Quantum mechanics is a fundamental

Chapter 8, p. 215

The time evolution of a quantum state is described by the

_{n}'' are the Hermite polynomials
:$H\_n(x)=(-1)^n\; e^\backslash frac\backslash left(e^\backslash right),$
and the corresponding energy levels are
:$E\_n\; =\; \backslash hbar\; \backslash omega\; \backslash left(n\; +\; \backslash right).$
This is another example illustrating the discretization of energy for bound states.

^{−35} m, and so lengths shorter than the Planck length are not physically meaningful in LQG.

photon
The photon ( el, φῶς, phōs, light) is a type of elementary particle
In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

), with a discrete amount of energy that depends on its frequency. In his paper "On the Quantum Theory of Radiation," Einstein expanded on the interaction between energy and matter to explain the absorption and emission of energy by atoms. Although overshadowed at the time by his general theory of relativity, this paper articulated the mechanism underlying the stimulated emission of radiation, which became the basis of the Werner Heisenberg
Werner Karl Heisenberg (; ; 5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the key pioneers of quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a de ...

, electron
The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family,
and are general ...

s in computer semiconductors, and therefore plays a crucial role in many modern technologies. While quantum mechanics was constructed to describe the world of the very small, it is also needed to explain some macroscopic phenomena such as superconductivity, superconductors and superfluids.

Draft of 4th edition.

*

Online copy

* * Gunther Ludwig, 1968. ''Wave Mechanics''. London: Pergamon Press. * George Mackey (2004). ''The mathematical foundations of quantum mechanics''. Dover Publications. . * * Albert Messiah, 1966. ''Quantum Mechanics'' (Vol. I), English translation from French by G.M. Temmer. North Holland, John Wiley & Sons. Cf. chpt. IV, section III

online

* * Eric R. Scerri, Scerri, Eric R., 2006. ''The Periodic Table: Its Story and Its Significance''. Oxford University Press. Considers the extent to which chemistry and the periodic system have been reduced to quantum mechanics. * * * * Martinus J. G. Veltman, Veltman, Martinus J.G. (2003), ''Facts and Mysteries in Elementary Particle Physics''. On Wikibooks

This Quantum World

Introduction to Quantum Theory at Quantiki.

Quantum Physics Made Relatively Simple

three video lectures by Hans Bethe ; Course material

Quantum Cook Book

an

PHYS 201: Fundamentals of Physics II

by Ramamurti Shankar, Yale OpenCourseware

The Modern Revolution in Physics

– an online textbook. * MIT OpenCourseWare

Chemistry

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Physics

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5½ Examples in Quantum Mechanics

Imperial College Quantum Mechanics Course.

;Philosophy * * {{DEFAULTSORT:Quantum Mechanics Quantum mechanics,

theory
A theory is a reason, rational type of abstraction, 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 ...

in physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succession of eve ...

that provides a description of the physical properties of nature
Nature, in the broadest sense, is the natural, physical, material world or universe
The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxy, galaxies, and all other forms of matter an ...

at the scale of atom
An atom is the smallest unit of ordinary 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 atom ...

s and subatomic particle
In physical sciences, subatomic particles are smaller than atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...

s. It is the foundation of all quantum physics including quantum chemistry
Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry
Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: thei ...

, 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 co ...

, quantum technology
Quantum technology is an emerging field of physics and engineering, which relies on the principles of quantum physics. Quantum computing, quantum sensors, quantum cryptography, quantum simulation, quantum metrology and quantum imaging are al ...

, and quantum information science
Quantum information science is an interdisciplinary field that seeks to understand the analysis, processing, and transmission of information using quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that p ...

.
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 ...

, the collection of theories that existed before the advent of quantum mechanics, describes many aspects of nature at an ordinary (macroscopic) scale, but is not sufficient for describing them at small (atomic and subatomic
In physics, physical sciences, subatomic particles are smaller than atoms. They can be composite particles, such as the neutron and proton; or elementary particles, which according to the standard model are not made of other particles. Particle p ...

) scales. Most theories in classical physics can be derived from quantum mechanics as an approximation valid at large (macroscopic) scale.
Quantum mechanics differs from classical physics in that energy
In physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regula ...

, momentum
In Newtonian mechanics, linear momentum, translational momentum, or simply momentum is the product of the mass
Mass is both a property
Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or w ...

, angular momentum
In , angular momentum (rarely, moment of momentum or rotational momentum) is the rotational equivalent of . It is an important quantity in physics because it is a —the total angular momentum of a closed system remains constant.
In three , the ...

, and other quantities of a bound
Bound or bounds may refer to:
Mathematics
* Bound variable
* Upper and lower bounds, observed limits of mathematical functions
Geography
*Bound Brook (Raritan River), a tributary of the Raritan River in New Jersey
*Bound Brook, New Jersey, a borou ...

system are restricted to discrete values ( quantization), objects have characteristics of both particle
In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which can be ascribed several physical property, physical or chemical property, chemical p ...

s and wave
In physics
Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

s (), and there are limits to how accurately the value of a physical quantity can be predicted prior to its measurement, given a complete set of initial conditions (the uncertainty principle
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 subatomic particles. It is the foundation of all quant ...

).
Quantum mechanics arose gradually from theories to explain observations which could not be reconciled with classical physics, such as Max Planck
Max Karl Ernst Ludwig Planck, (; ; 23 April 1858 – 4 October 1947) was a German
German(s) may refer to:
Common uses
* of or related to Germany
* Germans, Germanic ethnic group, citizens of Germany or people of German ancestry
* For citi ...

's solution in 1900 to the black-body radiation
Black-body radiation is the within or surrounding a body in with its environment, emitted by a (an idealized opaque, non-reflective body). It has a specific spectrum of wavelengths, inversely related to intensity that depend only on the bod ...

problem, and the correspondence between energy and frequency in Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born , widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the , but he also made important contributions to the develo ...

's which explained the photoelectric effect
The photoelectric effect is the emission of electron
The electron is a subatomic particle
In physical sciences, subatomic particles are smaller than atom
An atom is the smallest unit of ordinary matter
In classical physics and ...

. These early attempts to understand microscopic phenomena, now known as the "old quantum theory
The old quantum theory is a collection of results from the years 1900–1925 which predate modern quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties ...

", led to the full development of quantum mechanics in the mid-1920s by Niels Bohr
Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish
Danish may refer to:
* Something of, from, or related to the country of Denmark
* A national or citizen of Denmark, also called a "Dane", see Demographics of De ...

, Erwin Schrödinger
Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian-Irish who developed a number of fundamental results in : the provides a way to calculate the ...

, Werner Heisenberg
Werner Karl Heisenberg (; ; 5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the key pioneers of quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a de ...

, Max Born
Max Born (; 11 December 1882 – 5 January 1970) was a German physicist
A physicist is a scientist
A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of intere ...

and others. The modern theory is formulated in various specially developed mathematical formalisms. In one of them, a mathematical entity called the 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 ...

provides information, in the form of probability amplitude
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 subatomic particles. It is the foundation of all quantum ...

s, about what measurements of a particle's energy, momentum, and other physical properties may yield.
Overview and fundamental concepts

Quantum mechanics allows the calculation of properties and behaviour of physical systems. It is typically applied to microscopic systems: molecules, atoms and sub-atomic particles. It has been demonstrated to hold for complex molecules with thousands of atoms, but its application to human beings raises philosophical problems, such as Wigner's friend, and its application to the universe as a whole remains speculative. Predictions of quantum mechanics have been verified experimentally to an extremely high degree ofaccuracy
In a set of measurements, accuracy is closeness of the measurements to a specific value, while precision is the closeness of the measurements to each other.
''Accuracy'' has two definitions:
# More commonly, it is a description of ''systematic err ...

