In

general relativity, a

geodesic that is

perpendicular to itself; that is, its

inner product with itself is

zero, hence the term

null.

Obviously, null geodesics are

non-existant in

Euclidean geometry, but they do

exist in

physical four-dimensional geometries, that is, ones with

signature +/- 2.

Photons and other

null particles follow null geodesics.

Yet another bizarre and difficult-to-visualise concept in GR.