|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| {{About|perturbation theory as a general mathematical method|perturbation theory as applied to quantum mechanics|Perturbation theory (quantum mechanics)}}
| | There are a couple of methods by which you obtain traffic to the website. To be able to more in order to your site very rapidly there belonging to the option. Buy website traffic through the pay per click (buy web site traffic) applications. The top services from where you can find more traffic are Bing search, the search engines search advertising. The main advantage to make use of the top PPC ( pay per click) search engines are persons can buy website traffic from most used search engines.<br><br>3) Choosing the phrases the particular highest search (demand) will result, in almost every case, of high competing firms. As what we're looking for is high demand and low competition, choose keywords along with a monthly Google demand up to 100 or slightly elevated.<br><br><br><br>It very important to make sure on every step by moving one take a step back and be sure it works perfectly otherwise its not good.SEO is one big way that your web traffic can grow unlimited. Link building is also one essential element which will determine where rrt's going to appear on search vehicle.<br><br>If the keyword density is too low then also it may be a problem. In this case the positioning may end picked the actual search engine. Because of this you might have to using low blog traffic. The SEO experts therefore make sure that they take care of the ideal keyword density. There no exact definition for determining the regular count. But the experts know the guidelines that are issued the particular search vehicle. This and their own experience in connection with SEO Services India enable them to determine the density that always be apt for your site.<br><br>You must careful purchasing a cash if any money package, even if. As you know, the Internet is along with great money-making opportunities. However, it additionally be full of scammed who only want to take your dollars. They lurk around the online market place looking for innocent afflicted individuals.<br><br>I wrote about techniques you may use to increase web traffic before. This article is not about the techniques themselves, but about could should apply yourself that will get the most out of them.<br><br>This is where traffic generation comes into play. You'll find article, video, blog comment and forum signature you out there, it's another opportunity for many people to find you. Better internet property you can claim, the actual greater visitors down the road . send on the site. Utilizing the techniques mentioned above, the very best way, will dramatically increase the amount of eyes you may get your content in front of. Simple website math will a person that, more eyes equals more followers.<br><br>In case you have any inquiries with regards to exactly where and also how to utilize [http://www.trafficfaze.com/ premium traffic], you are able to e-mail us with our own web-site. |
| '''Perturbation theory''' comprises mathematical methods for finding an [[approximation theory|approximate solution]] to a problem, by starting from the exact solution of a related problem. Perturbation theory is applicable if the problem at hand cannot be solved exactly, but can be formulated by adding a "small" term to the mathematical description of the exactly solvable problem.
| |
| | |
| Perturbation theory leads to an expression for the desired solution in terms of a [[formal power series]] in some "small" parameter – known as a '''perturbation series''' – that quantifies the deviation from the exactly solvable problem. The leading term in this power series is the solution of the exactly solvable problem, while further terms describe the deviation in the solution, due to the deviation from the initial problem. Formally, we have for the approximation to the full solution A, a series in the small parameter (here called <math>\epsilon</math>), like the following:
| |
| | |
| :<math> A= A_0 + \epsilon^1 A_1 + \epsilon^2 A_2 + \cdots</math>
| |
| | |
| In this example, <math>A_0</math> would be the known solution to the exactly solvable initial problem and <math>A_1</math>, <math>A_2, </math>... represent the '''higher-order terms''' which may be found iteratively by some systematic procedure. For small <math>\epsilon</math> these higher-order terms in the series become successively smaller. An approximate "perturbation solution" is obtained by truncating the series, usually by keeping only the first two terms, the initial solution and the "first-order" perturbation correction:
| |
| | |
| :<math>A \approx A_0 + \epsilon A_1</math>
| |
| | |
| ==General description==
| |
| Perturbation theory is closely related to methods used in [[numerical analysis]]. The earliest use of what would now be called ''perturbation theory'' was to deal with the otherwise unsolvable mathematical problems of [[celestial mechanics]]: [[Isaac Newton|Newton]]'s solution for the [[orbit of the Moon]], which moves noticeably differently from a simple [[Kepler's laws of planetary motion|Keplerian ellipse]] because of the competing gravitation of the [[Earth]] and the [[Sun]].
