A causal loop is a paradox of time travel that occurs when a future event is the cause of a past event, which in turn is the cause of the future event. Both events then exist in spacetime, but their origin cannot be determined. A causal loop may involve an event, a person or object, or information.[1][2] A causal loop is also known as a boot-strap paradox, predestination paradox or ontological paradox in fiction.[3][4]