Page:The Meaning of Relativity - Albert Einstein (1922).djvu/53

only when the two points concerned come together, the other hand, from the vanishing of

$s^{2}=\sum \Delta x_{\nu }^{2}=\Delta x_{1}^{2}+\Delta x_{2}^{2}+\Delta x_{3}^{2}-\Delta t^{2}$

it cannot be concluded that the two space-time points fall together; the vanishing of this quantity $s^{2}$, is the invariant condition that the two space-time points can be connected by a light signal in vacuo. If $P$ is a point