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Matrix mechanics is a formulation ofquantum mechanics created byWerner Heisenberg,Max Born, andPascual Jordan in 1925. It was the first conceptually autonomous and logically consistent formulation of quantum mechanics. Its account ofquantum jumps supplanted theBohr model'selectron orbits. It did so by interpreting the physical properties of particles asmatrices that evolve in time. It is equivalent to theSchrödinger wave formulation of quantum mechanics, as manifest inDirac'sbra–ket notation.
In some contrast to the wave formulation, it produces spectra of (mostly energy) operators by purely algebraic,ladder operator methods.[1] Relying on these methods,Wolfgang Pauli derived the hydrogen atom spectrum in 1926,[2] before the development of wave mechanics.
In 1925,Werner Heisenberg,Max Born, andPascual Jordan formulated the matrix mechanics representation of quantum mechanics.
In 1925 Werner Heisenberg was working inGöttingen on the problem of calculating thespectral lines ofhydrogen. By May 1925 he began trying to describe atomic systems byobservables only. On June 7, after weeks of failing to alleviate hishay fever with aspirin and cocaine,[3] Heisenberg left for the pollen-freeNorth Sea island ofHeligoland. While there, in between climbing and memorizing poems fromGoethe'sWest-östlicher Diwan, he continued to ponder the spectral issue and eventually realised that adoptingnon-commuting observables might solve the problem. He later wrote:
It was about three o' clock at night when the final result of the calculation lay before me. At first I was deeply shaken. I was so excited that I could not think of sleep. So I left the house and awaited the sunrise on the top of a rock.[4]: 275
After Heisenberg returned to Göttingen, he showedWolfgang Pauli his calculations, commenting at one point:
Everything is still vague and unclear to me, but it seems as if the electrons will no more move on orbits.[5]
On July 9 Heisenberg gave the same paper of his calculations to Max Born, saying that "he had written a crazy paper and did not dare to send it in for publication, and that Born should read it and advise him" prior to publication. Heisenberg then departed for a while, leaving Born to analyse the paper.[6]
In the paper, Heisenberg formulated quantum theory without sharply-defined electron orbits, directly advocating for a re-interpretation of quantum theory that only focused on experimental observables like frequencies and transition probabilities.
Before Heisenberg's paper,Hendrik Kramers had calculated the relative intensities of spectral lines in theSommerfeld model by interpreting theFourier coefficients of the orbits as intensities. But his answer, like all other calculations in theold quantum theory, was only correct forlarge orbits.
Heisenberg, after a collaboration with Kramers,[7] began to believe that the transition probabilities describing quantum transitions would need a new interpretation different from classical mechanics because Heisenberg believed that the frequencies that should appear in a series describing the position of the electron shouldonly be the ones that are experimentally observed in quantum transitions (like through spectral lines), not the complete set of spatial frequencies that come from making a traditionalFourier series of classical orbits.
The quantities in Heisenberg's original formulation involved a series that described position as a series of "virtual oscillators" with two indices, with the two indices representing the initial and final states of a quantum transition.[8] Rather than following the multiplication rule as expected from multiplying Fourier series, Heisenberg formed a non-commutative multiplication rule to ensure that multiplying position states would preserve the frequencies that are only found in the quantum transitions.
When Born read the paper, he recognized the formulation, particularly the non-commutative multiplication rule, as one which could be transcribed and extended to the systematiclanguage of matrices,[9] which he had learned from his study underJakob Rosanes[10] atBreslau University. Born, with the help of his assistant and former student Pascual Jordan, began immediately to make the transcription and extension, and they submitted their results for publication; the paper was received for publication just 60 days after Heisenberg's paper.[11]
A follow-on paper was submitted for publication before the end of the year by all three authors.[12] (A brief review of Born's role in the development of the matrix mechanics formulation of quantum mechanics along with a discussion of the key formula involving the non-commutativity of the probability amplitudes can be found in an article byJeremy Bernstein.[13] A detailed historical and technical account can be found in Mehra and Rechenberg's bookThe Historical Development of Quantum Theory. Volume 3. The Formulation of Matrix Mechanics and Its Modifications 1925–1926.[14])
The three fundamental papers:
- W. Heisenberg,Über quantentheoretische Umdeutung kinematischer und mechanischer Beziehungen,Zeitschrift für Physik,33, 879-893, 1925 (received July 29, 1925). [English translation in: B. L. van der Waerden, editor,Sources of Quantum Mechanics (Dover Publications, 1968)ISBN 0-486-61881-1 (English title:Quantum-Theoretical Re-interpretation of Kinematic and Mechanical Relations).]
