It is possible to travel in time in this universe by several means, including moving faster than the speed of light. The Legion of Super-Heroes from 1,000 years into the future in particular have access to time-travel technology (although the threeboot Legion lacks time travel technology) while Rip Hunter is the present day authority of the technology. Originally, it was impossible to change the past, or to exist in two places at the same time (a time traveler appearing in an era where they already existed would become an ineffectual, invisible phantom while there). However that was all changed after the Anti-Monitor tried to change history at the beginning of time during the Crisis on Infinite Earths. Also, a number of alternate realities-known as Hypertime-now exist. A group calling itself the Linear Men formed to prevent anyone from changing history. In addition, an enormously powerful being called the Time Trapper, an enemy of the Legion, has been known to manipulate the timestream, even creating "pocket universes".