Yes, it's true, to define a velocity scale you need to plug in a spatial scale and a temporal scale. A human lifetime defines the temporal scale, but I technica

Yes, it's true, to define a velocity scale you need to plug in a spatial scale and a temporal scale. A human lifetime defines the temporal scale, but I technically didn't mention a spatial scale. However, we were talking about Fermi paradox and galactic colonization -- so a reasonable spatial scale would be that of the galaxy, and then we have defined a velocity scale and can make comparisons: c << (galactic diameter / human lifetime) c >> (galactic diameter / billion year evolutionary lead)