| |
| | |
| Perturbation methods start with a simplified form of the original problem, which is ''simple enough'' to be solved exactly. In [[celestial mechanics]], this is usually a [[Kepler's laws of planetary motion|Keplerian ellipse]]. Under non relativistic gravity, an ellipse is exactly correct when there are only two gravitating bodies (say, the [[Earth]] and the [[Moon]]) but not quite correct when there are three or more objects (say, the [[Earth]], [[Moon]], [[Sun]], and the rest of the [[solar system]]).
| |
| | |
| The solved, but simplified problem is then ''"perturbed"'' to make the conditions that the perturbed solution actually satisfies closer to the real problem, such as including the gravitational attraction of a third body (the [[Sun]]). The "conditions" are a formula (or several) that represent reality, often something arising from a physical law like [[Newton's second law|Newton's second law, the force-acceleration equation]]: | |
| :<math>\bold{F} = m \bold{a}</math>
| |
| In the case of the example, the force <math>\bold{F}</math> is calculated based on the number of gravitationally relevant bodies; the acceleration <math>\bold{a}</math> is obtained, using calculus, from the path of the [[Moon]] in its orbit. Both of these come in two forms: approximate values for force and acceleration, which result from simplifications, and hypothetical exact values for force and acceleration, which would require the complete answer to calculate.
| |
| | |
| The slight changes that result from accommodating the perturbation, which themselves may have been simplified yet again, are used as corrections to the approximate solution. Because of simplifications introduced along every step of the way, the corrections are never perfect, and the conditions met by the corrected solution do not perfectly match the equation demanded by reality. However, even only one cycle of corrections often provides an excellent approximate answer to what the real solution should be.
| |
| | |
| There is no requirement to stop at only one cycle of corrections. A partially corrected solution can be re-used as the new starting point for yet another cycle of perturbations and corrections. In principle, cycles of finding increasingly better corrections could go on indefinitely. In practice, one typically stops at one or two cycles of corrections. The usual difficulty with the method is that the corrections progressively make the new solutions very much more complicated, so each cycle is much more difficult to manage than the previous cycle of corrections. [[Isaac Newton]] is reported to have said, regarding the problem of the [[Moon]]'s orbit, that ''"It causeth my head to ache."''<ref>{{Citation | last1=Cropper | first1=William H. | title=Great Physicists: The Life and Times of Leading Physicists from Galileo to Hawking | publisher=[[Oxford University Press]] | isbn=978-0-19-517324-6 | year=2004 | page=34}}.</ref>
| |
| | |
| This general procedure is a widely used mathematical tool in advanced sciences and engineering: start with a simplified problem and gradually add corrections that make the formula that the corrected problem matches closer and closer to the formula that represents reality. It is the natural extension to [[function (mathematics)|mathematical functions]] of the "guess, check, and fix" method used by [[Methods of computing square roots#Babylonian method|older civilisations]] to compute certain numbers, such as square roots.
| |
| | |
| ==Examples==
| |
| Examples for the "mathematical description" are:
| |
| an [[algebraic equation]],
| |
| a [[differential equation]] (e.g., the [[equations of motion]] in [[celestial mechanics]] or a [[wave equation]]),
| |
| a [[Thermodynamic free energy|free energy]] (in [[statistical mechanics]]),
| |
| a [[Hamiltonian (quantum mechanics)|Hamiltonian]] operator (in [[quantum mechanics]]).
| |
| | |
| Examples for the kind of solution to be found perturbatively:
| |
| the solution of the equation (e.g., the [[trajectory]] of a particle),
| |
| the [[average|statistical average]] of some
| |
| physical quantity (e.g., average magnetization),
| |
| the [[ground state]] energy of a quantum mechanical
| |
| problem.
