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 I don't get it. 

So if someone would be to travel through time, their actions would have no effect?

But time travel itself affects the past  you're taking something from the present and putting it in a different point in time, thus, for the context of the earlier point in time you travel to, introducing matter/energy/whatever that didn't exist there before, thus (as per chaos theory) possibly screwing up everything. Any successful travel through time would have an effect, otherwise it wouldn't be time travel.

With that in mind  does the principle boil down to ""time travel is only possible if it has no effect, thus time travel (the possibility of which necessitates having an effect) is not possible""?

The matter and energy of which the time traveller (and his clothing, etc) consists would (if the universe is a closed system) have been in use by something else at an earlier point in time and thus a re-allocation of matter and energy already existent at that earlier point in time to recreate the time traveller doesn't seem to be a viable alternative either. But if the time traveller can neither be composed of matter and energy that didn't exist, nor of that which already existed but was ""in use"" already, what does he consist of?

Even if the universe is not a temporally closed system (i.e. matter/energy can vanish and re-appear at future points in time), for something to vanish in the present and re-appear in the past, it would have to vanish before its original appearance unless it can't exist twice at the same point in time (i.e. if atom A was sent back in time from a point Z to a point X, it would vanish at point Z (now) and appear at point X (back then) as A', but have to vanish again as time advances closer to the point Y (somewhere before point Z) where it was originally introduced as A, otherwise A and A' (which should be identical) would exist at the same point in time, which would definitely be an alteration of past events and thus incompatible with the principle.

I'm assuming the principle isn't based on the simplification of the idea that the only thing that ""matters"" is the action/inaction of humans (as in the interpretation of Schroedinger's experiment that the cat is in an unknown state where it is both alive and dead until the human interacts with it  which implies the human's perception determines the state of reality). If that is true, then the example from Twilight Zone where someone goes back in history to start the fire he travelled back in time to prevent is clearly contradictory as it introduces a paradox (a time travel having to have occurred in order for the time travel to occur) unless it actually changed the events (i.e. originally the fire would have been caused by something else), which would still be inconsistent with the principle which doesn't allow for any changes of the past.

I'm aware this post is getting a bit rant-y, but I'm seriously at a loss here. How does time travel NOT change the past, regardless of whether it has any consequences in the grand scheme of things or not? The very concept of travel backwards through time is that one appears at an earlier point of time, thus changing the past.

The only possibility I can think of for time travel not to change the past is a paradox in which the time travel happens because it has already occurred (i.e. the future representation (person A') of someone (person A) appears in the present and at some future point in time the same events cause person A to travel back in time that ""already"" caused person A' to go back in time, i.e. both of their time lines are identical), but in which there is no ""other"" time line in which the time traveller didn't already appear before travelling back in time (i.e. there is no time line in which person A travels back in time without person A' having travelled in time already, i.e. where person A doesn't eventually become person A', the time traveller).

All of this is obviously rendered null and void if the laws of physics are inconsistent and random stuff does appear out of thin air for no reason. —  (talk · contribs) "