- M. Born and P. Jordan,Zur Quantenmechanik,Zeitschrift für Physik,34, 858-888, 1925 (received September 27, 1925). [English translation in: B. L. van der Waerden, editor,Sources of Quantum Mechanics (Dover Publications, 1968)ISBN 0-486-61881-1 (English title:On Quantum Mechanics).]
- M. Born, W. Heisenberg, and P. Jordan,Zur Quantenmechanik II,Zeitschrift für Physik,35, 557-615, 1926 (received November 16, 1925). [English translation in: B. L. van der Waerden, editor,Sources of Quantum Mechanics (Dover Publications, 1968)ISBN 0-486-61881-1 (English title:On Quantum Mechanics II).]
Up until this time, matrices were seldom used by physicists; they were considered to belong to the realm of pure mathematics, thus requiring Born and Jordan's paper to introduce matrix algebra to physicists unaware of their use.Gustav Mie had used them in a paper on electrodynamics in 1912 and Born had used them in his work on the lattices theory of crystals in 1921. While matrices were used in these cases, the algebra of matrices with their multiplication did not enter the picture as they did in the matrix formulation of quantum mechanics.[15]
Born, however, had learned matrix algebra from Rosanes, as already noted, but Born had also learned Hilbert's theory of integral equations and quadratic forms for an infinite number of variables as was apparent from a citation by Born of Hilbert's workGrundzüge einer allgemeinen Theorie der Linearen Integralgleichungen published in 1912.[16][17]
Jordan, too, was well equipped for the task. For a number of years, he had been an assistant toRichard Courant at Göttingen in the preparation of Courant andDavid Hilbert's bookMethoden der mathematischen Physik I, which was published in 1924.[18] This book, fortuitously, contained a great many of the mathematical tools necessary for the continued development of quantum mechanics.
In 1926,John von Neumann became assistant to David Hilbert, and he would coin the termHilbert space to describe the algebra and analysis which were used in the development of quantum mechanics.[19][20]
A linchpin contribution to this formulation was achieved in Dirac's reinterpretation/synthesis paper of 1925,[21] which invented the language and framework usually employed today, in full display of the noncommutative structure of the entire construction.
Before matrix mechanics, the old quantum theory described the motion of a particle by a classical orbit, with well defined position and momentum X(t) , P(t) , with the restriction that the time integral over one period T of the momentum times the velocity must be a positive integer multiple of thePlanck constant(h ) as described by theSommerfeld-Wilson quantization conditionWhile this restriction correctly selects orbits with theright energy values En , the old quantum formalism did not describe time dependent processes, such as the emission or absorption of radiation.
When a classical particle is weakly coupled to a radiation field, so that the radiative damping can be neglected, it will emitradiation in a pattern that repeats itself every orbital period. The frequencies that make up the outgoing wave are then integer multiples of the orbital frequency, and this is a reflection of the fact that X(t) is periodic, so that itsFourier representation has frequencies 2 π n / T only.The coefficients Xn arecomplex numbers. The ones with negative frequencies must be thecomplex conjugates of the ones with positive frequencies, so that X(t) will always be real,
A quantum mechanical particle, on the other hand, cannot emit radiation continuously; it can only emit photons. Under theBohr model, for a quantum particle starting in quantum number n that then emits a photon by transitioning to orbit number m , the energy of the photon is En − Em , that gives a photon of frequency is En − Em / h .
For large n and m , but with n − m relatively small,Bohr'scorrespondence principle expects the same classical frequenciesIn the formula above, T is the classical period of either orbit n or orbit m , since the difference between them is higher order in h . But for small n and m , or if n − m is large, the frequencies are not integer multiples of any single frequency.
Since in classical mechanics, the frequencies that the particle emits are the same as the frequencies in the Fourier description of its motion, Heisenberg inferred that in the time-dependent description of the particle, there should be something oscillating with frequency En − Em / h . Heisenberg called this quantity Xnm , and demanded that it should reduce to the classicalFourier coefficients in the classical limit. For large values ofn and m but with n − m relatively small, Xnm is the (n −m)th Fourier coefficient of the classical motion at orbit n . Since Xnm has opposite frequency to Xmn , the condition that X is real becomes
By definition, Xnm only has the frequency En − Em /h , so its time evolution is may be described as:This is the original form of Heisenberg's equation of motion.
