mathematical formulation of quantum mechanics

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The mathematical formulations of quantum mechanics are those mathematical formalisms that permit a rigorous description of
quantum mechanics Quantum mechanics is a fundamental Scientific theory, 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 qua ...
. This mathematical formalism uses mainly a part of
functional analysis Functional analysis is a branch of mathematical analysis Analysis is the branch of mathematics dealing with continuous functions, limit (mathematics), limits, and related theories, such as Derivative, differentiation, Integral, integration, ...
, especially
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 naturally ...
s, which are a kind of
linear space In mathematics and physics, a vector space (also called a linear space) is a set (mathematics), set whose elements, often called ''vector (mathematics and physics), vectors'', may be Vector addition, added together and Scalar multiplication, mu ...
. Such are distinguished from mathematical formalisms for physics theories developed prior to the early 1900s by the use of abstract mathematical structures, such as infinite-dimensional
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 naturally ...
s ( ''L''2 space mainly), and
operators Operator may refer to: Mathematics * A symbol indicating a mathematical operation * Logical operator or logical connective in mathematical logic * Operator (mathematics), mapping that acts on elements of a space to produce elements of another sp ...
on these spaces. In brief, values of physical
observable In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...
s such as
energy In physics, energy (from Ancient Greek: wikt:ἐνέργεια#Ancient_Greek, ἐνέργεια, ''enérgeia'', “activity”) is the physical quantity, quantitative physical property, property that is #Energy transfer, transferred to a phy ...
and
momentum In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the Multiplication, product of the mass and velocity of an object. It is a Euclidean vector, vector quantity, possessing a magnitude and a dire ...
were no longer considered as values of functions on
phase space In Dynamical systems theory, dynamical system theory, a phase space is a Space (mathematics), space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. F ...
, but as
eigenvalue In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces ...
s; more precisely as spectral values of linear operators in Hilbert space. These formulations of quantum mechanics continue to be used today. At the heart of the description are ideas of ''
quantum state In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement in quantum mechanics, measurement on a system. Knowledge of the quantum state together with the rul ...
'' and ''quantum observables'', which are radically different from those used in previous models of physical reality. While the mathematics permits calculation of many quantities that can be measured experimentally, there is a definite theoretical limit to values that can be simultaneously measured. This limitation was first elucidated by Heisenberg through a
thought experiment A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences. History The ancient Greek ''deiknymi'' (), or thought experiment, "was the most anci ...
, and is represented mathematically in the new formalism by the non-commutativity of operators representing quantum observables. Prior to the development of quantum mechanics as a separate
theory A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may ...
, the mathematics used in physics consisted mainly of formal
mathematical analysis Analysis is the branch of mathematics dealing with continuous functions, limit (mathematics), limits, and related theories, such as Derivative, differentiation, Integral, integration, measure (mathematics), measure, infinite sequences, series (m ...
, beginning with
calculus Calculus, originally called infinitesimal calculus or "the calculus of infinitesimals", is the mathematics, mathematical study of continuous change, in the same way that geometry is the study of shape, and algebra is the study of generalizati ...
, and increasing in complexity up to
differential geometry Differential geometry is a Mathematics, mathematical discipline that studies the geometry of smooth shapes and smooth spaces, otherwise known as smooth manifolds. It uses the techniques of differential calculus, integral calculus, linear algebra ...
and
partial differential equation In mathematics, a partial differential equation (PDE) is an equation which imposes relations between the various partial derivatives of a Multivariable calculus, multivariable function. The function is often thought of as an "unknown" to be sol ...
s.
Probability theory Probability theory is the branch of mathematics concerned with probability. Although there are several different probability interpretations, probability theory treats the concept in a rigorous mathematical manner by expressing it through a set o ...
was used in
statistical mechanics In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...
. Geometric intuition played a strong role in the first two and, accordingly, theories of relativity were formulated entirely in terms of differential geometric concepts. The phenomenology of quantum physics arose roughly between 1895 and 1915, and for the 10 to 15 years before the development of quantum mechanics (around 1925) physicists continued to think of quantum theory within the confines of what is now called
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 ...
, and in particular within the same mathematical structures. The most sophisticated example of this is the Sommerfeld–Wilson–Ishiwara quantization rule, which was formulated entirely on the classical
phase space In Dynamical systems theory, dynamical system theory, a phase space is a Space (mathematics), space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. F ...
.

