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The Foldy–Wouthuysen transform is widely used in high energy physics. It was historically formulated by Leslie Lawrance Foldy and Siegfried Adolf Wouthuysen in 1949 to understand the nonrelativistic limit of the Dirac equation, the equation for the spin-1/2 particles. A detailed general discussion of the Foldy–Wouthuysen-type transformations in particle interpretation of relativistic wave equations is in.

The approach of the Foldy and Wouthuysen made use of a canonical transform that has now come to be known as the Foldy–Wouthuysen transformation. A brief account of the history of the transformation is to be found in the obituaries of Foldy and Wouthuysen; and the biographical memoir of Foldy. Before their work, there was some difficulty in understanding and gathering all the interaction terms of a given order, such as those for a Dirac particle immersed in an external field. With their procedure the physical interpretation of the terms was clear, and it became possible to apply their work in a systematic way to a number of problems that had previously defied solution. The Foldy–Wouthuysen transform was extended to the physically important cases of the spin-0 and the spin-1 particles, and even generalized to the case of arbitrary spins.

The Foldy–Wouthuysen (FW) transformation (after Lesley L. Foldy and Siegfried A. Wouthuysen) is a unitary transformation on a fermion wave function of the form:

where the unitary operator is the 4 × 4 matrix:

Above, $\widehat{p^i}\equiv p^i/\left|p\right|$ is the unit vector oriented in the direction of the fermion momentum. The above are related to the Dirac matrices by β = γ⁰ and αⁱ = γ⁰γⁱ, with i = 1, 2, 3. A straightforward series expansion applying the commutativity properties of the Dirac matrices demonstrates that (Template:EquationNote) above is true. The inverse

$U^{-1}=e^{-\beta \mathit{\alpha }\cdot \hat{p}\theta }=\cos \theta -\beta \mathit{\alpha }\cdot \hat{p}\sin \theta ,$

so it is clear that U⁻¹U = I, where I is a 4 × 4 identity matrix.

Foldy–Wouthuysen transformation of the Dirac Hamiltonian for a free fermion

This transformation is of particular interest when applied to the free-fermion Dirac Hamiltonian operator

${\hat{H}}_0\equiv \alpha \cdot p+\beta m$

in bi-unitary fashion, in the form:

Using the commutativity properties of the Dirac matrices, this can be massaged over into the double-angle expression:

This factors out into:

Choosing a particular representation: Newton-Wigner

Clearly, the FW transformation is a continuous transformation, that is, one may employ any value for θ which one chooses. Now comes the distinct question of choosing a particular value for θ, which amounts to choosing a particular transformed representation.

One particularly important representation, is that in which the transformed Hamiltonian operator $\hat{H}{\text{'}}_0$ is diagonalized. Clearly, a completely diagonalized representation can be obtained by choosing θ such that the α · p term in (Template:EquationNote) is made to vanish. Such a representation is specified by defining:

so that (Template:EquationNote) is reduced to the diagonalized (this presupposes that β is taken in the Dirac-Pauli representation (after Paul Dirac and Wolfgang Pauli) in which it is a diagonal matrix):

By elementary trigonometry, (Template:EquationNote) also implies that:

so that using (Template:EquationNote) in (Template:EquationNote) now leads following reduction to:

This calculation can be examined in further detail in the following link.

Prior to Foldy and Wouthuysen publishing their transformation, it was already known that (Template:EquationNote) is the Hamiltonian in the Newton-Wigner (NW) representation (named after Theodore Duddell Newton and Eugene Wigner) of the Dirac equation. What (Template:EquationNote) therefore tells us, is that by applying a FW transformation to the Dirac-Pauli representation of Dirac's equation, and then selecting the continuous transformation parameter θ so as to diagonalize the Hamiltonian, one arrives at the NW representation of Dirac's equation, because NW itself already contains the Hamiltonian specified in (Template:EquationNote). See this link

If one considers an "on shell" mass—fermion or otherwise—given by m² = p^σp_σ, and employs a Minkowski metric tensor for which diag(η) = (+1, −1, −1, −1), it should be apparent that the expression

$p^0=\sqrt{m^2+{\left|p\right|}^2}$

is equivalent to the E ≡ p⁰ component of the energy-momentum vector p^μ, so that (Template:EquationNote) is alternatively specified rather simply by $\hat{H}{\text{'}}_0=\beta E$.

