A null-line is the path of a light ray or other massless object through space-time.
The consequences of this statement are both fascinating and frightening.
To understand why, we must first consider what happens to a particle at light speed.
Albert Einstein said in his theory of relativity that an object travelling at the speed of light is not affected by time, that is to say it does not experience time. Something that does not experience time takes no time to travel any distance.
Although we may observe a particle of light, called a photon, travelling a set distance in a measured time, the particle itself has not aged. This is a fact.
If a particle of light does not experience time then distance has no meaning as it travels any distance in no time. If distance has no meaning, then neither does size or shape. A photon can therefore be thought of as a dimensionless point.
If the path taken by a photon has zero length and is traversed in zero time this is the same as saying that when you look along a null-line the universe has no size or shape, therefore it doesn't exist.
- A. Banks