  Dirac's hole theory (HT) and quantum field theory (QFT) are generally
considered to be equivalent to each other. However, it has been recently shown
that for a time independent perturbation different results are obtained when
the change in the vacuum energy is calculated. Here we shall extend this
discussion to include a time dependent perturbation. It will be shown that HT
and QFT yield different results for the change in the vacuum energy due to a
time dependent perturbation.
