Halliday & Matthiessen (1999: 402-3):
In the second type [of ideational metaphor] to be exemplified, a clause or clause complex is nominalised as a Medium participant in a clause whose Process simply means 'happen' (i.e., existence of a process: happen, occur, take place; begin, continue, stop). The Medium constitutes a thematic quantum of information, as in the following example:However, Einstein realised that if an observer was travelling alongside a light wave at the same speed as the light wave, the wave would essentially disappear, as no wave peaks or troughs would pass by the observer. But the disappearance of light waves because of the motion of an observer should not happen according to Maxwell, so Einstein concluded that either Maxwell's equations were wrong or that no observer could move at the speed of light.Here the metaphorical clause makes it possible to summarise the preceding clause complex the wave would essentially disappear, as no wave peaks or troughs would pass by the observer as a thematic nominalised participant, the disappearance of light waves because of the motion of an observer.