Imagine that a 24 year old person develops time travel in 2101. Let's call him Jerknose. Jerknose decides that he wants to test his time machine by traveling back to when he was 15 years old (2086), which he does, and successfully. To him, according to my linear thinking (which is what I must use, being a good scientist, because it is all that anyone has experience with), the past would be exactly the same as it was when he was there, except that there would be two of him.

Jerknose would most likely stay removed from the events of this time, and perhaps would be unable to do anything anyway as a foreign person in his past. After all, the past has already happened, and time travel would probably be like rewinding a video cassette. This assumption, that the past is unalterable, is the only experience we have and therefore it is the most sound notion of the past currently available.

Somehow, Jerknose has developed a way to revisit the past. It is my belief that he would be unable to return to true present because the time machine (at least the first model) would require an enormous power source just to transport a single human. There is absolutely no way that a time machine and it's power source could be transported as well. Jerknose would be doomed to live out his life watching the other himself grow up and develop the time machine. When the other himself develops the time machine and travels backwards, he will travel to a time when there were already two of him.