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WKB approximation

The WKB approximation, named after physicists Gregor Wentzel, Hendrik Anthony Kramers, and Léon Brillouin who independently developed it in 1926, is a semiclassical method for finding approximate solutions to the time-independent Schrödinger equation in quantum mechanics when the potential varies slowly compared to the local de Broglie wavelength of the particle. An earlier formulation of the technique appeared in 1924 by mathematician Harold Jeffreys for approximate solutions to classical linear differential equations with variable coefficients. In the WKB approach, the wave function is expressed as \psi(x) = \frac{A}{\sqrt{p(x)}} \exp\left(\pm \frac{i}{\hbar} \int p(x') \, dx'\right), where p(x) = \sqrt{2m(E - V(x))} is the classical momentum, providing a connection between quantum wave functions and classical trajectories.^[1] This ansatz arises from substituting \psi(x) = \exp(i S(x)/\hbar) into the Schrödinger equation and expanding S(x) in a semiclassical series in powers of \hbar, with the leading order satisfying the classical Hamilton-Jacobi equation.^[1] The approximation is valid in regions where the de Broglie wavelength \lambda(x) = h / p(x) changes slowly, specifically where |d\lambda / dx| \ll 1, allowing the neglect of higher-order \hbar terms; it breaks down near classical turning points where E = V(x), requiring special connection formulas involving Airy functions to match solutions across these points.^[1] Key applications include the quantization of energy levels for bound states in slowly varying potentials, yielding the condition \int_{x_1}^{x_2} k(x) \, dx = \left(n + \frac{1}{2}\right)\pi for large quantum numbers n, where k(x) = p(x)/\hbar, and calculating tunneling probabilities through barriers as T \approx \exp\left(-2 \int \kappa(x) \, dx\right), where \kappa(x) = \sqrt{2m(V(x) - E)} / \hbar.^[2] Beyond one-dimensional quantum problems, the WKB method extends to multidimensional systems, wave propagation in optics and acoustics, and asymptotic analysis in other fields involving slowly varying media.^[3]

History and Development

Origins in the 1920s

In the early 1920s, the old quantum theory, exemplified by the Bohr-Sommerfeld quantization rules, provided a framework for understanding atomic spectra but struggled with exact solutions for complex potentials, prompting the search for semiclassical approximations. The introduction of Erwin Schrödinger's wave equation in 1926 marked a pivotal shift toward wave mechanics, yet solving it exactly for most systems remained challenging, especially for scattering and bound-state problems where classical turning points complicated the analysis. This context spurred independent efforts to develop asymptotic methods that bridged classical mechanics and the new quantum framework, laying the groundwork for what became known as the WKB approximation.^[4] Gregor Wentzel derived an early form of the method in 1926, applying phase-integral techniques to the radial Schrödinger equation for scattering problems in central potentials. His approach emphasized continuous integration over phase space to approximate wave functions, providing a semiclassical quantization condition that generalized older integral methods while addressing quantum corrections near turning points. This work, published in Zeitschrift für Physik, focused on the validity of the approximation for slowly varying potentials, establishing its utility for continuous spectra.^[4] Independently, Hendrik Anthony Kramers extended the technique that same year to bound states, incorporating treatments of classical turning points to derive quantization rules for discrete energy levels. In his paper in Zeitschrift für Physik, Kramers introduced a key modification to the effective potential in the radial equation—replacing the centrifugal term l(l+1) with (l + 1/2)^2—to achieve first-order accuracy and connect the wave function's oscillatory behavior to half-integer quantum numbers, thus reconciling wave mechanics with empirical spectral data.^[4] Léon Brillouin contributed a complementary perspective in 1926 through two Comptes Rendus notes, highlighting the method's roots in the adiabatic invariant from classical mechanics and its application to electron motion in force fields. He framed the approximation as successive perturbations of the Hamilton-Jacobi equation, deriving phase-integral expressions that preserved quantization under slow variations, thereby underscoring the semiclassical limit's connection to classical action integrals for periodic orbits. Brillouin's emphasis on adiabaticity influenced later refinements, positioning the technique as a tool for transitioning between classical and quantum regimes.^[4]

