Choosing to be informative
Using the passive gives us the choice of not stating who carried out the action. This is an important factor, because in the active clause this information can’t be omitted. What conditions our choice, then, between a passive without an agent and one in which we keep the Agent in a by-phrase at the end? The answer is: informativeness. If the Agent is new important information, keep it. If not, omit it. In this extract from Stephen Hawking writing about black holes, there is an example of each type:
Although the concept of what we call a black hole goes back more than two hundred years, the name black hole was introduced only in 1967 by the American physicist John Wheeler. It was a stroke of genius: the name ensured that black holes entered the mythology of science fiction. It also stimulated scientific research by providing a definite name for something that previously had not had a satisfactory title. The importance in science of a good name should not be underestimated.
In this passage, Hawking gives credit to the originator of the term black hole, with the full name of the physicist encoded as an Agent by-phrase. The second passive has no Agent because it is generic and implied (by anyone working in science).
An additional motivation for the use of a passive with an Agent by-phrase occurs when the Agent is long. By putting it at the end we follow the principle of end-weight (‘shortest first, longest last’) as in the following examples, in which the Agent is ‘weightier’ than the passive Subject:
The front seats were filled by members of the families of the victims.
The goal was scored by Messi, the player with most goals to his credit this season.
It is clear that end-focus, end-weight and informativeness are closely linked. New participants introduced onto the scene of discourse need to be described and defined in more detail than known ones. They are, consequently ‘heavy’ and are better placed at the end, whereas the subject in a passive clause tends to be ‘light’ (the front seats, the goal), pronouns being the lightest.
Instead of an Agent, an event or a force of nature may occur in final position, as in the examples below, while Scotland’s railway network and the house will be considered important enough to become subject:
Scotland’s railway network has been paralyzed by the one-day strike.
The house was struck by lightning.