# History of the formalism

## The "old quantum theory" and the need for new mathematics

In the 1890s, Planck was able to derive the blackbody spectrum, which was later used to avoid the classical
ultraviolet catastrophe The ultraviolet catastrophe, also called the Rayleigh–Jeans catastrophe, was the prediction of late 19th century/early 20th century classical physics that an ideal black body at thermal equilibrium would emit an infinity, unbounded quantity o ...
by making the unorthodox assumption that, in the interaction of
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic field, electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, inf ...
with
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...
, energy could only be exchanged in discrete units which he called quanta. Planck postulated a direct proportionality between the frequency of radiation and the quantum of energy at that frequency. The proportionality constant, , is now called Planck's constant in his honor. In 1905,
Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born Theoretical physics, theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for d ...
explained certain features of the
photoelectric effect The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and Solid-stat ...
by assuming that Planck's energy quanta were actual particles, which were later dubbed
photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...
. All of these developments were phenomenological and challenged the theoretical physics of the time. Bohr and Sommerfeld went on to modify
classical mechanics Classical mechanics is a Theoretical physics, physical theory describing the motion of macroscopic objects, from projectiles to parts of Machine (mechanical), machinery, and astronomical objects, such as spacecraft, planets, stars, and galax ...
in an attempt to deduce 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 Syste ...
from first principles. They proposed that, of all closed classical orbits traced by a mechanical system in its
phase space In Dynamical systems theory, dynamical system theory, a phase space is a Space (mathematics), space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. F ...
, only the ones that enclosed an area which was a multiple of Planck's constant were actually allowed. The most sophisticated version of this formalism was the so-called Sommerfeld–Wilson–Ishiwara quantization. Although the Bohr model of the hydrogen atom could be explained in this way, the spectrum of the helium atom (classically an unsolvable 3-body problem) could not be predicted. The mathematical status of quantum theory remained uncertain for some time. In 1923, de Broglie proposed that
wave–particle duality Wave–particle duality is the concept in quantum mechanics that every particle or quantum entity may be described as either a particle or a wave. It expresses the inability of the classical physics, classical concepts "particle" or "wave" to fu ...
applied not only to photons but to electrons and every other physical system. The situation changed rapidly in the years 1925–1930, when working mathematical foundations were found through the groundbreaking work of Erwin Schrödinger,
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 Quantum mechanics is a fundamental Scientific theory, theory in physics ...
,
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 ...
,
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 matrix ...
, and the foundational work of
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 cover ...
,
Hermann Weyl Hermann Klaus Hugo Weyl, (; 9 November 1885 – 8 December 1955) was a German mathematician, theoretical physicist and philosopher. Although much of his working life was spent in Zürich, Switzerland, and then Princeton, New Jersey, he is assoc ...
and
Paul Dirac Paul Adrien Maurice Dirac (; 8 August 1902 – 20 October 1984) was an English theoretical physicist who is regarded as one of the most significant physicists of the 20th century. He was the Lucasian Professor of Mathematics at the Univer ...
, and it became possible to unify several different approaches in terms of a fresh set of ideas. The physical interpretation of the theory was also clarified in these years after
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 Quantum mechanics is a fundamental Scientific theory, theory in physics ...
discovered the uncertainty relations and
Niels Bohr Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danes, Danish physicist who made foundational contributions to understanding atomic structure and old quantum theory, quantum theory, for which he received the Nobel ...
introduced the idea of complementarity.

