In the general theory of relativity the geodesics of spacetime are defined by light rays; the path of a light ray defines the shortest possible path between two points. However, once we remove matter from spacetime there ceases to be a reference frame that seperates physical points, so the Universe would have no spatial extent.* No spatial extent infers no temporal extent; meaning the Universe would literally collapse in on itself instantaneously, becoming a point. There is thus a major difference between light, massless electromagnetic radiation, and the 'modes of light' we call particles of matter. Both are the same substance but expressed differently. How could we measure the speed of light in a Universe of 'pure light'? Does this mean what we call distance, speed and other dimensional measurements are simply projections of the material observer? Why this distinct boundary between the perspective of light, for which dimensional measurements have no meaning, and the perspective of matter? * The intervals of spacetime are defined by the presence of matter so that without matter we literally get no spatial extent NOTE: I recognise this post as speculative but would like some feedback. The problem has been bothering me for a long while and I wonder if anyone has encountered some form of solution...?