| |
| | |
| Examples for the exactly solvable problems to start with:
| |
| [[linear equation]]s, including linear equations of motion
| |
| ([[harmonic oscillator]], [[linear wave equation]]), statistical or quantum-mechanical systems of
| |
| non-interacting particles (or in general, Hamiltonians or free
| |
| energies containing only terms quadratic in all degrees of freedom).
| |
| | |
| Examples of "perturbations" to deal with:
| |
| Nonlinear contributions to the equations of motion, [[interaction]]s
| |
| between particles, terms of higher powers in the Hamiltonian/Free Energy.
| |
| | |
| For physical problems involving interactions between particles,
| |
| the terms of the perturbation series may be displayed (and
| |
| manipulated) using [[Feynman diagram]]s.
| |
| | |
| ==History==
| |
| Perturbation theory has its roots in early [[celestial mechanics]], where the theory of [[epicycles]] was used to make small corrections to the predicted paths of planets. Curiously, it was the need for more and more epicycles that eventually led to the 16th century [[Copernican revolution]] in the understanding of planetary orbits. The development of basic perturbation theory for [[differential equation]]s was fairly complete by the middle of the 19th century. It was at that time that [[Charles-Eugène Delaunay]] was studying the perturbative expansion for the [[Earth-Moon-Sun system]], and discovered the so-called "problem of small denominators". Here, the denominator appearing in the ''n''<nowiki>'th</nowiki> term of the perturbative expansion could become arbitrarily small, causing the ''n''<nowiki>'th</nowiki> correction to be as large or larger than the first-order correction. At the turn of the 20th century, this problem led [[Henri Poincaré]] to make one of the first deductions of the existence of [[chaos theory|chaos]], or what is prosaically called the "[[butterfly effect]]": that even a very small perturbation can have a very large effect on a system.
| |
| | |
| Perturbation theory saw a particularly dramatic expansion and evolution with the arrival of [[quantum mechanics]]. Although perturbation theory was used in the semi-classical theory of the [[Bohr atom]], the calculations were monstrously complicated, and subject to somewhat ambiguous interpretation. The discovery of Heisenberg's [[matrix mechanics]] allowed a vast simplification of the application of perturbation theory. Notable examples are the [[Stark effect]] and the [[Zeeman effect]], which have a simple enough theory to be included in standard undergraduate textbooks in quantum mechanics. Other early applications include the [[fine structure]] and the [[hyperfine structure]] in the [[hydrogen atom]].
| |
| | |
| In modern times, perturbation theory underlies much of [[quantum chemistry]] and [[quantum field theory]]. In chemistry, perturbation theory was used to obtain the first solutions for the [[helium atom]].
| |
| | |
| In the middle of the 20th century, [[Richard Feynman]] realized that the perturbative expansion could be given a dramatic and beautiful graphical representation in terms of what are now called [[Feynman diagrams]]. Although originally applied only in [[quantum field theory]], such diagrams now find increasing use in any area where perturbative expansions are studied. {{Citation needed|date=December 2008}}
| |
| | |
| A partial resolution of the small-divisor problem was given by the statement of the [[KAM theorem]] in 1954. Developed by [[Andrey Kolmogorov]], [[Vladimir Arnold]] and [[Jürgen Moser]], this theorem stated the conditions under which a system of partial differential equations will have only mildly chaotic behaviour under small perturbations.
| |
| | |
| In the late 20th century, broad dissatisfaction with perturbation theory in the quantum physics community, including not only the difficulty of going beyond second order in the expansion, but also questions about whether the perturbative expansion is even [[convergent series|convergent]], has led to a strong interest in the area of [[non-perturbative analysis]], that is, the study of [[exactly solvable model]]s. The prototypical model is the [[Korteweg–de Vries equation]], a highly non-linear equation for which the interesting solutions, the [[soliton]]s, cannot be reached by perturbation theory, even if the perturbations were carried out to infinite order. Much of the theoretical work in non-perturbative analysis goes under the name of [[quantum group]]s and [[non-commutative geometry]].