Given two arrays Xnm and Pnm describing two physical quantities, when modeling each as classical Fourier series, it is expected that their multiplication Xnk Pkm should also result in a new frequency as part of a new Fourier series. Whilst the Fourier coefficients of the product of two quantities is theconvolution of the Fourier coefficients of each one separately, Heisenbergchanged the multiplication rule to ensure that when multiplying each component, the new frequencies would only correspond to frequencies that already existed in the quantum orbit:
Born noticed thatthis is the law of matrix multiplication, so that the position, the momentum, the energy, all the observable quantities in the theory, are interpreted as matrices. Under this multiplication rule, the product depends on the order: X P is different from P X .
The X matrix is a complete description of the motion of a quantum mechanical particle. Because the frequencies in the quantum motion are not multiples of a common frequency, the matrix elementscannot be interpreted as the Fourier coefficients of a sharp classical trajectory. Nevertheless, as matrices, X(t) and P(t) satisfy the classical equations of motion; also see Ehrenfest's theorem, below.
When it was introduced by Werner Heisenberg, Max Born and Pascual Jordan in 1925, matrix mechanics was not immediately accepted and was a source of controversy, at first. Schrödinger's later introduction ofwave mechanics was greatly favored.
Part of the reason was that Heisenberg's formulation was in an odd mathematical language, for the time, while Schrödinger's formulation was based on familiar wave equations. But there was also a deeper sociological reason. Quantum mechanics had been developing by two paths, one led by Einstein, who emphasized the wave–particle duality he proposed for photons, and the other led by Bohr, that emphasized the discrete energy states and quantum jumps that Bohr discovered. De Broglie had reproduced the discrete energy states within Einstein's framework – the quantum condition is the standing wave condition, and this gave hope to those in the Einstein school that all the discrete aspects of quantum mechanics would be subsumed into a continuous wave mechanics.
Matrix mechanics, on the other hand, came from the Bohr school, which was concerned with discrete energy states and quantum jumps. Bohr's followers did not appreciate physical models that pictured electrons as waves, or as anything at all. They preferred to focus on the quantities that were directly connected to experiments.
In atomic physics,spectroscopy gave observational data on atomic transitions arising from the interactions of atoms with lightquanta. The Bohr school required that only those quantities that were in principle measurable by spectroscopy should appear in the theory. These quantities include the energy levels and their intensities but they do not include the exact location of a particle in its Bohr orbit. It is very hard to imagine an experiment that could determine whether an electron in the ground state of a hydrogen atom is to the right or to the left of the nucleus. It was a deep conviction that such questions did not have an answer.
The matrix formulation was built on the premise that all physical observables are represented by matrices, whose elements are indexed by two different energy levels.[22] The set ofeigenvalues of the matrix were eventually understood to be the set of all possible values that the observable can have. Since Heisenberg's matrices areHermitian, the eigenvalues are real.
If an observable is measured and the result is a certain eigenvalue, the correspondingeigenvector is the state of the system immediately after the measurement. The act of measurement in matrix mechanics collapses the state of the system. If one measures two observables simultaneously, the state of the system collapses to a common eigenvector of the two observables. Since most matrices don't have any eigenvectors in common, most observables can never be measured precisely at the same time. This is theuncertainty principle.
If two matrices share their eigenvectors, they can be simultaneously diagonalized. In the basis where they are both diagonal, it is clear that their product does not depend on their order because multiplication of diagonal matrices is just multiplication of numbers. The uncertainty principle, by contrast, is an expression of the fact that often two matricesA andB do not always commute, i.e., thatAB −BA does not necessarily equal 0. The fundamental commutation relation of matrix mechanics,implies then thatthere are no states that simultaneously have a definite position and momentum.
This principle of uncertainty holds for many other pairs of observables as well. For example, the energy does not commute with the position either, so it is impossible to precisely determine the position and energy of an electron in an atom.
In 1928,Albert Einstein nominated Heisenberg, Born, and Jordan for theNobel Prize in Physics.[23] The announcement of the Nobel Prize in Physics for 1932 was delayed until November 1933.[24] It was at that time that it was announced Heisenberg had won the Prize for 1932 "for the creation of quantum mechanics, the application of which has,inter alia, led to the discovery of the allotropic forms of hydrogen"[25] andErwin Schrödinger andPaul Adrien Maurice Dirac shared the 1933 Prize "for the discovery of new productive forms of atomic theory".[25]
On November 25, 1933, Born received a letter from Heisenberg in which he said he had been delayed in writing due to a "bad conscience" that he alone had received the Prize "for work done in Göttingen in collaboration – you, Jordan and I". Heisenberg went on to say that Born and Jordan's contribution to quantum mechanics cannot be changed by "a wrong decision from the outside".[26] In 1954, Heisenberg wrote an article honoringMax Planck for his insight in 1900. In the article, Heisenberg credited Born and Jordan for the final mathematical formulation of matrix mechanics and Heisenberg went on to stress how great their contributions were to quantum mechanics, which were not "adequately acknowledged in the public eye".[27]
Once Heisenberg introduced the matrices forX andP, he could find their matrix elements in special cases by guesswork, guided by the correspondence principle. Since the matrix elements are the quantum mechanical analogs of Fourier coefficients of the classical orbits, the simplest case is theharmonic oscillator, where the classical position and momentum,X(t) andP(t), are sinusoidal.
