Vacuum two-point function
= Vacuum two-point function
{title2=$\langle0|A(x)B(y)|0\rangle$}
A two-field <vacuum expectation value>. Its ordered, time-ordered and anticommutator versions are different distributions. For a free <Dirac field>, <fermionic time ordering> combines the positive-energy <particle> and <antiparticle> contributions into a <Dirac propagator>. Its nonzero value at spacelike separation does not violate <microcausality>, which constrains the field <anticommutator>.