So the time ordered product T(AB) just rearranges A and B so that the one with the earliest time co-ordinate goes to the right. Does anybody know what the time ordered product of two fields at equal time is? Because they're often written using a heaviside step function, it's difficult to tell - I can't seem to find a straight answer as to what the heaviside function does at zero (0, 1/2 or 1 seem to be the possibilities). What I'm actually trying to find is the Feynmann progagator, which is the time ordered product enclosed between to ground states: <0|T(AB)|0>. It would make my calculations really nice if it was zero, but I'd have thought it more likely that it's just the <0|AB|0> as though there was no T function there... Any help appreciated! Thanks.