In units where the mass and frequency of the oscillator are equal to one (seenondimensionalization), the energy of the oscillator is
Thelevel sets of H are the clockwise orbits, and they are nested circles in phase space. The classical orbit with energy E is
The old quantum condition dictates that the integral ofP dX over an orbit, which is the area of the circle in phase space, must be an integer multiple of thePlanck constant. The area of the circle of radius √2E is 2π E . Soor, innatural units where ħ ≡ 1 , the energy becomes somewhole number.
TheFourier components of X(t ) and P(t ) are simple, and even more so when they are re-expressed as a sum and difference of position X and momentum P :
Both A and A† have only a single frequency, and X and P can be recovered from the similar sum and difference of A and A† .
Since A(t ) has a classical Fourier series with only the lowest frequency, and the matrix element Amn is the(m −n)th Fourier coefficient of the classical orbit, the matrix forA is nonzero only on the line just above the diagonal, where it is equal to √2En . The matrix for A† is likewise only nonzero on the line below the diagonal, with the same elements. Thus,from A and A† , reconstruction yieldsandwhich, up to the choice of units, are the Heisenberg matrices for the harmonic oscillator. Both matrices areHermitian, since they are constructed from the Fourier coefficients of real quantities.
Finding X(t ) and P(t ) is direct, since they are quantum Fourier coefficients so they evolve simply with time, as
The matrix product of X and P is not hermitian, but has a real and imaginary part. The real part is one half the symmetric expression X P + P X , while the imaginary part is proportional to thecommutator, which is written asIn the special case of the harmonic oscillator, it is simple to verify explicitly that X P − P X is i ħ I , where I is theidentity matrix.
It is likewise simple to verify that the matrixis adiagonal matrix, witheigenvalues Ei .
The harmonic oscillator is an important case. Finding the matrices is easier than determining the general conditions from these special forms. For this reason, Heisenberg investigated theanharmonic oscillator, withHamiltonian
In this case, theX andP matrices are no longer simple off-diagonal matrices, since the corresponding classical orbits are slightly squashed and displaced, so that they have Fourier coefficients at every classical frequency. To determine the matrix elements, Heisenberg required that the classical equations of motion be obeyed as matrix equations,
He noticed that if this could be done, thenH, considered as a matrix function ofX andP, will have zero time derivative.whereA∗B is theanticommutator,
Given that all the off diagonal elements have a nonzero frequency;H being constant implies thatH is diagonal.It was clear to Heisenberg that in this system, the energy could be exactly conserved in an arbitrary quantum system, a very encouraging sign.
The process of emission and absorption of photons seemed to demand that the conservation of energy will hold at best on average. If a wave containing exactly one photon passes over some atoms, and one of them absorbs it, that atom needs to tell the others that they can't absorb the photon anymore. But if the atoms are far apart, any signal cannot reach the other atoms in time, and they might end up absorbing the same photon anyway and dissipating the energy to the environment. When the signal reached them, the other atoms would have to somehowrecall that energy. This paradox ledBohr, Kramers and Slater to abandon exact conservation of energy. Heisenberg's formalism, when extended to include the electromagnetic field, was obviously going to sidestep this problem, a hint that the interpretation of the theory will involvewavefunction collapse.
Demanding that the classical equations of motion are preserved is not a strong enough condition to determine the matrix elements. The Planck constant does not appear in the classical equations, so that the matrices could be constructed for many different values ofħ and still satisfy the equations of motion, but with different energy levels.
So, in order to implement his program, Heisenberg needed to use the old quantum condition to fix the energy levels, then fill in the matrices with Fourier coefficients of the classical equations, then alter the matrix coefficients and the energy levels slightly to make sure the classical equations are satisfied. This is clearly not satisfactory. The old quantum conditions refer to the area enclosed by the sharp classical orbits, which do not exist in the new formalism.
