stars event null
This null dual-cone represents the "line of sight" of a point in space. That is, when we look at the stars and say "The light from that star which I am receiving is X years old", we are looking down this line of sight: a null geodesic. We are looking at an event a distance 



d
=



x

1


2


+

x

2


2


+

x

3


2






{\textstyle d={\sqrt {x_{1}^{2}+x_{2}^{2}+x_{3}^{2}}}}

 away and a time d/c in the past. For this reason the null dual cone is also known as the "light cone". (The point in the lower left of the Fig. 10-2 represents the star, the origin represents the observer, and the line represents the null geodesic "line of sight".)