.
A fundamental feature of the theory is that it usually cannot predict with certainty what will happen, but only give probabilities. Mathematically, a probability is found by taking the square of the absolute value of a complex number
In mathematics, a complex number is an element of a number system that contains the real numbers and a specific element denoted , called the imaginary unit, and satisfying the equation . Moreover, every complex number can be expressed in the for ...

, known as a probability amplitude. This is known as the Born rule
The Born rule (also called Born's rule) is a key postulate 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 an ...

, named after physicist Max Born
Max Born (; 11 December 1882 – 5 January 1970) was a German physicist
A physicist is a scientist
A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of intere ...

. For example, a quantum particle like an electron
The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family,
and are general ...

can be described by a 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 ...

, which associates to each point in space a probability amplitude. Applying the Born rule to these amplitudes gives a probability density function
and probability density function of a normal distribution .
Image:visualisation_mode_median_mean.svg, 150px, Geometric visualisation of the mode (statistics), mode, median (statistics), median and mean (statistics), mean of an arbitrary probabilit ...

for the position that the electron will be found to have when an experiment is performed to measure it. This is the best the theory can do; it cannot say for certain where the electron will be found. The Schrödinger equation
The Schrödinger equation is a linear
Linearity is the property of a mathematical relationship (''function
Function or functionality may refer to:
Computing
* Function key
A function key is a key on a computer
A computer is a mac ...

relates the collection of probability amplitudes that pertain to one moment of time to the collection of probability amplitudes that pertain to another.
One consequence of the mathematical rules of quantum mechanics is a tradeoff in predictability between different measurable quantities. The most famous form of this uncertainty principle
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 subatomic particles. It is the foundation of all quant ...

says that no matter how a quantum particle is prepared or how carefully experiments upon it are arranged, it is impossible to have a precise prediction for a measurement of its position and also at the same time for a measurement of its momentum
In Newtonian mechanics, linear momentum, translational momentum, or simply momentum is the product of the mass
Mass is both a property
Property (''latin: Res Privata'') in the Abstract and concrete, abstract is what belongs to or w ...

.
Another consequence of the mathematical rules of quantum mechanics is the phenomenon of quantum interference, which is often illustrated with the double-slit experiment
In modern physics
Modern physics is a branch of physics either developed in the early 20th century and onward or branches greatly influenced by early 20th century physics. Notable branches of modern physics include quantum physics, special r ...

. In the basic version of this experiment, a coherent light source, such as a laser
A laser is a device that emits light
Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined as ...

beam, illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate. The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen – a result that would not be expected if light consisted of classical particles. However, the light is always found to be absorbed at the screen at discrete points, as individual particles rather than waves; the interference pattern appears via the varying density of these particle hits on the screen. Furthermore, versions of the experiment that include detectors at the slits find that each detected photon
The photon ( el, φῶς, phōs, light) is a type of elementary particle
In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

passes through one slit (as would a classical particle), and not through both slits (as would a wave). However, demonstrate that particles do not form the interference pattern if one detects which slit they pass through. Other atomic-scale entities, such as electrons
The electron is a subatomic particle
In physical sciences, subatomic particles are smaller than atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has m ...

, are found to exhibit the same behavior when fired towards a double slit. This behavior is known as .
Another counter-intuitive phenomenon predicted by quantum mechanics is quantum tunnelling
Quantum tunnelling or tunneling (US) is the quantum mechanical phenomenon where a wavefunction can propagate through a potential barrier.
The transmission through the barrier can be finite and depends exponentially on the barrier height and ba ...

: a particle that goes up against a potential barrier
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 subatomic particles. It is the foundation of all quantum ...

can cross it, even if its kinetic energy is smaller than the maximum of the potential. In classical mechanics this particle would be trapped. Quantum tunnelling has several important consequences, enabling radioactive decay
Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

, nuclear fusion
400 px, The nuclear binding energy curve. The formation of nuclei with masses up to iron-56 releases energy, as illustrated above.
Nuclear fusion is a nuclear reaction, reaction in which two or more atomic nuclei are combined to form one or m ...

in stars, and applications such as scanning tunnelling microscopy
A scanning tunneling microscope (STM) is a type of microscope
A microscope (from the grc, μικρός, ''mikrós'', "small" and , ''skopeîn'', "to look" or "see") is a laboratory instrument used to examine objects that are too small to ...

and the tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanics, quantum mechanical effect called quantum tunneling, tunneling. It was invented in August 1957 by Leo Esak ...

.
When quantum systems interact, the result can be the creation of quantum entanglement
Quantum entanglement is a physical phenomenon that occurs when a group of particle
In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which ...

: their properties become so intertwined that a description of the whole solely in terms of the individual parts is no longer possible. Erwin Schrödinger called entanglement "...''the'' characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought". Quantum entanglement enables the counter-intuitive properties of quantum pseudo-telepathy
Quantum pseudo-telepathy is the fact that in certain Bayesian games with asymmetric information, players who have access to a shared physical system in an entangled quantum state, and who are able to execute strategies that are contingent upon m ...

, and can be a valuable resource in communication protocols, such as quantum key distribution
Quantum key distribution (QKD) is a secure communication
Secure communication is when two entities are communicating and do not want a third party to listen in. In order for this to be the case, entities need to communicate in a way unsusceptible ...

and superdense coding
In quantum information theory
Quantum information is the information of the quantum state, state of a quantum system. It is the basic entity of study in quantum information theory, and can be manipulated using quantum information processing t ...

. Contrary to popular misconception, entanglement does not allow sending signals faster than light
Faster-than-light (also superluminal, FTL or supercausal) communications and travel are the conjectural propagation of information or matter faster than the speed of light.
The special theory of relativity implies that only particles with zero mas ...

, as demonstrated by the no-communication theorem
In physics, the no-communication theorem or no-signaling principle is a no-go theorem from quantum information theory which states that, during measurement of an quantum entanglement, entangled quantum state, it is not possible for one observer, b ...

.
Another possibility opened by entanglement is testing for " hidden variables", hypothetical properties more fundamental than the quantities addressed in quantum theory itself, knowledge of which would allow more exact predictions than quantum theory can provide. A collection of results, most significantly , have demonstrated that broad classes of such hidden-variable theories are in fact incompatible with quantum physics. According to Bell's theorem, if nature actually operates in accord with any theory of ''local'' hidden variables, then the results of a Bell test
A Bell test, also known as Bell inequality test or Bell experiment, is a real-world physics
Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior thr ...

will be constrained in a particular, quantifiable way. Many Bell tests have been performed, using entangled particles, and they have shown results incompatible with the constraints imposed by local hidden variables.
It is not possible to present these concepts in more than a superficial way without introducing the actual mathematics involved; understanding quantum mechanics requires not only manipulating complex numbers, but also 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 ...

, differential equation
In mathematics, a differential equation is an functional equation, equation that relates one or more function (mathematics), functions and their derivatives. In applications, the functions generally represent physical quantities, the derivatives ...

s, group theory
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ...