The most important thing that Heisenberg discovered is how to translate the old quantum condition into a simple statement in matrix mechanics.
To do this, he investigated the action integral as a matrix quantity,
There are several problems with this integral, all stemming from the incompatibility of the matrix formalism with the old picture of orbits. Which periodT should be used?Semiclassically, it should be eitherm orn, but the difference is orderħ, and an answer to orderħ is sought. Thequantum condition tells us thatJmn is2πn on the diagonal, so the fact thatJ is classically constant tells us that the off-diagonal elements are zero.
His crucial insight was to differentiate the quantum condition with respect ton. This idea only makes complete sense in the classical limit, wheren is not an integer but the continuousaction variableJ, but Heisenberg performed analogous manipulations with matrices, where the intermediate expressions are sometimes discrete differences and sometimes derivatives.
In the following discussion, for the sake of clarity, the differentiation will be performed on the classical variables, and the transition to matrix mechanics will be done afterwards, guided by the correspondence principle.
In the classical setting, the derivative is the derivative with respect toJ of the integral which definesJ, so it is tautologically equal to 1.where the derivativesdP/dJ anddX/dJ should be interpreted as differences with respect toJ at corresponding times on nearby orbits, exactly what would be obtained if the Fourier coefficients of the orbital motion were differentiated. (These derivatives are symplectically orthogonal in phase space to the time derivativesdP/dt anddX/dt).
The final expression is clarified by introducing the variable canonically conjugate toJ, which is called theangle variableθ: The derivative with respect to time is a derivative with respect toθ, up to a factor of2πT,So the quantum condition integral is the average value over one cycle of thePoisson bracket ofX andP.
An analogous differentiation of the Fourier series ofP dX demonstrates that the off-diagonal elements of the Poisson bracket are all zero. The Poisson bracket of two canonically conjugate variables, such asX andP, is the constant value 1, so this integral really is the average value of 1; so it is 1, as we knew all along, because it isdJ/dJ after all. But Heisenberg, Born and Jordan, unlike Dirac, were not familiar with the theory of Poisson brackets, so, for them, the differentiation effectively evaluated{X, P} inJ,θ coordinates.
The Poisson Bracket, unlike the action integral, does have a simple translation to matrix mechanics – it normally corresponds to the imaginary part of the product of two variables, thecommutator.
To see this, examine the (antisymmetrized) product of two matricesA andB in the correspondence limit, where the matrix elements are slowly varying functions of the index, keeping in mind that the answer is zero classically.
In the correspondence limit, when indicesm,n are large and nearby, whilek,r are small, the rate of change of the matrix elements in the diagonal direction is the matrix element of theJ derivative of the corresponding classical quantity. So it is possible to shift any matrix element diagonally through the correspondence,where the right hand side is really only the(m −n)th Fourier component ofdA/dJ at the orbit nearm to this semiclassical order, not a full well-defined matrix.
The semiclassical time derivative of a matrix element is obtained up to a factor ofi by multiplying by the distance from the diagonal,since the coefficientAm(m+k) is semiclassically thekth Fourier coefficient of themth classical orbit.
The imaginary part of the product ofA andB can be evaluated by shifting the matrix elements around so as to reproduce the classical answer, which is zero.
The leading nonzero residual is then given entirely by the shifting. Since all the matrix elements are at indices which have a small distance from the large index position(m,m), it helps to introduce two temporary notations:A[r,k] =A(m+r)(m+k) for the matrices, anddA/dJ[r] for therth Fourier components of classical quantities,
Flipping the summation variable in the first sum fromr tor′ =k −r, the matrix element becomes,and it is clear that the principal (classical) part cancels.
The leading quantum part, neglecting the higher order product of derivatives in the residual expression, is then equal toso that, finally,which can be identified withi times thekth classical Fourier component of the Poisson bracket.
Heisenberg's original differentiation trick was eventually extended to a full semiclassical derivation of the quantum condition, in collaboration with Born and Jordan.Once they were able to establish thatthis condition replaced and extended the old quantization rule, allowing the matrix elements ofP andX for an arbitrary system to be determined simply from the form of the Hamiltonian.
The new quantization rule wasassumed to be universally true, even though the derivation from the old quantum theory required semiclassical reasoning.(A full quantum treatment, however, for more elaborate arguments of the brackets, was appreciated in the 1940s to amount to extending Poisson brackets toMoyal brackets.)
