A von Neumann probe is a specific example of a hypothetical concept based on the work of Hungarian-born American mathematician and physicist John von Neumann. Von Neumann rigorously studied the concept of self-replicating machines that he called "Universal Assemblers", which are most often referred to as von Neumann machines. While von Neumann never applied his work to the idea of spacecraft, theoreticians since then have done so. The idea of self-replicating spacecraft has been applied—in theory—to several distinct "tasks", and the particular variant of this idea applied to the idea of space exploration is known as a von Neumann probe. Other variants include the Berserker and an automated seeder ship.