| |
| | |
| ==Perturbation orders==
| |
| The standard exposition of perturbation theory is given in terms of the order to which the perturbation is carried out: '''first-order perturbation theory''' or '''second-order perturbation theory''', and whether the perturbed states are degenerate (that is, [[singular perturbation|singular]]), in which case extra care must be taken, and the theory is slightly more difficult.
| |
| | |
| :''This section needs to be expanded to include the standard textbook examples of each of the three expansions.''
| |
| | |
| ==First-order non-singular perturbation theory==
| |
| This section develops, in simplified terms, the general theory for the perturbative solution to a [[differential equation]] to the first order. To keep the exposition simple, a crucial assumption is made: that the solutions to the unperturbed system are not ''[[degenerate form|degenerate]]'', so that the perturbation series can be inverted. There are ways of dealing with the degenerate (or ''[[singular perturbation|singular]]'') case; these require extra care.
| |
| | |
| Suppose one wants to solve a differential equation of the form
| |
| | |
| :<math>Dg(x)=\lambda g(x) </math>
| |
| | |
| where ''D'' is some specific [[differential operator]], and <math>\lambda</math> is an [[eigenvalue]]. Many problems involving ordinary or partial differential equations can be cast in this form. It is presumed that the differential operator can be written in the form
| |
| | |
| :<math>D=D^{(0)}+\epsilon D^{(1)} </math>
| |
| | |
| where <math>\epsilon</math> is presumed to be small, and that furthermore, the complete set of solutions for <math>D^{(0)}</math> are known. That is, one has a set of solutions <math>f^{(0)}_n(x)</math>, labelled by some arbitrary index ''n'', such that
| |
| | |
| :<math>D^{(0)} f^{(0)}_n (x)=\lambda^{(0)}_n f^{(0)}_n (x) </math>.
| |
| | |
| Furthermore, one assumes that the set of solutions <math>\{f^{(0)}_n (x)\}</math> form an [[orthonormal]] set:
| |
| | |
| :<math>\int f^{(0)}_m (x) f^{(0)}_n (x) \,dx = \delta_{mn}</math>
| |
| | |
| with <math>\delta_{mn}</math> the [[Kronecker delta]] function.
| |
| | |
| To zeroth order, one expects that the solutions <math>g(x)</math> are then somehow "close" to one of the unperturbed solutions <math>f^{(0)}_n (x) </math>. That is,
| |
| | |
| :<math>g(x)=f^{(0)}_n (x) + \mathcal{O}(\epsilon)</math>
| |
| | |
| and
| |
| | |
| :<math>\lambda=\lambda^{(0)}_n + \mathcal{O}(\epsilon)</math>.
| |
| | |
| where <math>\mathcal{O}</math> denotes the relative size, in [[big-O notation]], of the perturbation. To solve this problem, one assumes that the solution <math>g(x)</math> can be written as a linear combination of the <math>f^{(0)}_n (x) </math>:
| |
| | |
| :<math>g(x)=\sum_m c_m f^{(0)}_m (x)</math>
| |
| | |
| with all of the constants <math>c_m =\mathcal{O}(\epsilon)</math> except for ''n'', where <math>c_n =\mathcal{O}(1)</math>. Substituting this last expansion into the differential equation, taking the inner product of the result with <math> f^{(0)}_n (x) </math>, and making use of orthogonality, one obtains
| |
| | |
| :<math>c_n\lambda^{(0)}_n + \epsilon \sum_m c_m
| |
| \int f^{(0)}_n(x) D^{(1)} f^{(0)}_m(x)\,dx = \lambda c_n</math>
| |
| | |
| This can be trivially rewritten as a simple [[linear algebra]] problem of finding the [[eigenvalue]] of a [[matrix (mathematics)|matrix]], where
| |
| | |
| :<math>\sum_m A_{nm}c_m = \lambda c_n\!</math>
| |
| | |
| where the matrix elements <math>A_{nm}</math> are given by
| |
| | |
| :<math>A_{nm} = \delta_{nm}\lambda^{(0)}_n + \epsilon \int f^{(0)}_n(x) D^{(1)} f^{(0)}_m(x)\,dx </math>
| |
| | |
| Rather than solving this full matrix equation, one notes that, of all the <math>c_m</math> in the linear equation, only one, namely <math>c_n</math>, is not small. Thus, to the first order in <math>\epsilon</math>, the linear equation may be solved trivially as
| |
| | |
| :<math>\lambda = \lambda^{(0)}_n + \epsilon \int f^{(0)}_n(x) D^{(1)} f^{(0)}_n(x)\,dx </math>
| |
| | |
| since all of the other terms in the linear equation are of order <math>\mathcal{O}(\epsilon^2)</math>. The above gives the solution of the eigenvalue '''to first order in perturbation theory'''.