## The "new quantum theory"

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 Quantum mechanics is a fundamental Scientific theory, theory in physics ...
's
matrix mechanics Matrix mechanics is a formulation of quantum mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account of Atomic el ...
was the first successful attempt at replicating the observed quantization of atomic spectra. Later in the same year, Schrödinger created his wave mechanics. Schrödinger's formalism was considered easier to understand, visualize and calculate as it led to
differential equations In mathematics, a differential equation is an functional equation, equation that relates one or more unknown function (mathematics), functions and their derivatives. In applications, the functions generally represent physical quantities, the der ...
, which physicists were already familiar with solving. Within a year, it was shown that the two theories were equivalent. Schrödinger himself initially did not understand the fundamental probabilistic nature of quantum mechanics, as he thought that the absolute square of the wave function of an
electron The electron ( or ) is a subatomic particle with a negative one elementary charge, elementary electric charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are generally thought t ...
should be interpreted as the
charge density In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the Systeme Internat ...
of an object smeared out over an extended, possibly infinite, volume of space. It was
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 ...
who introduced the interpretation of the absolute square of the wave function as the probability distribution of the position of a ''pointlike'' object. Born's idea was soon taken over by Niels Bohr in Copenhagen who then became the "father" of the
Copenhagen interpretation The Copenhagen interpretation is a collection of views about the meaning of quantum mechanics, principally attributed to Niels Bohr and Werner Heisenberg. It is one of the oldest of numerous proposed interpretations of quantum mechanics, as fe ...
of quantum mechanics. Schrödinger's
wave function A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex number, complex-valued probability amplitude, and the probabilities for the possible results of ...
can be seen to be closely related to the classical
Hamilton–Jacobi equation In physics, the Hamilton–Jacobi equation, named after William Rowan Hamilton and Carl Gustav Jacob Jacobi, is an alternative formulation of classical mechanics, equivalent to other formulations such as Newton's laws of motion, Lagrangian mechan ...
. The correspondence to classical mechanics was even more explicit, although somewhat more formal, in Heisenberg's matrix mechanics. In his PhD thesis project,
Paul Dirac Paul Adrien Maurice Dirac (; 8 August 1902 – 20 October 1984) was an English theoretical physicist who is regarded as one of the most significant physicists of the 20th century. He was the Lucasian Professor of Mathematics at the Univer ...
discovered that the equation for the operators in the Heisenberg representation, as it is now called, closely translates to classical equations for the dynamics of certain quantities in the Hamiltonian formalism of classical mechanics, when one expresses them through Poisson brackets, a procedure now known as
canonical quantization In physics, canonical quantization is a procedure for quantization (physics), quantizing a classical theory, while attempting to preserve the formal structure, such as symmetry (physics), symmetries, of the classical theory, to the greatest extent ...
. To be more precise, already before Schrödinger, the young postdoctoral fellow
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 Quantum mechanics is a fundamental Scientific theory, theory in physics ...
invented his
matrix mechanics Matrix mechanics is a formulation of quantum mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account of Atomic el ...
, which was the first correct quantum mechanics–– the essential breakthrough. Heisenberg's
matrix mechanics Matrix mechanics is a formulation of quantum mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account of Atomic el ...
formulation was based on algebras of infinite matrices, a very radical formulation in light of the mathematics of classical physics, although he started from the index-terminology of the experimentalists of that time, not even aware that his "index-schemes" were matrices, as Born soon pointed out to him. In fact, in these early years,
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrix (mat ...
was not generally popular with physicists in its present form. Although Schrödinger himself after a year proved the equivalence of his wave-mechanics and Heisenberg's matrix mechanics, the reconciliation of the two approaches and their modern abstraction as motions in Hilbert space is generally attributed to
Paul Dirac Paul Adrien Maurice Dirac (; 8 August 1902 – 20 October 1984) was an English theoretical physicist who is regarded as one of the most significant physicists of the 20th century. He was the Lucasian Professor of Mathematics at the Univer ...
, who wrote a lucid account in his 1930 classic '' The Principles of Quantum Mechanics''. He is the third, and possibly most important, pillar of that field (he soon was the only one to have discovered a relativistic generalization of the theory). In his above-mentioned account, he introduced the
bra–ket notation In quantum mechanics, bra–ket notation, or Dirac notation, is used ubiquitously to denote quantum states. The notation uses angle brackets, and , and a vertical bar , to construct "bras" and "kets". A ket is of the form , v \rangle. Mathemat ...
, together with an abstract formulation in terms of the
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 naturally ...
used in
functional analysis Functional analysis is a branch of mathematical analysis Analysis is the branch of mathematics dealing with continuous functions, limit (mathematics), limits, and related theories, such as Derivative, differentiation, Integral, integration, ...
; he showed that Schrödinger's and Heisenberg's approaches were two different representations of the same theory, and found a third, most general one, which represented the dynamics of the system. His work was particularly fruitful in many types of generalizations of the field. The first complete mathematical formulation of this approach, known as the Dirac–von Neumann axioms, is generally credited to
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 cover ...
's 1932 book '' Mathematical Foundations of Quantum Mechanics'', although
Hermann Weyl Hermann Klaus Hugo Weyl, (; 9 November 1885 – 8 December 1955) was a German mathematician, theoretical physicist and philosopher. Although much of his working life was spent in Zürich, Switzerland, and then Princeton, New Jersey, he is assoc ...
had already referred to Hilbert spaces (which he called ''unitary spaces'') in his 1927 classic paper and book. It was developed in parallel with a new approach to the mathematical spectral theory based on
linear operator In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...
s rather than the
quadratic form In mathematics, a quadratic form is a polynomial with terms all of Degree of a polynomial, degree two ("form (mathematics), form" is another name for a homogeneous polynomial). For example, :4x^2 + 2xy - 3y^2 is a quadratic form in the variables ...
s that were
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 approach a generation earlier. Though theories of quantum mechanics continue to evolve to this day, there is a basic framework for the mathematical formulation of quantum mechanics which underlies most approaches and can be traced back to the mathematical work of
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 cover ...
. In other words, discussions about ''interpretation'' of the theory, and extensions to it, are now mostly conducted on the basis of shared assumptions about the mathematical foundations.

