motions somewhere.
The interpretation of the state vector, whose components are written , was furnished by Born. This interpretation is statistical: the result of a measurement of the physical quantity corresponding to the matrix is random, with an average value equal to
Alternatively, and equivalently, the state vector gives the
probability amplitude
In quantum mechanics, a probability amplitude is a complex number used for describing the behaviour of systems. The modulus squared of this quantity represents a probability density.
Probability amplitudes provide a relationship between the quan ...
for the quantum system to be in the energy state .
Once the state vector was introduced, matrix mechanics could be rotated to ''any basis'', where the matrix need no longer be diagonal. The Heisenberg equation of motion in its original form states that evolves in time like a Fourier component,
which can be recast in differential form
and it can be restated so that it is true in an arbitrary basis, by noting that the matrix is diagonal with diagonal values ,
This is now a matrix equation, so it holds in any basis. This is the modern form of the Heisenberg equation of motion.
Its formal solution is:
All these forms of the equation of motion above say the same thing, that is equivalent to , through a basis rotation by the
unitary matrix
In linear algebra, a complex square matrix is unitary if its conjugate transpose is also its inverse, that is, if
U^* U = UU^* = UU^ = I,
where is the identity matrix.
In physics, especially in quantum mechanics, the conjugate transpose is ...
, a systematic picture elucidated by Dirac in his
bra–ket notation.
Conversely, by rotating the basis for the state vector at each time by , the time dependence in the matrices can be undone. The matrices are now time independent, but the state vector rotates,
This is the
Schrödinger equation
The Schrödinger equation is a linear partial differential equation that governs the wave function of a quantum-mechanical system. It is a key result in quantum mechanics, and its discovery was a significant landmark in the development of th ...
for the state vector, and this time-dependent change of basis amounts to transformation to the
Schrödinger picture
In physics, the Schrödinger picture is a formulation of quantum mechanics in which the state vectors evolve in time, but the operators (observables and others) are mostly constant with respect to time (an exception is the Hamiltonian which may ...
, with ⟨''x'', ''ψ''⟩ = ''ψ''(''x'').
In quantum mechanics in the
Heisenberg picture
In physics, the Heisenberg picture (also called the Heisenberg representation) is a formulation (largely due to Werner Heisenberg in 1925) of quantum mechanics in which the operators (observables and others) incorporate a dependency on time, but ...
the
state vector, , ''ψ''⟩ does not change with time, while an observable ''A'' satisfies the
''Heisenberg equation of motion'',
The extra term is for operators such as
which have an ''explicit time dependence'', in addition to the time dependence from the unitary evolution discussed.
The
Heisenberg picture
In physics, the Heisenberg picture (also called the Heisenberg representation) is a formulation (largely due to Werner Heisenberg in 1925) of quantum mechanics in which the operators (observables and others) incorporate a dependency on time, but ...
does not distinguish time from space, so it is better suited to
relativistic theories than the Schrödinger equation. Moreover, the similarity to
classical physics
Classical physics is a group of physics theories that predate modern, more complete, or more widely applicable theories. If a currently accepted theory is considered to be modern, and its introduction represented a major paradigm shift, then the ...
is more manifest: the Hamiltonian equations of motion for classical mechanics are recovered by replacing the commutator above by the
Poisson bracket
In mathematics and classical mechanics, the Poisson bracket is an important binary operation in Hamiltonian mechanics, playing a central role in Hamilton's equations of motion, which govern the time evolution of a Hamiltonian dynamical system. Th ...
(see also below). By the
Stone–von Neumann theorem, the Heisenberg picture and the Schrödinger picture must be unitarily equivalent, as detailed below.
Further results
Matrix mechanics rapidly developed into modern quantum mechanics, and gave interesting physical results on the spectra of atoms.
Wave mechanics
Jordan noted that the commutation relations ensure that ''P acts as a differential operator''.
The operator identity