| |
| | |
| The function <math>g(x)</math> to first order is obtained through similar reasoning. Substituting
| |
| | |
| :<math>g(x)=f^{(0)}_n(x) + \epsilon f^{(1)}_n(x)</math>
| |
| | |
| so that
| |
| | |
| :<math>\left(D^{(0)} +\epsilon D^{(1)}\right)
| |
| \left( f^{(0)}_n(x) + \epsilon f^{(1)}_n(x) \right) =
| |
| \left( \lambda^{(0)}_n + \epsilon \lambda^{(1)}_n \right)
| |
| \left( f^{(0)}_n(x) + \epsilon f^{(1)}_n(x) \right)
| |
| </math>
| |
| | |
| gives an equation for <math>f^{(1)}_n(x)</math>. It may be solved integrating with the [[partition of unity]]
| |
| | |
| :<math>\delta(x-y)=\sum_n f^{(0)}_n(x) f^{(0)}_n(y) </math>
| |
| | |
| to give
| |
| | |
| :<math>f^{(1)}_n(x) = \sum_{m\,( \ne n)} \frac
| |
| {f^{(0)}_m (x)}
| |
| {\lambda^{(0)}_n- \lambda^{(0)}_m}
| |
| \int f^{(0)}_m(y) D^{(1)} f^{(0)}_n(y) \,dy</math>
| |
| | |
| which gives the exact solution to the perturbed differential equation '''to the first order in the perturbation <math>\epsilon</math>'''.
| |
| | |
| Several important observations can be made about the form of this solution. First, the sum over functions with differences of eigenvalues in the denominator resembles the [[Resolvent formalism|resolvent]] in [[Fredholm theory]]. This is no accident; the resolvent acts essentially as a kind of [[Green's function]] or [[propagator]], passing the perturbation along. Higher-order perturbations resemble this form, with an additional sum over a resolvent appearing at each order.
| |
| | |
| The form of this solution is sufficient to illustrate the idea behind the '''small-divisor problem'''. If, for whatever reason, two eigenvalues are close so that difference <math>\lambda^{(0)}_n- \lambda^{(0)}_m</math> become small, the corresponding term in the sum will become disproportionately large. In particular, if this happens in higher-order terms, the high-order perturbation may become as large or larger in magnitude than the first-order perturbation. Such a situation calls into question the validity of doing a perturbation to begin with. This can be understood to be a fairly catastrophic situation; it is frequently encountered in [[chaos theory|chaotic dynamical systems]], and requires the development of techniques other than perturbation theory to solve the problem.
| |
| | |
| Curiously, the situation is not at all bad if two or more eigenvalues are exactly equal. This case is referred to as '''[[singular perturbation|singular]]''' or '''degenerate perturbation theory'''. The degeneracy of eigenvalues indicates that the unperturbed system has some sort of [[symmetry]], and that the generators of the symmetry commute with the unperturbed differential operator. Typically, the perturbing term does not possess the symmetry; one says the perturbation ''lifts'' or ''breaks'' the degeneracy. In this case, the perturbation can still be performed; however, one must be careful to work in a basis for the unperturbed states so that these map one-to-one to the perturbed states, rather than being a mixture.