Correspondence between the Dirac-Pauli and Newton-Wigner representations, for a fermion at rest

Now let us consider a fermion "at rest," which we may define in this context as a fermion for which |p| = 0. From (Template:EquationNote) or (Template:EquationNote), this means that cos(2θ) = 1, so that θ = 0, ±π, ±2π and, from (Template:EquationNote), that the unitary operator U = ±I. Therefore, any operator O in the Dirac-Pauli representation upon which we perform a bi-unitary transformation, will be given, for an "at rest" fermion, by:

Contrasting the original Dirac-Pauli Hamiltonian Operator

${\hat{H}}_0\equiv \alpha \cdot p+\beta m$

with the NW Hamiltonian (Template:EquationNote), we do indeed find the |p| = 0 "at rest" correspondence:

The velocity operator in the Dirac-Pauli representation

Now, let us consider the velocity operator. To obtain this operator, we must commute the Hamiltonian operator ${\hat{H}}_0$ with the canonical position operators x$i$, i.e., we must calculate $\widehat{v_i}\equiv i[{\hat{H}}_0,x_i]$. One good way to approach this calculation, is to start by writing the scalar rest mass m as $m={\gamma }^0{\hat{H}}_0+{\gamma }^j{p}_j$, and then to mandate that the scalar rest mass commute with the x$i$. Thus, we may write:

where we have made use of the Heisenberg canonical commutation relationship $[x_i,p_j]=-i{\eta }_{ij}$ to reduce terms. Then, multiplying from the left by ${\gamma }^0$ and rearranging terms, we arrive at:

Because the canonical relationship $i[{\hat{H}}_0,{\hat{v}}_i]\ne 0$, the above provides the basis for computing an inherent, non-zero acceleration operator, which specifies the oscillatory motion known as Zitterbewegung.

The velocity operator in the Newton-Wigner representation

In the Newton-Wigner representation, we now wish to calculate ${\widehat{v_i}}^{\prime }\equiv i[\hat{H}{\text{'}}_0,x_i]$. If we use the result at the very end of section 2 above, $\hat{H}{\text{'}}_0=\beta p_0$, then this can be written instead as:

Using the above, we need simply to calculate $[p_0,x_i]$, then multiply by iβ.

The canonical calculation proceeds similarly to the calculation in section 4 above, but because of the square root expression in $p^0=\sqrt{m^2+|p{|}^2}$, one additional step is required.

First, to accommodate the square root, we will wish to require that the scalar square mass m² commute with the canonical coordinates x_i, which we write as:

where we again use the Heisenberg canonical relationship $[x_i,p_j]=-i{\eta }_{ij}$. Then, we need an expression for $[p_0,x_i]$ which will satisfy (Template:EquationNote). It is straightforward to verify that:

will satisfy (Template:EquationNote) when again employing $[x_i,p_j]=-i{\eta }_{ij}$. Now, we simply return the iβ factor via (Template:EquationNote), to arrive at:

This is understood to be the velocity operator in the Newton-Wigner representation. Because:

it is commonly thought that the Zitterbewegung motion arising out of (Template:EquationNote), vanishes when a fermion is transformed into the Newton-Wigner representation.

The velocity operators for a fermion at rest

Now, let us compare equations (Template:EquationNote) and (Template:EquationNote) for a fermion "at rest," defined earlier in section 3 as a fermion for which |p| = 0. Here, (Template:EquationNote) remains:

while (Template:EquationNote) becomes:

In equation (Template:EquationNote) we found that for a fermion at rest, O′ = O for any operator. One would expect this to include:

however, equations (Template:EquationNote) and (Template:EquationNote) for a |p| = 0 fermion appear to contradict (Template:EquationNote).

Similar alternatives - perturbative schemes

Starting with the one-particle Dirac equation written earlier with ${\hat{H}}_0$ and rewritten here as:

$(c\overrightarrow{\alpha }\cdot \overrightarrow{p}+\beta m_o{c}^2+IV)\mathrm{\Phi }=E\mathrm{\Phi }$

where I = I₄ is the 4 × 4 unit matrix. This Hamiltonian is rewritten, namely divided into two parts:

$\hat{H}\equiv {\hat{H}}_0+\lambda {\hat{H}}_1$

where

${\hat{H}}_0=c\overrightarrow{\alpha }\cdot \overrightarrow{p}+\beta m_o{c}^2+\frac{1}{2}(I+\beta )V$

and

${\hat{H}}_1=\frac{1}{{\alpha }^2}{\widehat{H^{\prime }}}_1=\frac{1}{2{\alpha }^2}(I-\beta )V$

where α ≈ 1/137 is the Fine-structure constant (not to be confused with the Dirac alpha matrices). Letting

${\varphi }^0=A\left(\begin{array}{c}{\mathrm{\Psi }}_1^0\\ {\mathrm{\Psi }}_2^0\end{array}\right)$

into the zero order equation for ${\hat{H}}_0$ and using a particular but known representation of the Dirac operators, yields:

${\mathrm{\Psi }}_2^0=c\frac{\overrightarrow{\sigma }\cdot \overrightarrow{p}}{(E_0+m_0{c}^2)}{\mathrm{\Psi }}_1^0$

where σ_i are the 2 × 2 Pauli matrices. Note that the potential V does not appear in the equation above. The equation for the other spinor is:

