A Minkowski diagram can, in physics, be used to determine what an observer can observe, or what about an observer can be observed. On that diagram, the "present" is represented by a plane, while the paths of light to the observer from the past and from the observer into the future are represented by two of these shapes (assuming relatively flat space-time). What are these two 3-dimensional shapes which are joined at the plane of the present?
(light) cones