In mathematical physics, the Gordon decomposition (named after Walter Gordon one of the discoverers of the Klein–Gordon equation) of the Dirac current is a splitting of the charge or particle-number current into a part that arises from the motion of the center of mass of the particles and a part that arises from gradients of the spin density. It makes explicit use of the Dirac equation and so it applies only to "on-shell" solutions of the Dirac equation.
For any solution of the massive Dirac equation,
the Lorentz covariant number-current may be expressed as
is the spinor generator of Lorentz transformations.
The corresponding momentum-space version for plane wave solutions and obeying
One sees that from Dirac's equation that
and, from the conjugate of Dirac's equation,
Adding these two equations yields
From Dirac algebra, one may show that Dirac matrices satisfy
Using this relation,
which amounts to just the Gordon decomposition, after some algebra.
The second, spin-dependent, part of the current coupled to the photon field, yields, up to an ignorable total divergence,
that is, an effective Pauli moment term, .
This decomposition of the current into a particle number-flux (first term) and bound spin contribution (second term) requires .
If one assumed that the given solution has energy so that , one might obtain a decomposition that is valid for both massive and massless cases .
Using the Dirac equation again, one finds that
Here , and
where is the vector of Pauli matrices.
With the particle-number density identified with , and for a near plane-wave
solution of finite extent, one may interpret the first term in the decomposition as the current , due to particles moving at speed .
The second term, is the current due to the gradients in the intrinsic magnetic moment density. The magnetic moment itself is found by integrating by parts to show that
For a single massive particle in its rest frame, where , the magnetic moment reduces to
where and is the Dirac value of the gyromagnetic ratio.
For a single massless particle obeying the right-handed Weyl equation, the spin-1/2 is locked to the direction of its kinetic momentum and the magnetic moment becomes
Angular momentum density
For the both massive and massless cases, one also has an expression for the momentum density as part of the symmetric Belinfante–Rosenfeld stress–energy tensor
Using the Dirac equation one may evaluate to find the energy density to be , and the momentum density,
If one used the non-symmetric canonical energy-momentum tensor
one would not find the bound spin-momentum contribution.
By an integration by parts one finds that the spin contribution to the total angular momentum is
This is what is expected, so the division by 2 in the spin contribution to the momentum density is necessary. The absence of a division by 2 in the formula for the current reflects the gyromagnetic ratio of the electron. In other words, a spin-density gradient is twice as effective at making an electric current as it is at contributing to the linear momentum.
Spin in Maxwell's equations
Motivated by the Riemann–Silberstein vector form of Maxwell's equations, Michael Berry uses the Gordon strategy to obtain gauge-invariant expressions for the intrinsic spin angular-momentum density for solutions to Maxwell's equations.
He assumes that the solutions are monochromatic and uses the phasor expressions , . The time average of the Poynting vector momentum density is then given by
We have used Maxwell's equations in passing from the first to the second and third lines, and in expression such as the scalar product is between the fields so that the vector character is determined by the .
and for a fluid with intrinsic angular momentum density we have
these identities suggest that the spin density can be identified as either
The two decompositions coincide when the field is paraxial. They also coincide when the field is a pure helicity state – i.e. when where the helicity takes the values for light that is right or left circularly polarized respectively. In other cases they may differ.