| |
| | |
| ==Perturbation theory of degenerate states==
| |
| | |
| One may notice that the problem occurs in the first order perturbation theory when
| |
| two or more eigenfunctions of the unperturbed system correspond to one eigenvalue i.e.
| |
| when the eigenvalue equation becomes
| |
| | |
| :<math>D^{(0)} f^{(0)}_{n,i} (x)=\lambda^{(0)}_n f^{(0)}_{n,i} (x) </math>.
| |
| | |
| and the index <math>i</math> labels many states with the same eigenvalue <math>\lambda^{(0)}_n</math>.
| |
| Expression for the eigenfunctions having the energy differences in the denominators
| |
| becomes infinite. In that case the degenerate perturbation theory must be applied.
| |
| The degeneracy must be removed first for higher order perturbation
| |
| theory. The function is first assumed to be the linear combination of
| |
| eigenfunctions with the same eigenvalue only
| |
| | |
| :<math>g(x)=\sum_k c_{n,k} f^{(0)}_{n,k}(x)</math>
| |
| | |
| which again from the orthogonality of <math>f^{(0)}_{n,k}</math> leads to the following equation
| |
| | |
| :<math>c_{n,i}\lambda^{(0)}_{n,i} + \epsilon \sum_k c_{n,k}
| |
| \int f^{(0)}_{n,i}(x) D^{(1)} f^{(0)}_{n,k} (x)\,dx = \lambda c_{n,i}</math>
| |
| | |
| for each <math>n</math>.
| |
| As for the majority of low quantum numbers <math>n</math> the <math>i</math> changes over small range | |
| of integers the later equation can be usually solved analytically as at most
| |
| 4x4 matrix equation. Once the degeneracy is removed the first and any order of the
| |
| perturbation theory may be further used with respect to the new functions.
| |
| | |
| == Example of second-order singular perturbation theory ==
| |
| | |
| Consider the following equation for the unknown variable <math>x</math>:
| |
| | |
| :<math>x=1+\epsilon x^5.</math>
| |
| | |
| For the initial problem with <math>\epsilon=0</math>, the solution is <math>x_0=1</math>. For small <math>\epsilon</math> the lowest-order approximation may be found by inserting the [[ansatz]]
| |
| | |
| :<math>x=x_0+\epsilon x_1 (+\cdots)</math>
| |
| | |
| into the equation and demanding the equation to be fulfilled up to terms that involve powers of <math>\epsilon</math> higher than the first. This yields <math>x_1=1</math>. In the same way, the higher orders may be found. However, even in this simple example it may be observed that for (arbitrarily) small <math>\epsilon>0</math> there are four other solutions to the equation (with very large magnitude). The reason we don't find these solutions in the above perturbation method is because these solutions diverge when <math>\epsilon\rightarrow 0</math> while the ansatz assumes regular behavior in this limit.
| |
| | |
| The four additional solutions can be found using the methods of '''[[singular perturbation]] theory'''. In this case this works as follows. Since the four solutions diverge at <math>\epsilon=0</math>, it makes sense to rescale <math>x</math>. We put
| |
| | |
| :<math>x = y\epsilon^{-\nu}</math>
| |
| | |
| such that in terms of <math>y</math> the solutions stay finite. This means that we need to choose the exponent <math>\nu</math> to match the rate at which the solutions diverge. In terms of <math>y</math> the equation reads:
| |
| | |
| :<math>\epsilon^{-\nu}y=1+\epsilon^{1-5\nu} y^5</math>
| |
| | |
| The 'right' value for <math>\nu</math> is obtained when the exponent of <math>\epsilon</math> in the prefactor of the term proportional to <math>y</math> is equal to the exponent of <math>\epsilon</math> in the prefactor of the term proportional to <math>y^5</math>, i.e. when <math>\nu=1/4</math>. This is called 'significant degeneration'. If we choose <math>\nu</math> larger, then the four solutions will collapse to zero in terms of <math>y</math> and they will become degenerate with the solution we found above. If we choose <math>\nu</math> smaller, then the four solutions will still diverge to infinity.