To make the transition to standard quantum mechanics, the most important further addition was thequantum state vector, now written|ψ⟩,which is the vector that the matrices act on. Without the state vector, it is not clear which particular motion the Heisenberg matrices are describing, since they include all the motions somewhere.
The interpretation of the state vector, whose components are writtenψm, was furnished by Born. This interpretation is statistical: the result of a measurement of the physical quantity corresponding to the matrixA is random, with an average value equal toAlternatively, and equivalently, the state vector gives theprobability amplitudeψn for the quantum system to be in the energy staten.
Once the state vector was introduced, matrix mechanics could be rotated toany basis, where theH matrix need no longer be diagonal. The Heisenberg equation of motion in its original form states thatAmn evolves in time like a Fourier component,which can be recast in differential formand it can be restated so that it is true in an arbitrary basis, by noting that theH matrix is diagonal with diagonal valuesEm,This is now a matrix equation, so it holds in any basis. This is the modern form of the Heisenberg equation of motion.
Its formal solution is:
All these forms of the equation of motion above say the same thing, thatA(t) is equivalent toA(0), through a basis rotation by theunitary matrixeiHt, a systematic picture elucidated by Dirac in his bra–ket notation.
Conversely, by rotating the basis for the state vector at each time byeiHt, the time dependence in the matrices can be undone. The matrices are now time independent, but the state vector rotates,This is theSchrödinger equation for the state vector, and this time-dependent change of basis amounts to transformation to theSchrödinger picture, with⟨x|ψ⟩ =ψ(x).
In quantum mechanics in theHeisenberg picture thestate vector,|ψ⟩ does not change with time, while an observableA satisfies theHeisenberg equation of motion,
The extra term is for operators such aswhich have anexplicit time dependence, in addition to the time dependence from the unitary evolution discussed.
The Heisenberg picture does not distinguish time from space, so it is better suited torelativistic theories than the Schrödinger equation. Moreover, the similarity toclassical physics is more manifest: the Hamiltonian equations of motion for classical mechanics are recovered by replacing the commutator above by thePoisson bracket (see also below). By theStone–von Neumann theorem, the Heisenberg picture and the Schrödinger picture must be unitarily equivalent, as detailed below.
Matrix mechanics rapidly developed into modern quantum mechanics, and gave interesting physical results on the spectra of atoms.
Jordan noted that the commutation relations ensure thatP acts as a differential operator.
The operator identityallows the evaluation of the commutator ofP with any power ofX, and it implies thatwhich, together with linearity, implies that aP-commutator effectively differentiates any analytic matrix function ofX.
Assuming limits are defined sensibly, this extends to arbitrary functions−but the extension need not be made explicit until a certain degree of mathematical rigor is required,
SinceX is a Hermitian matrix, it should be diagonalizable, and it will be clear from the eventual form ofP that every real number can be an eigenvalue. This makes some of the mathematics subtle, since there is a separate eigenvector for every point in space.
In the basis whereX is diagonal, an arbitrary state can be written as a superposition of states with eigenvaluesx,so thatψ(x) = ⟨x|ψ⟩, and the operatorX multiplies each eigenvector byx,
Define a linear operatorD which differentiatesψ,and note thatso that the operator−iD obeys the same commutation relation asP. Thus, the difference betweenP and−iD must commute withX,so it may be simultaneously diagonalized withX: its value acting on any eigenstate ofX is some functionf of the eigenvaluex.
This function must be real, because bothP and−iD are Hermitian,rotating each state|x⟩ by a phasef(x), that is, redefining the phase of the wavefunction:The operatoriD is redefined by an amount:which means that, in the rotated basis,P is equal to−iD.
Hence, there is always a basis for the eigenvalues ofX where the action ofP on any wavefunction is known:and the Hamiltonian in this basis is a linear differential operator on the state-vector components,
Thus, the equation of motion for the state vector is but a celebrated differential equation,
SinceD is a differential operator, in order for it to be sensibly defined, there must be eigenvalues ofX which neighbors every given value. This suggests that the only possibility is that the space of all eigenvalues ofX is all real numbers, and thatP isiD, up to a phase rotation.
To make this rigorous requires a sensible discussion of the limiting space of functions, and in this space this is theStone–von Neumann theorem: any operatorsX andP which obey the commutation relations can be made to act on a space of wavefunctions, withP a derivative operator. This implies that a Schrödinger picture is always available.
Matrix mechanics easily extends to many degrees of freedom in a natural way. Each degree of freedom has a separateX operator and a separate effective differential operatorP, and the wavefunction is a function of all the possible eigenvalues of the independent commutingX variables.