Key Contributors and Evolution

Gregor Wentzel (1898–1978), a German theoretical physicist, contributed significantly to the early development of the approximation through his 1926 paper, where he generalized quantization conditions for open paths, particularly applying the method to radial wave functions in central force problems in quantum mechanics. Hendrik Anthony Kramers (1894–1952), a Dutch physicist known for his work in quantum theory, independently derived the approximation in the same year, emphasizing its role in deriving quantization rules that bridged wave mechanics with the old quantum theory's half-integer conditions. Léon Brillouin (1889–1969), a French physicist prominent in solid-state physics, also developed the method concurrently, focusing on its application to the motion of particles in force fields and the connection between Schrödinger's wave mechanics and classical Hamilton-Jacobi theory, laying groundwork for semiclassical methods. Prior to these quantum applications, British mathematician and geophysicist Harold Jeffreys (1891–1989) had introduced a precursor method in 1924 for approximating solutions to linear second-order differential equations, originally motivated by problems in astronomical wave propagation and seismology. Although Jeffreys' work predated the Schrödinger equation, the 1926 contributions by Wentzel, Kramers, and Brillouin adapted and extended it to quantum contexts, incorporating treatments of turning points that enhanced its utility for bound states and tunneling. The method evolved through the 1930s and 1940s with refinements to connection formulas and validity conditions, often referred to initially as the "phase integral method" in astronomical and geophysical literature. By the 1950s, it gained widespread recognition under the acronym WKB, honoring Wentzel, Kramers, and Brillouin, while sometimes denoted as JWKB or WKBJ to acknowledge Jeffreys' foundational role. Its standardization as a core tool in quantum mechanics was solidified by inclusion in influential textbooks, such as Albert Messiah's Quantum Mechanics (1961), which dedicated a full chapter to the classical approximation and WKB method, establishing it as a standard pedagogical and research technique.

Mathematical Formulation

Derivation from the Schrödinger Equation

The derivation of the WKB approximation begins with the time-independent Schrödinger equation for a single particle of mass m in a one-dimensional potential V(x):

-\frac{\hbar^2}{2m} \frac{d^2 \psi}{dx^2} + V(x) \psi(x) = E \psi(x),

where E is the energy, \hbar is the reduced Planck's constant, and \psi(x) is the wave function. This equation is exact for the stationary states but difficult to solve analytically for arbitrary V(x). The WKB method provides an approximate solution under the semiclassical limit \hbar \to 0, where quantum effects are small compared to classical ones, particularly when the potential varies slowly on the scale of the local de Broglie wavelength.^[5] To proceed, define the local classical momentum p(x) = \sqrt{2m(E - V(x))} and the corresponding local wave number k(x) = p(x)/\hbar, assuming E > V(x) in the classically allowed region. The key assumption is that V(x) changes slowly, meaning |d\lambda/dx| \ll 1, where \lambda(x) = 2\pi / k(x) is the local wavelength; this ensures the wave function resembles a plane wave modulated by the slowly varying potential. The WKB ansatz posits a wave function of the form

\psi(x) = A(x) \exp\left( \frac{i}{\hbar} S(x) \right),

where A(x) is a slowly varying amplitude and S(x) is the phase function, both treated as functions of x but varying on scales much longer than \lambda(x). This form is motivated by the plane-wave solutions in constant potentials and the semiclassical correspondence principle.^[5]^[6] Substituting the ansatz into the Schrödinger equation yields, after computing the second derivative and neglecting terms of order \hbar relative to the leading contributions in the limit \hbar \to 0,

\left( \frac{dS}{dx} \right)^2 = p^2(x) = 2m \bigl( E - V(x) \bigr).

This is the eikonal equation, analogous to the Hamilton-Jacobi equation in classical mechanics, where S(x) represents the classical action along the trajectory. Solving it gives S(x) = \pm \int^x p(x') \, dx'. To find the amplitude A(x), consider the next-order terms in the expansion, leading to the transport equation

\frac{d}{dx} \bigl( A^2(x) p(x) \bigr) = 0,

which implies conservation of probability current and yields A(x) \propto 1 / \sqrt{p(x)}. Thus, the full leading-order WKB wave function is

\psi(x) \approx \frac{C}{\sqrt{p(x)}} \exp\left( \pm \frac{i}{\hbar} \int^x p(x') \, dx' \right),

where C is a normalization constant. This solution captures the oscillatory behavior in allowed regions, with the phase accumulating according to the classical action and the amplitude ensuring unit probability flux, valid in the semiclassical regime where \hbar is small and the potential is smooth.^[5]^[6]

