My idea is that if the past exists as a tangible place where things have already happened, then the future does as well. All actions that will ever be taken by every single time traveller who will ever travel into the past are already accounted for in our present day and in the future that the time travellers come from. So in my idea you can't change the past either, because any action you take has already been accounted for. If you choose not to go back and try to change something, then the present accounts for that too.

In some ways that sounds like predestination, but I don't see it quite that way. The choices still belong to the people who make them, and they still get to make their choices. It's just that if the future exists as a tangible place, then that person has already made all of the choices that they will ever make, and did so from the moment time began. It doesn't absolve people from making decisions, because if they don't make any then that's a choice they made too, and the future already reflects that choice.

To look at it a different way, let's say that the earliest a time traveller will ever go back is 400 million years ago. When the timestream first went forward, when 400 million years ago happened, that time traveller showed up there, and did whatever he/she did, and then those actions are reflected in the timestream going forward from that point. Anything that the time traveller did was already a part of the world the time traveller came from in the future. It's a time as an infinite line sort of idea, where all points are plotted and can't be altered.

I have an idea for a story based on this, where someone goes back in time trying to learn the secrets of a mysterious event that happens in our future and in her past. The actions she takes in the past end up ultimately contributing to causing said event in the first place, and in enabling her past self (in the future) to time travel.

Lost had a take on time travel that was similar to mine. It made me very happy to watch, because I had those ideas already.

I saw it explained once in another way. Say you set up an experiment to try to alter the past and create a paradox. In my idea, the creation of a paradox is impossible.

So in this experiment you have a billiards table. You set up a time portal. You are going to shoot the ball into the portal and make it appear one second in the past on a trajectory that will cause it to collide with its earlier self as it heads toward the hole. Instant paradox.

But when you shoot the ball, one second before it enters the time portal a new time portal opens up with the same ball coming out of it, only it's not coming out at the trajectory you planned, even though from your perspective the original ball hasn't even entered the time portal yet. The new ball's trajectory is slightly angled. It only strikes a glancing blow to the original ball, not the head-on blow you were planning. The original ball still enters the time portal, but does so at a slightly different angle. Thus causing it to appear one second in the past at the slightly different trajectory you just witnessed.

The very act of shooting the ball into the past to hit itself caused the change in trajectory that kept a paradox from happening. Because the action of shooting the ball one second into the past had already been accounted for, which is why you saw it appear from the portal before your own ball entered the portal.

That is very wibbly-wobbly, I'm sure, but I love the concept to pieces and like to play with it in my head.

Your parallel universe/timestream idea is my second favourite, though, and also seems quite probable to me. Of course, even though you aren't really changing the future if you go back and change something in your idea, you are changing the future for yourself. You will go back to a changed world. It's just for everyone else that it'll seem to have always been that way, and in the old reality you came from everyone will wonder why your mission failed and they'll never see you again.