to the period he spends in the past. His disappearances and reappearances are rarely witnessed by anyone else. Dan does not arrive in the same location he departed, but instead arrives near the person who he is supposed to help. His jumps are typically restricted to the area he leaves from, usually keeping him in the San Francisco area. Originally, when Dan arrives to the past, he is unconscious. But as the series goes on, Dan somehow gets a hold on the jumps. It becomes evident when Dan feels the jumps coming on and as the jump occurs, he is seen walking with the ripples being shown behind him. Dan is also trying to figure out the mechanics of his travels. He has questioned a physicist who seems to have known his father in the past, and has discussed the possibility of tachyon particles being able to cause time jumps. It is now clear that the scientist is aware of Dan's travels, as well as those of others like him. It has also been discovered that those born around the time of a rare passing comet (the fictional "Joseph-Lee" comet) have this ability. Dan's journeys appear to each have an innate purpose, which is not always apparent to him, but involves positively changing the destiny of a certain person. Likewise, fate seems to conspire against him if he tries to alter other events beyond his current charge. Alterations Dan makes to the timeline affect the present and the memories of everyone in it. However, his own memories are unaltered, and he recalls events as they originally occurred before his interference. Dan is not alone in his tendency to time-travel. His ex-fiancée Livia, thought to have been killed in a plane crash, actually traveled back to her "home time", 1948, from where she jumps forward in time. Dan meets Livia on his jumps into the past, as she jumps into her future to offer her experience and assistance with Dan's missions. As revealed in the episode "Perfidia", there was at least one other person who also jumps through time, named Evan. However, the death of that individual occurs shortly before Dan experiences his first jump in the first episode, bringing the whole series full circle, making Dan believe that the loss of Evan as a time traveler created a void which Dan filled. Livia and Dan also sensed that Livia's purpose