## Later developments

The application of the new quantum theory to electromagnetism resulted in
quantum field theory In theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict List of natural phenomena, natural phenomena. This is in c ...
, which was developed starting around 1930. Quantum field theory has driven the development of more sophisticated formulations of quantum mechanics, of which the ones presented here are simple special cases. *
Path integral formulation The path integral formulation is a description in quantum mechanics Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatom ...
* Phase-space formulation of quantum mechanics & geometric quantization * quantum field theory in curved spacetime *
axiomatic An axiom, postulate, or assumption is a statement (logic), statement that is taken to be truth, true, to serve as a premise or starting point for further reasoning and arguments. The word comes from the Ancient Greek word (), meaning 'that whi ...
, algebraic and constructive quantum field theory *
C*-algebra In mathematics, specifically in functional analysis, a C∗-algebra (pronounced "C-star") is a Banach algebra together with an involution (mathematics), involution satisfying the properties of the Hermitian adjoint, adjoint. A particular case is t ...
formalism Formalism may refer to: * Form (disambiguation) * Formal (disambiguation) * Legal formalism, legal positivist view that the substantive justice of a law is a question for the legislature rather than the judiciary * Formalism (linguistics) * Scient ...
* Generalized statistical model of quantum mechanics A related topic is the relationship to classical mechanics. Any new physical theory is supposed to reduce to successful old theories in some approximation. For quantum mechanics, this translates into the need to study the so-called classical limit of quantum mechanics. Also, as Bohr emphasized, human cognitive abilities and language are inextricably linked to the classical realm, and so classical descriptions are intuitively more accessible than quantum ones. In particular, quantization, namely the construction of a quantum theory whose classical limit is a given and known classical theory, becomes an important area of quantum physics in itself. Finally, some of the originators of quantum theory (notably Einstein and Schrödinger) were unhappy with what they thought were the philosophical implications of quantum mechanics. In particular, Einstein took the position that quantum mechanics must be incomplete, which motivated research into so-called hidden-variable theories. The issue of hidden variables has become in part an experimental issue with the help of
quantum optics Quantum optics is a branch of Atomic, molecular, and optical physics, atomic, molecular, and optical physics dealing with how individual quanta of light, known as Photon, photons, interact with atoms and molecules. It includes the study of the par ...
.

# Postulates of quantum mechanics

A physical system is generally described by three basic ingredients: states;
observable In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...
s; and dynamics (or law of time evolution) or, more generally, a group of physical symmetries. A classical description can be given in a fairly direct way by a
phase space In Dynamical systems theory, dynamical system theory, a phase space is a Space (mathematics), space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. F ...
model A model is an informative representation of an object, person or system. The term originally denoted the Plan_(drawing), plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin ''modulus'', a mea ...
of mechanics: states are points in a phase space formulated by
symplectic manifold In differential geometry, a subject of mathematics, a symplectic manifold is a Differentiable manifold#Definition, smooth manifold, M , equipped with a Closed and exact differential forms, closed nondegenerate form, nondegenerate Differential for ...
, observables are real-valued functions on it, time evolution is given by a one-parameter group of symplectic transformations of the phase space, and physical symmetries are realized by symplectic transformations. A quantum description normally consists of a
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 naturally ...
of states, observables are
self-adjoint operator In mathematics, a self-adjoint operator on an infinite-dimensional complex vector space ''V'' with inner product \langle\cdot,\cdot\rangle (equivalently, a Hermitian operator in the finite-dimensional case) is a linear map ''A'' (from ''V'' to itse ...
s on the space of states, time evolution is given by a
one-parameter group In mathematics, a one-parameter group or one-parameter subgroup usually means a continuous (topology), continuous group homomorphism :\varphi : \mathbb \rightarrow G from the real line \mathbb (as an Abelian group, additive group) to some other ...
of unitary transformations on the Hilbert space of states, and physical symmetries are realized by unitary transformations. (It is possible, to map this Hilbert-space picture to a phase space formulation, invertibly. See below.) The following summary of the mathematical framework of quantum mechanics can be partly traced back to the Dirac–von Neumann axioms.

