It's a unit vector, and physicists working with relativity habitually set C=1 to make the equations simpler. That being said, it isn't really a vector -- it's just a direction. The "equal to C" bit is a mathematical artifact of both vectors being normalized.
It's a unit vector, and physicists working with relativity habitually set C=1 to make the equations simpler. That being said, it isn't really a vector -- it's just a direction. The "equal to C" bit is a mathematical artifact of both vectors being normalized.