Types of Time Travel to the Past

For my own reference (and that of anyone else who's interested), I'm listing the types of time travel to the past that I'm aware of. Any other thoughts or things I've missed, let me know.

It Changes the Past

This is the most straightforward. You go to the past, and every action has a cascading effect on the future. Butterfly effect and all. And it's why, as Hawking said, time travel to the past isn't possible -- because every action changes the future which changes the action of the person who went to the past, which changes the future, etc., an infinite feedback loop resulting in travel to the past never being accomplished.

But, continuing in theory anyway....

It Can Change Minor Events, But Not Major Events

Like water droplets flowing through a pipe, each drop of water may move wherever it wants. But, ultimately, the flow of water will end up in the same place due to the constraints of the pipe. 

Thus, in this view, some unknown force causes major events to happen regardless of what changes happen in the past.

It Causes the Events of the Present

This is similar to the previous one, or maybe even the same as the previous one. Here, going to the past causes the events of the present, like the two are some interwoven tapestry. And trying to change an event of the present is impossible.

The classic example is that of trying to stop the Holocaust by going back in time and killing Hitler as an infant. The person goes back, finds the Hitler family, and kills the baby in his crib. But his mother, finding the child, takes him and sneaks into her neighbor's house, who also happened to have a baby boy at about the same time, and exchanges her dead baby for the neighbor's live one. The neighbor's baby then turns out to be Hitler. So the time traveler actually caused the events of the future, rather than stopping them.

It Creates Alternate Realities

This one is very common in science fiction. A person goes to the past, and reality splits in two -- one with the original timeline, and another with the new timeline with the changed events -- and the two timelines exist side-by-side, neither one being aware of the other.

Frankly, I find this view to be absurd. I mean, why should the timeline split only in one place? If a person goes back to the past, then every action they take changes the future. They step on a bug, and there should be two timelines, one in which the bug lives, and another where the bug dies. They look at someone, and there should be two timelines, one in which the other person looks back at them, and another where the other person keeps walking, looking straight ahead. And so on.

Really, with this view, reality should split into multiple timelines every moment, or whatever the smallest unit of time may be, resulting in an infinite number of timelines by just going back in time for a few seconds. So I find this view to be preposterous.

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