Asymptotic Series Expansion

The WKB approximation extends beyond its leading-order form through a systematic asymptotic series expansion in powers of \hbar, allowing for higher-order corrections to the wave function. The ansatz for the wave function is typically written as \psi(x) = \exp\left( \frac{i}{\hbar} S(x) \right), where the phase function S(x) is expanded as an asymptotic series: S(x) = S_0(x) + \hbar S_1(x) + \hbar^2 S_2(x) + \cdots. This expansion is substituted into the time-independent Schrödinger equation, -\frac{\hbar^2}{2m} \frac{d^2\psi}{dx^2} + V(x)\psi = E\psi, and terms are collected order by order in \hbar to yield a hierarchy of equations for the coefficients S_n(x).^[1]^[7] At leading order, the equation for S_0(x) reduces to the eikonal equation, (S_0'(x))^2 = p^2(x), where p(x) = \sqrt{2m(E - V(x))}, so S_0'(x) = \pm p(x) and S_0(x) = \pm \int^x p(x') \, dx'. The next-order term satisfies the recursive equation $2 S_0' S_1' - i S_0'' = 0, which, using the leading-order solution with the positive branch S_0' = p(x), simplifies to S_1'(x) = \frac{i}{2} \frac{p'(x)}{p(x)}, with the solution S_1(x) = \frac{i}{2} \ln p(x) + C, where C is a constant. Higher-order coefficients S_n(x) for n \geq 2 are determined similarly through increasingly complex recursive differential equations derived from equating coefficients of \hbar^{n-2} in the expanded Schrödinger equation, involving derivatives of lower-order terms.^[1]^[7] The resulting wave function incorporates these corrections, yielding the expanded form \psi(x) \approx \frac{1}{\sqrt{p(x)}} \exp\left( \frac{i}{\hbar} \int^x p(x') \, dx' \right) at leading order, where the amplitude \frac{1}{\sqrt{p(x)}} arises from the real part of the O(\hbar) correction i S_1 = -\frac{1}{2} \ln p(x), ensuring conservation of probability current. Subsequent terms in the series refine both the amplitude and phase, with the full expansion capturing semiclassical effects more accurately away from regions of rapid variation.^[1]^[7] This asymptotic series is divergent in the formal sense, meaning it does not converge for any finite \hbar, but partial sums provide excellent approximations when \hbar \ll 1 and the potential varies slowly compared to the de Broglie wavelength. The error in truncating at order n is generally bounded by the magnitude of the next term in the series, ensuring controlled accuracy in the semiclassical regime. Despite its non-convergent nature, the series proves particularly valuable for eigenvalue problems, where higher-order terms facilitate precise matching of solutions across different regions to determine quantized energy levels.^[1]^[7]

Core Principles in Quantum Mechanics

Semiclassical Wave Function Away from Turning Points

In the WKB approximation, classical turning points are defined as the locations x_t where the particle's total energy E equals the potential energy V(x_t), thereby demarcating the classically allowed regions where E > V(x) from the forbidden regions where E < V(x). These points play a crucial role in determining the qualitative behavior of the quantum wave function, as the local de Broglie wavelength diverges at x_t, rendering the approximation invalid nearby.^[8]^[5] Away from these turning points, in the classically allowed region where E > V(x), the semiclassical wave function takes an oscillatory form that captures the wavelike nature of the particle's motion. Specifically, it is given by

\psi(x) \approx \frac{C}{\sqrt{|p(x)|}} \sin\left( \frac{1}{\hbar} \int_{x_1}^x |p(x')| \, dx' + \frac{\pi}{4} \right),

where p(x) = \sqrt{2m(E - V(x))} is the classical momentum, x_1 is the left turning point, C is a normalization constant, and \hbar is the reduced Planck's constant. This expression arises from the asymptotic solution to the time-independent Schrödinger equation under the assumption of slowly varying potential, representing a superposition of left- and right-moving waves with locally varying amplitude and phase. The phase integral \int |p(x')| \, dx' / \hbar accumulates the action along the classical trajectory, aligning the quantum phase with the semiclassical limit.^[8]^[5] In the classically forbidden region where E < V(x), the wave function exhibits exponential behavior. For the right forbidden region (x > x_2), where x_2 is the right turning point, the decaying solution away from the turning point is