$c\sigma \cdot \overrightarrow{p}{\mathrm{\Psi }}_2^0+V{\mathrm{\Psi }}_1^0={ϵ}_0{\mathrm{\Psi }}_1^0$

where ${ϵ}_0=E_0-m_0{c}^2$. Eliminating ${\mathrm{\Psi }}_2^0$ gives:

$\{\frac{p^2}{2m_0}+(1+\frac{{ϵ}_0}{2m_0{c}^2})(V-{ϵ}_0)\}{\mathrm{\Psi }}_1^0\equiv 0.$

This is simply the non-relativistic equation for a system with a re-normalized potential and energy eigenvalue:

$E^{\prime }=(1+\frac{{ϵ}_0}{2m_0{c}^2}){ϵ}_0\text{and}{V}^{\prime }=(1+\frac{{ϵ}_0}{2m_0{c}^2})V$

The higher-order corrections can be obtained by conventional perturbation theory. This is known as Moore's decoupling technique. Though it resembles the FW transformation, it is computationally and conceptually much simpler. Though misunderstood at first, in part because the fine structure constant appears in both the equations and the order parameter λ requiring care in the "bookkeeping" of the perturbative scheme, Moore's decoupling technique was vindicated for the (relativistic) hydrogen atom using conventional Rayleigh Schrödinger perturbation theory and computer algebra and proven to converge to the correct solution.

It has been applied successfully to relativistic calculations on Alkali metals and represents one of many relativistic perturbative schemes investigated by Werner Kutzelnigg.

Other applications

The powerful machinery of the Foldy–Wouthuysen transform originally developed for the Dirac equation has found applications in many situations such as acoustics, and optics.

It has found applications in very diverse areas such as atomic systems synchrotron radiation and derivation of the Bloch equation for polarized beams.

The applications of the Foldy–Wouthuysen transformation in acoustics is very natural; comprehensive and mathematically rigorous accounts.

In the traditional scheme the purpose of expanding the light optics Hamiltonian

$\hat{H}=-{(n^2(r)-{\hat{p}}_{\perp }^2)}^{1/2}$

in a series using

$({\hat{p}}_{\perp }^2/n_0^2)$

as the expansion parameter is to understand the propagation of the quasiparaxial beam in terms of a series of approximations (paraxial + nonparaxial). Similar is the situation in the case of the charged-particle optics. Let us recall that in relativistic quantum mechanics too one has a similar problem of understanding the relativistic wave equations as the nonrelativistic approximation plus the relativistic correction terms in the quasirelativistic regime. For the Dirac equation (which is first order in time) this is done most conveniently using the Foldy–Wouthuysen transformation leading to an iterative diagonalization technique. The main framework of the newly developed formalisms of optics (both light optics and the charged-particle optics) is based on the transformation technique of the Foldy–Wouthuysen theory which casts the Dirac equation in a form displaying the different interaction terms between the Dirac particle and an applied electromagnetic field in a nonrelativistic and easily interpretable form.

In the Foldy–Wouthuysen theory the Dirac equation is decoupled through a canonical transformation into two two-component equations: one reduces to the Pauli equation in the nonrelativistic limit and the other describes the negative-energy states. It is possible to write a Dirac-like matrix representations of Maxwell's equations. In such a matrix form the Foldy–Wouthuysen can be applied.

There is a close algebraic analogy between the Helmholtz equation (governing scalar optics) and the Klein-Gordon Equation; and the matrix-form of the Maxwell's equations (governing vector optics) and the Dirac equation. So, it is natural to use the powerful machinery of standard quantum mechanics (particularly, the Foldy–Wouthuysen transform) in analyzing these systems.

The suggestion to employ the Foldy–Wouthuysen Transformation technique in the case of the Helmholtz equation was mentioned in the literature as a remark.

It was only in the recent works, that this idea was exploited to analyze the quasiparaxial approximations for specific beam optical system.

The Foldy–Wouthuysen technique is ideally suited for the Lie algebraic approach to optics. With all these plus points, the powerful and ambiguity-free expansion, the Foldy–Wouthuysen Transformation is still little used in optics. The technique of the Foldy–Wouthuysen Transformation results in what we call as the non-traditional prescriptions of Helmholtz optics and Maxwell optics respectively. The non-traditional approaches give rise to very interesting wavelength-dependent modifications of the paraxial and aberrating behaviour. The non-traditional formalism of Maxwell optics provides a unified framework of light beam optics and polarization. The non-traditional prescriptions of light optics are in close analogy with the quantum theory of charged-particle beam optics. In optics, it has enabled to see the deeper connections in the wavelength-dependent regime between light optics and charged-particle optics (see Electron optics).

See also

• Relativistic quantum mechanics