@ Nathan:
We are presuming that is possible to change the past (although, technically that concept doesn’t even make sense) or at least cause a branching of the timeline. What if your very presence caused things to start occurring differently from how you remembered them?

i have nothing to add to this discussion, but i want to recommend you to read mark twains “a connecticut yankee in king arthurs court” if you haven’t already.

@Kevin Benko: What you are talking about here is the Novikov Self Consistency Principle and yes – this is a valid interpretation.

Still even in that universe you could lose yourself in time. I mean, let’s face it – you may know who wins the Superbowl a given year, and who becomes the president. But then again, anyone can guess those things.

Tragic unforeseen events such as 9/11 would be good touch stones, but chances are that no such events occur during your mission. It has been proven in clinical trials that it is extremely easy to create fake memories that are distinguishable from the real ones. Over time the memories of the “future” would fade. On the other hand, the constant repetition and reinforcement of your “cover” story (after all you have a alternate story of your life – where you went to school, where you worked) may seem more and more real.

That would defeat any kind of hope for human free will: if Tim decides to change the past, the past already has himself trying to change stuff. By that order of thoughts, the “universe” already knows what Tim’s going to do, as the “past” events already happened before he entered the machine.

I think that one of Larry Niven’s earlier short story collections dealt with time-travel-related short stories, paradoxes, and multiverse explanations.

I would like to believe that IF time travel were a possibility, then if Tim [sic] Traveler went back in time, the past time-line would have already included Tim having traveled back to that time.

Hence, if Tim decided to attempt to change the events of the past, since the past already included some schmuck of a Time Traveler coming back and trying to mess things up, nothing would change since it had already happened that way.

I would think that the universe would be immune to paradoxes.

The even more freaky thing is trying to convince yourself that what you are doing even matters. If time is so “fragile” to changes, is anything you do going to matter? Throw in a “multiverse” vs a “universe” view of time and things get even messier. Its all very well if you assume a single “river of time” where you effectively hop in upstream or downstream. Its even worse if every time you time travel you branch off another river i.e. another universe. Every jump is almost like starting a new life – in either case – everything is either too fragile or too different.

I always liked the Quantum Leap TV series approach – you never physically traveled, you just “borrowed” a real person’s body for the time you were there. Thus no obvious paradoxes – just one person behaving really strangely for a period of time.

Still I think you’re right – retaining sanity and all the anchors to our reality we rely on must be really hard for anyone undergoing time travel.

Yeah, knowing specifics seems like it would keep you sane. E.g., if you’re a big football buff and know who wins which games by how much or when X player hits 3,000 career yards, etc., you could be pretty sure you’re not too crazy. If you go back to 1997 and start to question yourself, having 9/11 happen a few years later is going to be a pretty clear indicator. The only way to make you question yourself significantly would be if a side effect of the travel is to make your memories fuzzy. I can’t remember specifics of most things 10+ years ago, so if all my memory seemed that old, questioning myself would be more likely. What if you could only go back to the time you were born and only for ten years? Most people are unlikely to recall many details from that period of time.

Also, an interesting question is that if you can travel back in time (for yourself, not the government or another organization), what benefit could it have? Money, sure, but you’re probably pretty wealthy if you can actually build a time machine (assuming that’s the vehicle of travel).

@Nathan: True, but if we count in the “Butterfly Effect” thing your mere presence in the past could potentially alter these “touchstones”.

Lets say you one day you bump into a high school kid at the street. Because of this you nudge him a minute or two from his previous schedule. In the original time line, successfully crosses the street 20 minutes later. A drunk driver runs the red light after he is safely on the other side. Now that you nudged him over, the car clips him, breaks his leg, fractures his hip and etc.

The drunk driver is so shaken by this incident that he cleans up, and decides to turn his life around completely deviating from the original time line where he stays a drunk till hi dies due to a liver failure.

That kid was on his way to a football game where he got discovered by an NFL scout. 10 years later this kid became a famous quarterback and threw a legendary pass that won the Superbowl for his team.

Now he missed that game, he never got discovered, and due to his injuries he must end his Football career before it even started. As a result some other team wins the Superbowl that year.

Oh, and that drunk guy? He becomes a prominent political leader and launches his presidential campaign. He doesn’t win but his influence causes some changes on the political scene and a different person gets his party’s nomination. As a result, someone else becomes the president.

Yes, this is a little far fetched scenario – it sounds like something that would happen in a movie. But it is not out of the question.

My point is that relying on the “touchstones” may not be 100% reliable. Especially if your primary mission is something like “Prevent World War 3 by causing a major paradigm shift in US politics”. Or something like that.

I think that the knowledge of future events would keep time travelers sane. If I started to doubt my memories, I would just wait for some event that I knew would occur, like the outcome of a presidential election. As part of the time traveler’s preparation, presumably there would be some “touchstones” they would remember, like the winner of the Super Bowl or the World Cup for each year they’re going to be there. This knowledge of the future would serve to convince me that I’m not insane and focus me on my mission.