\psi(x) \approx \frac{D}{\sqrt{|p(x)|}} \exp\left( -\frac{1}{\hbar} \int_{x_2}^x |p(x')| \, dx' \right),

with p(x) = \sqrt{2m(V(x) - E)} (taken as positive real for the magnitude), and D a constant. This solution ensures the wave function remains normalizable and physically meaningful, with the exponential suppression quantifying the tunneling probability through barriers. The choice of the decaying form (rather than growing) is selected to satisfy boundary conditions at infinity.^[8]^[5] In the classically allowed region, the time-averaged amplitude of the WKB wave function, |\psi(x)|^2 \propto 1/|p(x)|, mirrors the classical probability density for finding the particle at position x. This correspondence arises because, in classical mechanics, the time spent in an interval dx is inversely proportional to the speed |p(x)|/m, leading to a probability density \rho(x) \propto 1/|p(x)| for ergodic motion. Thus, the semiclassical wave function provides a quantum analog of classical statistical mechanics, bridging the two descriptions in the limit of large quantum numbers.^[9]^[1] The validity of these semiclassical forms requires that the potential varies slowly compared to the local de Broglie wavelength, specifically the condition \left| \frac{d\lambda}{dx} \right| \ll 1, where \lambda(x) = 2\pi \hbar / |p(x)| is the local wavelength. This ensures the wavelength changes little over one oscillation period, justifying the neglect of higher-order derivatives in the asymptotic expansion. Equivalently, it implies \hbar |dV/dx| \ll [2m(E - V(x))]^{3/2} in allowed regions, preventing rapid variations that would invalidate the approximation.^[9]^[1]

Validity and Precision of the Approximation

The WKB approximation is valid in regions where the local de Broglie wavelength varies slowly with position, specifically when the condition \left| \frac{d}{dx} \left( \frac{\hbar}{p(x)} \right) \right| \ll 1 holds, with p(x) = \sqrt{2m(E - V(x))} being the classical momentum.^[10] This criterion ensures that the potential V(x) changes little over one de Broglie wavelength, allowing the semiclassical ansatz to capture the essential behavior of the wave function without significant distortion from rapid variations.^[1] Equivalently, the approximation applies when p(x)^2 \gg \hbar |p'(x)|, where p'(x) = dp/dx, meaning higher-order \hbar-corrections are negligible compared to the leading semiclassical terms.^[10] The leading-order error in the WKB approximation is of order O(\hbar), arising from the truncation of the asymptotic series expansion of the wave function.^[1] More precisely, the relative error in the wave function amplitude or phase is on the order of \sim \frac{\hbar |p''(x)|}{p(x)^3}, where p''(x) = d^2p/dx^2, reflecting the contribution from the next term in the expansion that involves the curvature of the momentum.^[10] This error becomes prominent when the potential's second derivative induces significant deviations from the zeroth-order solution, though it remains small under the slow-variation assumption. A notable comparison to exact solutions occurs in the quantum harmonic oscillator, where the WKB method, via the Bohr-Sommerfeld quantization rule, yields the exact energy levels E_n = (n + 1/2) \hbar \omega for integer n, despite providing only approximate wave functions.^[11] However, even here, the approximation breaks down near the turning points unless the energy is much larger than \hbar \omega, highlighting its limitations in capturing full wave function details.^[10] The approximation fails when |V'(x)| / |E - V(x)|^{3/2} becomes large, as this signals proximity to classical turning points where the momentum vanishes and the slow-variation condition is violated.^[1] In such regimes, the de Broglie wavelength diverges, invalidating the semiclassical expansion. As an asymptotic method, the precision of the WKB series improves as \hbar decreases, with higher-order terms becoming progressively smaller in the classical limit.^[10] Nonetheless, achieving uniform validity across the entire domain, particularly spanning turning points, necessitates supplementary connection formulas to match solutions between classically allowed and forbidden regions.^[1]

Handling Turning Points

Behavior Near Classical Turning Points

The standard WKB approximation fails near classical turning points, where the potential energy V(x_t) = E, the total energy of the system, causing the local momentum p(x) = \sqrt{2m(E - V(x))} to vanish.^[1] This leads to a singularity in the amplitude of the approximate wave function, as the prefactor p(x)^{-1/2} diverges, and the phase accumulates rapidly due to the infinite local de Broglie wavelength, violating the underlying assumption of slowly varying potential on the scale of the wavelength.^[7] Consequently, the semiclassical solutions in the oscillatory (allowed) and evanescent (forbidden) regions cannot be directly matched at x_t, requiring a specialized treatment to connect them smoothly.^[12] To analyze the behavior near a turning point, the potential is linearly approximated as V(x) \approx E + V'(x_t)(x - x_t), assuming a "simple" turning point where the derivative V'(x_t) \neq 0.^[13] Substituting this into the time-independent Schrödinger equation yields

-\frac{\hbar^2}{2m} \frac{d^2 \psi}{dx^2} + V'(x_t) (x - x_t) \psi = 0,

which, upon defining the scaled coordinate \xi = \left( \frac{2m |V'(x_t)|}{\hbar^2} \right)^{1/3} (x - x_t) (with appropriate sign for the slope), transforms into the Airy equation

\frac{d^2 \psi}{d\xi^2} = \xi \psi.

