Improbability is the modal probability employed by Douglas Adams in The Hitchhiker's Guide to the Galaxy. Improbability is modeled after mathematical probability, but is only related to real probability in the sense that irony is related to chance. In the fiction of Douglas Adams, improbability is portrayed by a succession of increasingly unlikely events. As a literary device it is closer to the concept of deus ex machina than probability. However, as the characters frequently comment about how improbable this or that is, it becomes a point of self-reference.
Improbability is based on a bizarre perception of quantum theory: if a certain elementary particle has a probability of being found in a certain place (such as within an atom) there is also a near infinitely improbable—but not quite impossible—chance of the particle being found anywhere else in the universe at the same time. This theory is employed to allow probability-powered engines known as improbability drives to let ships driven by such drives, such as the Heart of Gold, to travel interstellarly in a way similar to existing anywhere in the universe at the same time.
Improbability drives usually function on the theory that the likelihood of any event happening is as possible, while in no way as probable, as any other event. Improbability ships usually use this same theory, but instead these events are the ship transporting to certain destinations. The problem with an improbability drive is that unless one has a very powerful computer such as Eddie to do it, one must work out exactly how improbable it is for this event to happen for the drive to cause it to.