This has been a theory that’s been stuck in my mind for a long time, and it’s quite simple. If this has already been thought of by someone else, I can only claim ignorance as I’ve never heard it before. **disclaimer**
So much in nature has this tendency to stay the same, given no outside forces. Why wouldn’t ‘time’ be the same? I feel that given the proper technology, time travel would be trivial.
“But what about interfering in the past? Small changes that would affect the future? How can you know anything when something like the butterfly effect could be present?” you might ask.
Think of an event as a snowflake. One snowflake on its own is trivial. It can be melted or destroyed or what have you with no effort at all. But as snowflakes pile up, they can become substantial. Roll a snowball down a hill and eventually you could destroy the world, right?
In this vein of thought, think of how events are inter-linked. When something happens, the event in and of itself is often quite small, almost inconsequential. It’s the result of these events which is usually the important bit.
The longer time goes on, the more things that have possibly been influenced by this event. It’s in this way that you could say that the ‘weight of history’ bears down upon events, much like the snowball effect, and that the more time has passed, the more ‘temporal inertia’ a given event has, such that if one went back in time the universe would actively resist a change to any weighty events.
This only works in reverse, meaning that all this temporal energy flows backwards in time to the branch’s point of origin. For example: if you killed went to Rome and got robbed, and this caused you to lose your house somehow, getting robbed would have less temporal inertia than the decision to go to Rome. If you wanted to change history, you’d want to stop the mugging rather than the entire trip because that would take less energy on your part – the universe would resist it less (relatively, anyway).
What form would this universal ‘resistance’ take? Well think of it as an almost passive-active reorganization of small events with little to no temporal inertia of their own. If one went back in time to attempt to assassinate Hitler before 1933, one might find any number of things getting in his way: they might miss if firing a gun, a car might break down on the person’s way to get to where he meant to plant a bomb – that bomb might even end up being a dud. In extreme circumstances, the time traveller could be killed through some accident while attempting to change this historical event.
The universe can only put up so much of a fight though. Just like an asteroid threatening the Earth would be destroyed (and the Earth saved! yay!) by a big enough missile/laser/use of force, if you can put enough energy into changing the path of history, you certainly could. Using our Hitler example, just one man might find it impossible to do such a thing, but send back a large group of men, all with proper equipment etc, and it would be conceivable that at least one of these men, with his future-gun capable of shooting target-seeking smart-bullets, would be able to gun down Hitler outside of some nondescript bar in Bavaria, thus changing history forever.
With all this in mind, I’m of the opinion that time travel could be a safe and enjoyable new tourist industry, since anyone going back could only have little to no affect on their timeline.