^[1] The solutions are the Airy functions \mathrm{Ai}(\xi) and \mathrm{Bi}(\xi), where \mathrm{Ai}(\xi) remains finite and decays exponentially for \xi > 0 (forbidden region), while oscillating for \xi < 0 (allowed region), providing a uniform approximation that bridges the two regimes.^[12] Qualitatively, the wave function transitions from oscillatory to evanescent over a characteristic distance \Delta x \sim \hbar^{2/3} / (m |V'(x_t)|)^{1/3}, which is the scale over which the linear approximation holds and the WKB validity breaks down.^[7] For non-simple turning points, where V'(x_t) = 0 (e.g., quadratic extrema), higher-order expansions are needed, leading to more complex behaviors beyond the linear case.^[13] This local Airy function solution sets the stage for matching to the global WKB wave functions away from the turning point.^[1]

Airy Function Connections

Near a classical turning point, where the potential is approximately linear, the Schrödinger equation simplifies to the Airy equation, whose exact solutions are linear combinations of the Airy functions of the first and second kind: \psi(\xi) = c_1 \mathrm{Ai}(\xi) + c_2 \mathrm{Bi}(\xi), with \mathrm{Ai}(\xi) representing the decaying solution and \mathrm{Bi}(\xi) the growing one in the forbidden region.^[14] To connect these exact solutions to the WKB approximation, the asymptotic expansions of the Airy functions are matched to the WKB wave functions in the adjacent regions. In the oscillatory region where \xi < 0, the Airy function \mathrm{Ai}(\xi) asymptotes to

\mathrm{Ai}(\xi) \sim \frac{1}{\sqrt{\pi} (-\xi)^{1/4}} \sin\left( \zeta + \frac{\pi}{4} \right),

with \zeta = \frac{2}{3} (-\xi)^{3/2}, allowing seamless matching to the sinusoidal WKB form.^[14] In the forbidden region where \xi > 0, it asymptotes to the exponentially decaying form

\mathrm{Ai}(\xi) \sim \frac{1}{2\sqrt{\pi} \xi^{1/4}} \exp\left( -\frac{2}{3} \xi^{3/2} \right),

which corresponds to the WKB evanescent wave.^[14] The connection rule ensures continuity of the wave function and its derivative by matching an incoming oscillatory wave from the allowed region to the decaying exponential in the forbidden region, while setting the coefficient of the growing \mathrm{Bi}(\xi) term to zero for physically relevant bound states.^[14] This matching introduces a characteristic phase shift of \pi/4 at each turning point, which accumulates to contribute to the overall quantization condition.^[14]

Quantization and Connection Formulas

Bohr-Sommerfeld Quantization Rule

The Bohr-Sommerfeld quantization rule emerges as a key outcome of the WKB approximation for bound states in one-dimensional quantum systems, providing a semiclassical condition on the energy levels. For a particle confined between two classical turning points x_1 and x_2, where the local momentum p(x) = \sqrt{2m(E - V(x))} vanishes, the rule states that the phase integral must satisfy