## Description of the state of a system

Each isolated physical system is associated with a (topologically) Separable space, separable complex number, complex
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 naturally ...
with inner product . Ray (quantum theory), Rays (that is, subspaces of ''complex'' dimension 1) in are associated with quantum state, quantum states of the system. In other words, quantum states can be identified with equivalence classes (rays) of vectors of length 1 in , where two vectors represent the same state if they differ only by a phase factor. ''Separability'' is a mathematically convenient hypothesis, with the physical interpretation that countably many observations are enough to uniquely determine the state. A quantum mechanical state is a ''ray'' in projective Hilbert space, not a ''vector''. Many textbooks fail to make this distinction, which could be partly a result of the fact that the Schrödinger equation itself involves Hilbert-space "vectors", with the result that the imprecise use of "state vector" rather than ''ray'' is very difficult to avoid. Accompanying Postulate I is the composite system postulate: In the presence of quantum entanglement, the quantum state of the composite system cannot be factored as a tensor product of states of its local constituents; Instead, it is expressed as a sum, or Quantum superposition, superposition, of tensor products of states of component subsystems. A subsystem in an entangled composite system generally can't be described by a state vector (or a ray), but instead is described by a density matrix, density operator; Such quantum state is known as a Mixed state (physics), mixed state. The density operator of a mixed state is a trace class, nonnegative (Positive semi-definite matrix, positive semi-definite) Self-adjoint operator, self-adjoint operator normalized to be of Trace (linear algebra), trace 1. In turn, any density operator of a mixed state can be represented as a subsystem of a larger composite system in a pure state (see Schrödinger–HJW theorem, purification theorem). In the absence of quantum entanglement, the quantum state of the composite system is called a separable state. The density matrix of a bipartite system in a separable state can be expressed as $\rho=\sum_k p_k \rho_1^k \otimes \rho_2^k$, where $\; \sum_k p_k = 1$. If there is only a single non-zero $p_k$, then the state can be expressed just as $\rho = \rho_1 \otimes \rho_2 ,$ and is called simply separable or product state.

## Measurement on a system

### Description of physical quantities

Physical
observable In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Physical scien ...
s are represented by Hermitian matrix, Hermitian matrices on . Since these operators are Hermitian, their Eigenvalue, eigenvalues are always real, and represent the possible outcomes/results from measuring the corresponding observable. If the spectrum of the observable is Discrete spectrum, discrete, then the possible results are ''quantized''.

### Results of measurement

By spectral theory, we can associate a probability measure to the values of in any state . We can also show that the possible values of the observable in any state must belong to the spectrum of an operator, spectrum of . The expected value, expectation value (in the sense of probability theory) of the observable for the system in state represented by the unit vector ∈ ''H'' is $\langle\psi, A, \psi\rangle$. If we represent the state in the basis formed by the eigenvectors of , then the square of the modulus of the component attached to a given eigenvector is the probability of observing its corresponding eigenvalue. For a mixed state , the expected value of in the state is $\operatorname\left(A\rho\right)$, and the probability of obtaining an eigenvalue $a_n$ in a discrete, nondegenerate spectrum of the corresponding observable $A$ is given by $\mathbb P\left(a_n\right)=\operatorname\left(, a_n\rangle\langle a_n, \rho\right)=\langle a_n, \rho, a_n\rangle$. If the eigenvalue $a_n$ has degenerate, orthonormal eigenvectors $\$, then the Projection (linear algebra), projection operator onto the eigensubspace can be defined as the identity operator in the eigensubspace: $P_n=, a_\rangle\langle a_, +, a_\rangle\langle a_, + \dots + , a_\rangle\langle a_, ,$ and then $\mathbb P\left(a_n\right)=\operatorname\left(P_n\rho\right)$. Postulates II.a and II.b are collectively known as the Born rule of quantum mechanics.

### Effect of measurement on the state

When a measurement is performed, only one result is obtained (according to some interpretations of quantum mechanics). This is modeled mathematically as the processing of additional information from the measurement, confining the probabilities of an immediate second measurement of the same observable. In the case of a discrete, non-degenerate spectrum, two sequential measurements of the same observable will always give the same value assuming the second immediately follows the first. Therefore the state vector must change as a result of measurement, and ''collapse'' onto the eigensubspace associated with the eigenvalue measured. For a mixed state , after obtaining an eigenvalue $a_n$ in a discrete, nondegenerate spectrum of the corresponding observable $A$, the updated state is given by $\rho'=\frac$. If the eigenvalue $a_n$ has degenerate, orthonormal eigenvectors $\$, then the Projection (linear algebra), projection operator onto the eigensubspace is $P_n=, a_\rangle\langle a_, +, a_\rangle\langle a_, + \dots + , a_\rangle\langle a_,$. Postulates II.c is sometimes called the "state update rule" or "collapse rule"; Together with the Born rule (Postulates II.a and II.b), they form a complete representation of Measurement in quantum mechanics, measurements, and are sometimes collectively called the measurement postulate(s). Note that the Projection-valued measure, projection-valued measures (PVM) described in the measurement postulate(s) can be generalized to POVM, positive operator-valued measures (POVM), which is the most general kind of measurement in quantum mechanics. A POVM can be understood as the effect on a component subsystem when a PVM is performed on a larger, composite system (see Naimark's dilation theorem).

