It's not true. It's a mashup of a few ideas that don't belong together. 1. an object at rest has a world line which points (has tangent vectors) in the time di

It's not true. It's a mashup of a few ideas that don't belong together. 1. an object at rest has a world line which points (has tangent vectors) in the time dimension 2. light has proper time (dtau) = 0, so clocks moving at the speed of light dont tick 3. the magnitude^2 of an objects 4-velocity is c^2 (objects move through spacetime at c) 4. light has no 4-velocity (because dtau=0, you cant divide by zero) You can't say (3) means objects have 4-velocity c in spacetime and light has 3-velocity c in space and so that means the time component of 4-velocity for light is zero. Because light has no 4-velocity.