Long range forces induced by neutrinos at finite temperature

We revisit and extend previous work on neutrino mediated long range forces in a background at finite temperature. For Dirac neutrinos, we correct exis ting results. We also give new results concerning spin-independent as well as spin-dependent long range forces associated to Majorana neutrinos. An in teresting outcome of the investigation is that, for both types of neutrinos whether massless or not, the effect of the relic neutrino heat bath is to convert those forces into attractive ones in the supra-millimeter scale whi le they stay repulsive within the sub-millimeter scale. (C) 1999 Elsevier S cience B.V. All rights reserved.