  Considering what the world would be like if backwards causation were possible
is usually mind-bending. Here I discuss something that is easier to study: a
toy model that incorporates a very restricted sort of backwards causation. It
defines particle world lines by means of a kind of differential delay equation
with negative delay. The model presumably prohibits signalling to the past and
superluminal signalling, but allows nonlocality while being fully covariant.
And that is what constitutes the model's value: it is an explicit example of
the possibility of Lorentz invariant nonlocality. That is surprising in so far
as many authors thought that nonlocality, in particular nonlocal laws for
particle world lines, must conflict with relativity. The development of this
model was inspired by the search for a fully covariant version of Bohmian
mechanics.