## Time evolution of a system

Though it is possible to derive the Schrödinger equation, which describes how a state vector evolves in time, most texts assert the equation as a postulate. Common derivations include using the DeBroglie hypothesis or Relation between Schrödinger's equation and the path integral formulation of quantum mechanics, path integrals. Equivalently, the time evolution postulate can be stated as: For a closed system in a mixed state , the time evolution is $\rho\left(t\right)=U\left(t;t_0\right)\rho\left(t_0\right) U^\dagger\left(t;t_0\right)$. The evolution of an open quantum system can be described by Quantum operation, quantum operations (in an Quantum operation#Statement of the theorem, operator sum formalism) and Quantum instrument, quantum instruments, and generally does not have to be unitary.

## Other implications of the postulates

* Physical symmetries act on the Hilbert space of quantum states unitary operator, unitarily or antiunitary, antiunitarily due to Wigner's theorem (supersymmetry is another matter entirely). * Density operators are those that are in the closure of the convex hull of the one-dimensional orthogonal projectors. Conversely, one-dimensional orthogonal projectors are extreme points of the set of density operators. Physicists also call one-dimensional orthogonal projectors ''pure states'' and other density operators ''mixed states''. * One can in this formalism state Heisenberg's uncertainty principle and prove it as a theorem, although the exact historical sequence of events, concerning who derived what and under which framework, is the subject of historical investigations outside the scope of this article. * Recent research has shown that the composite system postulate (tensor product postulate) can be derived from the state postulate (Postulate I) and the measurement postulates (Postulates II); Moreover, it has also been shown that the measurement postulates (Postulates II) can be derived from "unitary quantum mechanics", which includes only the state postulate (Postulate I), the composite system postulate (tensor product postulate) and the unitary evolution postulate (Postulate III). Furthermore, to the postulates of quantum mechanics one should also add basic statements on the properties of spin (physics), spin and Pauli's Pauli exclusion principle, exclusion principle, see below.

## Spin

In addition to their other properties, all particles possess a quantity called Spin (physics), spin, an ''intrinsic angular momentum''. Despite the name, particles do not literally spin around an axis, and quantum mechanical spin has no correspondence in classical physics. In the position representation, a spinless wavefunction has position and time as continuous variables, . For spin wavefunctions the spin is an additional discrete variable: , where takes the values; $\sigma = -S \hbar , -(S-1) \hbar , \dots, 0, \dots ,+(S-1) \hbar ,+S \hbar \,.$ That is, the state of a single particle with spin is represented by a -component spinor of complex-valued wave functions. Two classes of particles with ''very different'' behaviour are bosons which have integer spin (), and fermions possessing half-integer spin ().

## Pauli's principle

The property of spin relates to another basic property concerning systems of identical particles: Pauli's Pauli exclusion principle, exclusion principle, which is a consequence of the following permutation behaviour of an -particle wave function; again in the position representation one must postulate that for the transposition of any two of the particles one always should have i.e., on Transposition (mathematics), transposition of the arguments of any two particles the wavefunction should ''reproduce'', apart from a prefactor which is for bosons, but () for fermions. Electrons are fermions with ; quanta of light are bosons with . In nonrelativistic quantum mechanics all particles are either bosons or fermions; in relativistic quantum theories also Supersymmetry, "supersymmetric" theories exist, where a particle is a linear combination of a bosonic and a fermionic part. Only in dimension can one construct entities where is replaced by an arbitrary complex number with magnitude 1, called anyons. Although ''spin'' and the ''Pauli principle'' can only be derived from relativistic generalizations of quantum mechanics, the properties mentioned in the last two paragraphs belong to the basic postulates already in the non-relativistic limit. Especially, many important properties in natural science, e.g. the periodic system of chemistry, are consequences of the two properties.

# Mathematical structure of quantum mechanics

Summary:

## Representations

The original form of the Schrödinger equation depends on choosing a particular representation of Werner Heisenberg, Heisenberg's canonical commutation relations. The Stone–von Neumann theorem dictates that all irreducible representations of the finite-dimensional Heisenberg commutation relations are unitarily equivalent. A systematic understanding of its consequences has led to the phase space formulation of quantum mechanics, which works in full
phase space In Dynamical systems theory, dynamical system theory, a phase space is a Space (mathematics), space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. F ...
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 naturally ...
, so then with a more intuitive link to the classical limit thereof. This picture also simplifies considerations of quantization, the deformation extension from classical to quantum mechanics. The quantum harmonic oscillator is an exactly solvable system where the different representations are easily compared. There, apart from the Heisenberg, or Schrödinger (position or momentum), or phase-space representations, one also encounters the Fock (number) representation and the Oscillator representation, Segal–Bargmann (Fock-space or coherent state) representation (named after Irving Segal and Valentine Bargmann). All four are unitarily equivalent.