, and other more advanced subjects. Accordingly, this article will present a mathematical formulation of quantum mechanics and survey its application to some useful and oft-studied examples.
Mathematical formulation

In the mathematically rigorous formulation of quantum mechanics, the state of a quantum mechanical system is a vector $\backslash psi$ belonging to a ( separable) complexHilbert space
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ge ...

$\backslash mathcal\; H$. This vector is postulated to be normalized under the Hilbert space inner product, that is, it obeys $\backslash langle\; \backslash psi,\backslash psi\; \backslash rangle\; =\; 1$, and it is well-defined up to a complex number of modulus 1 (the global phase), that is, $\backslash psi$ and $e^\backslash psi$ represent the same physical system. In other words, the possible states are points in the projective space
In mathematics
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...

of a Hilbert space, usually called the complex projective space
In mathematics, complex projective space is the projective space with respect to the field of complex numbers. By analogy, whereas the points of a real projective space label the lines through the origin of a real Euclidean space, the points of a ...

. The exact nature of this Hilbert space is dependent on the system – for example, for describing position and momentum the Hilbert space is the space of complex square-integrable
In mathematics
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...

functions $L^2(\backslash mathbb\; C)$, while the Hilbert space for the spin
Spin or spinning may refer to:
Businesses
* or South Pacific Island Network
* , an American scooter-sharing system
* , a chain of table tennis lounges
Computing
* , 's tool for formal verification of distributed software systems
* , a Mach-like ...

of a single proton is simply the space of two-dimensional complex vectors $\backslash mathbb\; C^2$ with the usual inner product.
Physical quantities of interestposition, momentum, energy, spinare represented by observables, which are Hermitian{{short description, Wikipedia list article
Numerous things are named after the French mathematician Charles Hermite
Charles Hermite () FRS FRSE
Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the R ...

(more precisely, self-adjointIn mathematics
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...

) linear operators acting on the Hilbert space. A quantum state can be an eigenvector
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 an ...

of an observable, in which case it is called an eigenstate
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement
'
Measurement is the number, numerical quantification (science), quantification of the variable an ...

, and the associated 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 an ...

corresponds to the value of the observable in that eigenstate. More generally, a quantum state will be a linear combination of the eigenstates, known as a quantum superposition
Quantum superposition is a fundamental principle 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 sub ...

. When an observable is measured, the result will be one of its eigenvalues with probability given by the Born rule
The Born rule (also called Born's rule) is a key postulate 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 an ...

: in the simplest case the eigenvalue $\backslash lambda$ is non-degenerate and the probability is given by $,\; \backslash langle\; \backslash vec\backslash lambda,\backslash psi\backslash rangle,\; ^2$, where $\backslash vec\backslash lambda$ is its associated eigenvector. More generally, the eigenvalue is degenerate and the probability is given by $\backslash langle\; \backslash psi,P\_\backslash lambda\backslash psi\backslash rangle$, where $P\_\backslash lambda$ is the projector onto its associated eigenspace. In the continuous case, these formulas give instead the probability density
and probability density function of a normal distribution
In probability theory, a normal (or Gaussian or Gauss or Laplace–Gauss) distribution is a type of continuous probability distribution for a real number, real-valued random variable. ...

.
After the measurement, if result $\backslash lambda$ was obtained, the quantum state is postulated to collapse
Collapse or its variants may refer to:
Concepts
* Collapse (structural)
* Collapse (topology), a mathematical concept
* Collapsing manifold
* Collapse, the action of collapsing or telescoping objects
* Ecosystem collapse
An ecosystem
...

to $\backslash vec\backslash lambda$, in the non-degenerate case, or to $P\_\backslash lambda\backslash psi/\backslash sqrt$, in the general case. The probabilistic
Probability is the branch of mathematics
Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathe ...

nature of quantum mechanics thus stems from the act of measurement. This is one of the most difficult aspects of quantum systems to understand. It was the central topic in the famous Bohr–Einstein debates, in which the two scientists attempted to clarify these fundamental principles by way of thought experiment
A thought experiment is a hypothetical situation in which a hypothesis
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can t ...

s. In the decades after the formulation of quantum mechanics, the question of what constitutes a "measurement" has been extensively studied. Newer interpretations of quantum mechanics
An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties of ...

have been formulated that do away with the concept of "wave function collapse
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 subatomic particles. It is the foundation of all quantu ...

" (see, for example, the many-worlds interpretation
The many-worlds interpretation (MWI) is an that asserts that the is , and that there is no . This implies that all outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations ...

). The basic idea is that when a quantum system interacts with a measuring apparatus, their respective wave functions become entangled so that the original quantum system ceases to exist as an independent entity. For details, see the article on measurement in quantum mechanics
In quantum physics
Quantum mechanics is a fundamental theory
A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is o ...

.Chapter 8, p. 215

The time evolution of a quantum state is described by the

Schrödinger equation
The Schrödinger equation is a linear
Linearity is the property of a mathematical relationship (''function
Function or functionality may refer to:
Computing
* Function key
A function key is a key on a computer
A computer is a mac ...

:
:$i\backslash hbar\; \backslash psi\; (t)\; =H\; \backslash psi\; (t).$
Here $H$ denotes the Hamiltonian, the observable corresponding to the of the system, and $\backslash hbar$ is the reduced Planck constant
The Planck constant, or Planck's constant, is a fundamental physical constant
A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature an ...

. The constant $i\backslash hbar$ is introduced so that the Hamiltonian is reduced to the classical Hamiltonian in cases where the quantum system can be approximated by a classical system; the ability to make such an approximation in certain limits is called the correspondence principle
In physics
Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through S ...

.
The solution of this differential equation is given by
:$\backslash psi(t)\; =\; e^\backslash psi(0).$
The operator $U(t)\; =\; e^$ is known as the time-evolution operator, and has the crucial property that it is unitary
Unitary may refer to:
* Unitary construction, in automotive design a common term for unibody (unitary body/chassis) construction
* Lethal Unitary Chemical Agents and Munitions (Unitary), as chemical weapons opposite of Binary
* Unitarianism, in Chr ...

. This time evolution is deterministic
Determinism is the philosophical
Philosophy (from , ) is the study of general and fundamental questions, such as those about existence
Existence is the ability of an entity to interact with physical or mental reality
Reality is the ...

in the sense that – given an initial quantum state $\backslash psi(0)$ – it makes a definite prediction of what the quantum state $\backslash psi(t)$ will be at any later time.
Some wave functions produce probability distributions that are independent of time, such as eigenstates of the Hamiltonian. Many systems that are treated dynamically in classical mechanics are described by such "static" wave functions. For example, a single electron
The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family,
and are general ...

in an unexcited atom
An atom is the smallest unit of ordinary 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 atom ...

is pictured classically as a particle moving in a circular trajectory around the atomic nucleus
The atomic nucleus is the small, dense region consisting of s and s at the center of an , discovered in 1911 by based on the 1909 . After the discovery of the neutron in 1932, models for a nucleus composed of protons and neutrons were quickl ...

, whereas in quantum mechanics, it is described by a static wave function surrounding the nucleus. For example, the electron wave function for an unexcited hydrogen atom is a spherically symmetric function known as an ''s'' orbital ( Fig. 1).
Analytic solutions of the Schrödinger equation are known for very few relatively simple model Hamiltonians including the quantum harmonic oscillator
The quantum harmonic oscillator is the quantum-mechanical analog of the classical harmonic oscillator. Because an arbitrary smooth potential can usually be approximated as a harmonic potential at the vicinity of a stable equilibrium point, ...

