Dr. Paul Manheim was a human scientist, widely renowned as one of the greatest minds of the 24th century. He was considered a visionary in the field of temporal mechanics, despite the fact that many of his theories did not meet with mainstream acceptance.
In 2349, Manheim left Earth with a team of other scientists to test his theories on the remote planetoid Vandor IV. After twenty-five years, he reached a level of success when, in 2364, he was able to open a "window" into one of what he believed to be infinite parallel dimensions. Unfortunately, his consciousness became temporarily trapped between the two states of existence, and one of his labs was destroyed. Manheim managed to send a distress call, which was answered by the USS Enterprise. Lt. Commander Data was able to close the "window," putting a stop to the propagating Manheim effect which threatened the galaxy, and saving Dr. Manheim's life. Manheim remained behind on Vandor IV with his wife, Jenice, in order to continue his work. (TNG: "We'll Always Have Paris")
Shortly after this incident, the USS Scandalon was sent to investigate Manheim's work. During this time, Timothy Sinclair and Steve Tecklenberg were both exposed to alternate dimensions. Later on, the Department of Temporal Investigations oversaw Manheim's continuing research. (Star Trek: Pendragon: "Reflections", "Wings As Eagles")
Sometime during his career, he worked with Hudec to create the Manheim-Hudec scale, a measurement of intensity when changes in the space-time continuum take place. (Star Trek: The Cantabrian Expeditions: "The Fire In Which We Burn")