| |
| | |
| Putting <math>\nu=1/4</math> in the above equation yields:
| |
| | |
| :<math>y=\epsilon^{1/4}+y^5</math>
| |
| | |
| This equation can be solved using ordinary perturbation theory in the same way as regular expansion for <math>x</math> was obtained. Since the expansion parameter is now <math>\epsilon^{1/4}</math> we put:
| |
| | |
| :<math>y=y_0 + \epsilon^{1/4}y_1 + \epsilon^{1/2}y_2 \cdots</math>
| |
| | |
| There are 5 solutions for <math>y_0</math>: 0, 1, -1, i and -i. We must disregard the solution <math>y=0</math>. The case <math>y=0</math> corresponds to the original regular solution which appears to be at zero for <math>\epsilon=0</math>, because in the limit <math>\epsilon\rightarrow 0</math> we are rescaling by an infinite amount. The next term is <math>y_1= -1/4</math>. In terms of <math>x</math> the four solutions are thus given as:
| |
| | |
| <math>x = \epsilon^{-1/4}\left[y_0 - 1/4\epsilon^{1/4} +\cdots\right]</math>
| |
| | |
| == Example of degenerate perturbation theory – Stark effect in resonant rotating wave ==
| |
| | |
| Let us consider the Hydrogen atom rotating with a constant angular frequency <math>\omega</math> in an electric field. The Hamiltonian is given by:
| |
| :<math>H=H_0 + \epsilon x</math>
| |
| where the unperturbed Hamiltonian is
| |
| :<math>{{}\over{}}H_0={\mathbf p}^2/2 - 1/r - \omega L_z</math>,
| |
| and <math>L_z</math> is the operator for the <math>z</math>-component of angular momentum: <math>L_z=i\frac{\partial}{\partial\phi}</math>. The perturbation <math>\epsilon x</math> can be seen as the strength of the applied electric field multiplied by one of the space coordinates (This calculation is in [[atomic units]], so that every quantity is dimensionless).
| |
| | |
| | |
| The eigenvalues of <math>H_0</math> are
| |
| | |
| :<math>{{}\over{}} E_{n,m}=-1/2n^2 - m \omega </math>
| |
| | |
| For the lowest energy eigenstates of Hydrogen <math>\left|n,l,m\right\rangle</math>, <math>\left|1,0,0\right\rangle</math> and
| |
| <math>\left|2,1,1\right\rangle</math> in the resonance <math>E_{2,1}-E_{1,0}=0</math> their energies are therefore equal <math>E_{1,0}=E_{2,1}=-1/2</math>,
| |
| while the eigenstates are different.
| |
| | |
| The eigenvalue equation for the Hamiltonian takes the form
| |
| | |
| : <math>\begin{bmatrix} E_{1,0} & {\epsilon} d\\ {\epsilon} d & E_{1,0} \end{bmatrix}\begin{bmatrix}a\\b\end{bmatrix} = E \begin{bmatrix}a\\b\end{bmatrix}</math>
| |
| where
| |
| : <math>d= \frac {128}{243}a_0 </math>
| |
| | |
| which leads to the quadratic equation which can be readily solved
| |
| | |
| :<math>{{}\over{}} (E_{1,0}-E)^2 - d^2 \epsilon^2 = 0 </math>
| |
| | |
| with the solution
| |
| : <math>|\chi 1 \rangle= (|1,0,0 \rangle + |2,1,1 \rangle)/ \sqrt{2} \frac {}{} </math>
| |
| | |
| : <math>E(1)=E_{1,0}+d {\epsilon} \frac {}{} </math>
| |
| | |
| : <math>|\chi 2 \rangle= (|1,0,0 \rangle - |2,1,1 \rangle)/ \sqrt{2} \frac {}{} </math>
| |
| | |
| : <math>E(2)=E_{1,0} -d {\epsilon} \frac {}{} </math>
| |
| | |
| These states are the Stark states in the rotating frame, they are Trojan (higher eigenvalue) and anti-Trojan wavepackets.
