• Energy Research

We now return to the problem of the negative-energy solutions that appeared when we solved the free particle Dirac equation in the previous chapter. The energy eigenvalues obtained there were either E+ = +√(m2c4 + p2c2) or E− = − √(m2c4 + p2c2) . The minimum absolute values we can have are therefore |E| = mc2 for p = 0 (that is, the particle at rest). As the momentum of the particle increases, we generate a continuum of solutions, either below − mc2 or above + mc2. This is a general feature of all solutions we will obtain from the Dirac equation—we will have continuum solutions on both sides of an energy gap stretching from − mc2 to + mc2, in addition to any discrete solutions. Classically and nonrelativistically we would expect a free particle to have a positive energy. The addition of a rest mass term mc2, which is definitely also positive, should not change this. However, the fact that we now have negative-energy states means that a single particle in a positive-energy state could spontaneously fall to a negative energy state with the emission of a photon. The interaction with the radiation field occurs via the operator α • A. The radiative transition moment therefore connects the large component of the positive-energy solution with the small component of the negative-energy solution. As we have seen, both of these should be large in magnitude, giving a large transition moment. Calculations (Bjorken and Drell 1964) show that the transition rate into the highest mc2 section of the negative continuum is approximately 108 s−1, and the rate of decay into the whole continuum is infinite. Any bound state would therefore immediately dissolve into the negative continuum with the emission of photons. This is clearly an unphysical situation. To resolve this dilemma, Dirac postulated in 1930 that the negative-energy states are fully occupied. The implications of this postulate are significant and wide-ranging.

© 2015 World Scientific Publishing Company. We consider behavior of equatorial geodesics with the negative energy in the ergoregion of a generic rotating "dirty" (surrounded by matter) black hole. It is shown that under very simple and generic conditions on the metric coefficients, there are no such circular orbits. This entails that such geodesic must originate and terminate under the event horizon. These results generalize the observation made for the Kerr metric in A. A. Grib, Yu. V. Pavlov and V. D. Vertogradov, Mod. Phys. Lett.29, 1450110 (2014), arXiv:1304.7360.