, the particle in a box
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 subatomic particles. It is the foundation of all quantum ...

, the dihydrogen cation, and the hydrogen atom. Even the helium atom – which contains just two electrons – has defied all attempts at a fully analytic treatment.
However, there are techniques for finding approximate solutions. One method, called perturbation theory (quantum mechanics), perturbation theory, uses the analytic result for a simple quantum mechanical model to create a result for a related but more complicated model by (for example) the addition of a weak potential energy. Another method is called "semi-classical equation of motion", which applies to systems for which quantum mechanics produces only small deviations from classical behavior. These deviations can then be computed based on the classical motion. This approach is particularly important in the field of quantum chaos.
Uncertainty principle

One consequence of the basic quantum formalism is theuncertainty principle
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 subatomic particles. It is the foundation of all quant ...

. In its most familiar form, this states that no preparation of a quantum particle can imply simultaneously precise predictions both for a measurement of its position and for a measurement of its momentum. Both position and momentum are observables, meaning that they are represented by Hermitian operators. The position operator $\backslash hat$ and momentum operator $\backslash hat$ do not commute, but rather satisfy the canonical commutation relation:
:$[\backslash hat,\; \backslash hat]\; =\; i\backslash hbar.$
Given a quantum state, the Born rule lets us compute expectation values for both $X$ and $P$, and moreover for powers of them. Defining
the uncertainty for an observable by a standard deviation, we have
:$\backslash sigma\_X=\backslash sqrt,$
and likewise for the momentum:
:$\backslash sigma\_P=\backslash sqrt.$
The uncertainty principle states that
:$\backslash sigma\_X\; \backslash sigma\_P\; \backslash geq\; \backslash frac.$
Either standard deviation can in principle be made arbitrarily small, but not both simultaneously.Section 3.2 of . This fact is experimentally well-known for example in quantum optics; see e.g. chap. 2 and Fig. 2.1 This inequality generalizes to arbitrary pairs of self-adjoint operators $A$ and $B$. The commutator of these two operators is
:$[A,B]=AB-BA,$
and this provides the lower bound on the product of standard deviations:
:$\backslash sigma\_A\; \backslash sigma\_B\; \backslash geq\; \backslash frac\backslash left,\; \backslash langle[A,B]\backslash rangle\; \backslash .$
Another consequence of the canonical commutation relation is that the position and momentum operators are Fourier transform#Uncertainty principle, Fourier transforms of each other, so that a description of an object according to its momentum is the Fourier transform of its description according to its position. The fact that dependence in momentum is the Fourier transform of the dependence in position means that the momentum operator is equivalent (up to an $i/\backslash hbar$ factor) to taking the derivative according to the position, since in Fourier analysis Fourier transform#Differentiation, differentiation corresponds to multiplication in the dual space. This is why in quantum equations in position space, the momentum $p\_i$ is replaced by $-i\; \backslash hbar\; \backslash frac$, and in particular in the Schrödinger equation#Equation, non-relativistic Schrödinger equation in position space the momentum-squared term is replaced with a Laplacian times $-\backslash hbar^2$.
Composite systems and entanglement

When two different quantum systems are considered together, the Hilbert space of the combined system is the tensor product of the Hilbert spaces of the two components. For example, let and be two quantum systems, with Hilbert spaces $\backslash mathcal\; H\_A$ and $\backslash mathcal\; H\_B$, respectively. The Hilbert space of the composite system is then : $\backslash mathcal\; H\_\; =\; \backslash mathcal\; H\_A\; \backslash otimes\; \backslash mathcal\; H\_B.$ If the state for the first system is the vector $\backslash psi\_A$ and the state for the second system is $\backslash psi\_B$, then the state of the composite system is : $\backslash psi\_A\; \backslash otimes\; \backslash psi\_B.$ Not all states in the joint Hilbert space $\backslash mathcal\; H\_$ can be written in this form, however, because the superposition principle implies that linear combinations of these "separable" or "product states" are also valid. For example, if $\backslash psi\_A$ and $\backslash phi\_A$ are both possible states for system $A$, and likewise $\backslash psi\_B$ and $\backslash phi\_B$ are both possible states for system $B$, then : $\backslash tfrac\; \backslash left\; (\; \backslash psi\_A\; \backslash otimes\; \backslash psi\_B\; +\; \backslash phi\_A\; \backslash otimes\; \backslash phi\_B\; \backslash right\; )$ is a valid joint state that is not separable. States that are not separable are called quantum entanglement, entangled. If the state for a composite system is entangled, it is impossible to describe either component system or system by a state vector. One can instead define reduced density matrix, reduced density matrices that describe the statistics that can be obtained by making measurements on either component system alone. This necessarily causes a loss of information, though: knowing the reduced density matrices of the individual systems is not enough to reconstruct the state of the composite system. Just as density matrices specify the state of a subsystem of a larger system, analogously, POVM, positive operator-valued measures (POVMs) describe the effect on a subsystem of a measurement performed on a larger system. POVMs are extensively used in quantum information theory. As described above, entanglement is a key feature of models of measurement processes in which an apparatus becomes entangled with the system being measured. Systems interacting with the environment in which they reside generally become entangled with that environment, a phenomenon known as quantum decoherence. This can explain why, in practice, quantum effects are difficult to observe in systems larger than microscopic.Equivalence between formulations

There are many mathematically equivalent formulations of quantum mechanics. One of the oldest and most common is the "transformation theory (quantum mechanics), transformation theory" proposed by Paul Dirac, which unifies and generalizes the two earliest formulations of quantum mechanics – matrix mechanics (invented byWerner Heisenberg
Werner Karl Heisenberg (; ; 5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the key pioneers of quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a de ...

) and Schrödinger equation, wave mechanics (invented by Erwin Schrödinger
Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian-Irish who developed a number of fundamental results in : the provides a way to calculate the ...

). An alternative formulation of quantum mechanics is Feynman's path integral formulation, in which a quantum-mechanical amplitude is considered as a sum over all possible classical and non-classical paths between the initial and final states. This is the quantum-mechanical counterpart of the action principle in classical mechanics.
Symmetries and conservation laws

The Hamiltonian $H$ is known as the ''generator'' of time evolution, since it defines a unitary time-evolution operator $U(t)\; =\; e^$ for each value of $t$. From this relation between $U(t)$ and $H$, it follows that any observable $A$ that commutes with $H$ will be ''conserved'': its expectation value will not change over time. This statement generalizes, as mathematically, any Hermitian operator $A$ can generate a family of unitary operators parameterized by a variable $t$. Under the evolution generated by $A$, any observable $B$ that commutes with $A$ will be conserved. Moreover, if $B$ is conserved by evolution under $A$, then $A$ is conserved under the evolution generated by $B$. This implies a quantum version of the result proven by Emmy Noether in classical (Lagrangian mechanics, Lagrangian) mechanics: for every Differentiable function, differentiable Symmetry (physics), symmetry of a Hamiltonian, there exists a corresponding conservation law.Examples