\int_{x_1}^{x_2} p(x) \, dx = \left(n + \frac{1}{2}\right) \pi \hbar,

with n = 0, 1, 2, \dots.^[7]^[13] This condition ensures the wave function is single-valued and matches smoothly across the classically allowed region, incorporating quantum corrections to the classical action.^[15] The derivation arises from the WKB wave function in the allowed region, \psi(x) \propto p(x)^{-1/2} \exp\left(\pm \frac{i}{\hbar} \int^x p(x') \, dx'\right), which accumulates phase as the particle traverses between turning points. Near each turning point, the approximation breaks down, but connection formulas introduce a phase shift of \pi/4 per turning point due to the transition to evanescent behavior in the forbidden regions.^[13] For two turning points, these shifts total \pi/2, corresponding to a Maslov index of 1, which modifies the naive integer quantization to include the +1/2 correction.^[7] This total phase accumulation of $2\pi(n + 1/2) around the "orbit" guarantees constructive interference for bound states.^[16] In terms of classical mechanics, the rule relates directly to the action variable J = \frac{1}{2\pi} \oint p \, dq, where the closed integral \oint p \, dq = 2 \int_{x_1}^{x_2} p(x) \, dx encloses the phase-space orbit. Thus, J = (n + 1/2) \hbar, quantizing the action in units of \hbar with the semiclassical shift.^[7]^[13] This represents an improvement over the original Bohr-Sommerfeld rule from old quantum theory, which used integer multiples \oint p \, dq = n h without the $1/2 correction, leading to inaccuracies for low-lying states.^[17]^[16] The rule is exact for the quantum harmonic oscillator, yielding energies E_n = (n + 1/2) \hbar \omega.^[7]^[13] For other potentials, such as anharmonic ones, it provides a good approximation for large n but deviates for small n due to higher-order WKB corrections.^[15]

General Connection Conditions

In the WKB approximation, general connection conditions enable the matching of semiclassical wave functions across classical turning points, where the standard asymptotic form breaks down. For a simple left turning point at position x = a, the potential near a is linearly approximated as V(x) \approx E + F(a - x), with F > 0 the force magnitude, transforming the Schrödinger equation into the Airy equation in a scaled variable \zeta = (2mF/\hbar^2)^{1/3} (x - a). The physically relevant solution in the forbidden region (x < a, where E < V(x)) is the exponentially decaying WKB form \psi(x) \sim \frac{C}{\sqrt{\kappa(x)}} \exp\left( -\frac{1}{\hbar} \int_x^a \kappa(x') \, dx' \right), with \kappa(x) = \sqrt{2m(V(x) - E)}; this connects to the oscillatory form in the allowed region (x > a, E > V(x)) as \psi(x) \sim \frac{2C}{\sqrt{p(x)}} \cos\left( \frac{1}{\hbar} \int_a^x p(x') \, dx' - \frac{\pi}{4} \right), where p(x) = \sqrt{2m(E - V(x))}, via the Airy function \mathrm{Ai}(\zeta) that provides the uniform approximation bridging both regimes.^[18] A symmetric connection applies for a right turning point at x = b, where the decaying solution to the right (x > b) matches to an oscillatory form to the left with a phase shift of -\pi/4, ensuring continuity and derivative matching through the Airy function.^[13] These conditions extend to tunneling through a potential barrier defined by two turning points x_1 < x_2, where the incident wave from the left allowed region connects through the forbidden barrier to the transmitted wave on the right. The transmission coefficient is then T \approx \exp\left( -\frac{2}{\hbar} \int_{x_1}^{x_2} |p(x)| \, dx \right), with p(x) = i \kappa(x) in the classically forbidden region, capturing the exponential suppression of transmission for thick barriers.^[13] For such barriers, the reflection coefficient approximates R \approx 1 (in amplitude, with |R|^2 \approx 1), as the growing exponential solution in the forbidden region is negligible compared to the decaying one, leading to near-total reflection modulated by weak transmission.^[18] For scenarios involving multiple turning points, such as complex potentials or higher-dimensional reductions, the connection formulas employ analytic continuation into the complex x-plane, where turning points are branch points of the semiclassical phase. Stokes lines, defined as loci where the WKB integrand's imaginary part vanishes (emanating from turning points at angles multiples of \pi/3), delineate sectors of dominant exponential behavior, allowing precise matching of solutions along steepest-descent contours that avoid anti-Stokes lines and ensure subdominant terms are properly included or excluded.^[19] In radial quantum mechanical problems, the Langer modification refines these connections by transforming the effective potential to replace the centrifugal barrier l(l+1)\hbar^2/(2mr^2) with (l + 1/2)^2 \hbar^2/(2mr^2), treating the origin as an effective regular turning point for improved asymptotic accuracy across the entire domain.^[20]

Applications and Examples

Quantum Tunneling

In the context of quantum tunneling, the WKB approximation is employed to describe the penetration of a particle through a potential barrier where the classical energy E is less than the potential V(x) in the interval between turning points x_1 and x_2, defined by V(x_1) = V(x_2) = E and V(x) > E for x_1 < x < x_2. In this forbidden region, the local momentum becomes imaginary, p(x) = i \sqrt{2m(V(x) - E)}, leading to an evanescent WKB wave function of the form