## Time as an operator

The framework presented so far singles out time as ''the'' parameter that everything depends on. It is possible to formulate mechanics in such a way that time becomes itself an observable associated with a self-adjoint operator. At the classical level, it is possible to arbitrarily parameterize the trajectories of particles in terms of an unphysical parameter , and in that case the time ''t'' becomes an additional generalized coordinate of the physical system. At the quantum level, translations in would be generated by a "Hamiltonian" , where ''E'' is the energy operator and is the "ordinary" Hamiltonian. However, since ''s'' is an unphysical parameter, ''physical'' states must be left invariant by "''s''-evolution", and so the physical state space is the kernel of (this requires the use of a rigged Hilbert space and a renormalization of the norm). This is related to the Dirac bracket, quantization of constrained systems and quantization of gauge theories. It is also possible to formulate a quantum theory of "events" where time becomes an observable (see D. Edwards).

# The problem of measurement

The picture given in the preceding paragraphs is sufficient for description of a completely isolated system. However, it fails to account for one of the main differences between quantum mechanics and classical mechanics, that is, the effects of measurement.G. Greenstein and A. Zajonc
/ref> The von Neumann description of quantum measurement of an observable , when the system is prepared in a pure state is the following (note, however, that von Neumann's description dates back to the 1930s and is based on experiments as performed during that time – more specifically the Compton scattering, Compton–Simon experiment; it is not applicable to most present-day measurements within the quantum domain): * Let have spectral resolution $A = \int \lambda \, d \operatorname_A(\lambda),$ where is the resolution of the identity (also called projection-valued measure) associated with . Then the probability of the measurement outcome lying in an interval of is . In other words, the probability is obtained by integrating the characteristic function of against the countably additive measure $\langle \psi \mid \operatorname_A \psi \rangle.$ * If the measured value is contained in , then immediately after the measurement, the system will be in the (generally non-normalized) state . If the measured value does not lie in , replace by its complement for the above state. For example, suppose the state space is the -dimensional complex Hilbert space and is a Hermitian matrix with eigenvalues , with corresponding eigenvectors . The projection-valued measure associated with , , is then $\operatorname_A (B) = , \psi_i\rangle \langle \psi_i, ,$ where is a Borel set containing only the single eigenvalue . If the system is prepared in state $, \psi \rangle$ Then the probability of a measurement returning the value can be calculated by integrating the spectral measure $\langle \psi \mid \operatorname_A \psi \rangle$ over . This gives trivially $\langle \psi, \psi_i\rangle \langle \psi_i \mid \psi \rangle = , \langle \psi \mid \psi_i\rangle , ^2.$ The characteristic property of the von Neumann measurement scheme is that repeating the same measurement will give the same results. This is also called the ''projection postulate''. A more general formulation replaces the projection-valued measure with a POVM, positive-operator valued measure (POVM). To illustrate, take again the finite-dimensional case. Here we would replace the rank-1 projections $, \psi_i\rangle \langle \psi_i,$ by a finite set of positive operators $F_i F_i^*$ whose sum is still the identity operator as before (the resolution of identity). Just as a set of possible outcomes is associated to a projection-valued measure, the same can be said for a POVM. Suppose the measurement outcome is . Instead of collapsing to the (unnormalized) state $, \psi_i\rangle \langle \psi_i , \psi\rangle$ after the measurement, the system now will be in the state $F_i , \psi\rangle.$ Since the operators need not be mutually orthogonal projections, the projection postulate of von Neumann no longer holds. The same formulation applies to general mixed state (physics), mixed states. In von Neumann's approach, the state transformation due to measurement is distinct from that due to time evolution in several ways. For example, time evolution is deterministic and unitary whereas measurement is non-deterministic and non-unitary. However, since both types of state transformation take one quantum state to another, this difference was viewed by many as unsatisfactory. The POVM formalism views measurement as one among many other quantum operations, which are described by completely positive maps which do not increase the trace. In any case it seems that the above-mentioned problems can only be resolved if the time evolution included not only the quantum system, but also, and essentially, the classical measurement apparatus (see above).

## The ''relative state'' interpretation

An alternative interpretation of measurement is Everett's many-worlds interpretation, relative state interpretation, which was later dubbed the "many-worlds interpretation" of quantum physics.