Free particle

The simplest example of quantum system with a position degree of freedom is a free particle in a single spatial dimension. A free particle is one which is not subject to external influences, so that its Hamiltonian consists only of its kinetic energy: :$H\; =\; \backslash fracP^2\; =\; -\; \backslash frac\; \backslash frac\; .$ The general solution of the Schrödinger equation is given by :$\backslash psi\; (x,t)=\backslash frac\; \backslash int\; \_^\backslash infty(k,0)e^\backslash mathrmk,$ which is a superposition of all possible plane waves $e^$, which are eigenstates of the momentum operator with momentum $p\; =\; \backslash hbar\; k$. The coefficients of the superposition are $\backslash hat\; (k,0)$, which is the Fourier transform of the initial quantum state $\backslash psi(x,0)$. It is not possible for the solution to be a single momentum eigenstate, or a single position eigenstate, as these are not normalizable quantum states. Instead, we can consider a Gaussian wave packet: :$\backslash psi(x,0)\; =\; \backslash frace^$ which has Fourier transform, and therefore momentum distribution :$\backslash hat\; \backslash psi(k,0)\; =\; \backslash sqrt[4]e^.$ We see that as we make $a$ smaller the spread in position gets smaller, but the spread in momentum gets larger. Conversely, by making $a$ larger we make the spread in momentum smaller, but the spread in position gets larger. This illustrates the uncertainty principle. As we let the Gaussian wave packet evolve in time, we see that its center moves through space at a constant velocity (like a classical particle with no forces acting on it). However, the wave packet will also spread out as time progresses, which means that the position becomes more and more uncertain. The uncertainty in momentum, however, stays constant.Particle in a box

The particle in a one-dimensional potential energy box is the most mathematically simple example where restraints lead to the quantization of energy levels. The box is defined as having zero potential energy everywhere ''inside'' a certain region, and therefore infinite potential energy everywhere ''outside'' that region. For the one-dimensional case in the $x$ direction, the time-independent Schrödinger equation may be written : $-\; \backslash frac\; \backslash frac\; =\; E\; \backslash psi.$ With the differential operator defined by : $\backslash hat\_x\; =\; -i\backslash hbar\backslash frac$ the previous equation is evocative of the Kinetic energy#Kinetic energy of rigid bodies, classic kinetic energy analogue, : $\backslash frac\; \backslash hat\_x^2\; =\; E,$ with state $\backslash psi$ in this case having energy $E$ coincident with the kinetic energy of the particle. The general solutions of the Schrödinger equation for the particle in a box are : $\backslash psi(x)\; =\; A\; e^\; +\; B\; e\; ^\; \backslash qquad\backslash qquad\; E\; =\; \backslash frac$ or, from Euler's formula, : $\backslash psi(x)\; =\; C\; \backslash sin(kx)\; +\; D\; \backslash cos(kx).\backslash !$ The infinite potential walls of the box determine the values of $C,\; D,$ and $k$ at $x=0$ and $x=L$ where $\backslash psi$ must be zero. Thus, at $x=0$, :$\backslash psi(0)\; =\; 0\; =\; C\backslash sin(0)\; +\; D\backslash cos(0)\; =\; D$ and $D=0$. At $x=L$, :$\backslash psi(L)\; =\; 0\; =\; C\backslash sin(kL),$ in which $C$ cannot be zero as this would conflict with the postulate that $\backslash psi$ has norm 1. Therefore, since $\backslash sin(kL)=0$, $kL$ must be an integer multiple of $\backslash pi$, :$k\; =\; \backslash frac\backslash qquad\backslash qquad\; n=1,2,3,\backslash ldots.$ This constraint on $k$ implies a constraint on the energy levels, yielding $E\_n\; =\; \backslash frac\; =\; \backslash frac.$ A finite potential well is the generalization of the infinite potential well problem to potential wells having finite depth. The finite potential well problem is mathematically more complicated than the infinite particle-in-a-box problem as the wave function is not pinned to zero at the walls of the well. Instead, the wave function must satisfy more complicated mathematical boundary conditions as it is nonzero in regions outside the well. Another related problem is that of the rectangular potential barrier, which furnishes a model for the quantum tunneling effect that plays an important role in the performance of modern technologies such as flash memory and scanning tunneling microscope, scanning tunneling microscopy.Harmonic oscillator

As in the classical case, the potential for the quantum harmonic oscillator is given by :$V(x)=\backslash fracm\backslash omega^2x^2.$ This problem can either be treated by directly solving the Schrödinger equation, which is not trivial, or by using the more elegant "ladder method" first proposed by Paul Dirac. Theeigenstate
In quantum physics, a quantum state is a mathematical entity that provides a probability distribution for the outcomes of each possible measurement
'
Measurement is the number, numerical quantification (science), quantification of the variable an ...

s are given by
:$\backslash psi\_n(x)\; =\; \backslash sqrt\; \backslash cdot\; \backslash left(\backslash frac\backslash right)^\; \backslash cdot\; e^\; \backslash cdot\; H\_n\backslash left(\backslash sqrt\; x\; \backslash right),\; \backslash qquad$
:$n\; =\; 0,1,2,\backslash ldots.$
where ''HMach–Zehnder interferometer

The Mach–Zehnder interferometer (MZI) illustrates the concepts of superposition and interference with linear algebra in dimension 2, rather than differential equations. It can be seen as a simplified version of the double-slit experiment, but it is of interest in its own right, for example in the delayed choice quantum eraser, the Elitzur–Vaidman bomb tester, and in studies of quantum entanglement. We can model a photon going through the interferometer by considering that at each point it can be in a superposition of only two paths: the "lower" path which starts from the left, goes straight through both beam splitters, and ends at the top, and the "upper" path which starts from the bottom, goes straight through both beam splitters, and ends at the right. The quantum state of the photon is therefore a vector $\backslash psi\; \backslash in\; \backslash mathbb^2$ that is a superposition of the "lower" path $\backslash psi\_l\; =\; \backslash begin\; 1\; \backslash \backslash \; 0\; \backslash end$ and the "upper" path $\backslash psi\_u\; =\; \backslash begin\; 0\; \backslash \backslash \; 1\; \backslash end$, that is, $\backslash psi\; =\; \backslash alpha\; \backslash psi\_l\; +\; \backslash beta\; \backslash psi\_u$ for complex $\backslash alpha,\backslash beta$. In order to respect the postulate that $\backslash langle\; \backslash psi,\backslash psi\backslash rangle\; =\; 1$ we require that $,\; \backslash alpha,\; ^2+,\; \backslash beta,\; ^2\; =\; 1$. Both beam splitters are modelled as the unitary matrix $B\; =\; \backslash frac1\backslash begin\; 1\; \&\; i\; \backslash \backslash \; i\; \&\; 1\; \backslash end$, which means that when a photon meets the beam splitter it will either stay on the same path with a probability amplitude of $1/\backslash sqrt$, or be reflected to the other path with a probability amplitude of $i/\backslash sqrt$. The phase shifter on the upper arm is modelled as the unitary matrix $P\; =\; \backslash begin\; 1\; \&\; 0\; \backslash \backslash \; 0\; \&\; e^\; \backslash end$, which means that if the photon is on the "upper" path it will gain a relative phase of $\backslash Delta\backslash Phi$, and it will stay unchanged if it is in the lower path. A photon that enters the interferometer from the left will then be acted upon with a beam splitter $B$, a phase shifter $P$, and another beam splitter $B$, and so end up in the state :$BPB\backslash psi\_l\; =\; ie^\; \backslash begin\; -\backslash sin(\backslash Delta\backslash Phi/2)\; \backslash \backslash \; \backslash cos(\backslash Delta\backslash Phi/2)\; \backslash end,$ and the probabilities that it will be detected at the right or at the top are given respectively by :$p(u)\; =\; ,\; \backslash langle\; \backslash psi\_u,\; BPB\backslash psi\_l\; \backslash rangle,\; ^2\; =\; \backslash cos^2\; \backslash frac,$ :$p(l)\; =\; ,\; \backslash langle\; \backslash psi\_l,\; BPB\backslash psi\_l\; \backslash rangle,\; ^2\; =\; \backslash sin^2\; \backslash frac.$ One can therefore use the Mach–Zehnder interferometer to estimate the Phase (waves), phase shift by estimating these probabilities. It is interesting to consider what would happen if the photon were definitely in either the "lower" or "upper" paths between the beam splitters. This can be accomplished by blocking one of the paths, or equivalently by removing the first beam splitter (and feeding the photon from the left or the bottom, as desired). In both cases there will be no interference between the paths anymore, and the probabilities are given by $p(u)=p(l)\; =\; 1/2$, independently of the phase $\backslash Delta\backslash Phi$. From this we can conclude that the photon does not take one path or another after the first beam splitter, but rather that it is in a genuine quantum superposition of the two paths.Applications

