where the relations are taken at equal time t , and x → → and y → → are the space variables. The equal-time requirement imposes that x → → − − y → → is a spacelike quantity. The non-zero value of the commutator [ ϕ ϕ ( t , x → → ) , π π ( t , y → → ) ] expresses the fact that when ϕ ϕ and π π are separated by a spacelike distance, they cannot communicate with each other and thus commute, except when their separation x → → − − y → → → → 0 .