= Spacelike distance between daughter decay events
{title2=$d^2/c^2=(M^2/m^2)\tau_1\tau_2-(\tau_1+\tau_2)^2$}
For a parent of mass $M$ decaying into two freely propagating daughters of equal mass $m$, whose proper survival times are $\tau_1,\tau_2$, the separation of their decay events has the displayed spacelike squared length when positive. It follows from their invariant relative four-velocity contraction. The distance in a simultaneous inertial frame is therefore independent of the laboratory boost of the parent; when the expression is not positive such a simultaneous frame does not exist.
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