Quantum mechanics has had enormous success in explaining many of the features of our universe, with regards to small-scale and discrete quantities and interactions which cannot be explained by Classical physics, classical methods. Quantum mechanics is often the only theory that can reveal the individual behaviors of thesubatomic particle
In physical sciences, subatomic particles are smaller than atom
An atom is the smallest unit of ordinary matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ...

s that make up all forms of matter (electron
The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family,
and are general ...

s, protons, neutrons, photon
The photon ( el, φῶς, phōs, light) is a type of elementary particle
In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

s, and others). Solid-state physics and materials science are dependent upon quantum mechanics.
In many aspects modern technology operates at a scale where quantum effects are significant. Important applications of quantum theory include quantum chemistry
Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry
Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: thei ...

, quantum optics, quantum computing, superconducting magnets, light-emitting diodes, the optical amplifier and the laser
A laser is a device that emits light
Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined as ...

, the transistor and semiconductors such as the microprocessor, medical imaging, medical and research imaging such as magnetic resonance imaging and electron microscope, electron microscopy. Explanations for many biological and physical phenomena are rooted in the nature of the chemical bond, most notably the macro-molecule DNA.
Relation to other scientific theories

Classical mechanics

The rules of quantum mechanics assert that the state space of a system is aHilbert space
In mathematics
Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ge ...

and that observables of the system are Hermitian operators acting on vectors in that space – although they do not tell us which Hilbert space or which operators. These can be chosen appropriately in order to obtain a quantitative description of a quantum system, a necessary step in making physical predictions. An important guide for making these choices is the correspondence principle
In physics
Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), motion and behavior through S ...

, a heuristic which states that the predictions of quantum mechanics reduce to those of classical mechanics in the regime of large quantum numbers. One can also start from an established classical model of a particular system, and then try to guess the underlying quantum model that would give rise to the classical model in the correspondence limit. This approach is known as Canonical quantization, quantization.
When quantum mechanics was originally formulated, it was applied to models whose correspondence limit was theory of relativity, non-relativistic classical mechanics. For instance, the well-known model of the quantum harmonic oscillator
The quantum harmonic oscillator is the quantum-mechanical analog of the classical harmonic oscillator. Because an arbitrary smooth potential can usually be approximated as a harmonic potential at the vicinity of a stable equilibrium point, ...

uses an explicitly non-relativistic expression for the kinetic energy of the oscillator, and is thus a quantum version of the harmonic oscillator, classical harmonic oscillator.
Complications arise with Chaos theory, chaotic systems, which do not have good quantum numbers, and quantum chaos studies the relationship between classical and quantum descriptions in these systems.
Quantum decoherence is a mechanism through which quantum systems lose quantum coherence, coherence, and thus become incapable of displaying many typically quantum effects: quantum superposition
Quantum superposition is a fundamental principle 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 sub ...

s become simply probabilistic mixtures, and quantum entanglement
Quantum entanglement is a physical phenomenon that occurs when a group of particle
In the Outline of physical science, physical sciences, a particle (or corpuscule in older texts) is a small wikt:local, localized physical body, object to which ...

becomes simply classical correlations. Quantum coherence is not typically evident at macroscopic scales, except maybe at temperatures approaching absolute zero at which quantum behavior may manifest macroscopically.
Many macroscopic properties of a classical system are a direct consequence of the quantum behavior of its parts. For example, the stability of bulk matter (consisting of atoms and molecules which would quickly collapse under electric forces alone), the rigidity of solids, and the mechanical, thermal, chemical, optical and magnetic properties of matter are all results of the interaction of electric charges under the rules of quantum mechanics.
Special relativity and electrodynamics

Early attempts to merge quantum mechanics with special relativity involved the replacement of the Schrödinger equation with a covariant equation such as the Klein–Gordon equation or the Dirac equation. While these theories were successful in explaining many experimental results, they had certain unsatisfactory qualities stemming from their neglect of the relativistic creation and annihilation of particles. A fully relativistic quantum theory required the development ofquantum 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 co ...

, which applies quantization to a field (rather than a fixed set of particles). The first complete quantum field theory, quantum electrodynamics, provides a fully quantum description of the electromagnetism, electromagnetic interaction. Quantum electrodynamics is, along with general relativity, one of the most accurate physical theories ever devised.
The full apparatus of quantum field theory is often unnecessary for describing electrodynamic systems. A simpler approach, one that has been used since the inception of quantum mechanics, is to treat electric charge, charged particles as quantum mechanical objects being acted on by a classical electromagnetic field. For example, the elementary quantum model of the hydrogen atom describes the electric field of the hydrogen atom using a classical $\backslash textstyle\; -e^2/(4\; \backslash pi\backslash epsilon\_r)$ Electric potential, Coulomb potential. This "semi-classical" approach fails if quantum fluctuations in the electromagnetic field play an important role, such as in the emission of photon
The photon ( el, φῶς, phōs, light) is a type of elementary particle
In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

s by charged particles.
Field (physics), Quantum field theories for the strong nuclear force and the weak nuclear force have also been developed. The quantum field theory of the strong nuclear force is called quantum chromodynamics, and describes the interactions of subnuclear particles such as quarks and gluons. The weak nuclear force and the electromagnetic force were unified, in their quantized forms, into a single quantum field theory (known as electroweak theory), by the physicists Abdus Salam, Sheldon Glashow and Steven Weinberg.
Relation to general relativity

Even though the predictions of both quantum theory and general relativity have been supported by rigorous and repeated empirical evidence, their abstract formalisms contradict each other and they have proven extremely difficult to incorporate into one consistent, cohesive model. Gravity is negligible in many areas of particle physics, so that unification between general relativity and quantum mechanics is not an urgent issue in those particular applications. However, the lack of a correct theory of quantum gravity is an important issue in physical cosmology and the search by physicists for an elegant "theory of everything, Theory of Everything" (TOE). Consequently, resolving the inconsistencies between both theories has been a major goal of 20th- and 21st-century physics. This TOE would combine not only the models of subatomic physics but also derive the four fundamental forces of nature from a single force or phenomenon. One proposal for doing so is string theory, which posits that the Point particle, point-like particles of particle physics are replaced by Dimension (mathematics and physics), one-dimensional objects called String (physics), strings. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its mass, charge (physics), charge, and other properties determined by the vibrational state of the string. In string theory, one of the many vibrational states of the string corresponds to the graviton, a quantum mechanical particle that carries gravitational force. Another popular theory is loop quantum gravity (LQG), which describes quantum properties of gravity and is thus a theory of quantum spacetime. LQG is an attempt to merge and adapt standard quantum mechanics and standard general relativity. This theory describes space as an extremely fine fabric "woven" of finite loops called spin networks. The evolution of a spin network over time is called a spin foam. The characteristic length scale of a spin foam is the Planck length, approximately 1.616×10Philosophical implications

