   : Introduction to total angular : lecture_7 : lecture_7

# Solution of the Dirac equation for a free particle

The Dirac Hamiltonian takes the form where Using , in the coordinate basis, the Dirac equation for a free particle reads Since the operator on the left side is a 4 4 matrix, the wave function is actually a four-component vector of functions of and : which is called a four-component Dirac spinor. In order to generate an eigenvalue problem, we look for a solution of the form which, when substituted into the Dirac equation gives the eigenvalue equation Note that, since is only a function of , then so that the eigenvalues of can be used to characterize the states. In particular, we look for free-particle (plane-wave) solutions of the form: where is a four-component vector which satisfies Since the matrix on the left is expressible in terms of 2 2 blocks, we look for in the form of a vector composed of two two-component vectors: Therefore, writing the equation in matrix form, we find or which yields two equations      From the second equation: Note, one could also solve the first for and obtain Using the first of these, then a single equation for can be obtained However, Hence, we have the condition Since , the equation is only satisfied if the quantity in the brackets vanishes, which yields the eigenvalues We see that the eigenvalues can be positive or negative. A plot of the energy levels is shown below: There is a continuum for (turquoise) and for (periwinkle). There is also a gap between and .

We will show that for , an appropriate solution is to take If this is the case, then However, so that so that the full solution is Note that when , the third and fourth components of vanish. In this case, energy is just and the full time-dependent wave function becomes which are both forward propagating solutions. These correspond to particle solutions, in particular, a spin-1/2 particle propagating forward in time with an energy equal to the rest mass energy.

When , we take so that By the same reasoning, the solution for is so that in the limit , and , which describes particles moving backward in times. Thus, the interpretation is that the negative energy solutions correspond to anti-particles, the the components, and of correspond to the particle and anti-particle components, respectively. Thus, the Dirac equation no only describes spin but it also includes particle and the corresponding anti-particle solutions!

In the non-relativistic limit, for , we have so that since , it follows that Neglecting it, and recalling that for , the eigenfunctions reduce to The lower component has become reduntant, and the eigenfunctions just correspond to those of a free particle with an attached spin eigenfunction or for or , respectively. For , the lower component, is called the minor component and the upper component is called the major component.   : Introduction to total angular : lecture_7 : lecture_7
Mark Tuckerman 平成17年2月18日