WKB approximation: Difference between revisions

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{{short description|Solution method for linear differential equations}}

{{Redirect2|WKB|WKBJ|other uses|WKB (disambiguation)|the television station in Live Oak, Florida|WKBJ-LD}}

In [[mathematical physics]], the '''WKB approximation''' or '''WKB method''' is a methodtechnique for finding approximate solutions to [[Linear differential equation|linear differential equations]] with spatially varying coefficients. It is typically used for a [[Semiclassical physics|semiclassical]] calculation in [[quantum mechanics]] in which the [[Wavewave function|wavefunction]] is recast as an exponential function, semiclassically expanded, and then either the amplitude or the phase is taken to be changing slowly.

The name is an initialism for '''Wentzel–Kramers–Brillouin'''. It is also known as the '''LG''' or '''Liouville–Green method'''. Other often-used letter combinations include '''JWKB''' and '''WKBJ''', where the "J" stands for Jeffreys.

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For the equation

<math display="block"> \epsilon^2 \frac{d^2 y}{dx^2} = Q(x) y, </math>

with {{math|''Q''(''x'') <0}} an [[analytic function]], the value <math>n_\max</math> and the magnitude of the last term can be estimated as follows:<ref>{{cite journal| last=Winitzki |first=S. |year=2005 |arxiv=gr-qc/0510001 |title=Cosmological particle production and the precision of the WKB approximation |journal=Phys. Rev. D |volume=72 |issue=10 |pages=104011, 14&nbsp;pp |doi=10.1103/PhysRevD.72.104011 |bibcode = 2005PhRvD..72j4011W |s2cid=119152049 }}</ref>

<math display="block">n_\max \approx 2\epsilon^{-1} \left| \int_{x_0}^{x_{\ast}} \sqrt{-Q(z)}\,dz \right| , </math>

<math display="block">\delta^{n_\max}S_{n_\max}(x_0) \approx \sqrt{\frac{2\pi}{n_\max}} \exp[-n_\max], </math>

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the number {{math|''n''_max}} will be large, and the minimum error of the asymptotic series will be exponentially small.

== Application in non -relativistic quantum mechanics ==

[[File:WKB approximation example.svg|thumb|WKB approximation to the indicated potential. Vertical lines show the turning points]]

[[File:WKB approximation to probability density.svg|thumb|Probability density for the approximate wave function. Vertical lines show the turning points]]

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2\nabla S_0 \cdot \nabla S_1 - i \nabla^2 S_0 = 0

\end{align}</math>

which can be solved for 1D systems, first equation resulting in:<math display="block">S_0(x) = \pm \int \sqrt{ \frac{2m}{\hbar^2} \left( E - V(x)\right) } \,dx=\pm\frac{1}{\hbar}\int p(x) \,dx </math>and the second equation computed for the possible values of the above, is generally expressed as:<math display="block">\Psi(x) \approx C_+ \frac{ e^{+ \frac i \hbar \int p(x)\,dx} }{\sqrt{|p(x)| }} + C_- \frac{ e^{- \frac i \hbar \int p(x)\,dx} }{\sqrt{|p(x)| }} </math>

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Hence, when <math>E > V(x)</math>, the wavefunction can be chosen to be expressed as:<math display="block">\Psi(x') \approx C \frac{\cos{(\frac 1 \hbar \int |p(x)|\,dx} + \alpha) }{\sqrt{|p(x)| }} + D \frac{ \sin{(- \frac 1 \hbar \int |p(x)|\,dx} +\alpha)}{\sqrt{|p(x)| }} </math>and for <math>V(x) > E</math>,<math display="block">\Psi(x') \approx \frac{ C_{+} e^{+- \frac{i1}{\hbar} \int |p(x)|\,dx}}{\sqrt{|p(x)|}} + \frac{ C_{-} e^{-+ \frac{i1}{\hbar} \int |p(x)|\,dx} }{ \sqrt{|p(x)|} } . </math>The integration in this solution is computed between the classical turning point and the arbitrary position x'.

=== Validity of WKB solutions ===

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==== First classical turning point ====

For <math>U_1 < 0</math> ie. decreasing potential condition or <math>x=x_1

</math> in the given example shown by the figure, we require the exponential function to decay for negative values of x so that wavefunction for it to go to zero. Considering Bairy functions to be the required connection formula, we get:<ref name=":3">{{Cite journal |last1=Ramkarthik |first1=M. S. |last2=Pereira |first2=Elizabeth Louis |date=2021-06-01 |title=Airy Functions Demystified — II |url=https://doi.org/10.1007/s12045-021-1179-z |journal=Resonance |language=en |volume=26 |issue=6 |pages=757–789 |doi=10.1007/s12045-021-1179-z |issn=0973-712X|url-access=subscription }}</ref>

<math display="block">\begin{align}

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Either way, the condition on the energy is a version of the [[Bohr–Sommerfeld quantization]] condition, with a "[[Lagrangian Grassmannian#Maslov index|Maslov correction]]" equal to 1/2.<ref>{{harvnb|Hall|2013}} Section 15.2</ref>

It is possible to show that after piecing together the approximations in the various regions, one obtains a good approximation to the actual [[eigenfunction]]. In particular, the Maslov-corrected Bohr–Sommerfeld energies are good approximations to the actual eigenvalues of the Schrödinger operator.<ref>{{harvnb|Hall|2013}} Theorem 15.8</ref> Specifically, the error in the energies is small compared to the typical spacing of the quantum energy levels. Thus, although the "old quantum theory" of Bohr and Sommerfeld was ultimately replaced by the Schrödinger equation, some vestige of that theory remains, as an approximation to the eigenvalues of the appropriate Schrödinger operator.

==== General connection conditions ====

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<math display="block">E = {\left(3\left(n-\frac 1 4\right)\pi\right)^{\frac 2 3} \over 2}(mg^2\hbar^2)^{\frac 1 3}. </math>

This result is also consistent with the use of equation from [[bound state]] of one rigid wall without needing to consider an alternative potential.

=== Quantum Tunneling ===