\psi(x) \approx \frac{C}{\sqrt{|p(x)|}} \exp\left( -\frac{1}{\hbar} \int_{x_1}^x |p(x')| \, dx' \right),

which exhibits exponential decay away from the left turning point. On the right side of the barrier, a growing exponential term is discarded to ensure physical boundedness, while the connection formulas link these solutions to the oscillatory WKB forms in the classically allowed regions outside the barrier. The tunneling probability P, or transmission coefficient, is derived by matching the WKB wave functions across the turning points using the general connection conditions, resulting in an expression dominated by an exponential suppression factor with a polynomial prefactor. For a general smooth barrier, P \approx \exp\left( -\frac{2}{\hbar} \int_{x_1}^{x_2} \sqrt{2m(V(x) - E)} \, dx \right), where the integral represents twice the imaginary action in the forbidden region. For the specific case of a rectangular barrier of height V_0 and width $2a, where V(x) = V_0 for |x| < a and $0 otherwise (with E < V_0), the approximation yields

P \approx 16 \frac{E(V_0 - E)}{V_0^2} \exp\left( -\frac{2\sqrt{2m}}{\hbar} \int_{x_1}^{x_2} \sqrt{V(x) - E} \, dx \right),

with the integral evaluating to $2a \sqrt{2m(V_0 - E)}. This prefactor emerges from the asymptotic matching of incident and transmitted plane waves to the evanescent solutions inside the barrier, approximating the exact transmission coefficient for thick barriers where \kappa a \gg 1 and \kappa = \sqrt{2m(V_0 - E)} / \hbar. The exponential term in the probability arises from the imaginary momentum in the classically inaccessible region, quantifying the severe suppression of tunneling for high or wide barriers, while the prefactor accounts for the relative amplitudes of the waves on either side. This formulation is particularly accurate for thick barriers, where the WKB validity condition |d\lambda/dx| \ll 1 (with \lambda the de Broglie wavelength) holds well away from the turning points. A seminal application is George Gamow's 1928 theory of alpha decay, where the WKB-like integral describes the tunneling of alpha particles through the Coulomb barrier in heavy nuclei, successfully explaining the dependence of decay rates on atomic number and providing quantitative agreement for barriers much thicker than the nuclear radius.

Bound States in Potentials

The WKB approximation provides a semiclassical method for determining the energy levels of bound states in one-dimensional potentials by applying the Bohr-Sommerfeld quantization rule, which relates the classical action integral between turning points to the quantum number n. For a general potential well V(x) with two turning points x_1 < x_2 where E = V(x_1) = V(x_2), the rule states that \int_{x_1}^{x_2} \sqrt{2m(E - V(x))} \, dx = \left(n + \frac{1}{2}\right) \pi \hbar, with n = 0, 1, 2, \dots.^[7] This condition arises from matching the WKB wave functions across the turning points, incorporating a \pi/4 phase shift at each permeable turning point, leading to an effective Maslov index correction of $1/2.^[13] For a finite square well potential, where V(x) = 0 for |x| < a and V(x) = V_0 for |x| > a with E < V_0, the integral simplifies to \sqrt{2mE} \cdot 2a = \left(n + \frac{1}{2}\right) \pi \hbar, yielding approximate energies E_n \approx \frac{(n + 1/2)^2 \pi^2 \hbar^2}{8 m a^2} for deep wells where turning points are near the well edges.^[21] In potentials with rigid walls, where the wave function must vanish abruptly, the standard n + 1/2 correction is modified due to a phase shift of \pi upon reflection at the wall, rather than the \pi/2 at a classical turning point. For the infinite square well, V(x) = 0 for $0 < x < a and infinite elsewhere, the WKB quantization becomes \int_0^a \sqrt{2mE} \, dx = n \pi \hbar, resulting in exact energies E_n = \frac{n^2 \pi^2 \hbar^2}{2 m a^2} for n = 1, 2, \dots, matching the precise quantum mechanical spectrum without the $1/2 term because the two rigid walls contribute phase shifts that cancel the turning-point corrections.^[7] This adjustment reflects the absence of wave function penetration beyond the walls, altering the effective quantization condition to n \pi \hbar.^[13] For potentials with one rigid wall, such as a linear potential V(x) = m g x for x > 0 with an infinite wall at x = 0 (the quantum bouncing ball), the quantization rule incorporates a combined phase shift of $3\pi/4 at the wall and turning point. The condition is \int_0^{x_2} \sqrt{2m(E - m g x)} \, dx = \left(n + \frac{3}{4}\right) \pi \hbar, where x_2 = E/(m g) is the upper turning point.^[22] Solving this yields E_n = \left[ \frac{(n + 3/4) \pi \hbar}{2^{1/2}} \left( \frac{m g^2}{\hbar^2} \right)^{1/3} \right]^{2/3}, or proportionally E_n \propto (n + 3/4)^{2/3}.^[13] This semiclassical result approximates the exact energies, which are determined by the zeros of the Airy function \mathrm{Ai}(z_n) = 0 at the wall, with the first few zeros z_1 \approx -2.338, z_2 \approx -4.088, and energies scaling as E_n \propto (-z_n)^{2/3}.^[21] The $3/4 correction improves accuracy for low-lying states in such asymmetric potentials.^[7]

