In Conway's Game of Life (and related cellular automata), the speed of light is a propagation rate across the grid of exactly one step (either horizontally, vertically or diagonally) per generation. In a single generation, a cell can only influence its nearest neighbours, and so the speed of light (by analogy with the speed of light in physics) is the maximum rate at which information can propagate. It is therefore an upper bound to the speed at which any pattern can move.
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Notation
As in physics, the speed of light is represented with the letter
Lightspeed propagation
While
Faster than light propagation
Certain patterns can appear to move at a speed greater than one cell per generation, but like faster than light phenomena in physics this is illusory.
An example is the "Star Gate", an arrangement of three converging gliders that will mutually annihilate on collision. If a lightweight spaceship (LWSS) hits the colliding gliders, it will appear to move forwards by 11 cells in only 6 generations, and thus travel faster than light. This illusion happens because the glider annihilation reaction proceeds by the creation and soon-after destruction of another LWSS. When the incoming LWSS hits the colliding gliders, it isn't transported, but instead modifies the reaction so that the newly created LWSS can survive. The only signal being transmitted is that determining whether the outgoing LWSS should survive or not. This does not need to reach its destination until after the LWSS has been "transported", and so no information needs to travel faster than light.