[Talisker]

There is a principle in physics, called the Principle of Self-Consistency. This principle states that if you could go back in time (by whatever method), you would not be able to change anything. In other words, what has happened cannot be changed.

That principle's only consistent in a world where people cannot go back in time - by definition. But if someone actually did go back in time, then the reality of that ability would mean a world where time travel was possible. And in that case, only experimentation could resolve whether history could be changed.

[Leaning Right]

That principle's only consistent in a world where people cannot go back in time - by definition.

Actually, no. The Principle most certainly allows for backwards time-travel. It just requires that nature interfere as to prohibit the changing of history.

The classic example is this: Suppose you time-travel back to Germany in the 1930's. You decide to shoot Hitler. You will not be able to do it! Your rifle might jam. Or the cartridge might be a dud, etc.