Semiclassical Approximations in Other Fields

The WKB approximation, equivalently termed the Liouville-Green method, finds broad application in wave phenomena across physics, providing asymptotic solutions for linear differential equations with slowly varying coefficients, such as the general second-order ordinary differential equation y'' + k^2(x) y = 0, where k(x) changes gradually over the wavelength scale. This universality arises from the method's reliance on high-frequency or semiclassical limits, enabling approximate analytic solutions in regimes where exact treatments are intractable.^[23] In optics, the WKB approximation underpins the ray theory of geometrical optics, emerging as the short-wavelength limit analogous to \hbar \to 0 in quantum mechanics. Here, wave solutions take the form of rapidly oscillating phases modulated by slowly varying amplitudes, leading to ray paths that follow the eikonal equation. Classical turning points correspond to caustics—envelopes of rays where intensity diverges—such as in rainbow formation, where uniform WKB extensions employing Airy functions capture the oscillatory behavior and finite amplitude near these singularities.^[24]^[25]^[26] Acoustic wave propagation similarly benefits from WKB in high-frequency regimes, approximating solutions to the Helmholtz equation \nabla^2 \psi + k^2(x) \psi = 0 in media with gradual variations, such as stratified atmospheres or oceans. For instance, in underwater sound modeling, WKB yields normal modes and ray paths that account for refraction due to sound-speed gradients, facilitating efficient computation of propagation losses over long distances.^[27]^[28] The method's origins trace to Harold Jeffreys' 1924 application of asymptotic approximations to geophysical problems, including tidal friction in the Earth-Moon system, where it modeled secular perturbations in orbital motion due to dissipative ocean tides. In modern plasma physics, WKB analyzes wave damping and evanescence in inhomogeneous plasmas, such as surface waves at density gradients, by deriving dispersion relations and attenuation rates under the slowly varying plasma parameter assumption.^[29]^[30]

References

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Feb 3, 2019 · The WKB approximation provides approximate solutions for linear differential equations with coefficients that have slow spatial variation.
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The initials stand for Wentzel, Kramers and Brillouin, who first applied the method to the Schrödinger equation in the 1920's. The WKB approximation is also ...
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And because the momentum goes to zero the wavelength gets very long and the approximation is only valid if the wavelength is short.
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None
### Summary of WKB Connection Formulas from MIT Notes
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The first full analysis of the situation was by Langer (1937) and led to the {Z(Z+ 1)>'” + Z+ + substitution being called the Langer modijcation. Langer ...<|separator|>
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After tunnelling, the particle emerges at distance x = x* defined by E = ↵/x*. For x>x*, the wavefunction again oscillates, with a form given by the WKB ...
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Nov 26, 2007 · This imposes a quantisation condition on the energy. Only the eigenenergies. {En} for which Eq (3.29) is satisfied yield a quantum state.
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The non-uniform wave equations (1063)-(1064) is usually called the WKB solution, in honor of G. Wentzel, HA Kramers, and L. Brillouin.
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So, another way to think about the WKB approxi- mation is that it works when the wavelength of the wave changes GRADUALLY as the wave propagates through the ...
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These ray-based propagation laws are called the laws of geometric optics. In this section we shall develop and study the eikonal approximation and its resulting.
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The geometrical-optics (GO) approximation, sometimes called the Wentzel–Kramers–Brillouin (WKB) approximation, is commonly used to model wave propagation in ...
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[55]. To derive this approximation, we return to the Helmholtz equation in two-dimensions: ... WKB solution must match our normal solution, Eq. ( gif ) ...
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In this thesis, the use of the Wentzel-Kramers-Brillouin (WKB) Theory to obtain the solution to the Helmholtz Equation governing the acoustic normal modes is ...
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