In particular, this means that a system ofN interacting particles in 3 dimensions is described by one vector whose components in a basis where all theX are diagonal is a mathematical function of3N-dimensional spacedescribing all their possible positions, effectively amuch bigger collection of values than the mere collection ofN three-dimensional wavefunctions in one physical space. Schrödinger came to the same conclusion independently, and eventually proved the equivalence of his own formalism to Heisenberg's.
Since the wavefunction is a property of the whole system, not of any one part, the description in quantum mechanics is not entirely local. The description of several quantum particles has them correlated, orentangled. This entanglement leads to strange correlations between distant particles which violate the classicalBell's inequality.
Even if the particles can only be in just two positions, the wavefunction forN particles requires2N complex numbers, one for each total configuration of positions. This is exponentially many numbers inN, so simulating quantum mechanics on a computer requires exponential resources. Conversely, this suggests that it might be possible to find quantum systems of sizeN which physically compute the answers to problems which classically require2N bits to solve. This is the aspiration behindquantum computing.
For the time-independent operatorsX andP,∂A/∂t = 0 so the Heisenberg equation above reduces to:[28]where the square brackets[ , ] denote the commutator. For a Hamiltonian which isp2/2m +V(x), theX andP operators satisfy:where the first is classically thevelocity, and second is classically theforce, orpotential gradient. These reproduce Hamilton's form ofNewton's laws of motion. In the Heisenberg picture, theX andP operators satisfy the classical equations of motion. You can take the expectation value of both sides of the equation to see that, in any state|ψ⟩:
So Newton's laws are exactly obeyed by the expected values of the operators in any given state. This isEhrenfest's theorem, which is an obvious corollary of the Heisenberg equations of motion, but is less trivial in the Schrödinger picture, where Ehrenfest discovered it.
In classical mechanics, a canonical transformation of phase space coordinates is one which preserves the structure of the Poisson brackets. The new variablesx′,p′ have the same Poisson brackets with each other as the original variablesx,p. Time evolution is a canonical transformation, since the phase space at any time is just as good a choice of variables as the phase space at any other time.
The Hamiltonian flow is thecanonical transformation:
Since the Hamiltonian can be an arbitrary function ofx andp, there are such infinitesimal canonical transformations corresponding toevery classical quantityG, whereG serves as the Hamiltonian to generate a flow of points in phase space for an increment of times,
For a general functionA(x,p) on phase space, its infinitesimal change at every stepds under this map isThe quantityG is called theinfinitesimal generator of the canonical transformation.
In quantum mechanics, the quantum analogG is now a Hermitian matrix, and the equations of motion are given by commutators,
The infinitesimal canonical motions can be formally integrated, just as the Heisenberg equation of motion were integrated,whereU =eiGs ands is an arbitrary parameter.
The definition of a quantum canonical transformation is thus an arbitrary unitary change of basis on the space of all state vectors.U is an arbitrary unitary matrix, a complex rotation in phase space,These transformations leave the sum of the absolute square of the wavefunction componentsinvariant, while they take states which are multiples of each other (including states which are imaginary multiples of each other) to states which are thesame multiple of each other.
The interpretation of the matrices is that they act asgenerators of motions on the space of states.
For example, the motion generated byP can be found by solving the Heisenberg equation of motion usingP as a Hamiltonian,These are translations of the matrixX by a multiple of the identity matrix,This is the interpretation of the derivative operatorD:eiPs =eD,the exponential of a derivative operator is a translation (so Lagrange'sshift operator).
TheX operator likewise generates translations inP. The Hamiltonian generatestranslations in time, the angular momentum generatesrotations in physical space, and the operatorX2 +P2 generatesrotations in phase space.
When a transformation, like a rotation in physical space, commutes with the Hamiltonian, the transformation is called asymmetry (behind a degeneracy) of the Hamiltonian – the Hamiltonian expressed in terms of rotated coordinates is the same as the original Hamiltonian. This means that the change in the Hamiltonian under the infinitesimal symmetry generatorL vanishes,
It then follows that the change in the generator undertime translation also vanishes,so that the matrixL is constant in time: it is conserved.
The one-to-one association of infinitesimal symmetry generators and conservation laws was discovered byEmmy Noether for classical mechanics, where the commutators arePoisson brackets, but the quantum-mechanical reasoning is identical. In quantum mechanics, any unitary symmetry transformation yields a conservation law, since if the matrix U has the property thatso it follows thatand that the time derivative ofU is zero – it is conserved.
