A concept from NASA's Breakthrough Propulsion Physics Program, the Differential Sail is a method of propulsion that operates by tapping into the Zero Point Energy Field. As the Heisenberg uncertainty principle implies that there is no such thing as an exact amount of energy in an exact location, vacuum fluctuations are known to lead to discernable effects such as the Casimir effect. The differential sail is a concept that would utilize cosmic background radiation as its source of varying energy to create a gradient in pressure in space.