  It is usually assumed that the infrared divergences that occur for virtual
photons in Feynman diagrams cancel against the corresponding divergences in
phase space integrals for the emission of real soft photons. If this were the
case then why do the corresponding phase space integrals for the absorption of
real soft photons not diverge too, as one might expect on grounds of
time-reversal symmetry, and if so what cancels these divergences? We show that
this apparent time asymmetry is an artifact of the way that the diagrams are
calculated, and we introduce a procedure for such calculations that is
manifestly time symmetric. Moreover, we show that it is not mathematically
meaningful to distinguish arbitrarily soft massless particles of positive and
negative energy.