Since its inception, the many counter-intuitive aspects and results of quantum mechanics have provoked strong philosophy, philosophical debates and many interpretations of quantum mechanics, interpretations. The arguments centre on the probabilistic nature of quantum mechanics, the difficulties with wavefunction collapse and the related measurement problem, and quantum nonlocality. Perhaps the only consensus that exists about these issues is that there is no consensus. Richard Feynman once said, "I think I can safely say that nobody understands quantum mechanics." According to Steven Weinberg, "There is now in my opinion no entirely satisfactory interpretation of quantum mechanics." The views ofNiels Bohr
Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish
Danish may refer to:
* Something of, from, or related to the country of Denmark
* A national or citizen of Denmark, also called a "Dane", see Demographics of De ...

, Werner Heisenberg
Werner Karl Heisenberg (; ; 5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the key pioneers of quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a de ...

and other physicists are often grouped together as the "Copenhagen interpretation". According to these views, the probabilistic nature of quantum mechanics is not a ''temporary'' feature which will eventually be replaced by a deterministic theory, but is instead a ''final'' renunciation of the classical idea of "causality". Bohr in particular emphasized that any well-defined application of the quantum mechanical formalism must always make reference to the experimental arrangement, due to the complementarity (physics), complementary nature of evidence obtained under different experimental situations. Copenhagen-type interpretations remain popular in the 21st century.
Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born , widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the , but he also made important contributions to the develo ...

, himself one of the founders of Old quantum theory, quantum theory, was troubled by its apparent failure to respect some cherished metaphysical principles, such as determinism and principle of locality, locality. Einstein's long-running exchanges with Bohr about the meaning and status of quantum mechanics are now known as the Bohr–Einstein debates. Einstein believed that underlying quantum mechanics must be a theory that explicitly forbids action at a distance. He argued that quantum mechanics was incomplete, a theory that was valid but not fundamental, analogous to how thermodynamics is valid, but the fundamental theory behind it is statistical mechanics. In 1935, Einstein and his collaborators Boris Podolsky and Nathan Rosen published an argument that the principle of locality implies the incompleteness of quantum mechanics, a thought experiment
A thought experiment is a hypothetical situation in which a hypothesis
A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can t ...

later termed the Einstein–Podolsky–Rosen paradox. In 1964, John Stewart Bell, John Bell showed that EPR's principle of locality, together with determinism, was actually incompatible with quantum mechanics: they implied constraints on the correlations produced by distance systems, now known as Bell inequalities, that can be violated by entangled particles. Since then Bell test, several experiments have been performed to obtain these correlations, with the result that they do in fact violate Bell inequalities, and thus falsify the conjunction of locality with determinism.
Bohmian mechanics shows that it is possible to reformulate quantum mechanics to make it deterministic, at the price of making it explicitly nonlocal. It attributes not only a wave function to a physical system, but in addition a real position, that evolves deterministically under a nonlocal guiding equation. The evolution of a physical system is given at all times by the Schrödinger equation
The Schrödinger equation is a linear
Linearity is the property of a mathematical relationship (''function
Function or functionality may refer to:
Computing
* Function key
A function key is a key on a computer
A computer is a mac ...

together with the guiding equation; there is never a collapse of the wave function. This solves the measurement problem.
Everett's many-worlds interpretation
The many-worlds interpretation (MWI) is an that asserts that the is , and that there is no . This implies that all outcomes of quantum measurements are physically realized in some "world" or universe. In contrast to some other interpretations ...

, formulated in 1956, holds that ''all'' the possibilities described by quantum theory ''simultaneously'' occur in a multiverse composed of mostly independent parallel universes. This is a consequence of removing the axiom of the collapse of the wave packet. All possible states of the measured system and the measuring apparatus, together with the observer, are present in a real physical quantum superposition
Quantum superposition is a fundamental principle 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 sub ...

. While the multiverse is deterministic, we perceive non-deterministic behavior governed by probabilities, because we don't observe the multiverse as a whole, but only one parallel universe at a time. Exactly how this is supposed to work has been the subject of much debate. Several attempts have been made to make sense of this and derive the Born rule, with no consensus on whether they have been successful.
Relational quantum mechanics appeared in the late 1990s as a modern derivative of Copenhagen-type ideas, and QBism was developed some years later.
History

Quantum mechanics was developed in the early decades of the 20th century, driven by the need to explain phenomena that, in some cases, had been observed in earlier times. Scientific inquiry into the wave nature of light began in the 17th and 18th centuries, when scientists such as Robert Hooke, Christiaan Huygens and Leonhard Euler proposed a wave theory of light based on experimental observations. In 1803 English polymath Thomas Young (scientist), Thomas Young described the famous Young's interference experiment, double-slit experiment. This experiment played a major role in the general acceptance of the wave theory of light. During the early 19th century, chemistry, chemical research by John Dalton and Amedeo Avogadro lent weight to the atomic theory of matter, an idea that James Clerk Maxwell, Ludwig Boltzmann and others built upon to establish the kinetic theory of gases. The successes of kinetic theory gave further credence to the idea that matter is composed of atoms, yet the theory also had shortcomings that would only be resolved by the development of quantum mechanics. While the early conception of atoms from Greek philosophy had been that they were indivisible units the word "atom" deriving from the Greek for "uncuttable" the 19th century saw the formulation of hypotheses about subatomic structure. One important discovery in that regard was Michael Faraday's 1838 observation of a glow caused by an electrical discharge inside a glass tube containing gas at low pressure. Julius Plücker, Johann Wilhelm Hittorf and Eugen Goldstein carried on and improved upon Faraday's work, leading to the identification of cathode rays, which J. J. Thomson found to consist of subatomic particles that would be called electrons. Theblack-body radiation
Black-body radiation is the within or surrounding a body in with its environment, emitted by a (an idealized opaque, non-reflective body). It has a specific spectrum of wavelengths, inversely related to intensity that depend only on the bod ...

problem was discovered by Gustav Kirchhoff in 1859. In 1900, Max Planck
Max Karl Ernst Ludwig Planck, (; ; 23 April 1858 – 4 October 1947) was a German
German(s) may refer to:
Common uses
* of or related to Germany
* Germans, Germanic ethnic group, citizens of Germany or people of German ancestry
* For citi ...

proposed the hypothesis that energy is radiated and absorbed in discrete "quanta" (or energy packets), yielding a calculation that precisely matched the observed patterns of black-body radiation. The word ''quantum'' derives from the Latin language, Latin, meaning "how great" or "how much". According to Planck, quantities of energy could be thought of as divided into "elements" whose size (''E'') would be proportional to their frequency (''ν''):
:$E\; =\; h\; \backslash nu\backslash $,
where ''h'' is Planck constant, Planck's constant. Planck cautiously insisted that this was only an aspect of the processes of absorption and emission of radiation and was not the ''physical reality'' of the radiation. In fact, he considered his quantum hypothesis a mathematical trick to get the right answer rather than a sizable discovery. However, in 1905 Albert Einstein
Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born , widely acknowledged to be one of the greatest physicists of all time. Einstein is known for developing the , but he also made important contributions to the develo ...

interpreted Planck's quantum hypothesis local realism, realistically and used it to explain the photoelectric effect
The photoelectric effect is the emission of electron
The electron is a subatomic particle
In physical sciences, subatomic particles are smaller than atom
An atom is the smallest unit of ordinary matter
In classical physics and ...