The eigenvalues of unitary matrices are pure phases, so that the value of a unitary conserved quantity is a complex number of unit magnitude, not a real number. Another way of saying this is that a unitary matrix is the exponential ofi times a Hermitian matrix, so that the additive conserved real quantity, the phase, is only well-defined up to an integer multiple of2π. Only when the unitary symmetry matrix is part of a family that comes arbitrarily close to the identity are the conserved real quantities single-valued, and then the demand that they are conserved become a much more exacting constraint.
Symmetries which can be continuously connected to the identity are calledcontinuous, and translations, rotations, and boosts are examples. Symmetries which cannot be continuously connected to the identity arediscrete, and the operation of space-inversion, orparity, andcharge conjugation are examples.
The interpretation of the matrices as generators of canonical transformations is due to Paul Dirac.[29] The correspondence between symmetries and matrices was shown byEugene Wigner to be complete, ifantiunitary matrices which describe symmetries which include time-reversal are included.
It was physically clear to Heisenberg that the absolute squares of the matrix elements ofX, which are the Fourier coefficients of the oscillation, would yield the rate of emission of electromagnetic radiation.
In the classical limit of large orbits, if a charge with positionX(t) and chargeq is oscillating next to an equal and opposite charge at position 0, the instantaneous dipole moment isq X(t), and the time variation of this moment translates directly into the space-time variation of the vector potential, which yields nested outgoing spherical waves.
For atoms, the wavelength of the emitted light is about 10,000 times the atomic radius, and the dipole moment is the only contribution to the radiative field, while all other details of the atomic charge distribution can be ignored.
Ignoring back-reaction, the power radiated in each outgoing mode is a sum of separate contributions from the square of each independent time Fourier mode ofd,
Now, in Heisenberg's representation, the Fourier coefficients of the dipole moment are the matrix elements ofX. This correspondence allowed Heisenberg to provide the rule for the transition intensities, the fraction of the time that, starting from an initial statei, a photon is emitted and the atom jumps to a final statej,
This then allowed the magnitude of the matrix elements to be interpreted statistically:they give the intensity of the spectral lines, the probability for quantum jumps from the emission of dipole radiation.
Since the transition rates are given by the matrix elements ofX, whereverXij is zero, the corresponding transition should be absent. These were called theselection rules, which were a puzzle until the advent of matrix mechanics.
An arbitrary state of the hydrogen atom, ignoring spin, is labelled by|n;l,m⟩, where the value ofl is a measure of the total orbital angular momentum andm is itsz-component, which defines the orbit orientation. The components of the angular momentumpseudovector arewhere the products in this expression are independent of order and real, because different components ofX andP commute.
The commutation relations ofL with all three coordinate matricesX,Y,Z (or with any vector) are easy to find,which confirms that the operatorL generates rotations between the three components of the vector of coordinate matricesX.
From this, the commutator ofLz and the coordinate matricesX,Y,Z can be read off,
This means that the quantitiesX +iY andX −iY have a simple commutation rule,
Just like the matrix elements ofX +iP andX −iP for the harmonic oscillator Hamiltonian, this commutation law implies that these operators only have certain off diagonal matrix elements in states of definitem,meaning that the matrix(X +iY) takes an eigenvector ofLz with eigenvaluem to an eigenvector with eigenvaluem + 1. Similarly,(X −iY) decreasem by one unit, whileZ does not change the value ofm.
So, in a basis of|l,m⟩ states whereL2 andLz have definite values, the matrix elements of any of the three components of the position are zero, except whenm is the same or changes by one unit.
This places a constraint on the change in total angular momentum. Any state can be rotated so that its angular momentum is in thez-direction as much as possible, wherem =l. The matrix element of the position acting on|l,m⟩ can only produce values ofm which are bigger by one unit, so that if the coordinates are rotated so that the final state is|l′,l′⟩, the value ofl′ can be at most one bigger than the biggest value ofl that occurs in the initial state. Sol′ is at mostl + 1.
The matrix elements vanish forl′ >l + 1, and the reverse matrix element is determined by Hermiticity, so these vanish also whenl′ <l − 1: Dipole transitions are forbidden with a change in angular momentum of more than one unit.
The Heisenberg equation of motion determines the matrix elements ofP in the Heisenberg basis from the matrix elements ofX.which turns the diagonal part of the commutation relation into a sum rule for the magnitude of the matrix elements:
This yields a relation for the sum of the spectroscopic intensities to and from any given state, although to be absolutely correct, contributions from the radiative capture probability for unbound scattering states must be included in the sum:
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