A universal constructor is a pattern that is able to construct almost any pattern that has a glider synthesis. This definition is a bit vague, as a precise definition seems impossible because it has not been proven that all possible glider fleets are constructible. In any case, a universal constructor ought to be able to construct itself in order to qualify as such. John Conway proved that such a pattern exists in Life, and an outline of the proof can be found in Winning Ways for Your Mathematical Plays and The Recursive Universe. The key mechanism for the production of gliders with any given path and timing is known as side-tracking, and is based on the kickback reaction. A universal constructor designed in this way can also function as a universal destructor -- it can delete almost any pattern that can be deleted by gliders.
With a universal computer
A universal constructor is most useful when attached to a universal computer, which can be programmed to control the constructor to produce the desired pattern of gliders. The existence of a universal constructor/destructor together with a universal computer has a number of theoretical consequences.
- The constructor could be programmed to make copies of itself; such a pattern is known as a replicator.
- The constructor could be programmed to make just one copy of itself, translated by a certain amount, and then delete itself. Such a pattern would be a (very large, very high period) spaceship. Any translation is possible (except that it must not be too small), so that the spaceship could travel in any direction. It could also travel slower than any given speed, since we could program it to perform some time-wasting task (such as repeatedly constructing and deleting a block) before copying itself. Of course, we could also choose for it to leave some debris behind, thus making a puffer.
- It is also possible to show that the existence of a universal constructor implies the existence of a stable reflector. This proof is not so easy, however, and is no longer of much significance now that explicit examples of such reflectors are known.