| |
| | |
| ==Commentary==
| |
| Both regular and singular perturbation theory are frequently used in physics and engineering. Regular perturbation theory may only be used to find those solutions of a problem that evolve smoothly out of the initial solution when changing the parameter (that are "[[adiabatic]]ally connected" to the initial solution). A well-known example from physics where regular perturbation theory fails is in fluid dynamics when one treats the viscosity as a small parameter. Close to a boundary, the fluid velocity goes to zero, even for very small viscosity (the [[no-slip condition]]). For zero viscosity, it is not possible to impose this boundary condition and a regular perturbative expansion amounts to an expansion about an unrealistic physical solution. Singular perturbation theory can, however, be applied here and this amounts to 'zooming in' at the boundaries (using the [[method of matched asymptotic expansions]]).
| |
| | |
| Perturbation theory can fail when the system can transition to a different "phase" of matter, with a qualitatively different behaviour, that cannot be modelled by the physical formulas put into the perturbation theory (e.g., a solid crystal melting into a liquid). In some cases, this failure manifests itself by divergent behavior of the perturbation series. Such divergent series can sometimes be resummed using techniques such as [[Borel resummation]].
| |
| | |
| Perturbation techniques can be also used to find approximate solutions to non-linear differential equations. Examples of techniques used to find approximate solutions to these types of problems are the [[Lindstedt–Poincaré technique]], the [[method of harmonic balancing]], and the [[method of multiple time scales]].
| |
| | |
| There is absolutely no guarantee that perturbative methods result in a convergent solution. In fact, [[asymptotic series]] are the norm.
| |
| | |
| ==Perturbation theory in chemistry==
| |
| | |
| Many of the [[ab initio quantum chemistry methods]] use perturbation theory directly or are closely related methods. [[Møller–Plesset perturbation theory]] uses the difference between the [[Hartree–Fock]] Hamiltonian and the exact non-relativistic Hamiltonian as the perturbation. The zero-order energy is the sum of orbital energies. The first-order energy is the Hartree–Fock energy and electron correlation is included at second-order or higher. Calculations to second, third or fourth order are very common and the code is included in most [[Computational chemistry#Software packages|ab initio quantum chemistry programs]]. A related but more accurate method is the [[coupled cluster]] method.
| |
| | |
| ==See also==
| |
| * [[Cosmological perturbation theory]]
| |
| * [[Dynamic nuclear polarisation]]
| |
| * Alternative approach to perturbation theory<ref>J.
| |
| Martínez-Carranza, F. Soto-Eguibar and H. Moya-Cessa, Eur. Phys.
| |
| J. D, 66, 22(2012). “ALTERNATIVE ANALYSIS TO PERTURBATION THEORY.”
| |
| http://dx.doi.org/10.1140/epjd/e2011-20654-5 </ref>
| |
| * [[Eigenvalue perturbation]]
| |
| * [[Interval FEM]]
| |
| * [[Orders of approximation]]
| |
| * [[Structural stability]]
| |
| | |
| == References ==
| |
| <references/>
| |
| | |
| == External links ==
| |
| * [http://www.cims.nyu.edu/~eve2/reg_pert.pdf Introduction to regular perturbation theory] by Eric Vanden-Eijnden (PDF)
| |
| * [http://www.scholarpedia.org/article/Multiple_Scale_Analysis Perturbation Method of Multiple Scales]
| |
| | |
| {{DEFAULTSORT:Perturbation theory}}
| |
| [[Category:Perturbation theory| ]]
| |
| [[Category:Concepts in physics]]
| |
| [[Category:Functional analysis]]
| |
| [[Category:Ordinary differential equations]]
| |
| [[Category:Mathematical physics]]
| |
| [[Category:Computational chemistry]]
| |
| [[Category:Asymptotic analysis]]
| |