, in which shining light on certain materials can eject electrons from the material. Niels Bohr
Niels Henrik David Bohr (; 7 October 1885 – 18 November 1962) was a Danish
Danish may refer to:
* Something of, from, or related to the country of Denmark
* A national or citizen of Denmark, also called a "Dane", see Demographics of De ...

then developed Planck's ideas about radiation into a Bohr model, model of the hydrogen atom that successfully predicted the spectral lines of hydrogen. Einstein further developed this idea to show that an electromagnetic wave such as light could also be described as a particle (later called the laser
A laser is a device that emits light
Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined as ...

.
This phase is known as the old quantum theory
The old quantum theory is a collection of results from the years 1900–1925 which predate modern quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that provides a description of the physical properties ...

. Never complete or self-consistent, the old quantum theory was rather a set of heuristic corrections to classical mechanics. The theory is now understood as a WKB approximation#Application to the Schr.C3.B6dinger equation, semi-classical approximation to modern quantum mechanics. Notable results from this period include, in addition to the work of Planck, Einstein and Bohr mentioned above, Einstein and Peter Debye's work on the specific heat of solids, Bohr and Hendrika Johanna van Leeuwen's Bohr–Van Leeuwen theorem, proof that classical physics cannot account for diamagnetism, and Arnold Sommerfeld's extension of the Bohr model to include special-relativistic effects.
In the mid-1920s quantum mechanics was developed to become the standard formulation for atomic physics. In 1923, the French physicist Louis-Victor de Broglie, Louis de Broglie put forward his theory of matter waves by stating that particles can exhibit wave characteristics and vice versa. Building on de Broglie's approach, modern quantum mechanics was born in 1925, when the German physicists Max Born
Max Born (; 11 December 1882 – 5 January 1970) was a German physicist
A physicist is a scientist
A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branches of science, area of intere ...

, and Pascual JordanDavid Edwards,"The Mathematical Foundations of Quantum Mechanics", ''Synthese'', Volume 42, Number 1/September, 1979, pp. 1–70.D. Edwards, "The Mathematical Foundations of Quantum Field Theory: Fermions, Gauge Fields, and Super-symmetry, Part I: Lattice Field Theories", ''International J. of Theor. Phys.'', Vol. 20, No. 7 (1981). developed matrix mechanics and the Austrian physicist Erwin Schrödinger
Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian-Irish who developed a number of fundamental results in : the provides a way to calculate the ...

invented Schrödinger equation, wave mechanics. Born introduced the probabilistic interpretation of Schrödinger's wave function in July 1926. Thus, the entire field of quantum physics emerged, leading to its wider acceptance at the Fifth Solvay Conference in 1927.
By 1930 quantum mechanics had been further unified and formalized by David Hilbert, Paul Dirac and John von Neumann with greater emphasis on measurement in quantum mechanics, measurement, the statistical nature of our knowledge of reality, and Interpretations of quantum mechanics, philosophical speculation about the 'observer'. It has since permeated many disciplines, including quantum chemistry, quantum electronics, quantum optics, and quantum information science
Quantum information science is an interdisciplinary field that seeks to understand the analysis, processing, and transmission of information using quantum mechanics
Quantum mechanics is a fundamental Scientific theory, theory in physics that p ...

. It also provides a useful framework for many features of the modern periodic table, periodic table of elements, and describes the behaviors of atoms during chemical bonding and the flow of See also

* Bra–ket notation * Einstein's thought experiments * List of textbooks on classical and quantum mechanics * Macroscopic quantum phenomena * Phase-space formulation * Regularization (physics) * Two-state quantum systemExplanatory notes

References

Further reading

The following titles, all by working physicists, attempt to communicate quantum theory to lay people, using a minimum of technical apparatus. * Marvin Chester, Chester, Marvin (1987). ''Primer of Quantum Mechanics''. John Wiley. * * Richard Feynman, 1985. ''QED: The Strange Theory of Light and Matter'', Princeton University Press. . Four elementary lectures on quantum electrodynamics andquantum 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 co ...

, yet containing many insights for the expert.
* Giancarlo Ghirardi, Ghirardi, GianCarlo, 2004. ''Sneaking a Look at God's Cards'', Gerald Malsbary, trans. Princeton Univ. Press. The most technical of the works cited here. Passages using algebra, trigonometry, and bra–ket notation can be passed over on a first reading.
* N. David Mermin, 1990, "Spooky actions at a distance: mysteries of the QT" in his ''Boojums All the Way Through''. Cambridge University Press: 110–76.
* Victor Stenger, 2000. ''Timeless Reality: Symmetry, Simplicity, and Multiple Universes''. Buffalo, NY: Prometheus Books. Chpts. 5–8. Includes cosmological and philosophical considerations.
More technical:
*
*
*
* Bryce DeWitt, R. Neill Graham, eds., 1973. ''The Many-Worlds Interpretation of Quantum Mechanics'', Princeton Series in Physics, Princeton University Press.
*
*
* Daniel Greenberger, D. Greenberger, Klaus Hentschel, K. Hentschel, F. Weinert, eds., 2009. ''Compendium of quantum physics, Concepts, experiments, history and philosophy'', Springer-Verlag, Berlin, Heidelberg.
* A standard undergraduate text.
* Max Jammer, 1966. ''The Conceptual Development of Quantum Mechanics''. McGraw Hill.
* Hagen Kleinert, 2004. ''Path Integrals in Quantum Mechanics, Statistics, Polymer Physics, and Financial Markets'', 3rd ed. Singapore: World ScientificDraft of 4th edition.

*

Online copy

* * Gunther Ludwig, 1968. ''Wave Mechanics''. London: Pergamon Press. * George Mackey (2004). ''The mathematical foundations of quantum mechanics''. Dover Publications. . * * Albert Messiah, 1966. ''Quantum Mechanics'' (Vol. I), English translation from French by G.M. Temmer. North Holland, John Wiley & Sons. Cf. chpt. IV, section III

online

* * Eric R. Scerri, Scerri, Eric R., 2006. ''The Periodic Table: Its Story and Its Significance''. Oxford University Press. Considers the extent to which chemistry and the periodic system have been reduced to quantum mechanics. * * * * Martinus J. G. Veltman, Veltman, Martinus J.G. (2003), ''Facts and Mysteries in Elementary Particle Physics''. On Wikibooks

This Quantum World

External links

* J. O'Connor and E. F. RobertsonIntroduction to Quantum Theory at Quantiki.

Quantum Physics Made Relatively Simple

three video lectures by Hans Bethe ; Course material

Quantum Cook Book

an

PHYS 201: Fundamentals of Physics II

by Ramamurti Shankar, Yale OpenCourseware

The Modern Revolution in Physics

– an online textbook. * MIT OpenCourseWare

Chemistry

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Physics

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5½ Examples in Quantum Mechanics

Imperial College Quantum Mechanics Course.

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