# List of mathematical tools

Part of the folklore of the subject concerns the mathematical physics textbook Methods of Mathematical Physics put together by Richard Courant from
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 Göttingen University courses. The story is told (by mathematicians) that physicists had dismissed the material as not interesting in the current research areas, until the advent of Schrödinger's equation. At that point it was realised that the mathematics of the new quantum mechanics was already laid out in it. It is also said that Heisenberg had consulted Hilbert about his
matrix mechanics Matrix mechanics is a formulation of quantum mechanics created by Werner Heisenberg, Max Born, and Pascual Jordan in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account of Atomic el ...
, and Hilbert observed that his own experience with infinite-dimensional matrices had derived from differential equations, advice which Heisenberg ignored, missing the opportunity to unify the theory as Weyl and Dirac did a few years later. Whatever the basis of the anecdotes, the mathematics of the theory was conventional at the time, whereas the physics was radically new. The main tools include: *
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrix (mat ...
: complex numbers, eigenvectors,
eigenvalue In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces ...
s *
functional analysis Functional analysis is a branch of mathematical analysis Analysis is the branch of mathematics dealing with continuous functions, limit (mathematics), limits, and related theories, such as Derivative, differentiation, Integral, integration, ...
:
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 naturally ...
s,
linear operator In mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in ...
s, spectral theory *
differential equations In mathematics, a differential equation is an functional equation, equation that relates one or more unknown function (mathematics), functions and their derivatives. In applications, the functions generally represent physical quantities, the der ...
: partial differential equations, separation of variables, ordinary differential equations, Sturm–Liouville theory, eigenfunctions * harmonic analysis: Fourier transforms

# References

* John von Neumann, J. von Neumann, ''Mathematical Foundations of Quantum Mechanics'' (1932), Princeton University Press, 1955. Reprinted in paperback form. * Hermann Weyl, H. Weyl, ''The Theory of Groups and Quantum Mechanics'', Dover Publications, 1950. * Andrew Gleason, A. Gleason, ''Measures on the Closed Subspaces of a Hilbert Space'', Journal of Mathematics and Mechanics, 1957. * George Mackey, G. Mackey, ''Mathematical Foundations of Quantum Mechanics'', W. A. Benjamin, 1963 (paperback reprint by Dover 2004). * R. F. Streater and A. S. Wightman, ''PCT, Spin and Statistics and All That'', Benjamin 1964 (Reprinted by Princeton University Press) * R. Jost, ''The General Theory of Quantized Fields'', American Mathematical Society, 1965. * J. M. Jauch, ''Foundations of quantum mechanics'', Addison-Wesley Publ. Cy., Reading, Massachusetts, 1968. * G. Emch, ''Algebraic Methods in Statistical Mechanics and Quantum Field Theory'', Wiley-Interscience, 1972. * Michael C. Reed, M. Reed and Barry Simon, B. Simon, ''Methods of Mathematical Physics'', vols I–IV, Academic Press 1972. * Thomas Samuel Kuhn, T. S. Kuhn, ''Black-Body Theory and the Quantum Discontinuity, 1894–1912'', Clarendon Press, Oxford and Oxford University Press, New York, 1978. * D. Edwards, ''The Mathematical Foundations of Quantum Mechanics'', Synthese, 42 (1979),pp. 1–70. * R. Shankar, "Principles of Quantum Mechanics", Springer, 1980. * E. Prugovecki, ''Quantum Mechanics in Hilbert Space'', Dover, 1981. * S. Auyang, ''How is Quantum Field Theory Possible?'', Oxford University Press, 1995. * N. Weaver, ''Mathematical Quantization'', Chapman & Hall/CRC 2001. * G. Giachetta, L. Mangiarotti, Gennadi Sardanashvily, G. Sardanashvily, ''Geometric and Algebraic Topological Methods in Quantum Mechanics'', World Scientific, 2005. * D. McMahon, ''Quantum Mechanics Demystified'', 2nd Ed., McGraw-Hill Professional, 2005. * Gerald Teschl, G. Teschl, ''Mathematical Methods in Quantum Mechanics with Applications to Schrödinger Operators'', https://www.mat.univie.ac.at/~gerald/ftp/book-schroe/, American Mathematical Society, 2009. * V. Moretti, ''Spectral Theory and Quantum Mechanics: Mathematical Foundations of Quantum Theories, Symmetries and Introduction to the Algebraic Formulation'', 2nd Edition, Springer, 2018. * B. C. Hall, ''Quantum Theory for Mathematicians'', Springer, 2013. * V. Moretti, ''Fundamental Mathematical Structures of Quantum Theory'', Springer, 2019. * K. Landsman, ''Foundations of Quantum Theory'', Springer 2017 {{Functional analysis Quantum mechanics, Mathematical physics History of physics