Dirac's theory of the electron predicted that there were identical particles of equal mass but of negative energy. He appealed to the Pauli exclusion principle and proposed that there was a negative energy 'sea' of electrons that was full up to -2mc^2 in order to answer critics that positive energy electrons described by the Dirac electron theory would simply decay down to -infinity. With this description pair creation is described by absortion of a photon (where the energy of the photon E_p > 2mc^2) by a negative energy electron that scatters up to a poitive energy state leaving behind a hole. This hole is the negative energy 'sea' has equal but opposite charge to the electron and is commonly known as a positron. I believe that this description is somewhat old hat and not used anymore in modern QFT circles. Can anybody give me a not too technical explanation to why the 'old hat' qualitative explanation is unsatisfactory and how Feynman's resolution works, i.e. negative-energy particles can only travel backward in time.