Null motion of a free string endpoint
= Null motion of a free string endpoint
{title2=$\dot X^2|_{\partial\Sigma}=0$}
In a boundary-adapted <temporal gauge>, a free <open string> endpoint has $X'^m=0$. The <Nambu–Goto phase-space constraints> then imply $P^2=0$, while $\dot X=eP$ implies $\dot X^2=0$. For $X^0=t$, its spatial speed is one. This null boundary is compatible with a timelike interior <worldsheet>.