# 11.3: Lorentz Invariance of Dirac Equation

Consider two inertial frames, and . Let the and be the space-time coordinates of a given event in each frame, respectively. These coordinates are related via a Lorentz transformation, which takes the general form

(1145) |

where the are real numerical coefficients that are independent of the . We also have

(1146) |

Now, since [see Equation (1102)]

(1147) |

it follows that

Moreover, it is easily shown that

(1149) | ||

(1150) |

By definition, a 4-vector has analogous transformation properties to the . Thus,

(1151) | ||

(1152) |

etc.

In frame , the Dirac equation is written

Let be the wavefunction in frame . Suppose that

(1154) |

where is a transformation matrix that is independent of the . (Hence, commutes with the and the .) Multiplying (1153) by , we obtain

(1155) |

Hence, given that the and are the covariant components of 4-vectors, we obtain

Suppose that

which is equivalent to

Here, we have assumed that the commute with and the (since they are just numbers). If (1157) holds then (1156) becomes

A comparison of this equation with (1153) reveals that the Dirac equation takes the same form in frames and . In other words, the Dirac equation is Lorentz invariant. Incidentally, it is clear from (1153) and (1159) that the matrices are the same in all inertial frames.

It remains to find a transformation matrix that satisfies (1158). Consider an infinitesimal Lorentz transformation, for which

where the are real numerical coefficients that are independent of the , and are also small compared to unity. To first order in small quantities, (1148) yields

Let us write

where the are matrices. To first order in small quantities,

Moreover, it follows from (1161) that

(1164) |

To first order in small quantities, Equations (1158), (1160), (1162), and (1163) yield

(1165) |

Hence, making use of the symmetry property (1161), we obtain

(1166) |

where . Since this equation must hold for arbitrary , we deduce that

Making use of the anti-commutation relations (1122), it can be shown that a suitable solution of the above equation is

Hence,

(1169) | ||

(1170) |

Now that we have found the correct transformation rules for an infinitesimal Lorentz transformation, we can easily find those for a finite transformation by building it up from a large number of successive infinitesimal transforms.

Making use of (1127), as well as , the Hermitian conjugate of (1169) can be shown to take the form

(1171) |

Hence, (1158) yields

(1172) |

It follows that

(1173) |

or

(1174) |

which implies that

(1175) |

where the are defined in Equation (1139). This proves that the transform as the contravariant components of a 4-vector.

### Contributors

- Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)