If we can try to change the past and always fail for some reason then Hitler's home town would have been littered with jammed machine guns, unexploded bombs, and dead commando squads. Since we have no record of such things then the reasons for the failures must have happened elsewhen, such as just before each time travel attempt. That makes attempted time travel very dangerous.

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Life is like a box of chocolates. All of your choices are bad for you.

Some nice leaps in logic there...

Assuming there are no records of jammed machine guns and so on (do we actually know this?), it is trivially easy to come up with reasons.

There may be a practical limit to the number of travellers who can visit a given era. (The rarity of time travellers supports this idea, although it is by no means conclusive.)

People from an advanced civilisation that has time travel technology might have no motive to go back in time and kill Hitler prematurely.

And even if they do have a motive, they are hardly going to act on it if they know that they are destined to fail.

Not everyone in the future would need to be motivated. There would probably be some who would want to try it. A near certainty of failure just doesn't impress some people.

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Life is like a box of chocolates. All of your choices are bad for you.

I never liked the idea that the universe conspires to stop you from changing things. And it leads to ridiculous results. Say you go back to kill your grandfather. And since he wasn't killed your gun misfires, so you try pistol whipping him, but you slip on the carpet, Your take out your dagger, but it slips from your hand, you take out your rocket launcher, but that very RPG turns out to be a dud. You drop an anvil but it falls to one side. you stick a stick of radium in his bed, but it turns to lead before your eyes, either that or all the rays and particles miss hitting his cells, but that leaves him with an inexplicable glowing rod. You open a magnetic containment tank and release antimatter, but it fail to react with the normal particles, so it doesn't go boom. You think a while and then go back to the future, then go back in time to when you are waiting, you do this again and again till there is as much temporal clones of you. But wait if you can't change the past, you can't change the past of the time you are thinking so there can be no temporal clones, the machine breaks down, if more then one of you tries to enter the past at once. If there is only one history, then that implies there is something like a God out there, or something even crazier is going on. A single history universe is worse then "Duck Amuck".

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...or you just can't time-travel.

Re RC:

"Oh brother, I'm a buzzboy!"

Shameless plug. Carry on...

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"Call me old-fashioned, but I think fire is magic. And it scares me a lot."

--The State

So far from what I have seen, the physics say you can, just in a relatively constrained way. Of course I am not doing the math, so I have only other researchers work to go by, so I don't know myself. But I have seen some very promising developments. Like the possibility that light is the best thing yet for bending space and hence, time.

"Daffy Duck he had a farm, yi aye yi aye yo.... And on this farm he had an *igloo*, yi aye... yi... aye... er?"
Ah, good old Cartoons. I love them so.

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I don't quite see the distinction between being motivated and wanting to try it.
It's not a near certainty of failure.

Can you crush diamonds with your teeth? Of course not - diamonds are a lot harder than teeth. If you try it, at best you'll simply fail; at worst you'll do your teeth some serious damage. Yet surely there must be people queuing up to get past the security guards at the Big Diamond Museum in order to have a go?

It doesn't have to conspire. There are known outcomes, and they take into account the efforts of meddling time travellers.

And it doesn't have to be complicated or cartoon-like, although you can probably envisage the wannabe grandfather killer slipping on a banana skin on his way to the time machine. Or he gets the coordinates wrong. Or he succeeds in killing the man, only to discover the man wasn't his grandfather after all. Or he's gunned down by a time traveller from further in the future.

It's trivially easy to come up with scenarios that keep time consistent.

Not at all.

Forget time travel for a moment, and imagine this:

You see someone's death certificate. Then a bit later you discover that the events leading up to his death were recorded on a video camera. You watch the video of a medical team doing what they can to save his life. No matter how promising their actions appear to be, you know how it's going to end. It might appear contrived, it might appear as if the universe is conspiring to kill that man, but of course it isn't really. It merely seems that way because you've seen events in the wrong order.

The only difference with time travel is that you are potentially a participant in a situation with a known outcome.

Time travel in a Many-Worlds universe is pointless, in most cases; if you send someone into the past they immediately change the timeline, and they simply disappear from your timeline. It is a bit like sending someone into oblivion.

Only in the vanishingly rare circumstance where the time traveller doesn't affect anything would you ever see them again; and you would acheive little in such a situation. The time traveller might, for instance, travel to an inaccessible desert, or cave, and affect the enironment to such a small extent that the new timeline he creates will easily converge with our own.
That is, assuming that such convergence is possible; a past timeline with few historical differences would converge with another similar one more easily than two completely different timelines.

This page describes the process of path-integration in quantum physics;
http://en.wikipedia.org/wiki/Sum-over-histories
In my imagination this is something like the process which allows the convergence of separate timelines; however I suspect that in reality the process would be even stranger, if it were even possible.

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People aren't motivated to bite diamonds. Preventing the second world war would draw more interest.If you see someone's death certificate then you can bribe someone to print and sign one, or make a fake one yourself. If you see a film of a medical team working to save his life then you alter some other film to look like the one you saw. Since the evidence has been faked it has nothing to do with whether or not the person in question actually dies. He might die just as anyone else might die, but he has no more chance of dying than anyone else since the evidence is not real. You don't need to change the past. Your faking of the evidence just becomes part of what already happened.

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Life is like a box of chocolates. All of your choices are bad for you.

You're the one who said that the past is immutable for the time traveller. Now you're saying the past could be different than the time traveller remembers it. Which is it?

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SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

But the similarity remains: Whether it's crushing diamonds with your teeth, or preventing WWII, the best case scenario is that you return home knowing your efforts ended in abject failure; a more likely outcome is that you did yourself some serious damage or got yourself killed.

Now I don't know about everyone in the future, but I'd find that quite offputting.

I agree that you can't always be sure of what really happened in the past, and if someone has faked evidence or documents, then you can be less sure of what will result from a time traveller's actions in the past. In theory you could get a stunt double of Abraham Lincoln to go to Ford Theatre and get the real prez to live out his days on a tropical island, although I doubt that that's what happened.

But that wasn't my point. My point was, if you know an outcome before you witness the events leading up to it, the events can have a "contrived" feel about them even if no time travel is involved. For instance, if you watch a recorded football game after you've been told the final score, it's a different experience to watching it "live".

I don't think it will be that simple. There will be people who claim to have changed the past just like there are people now who claim to be abducted by flying saucer aliens. Others will claim "conspiracy to protect the timeline" and try to change the past anyway. For some people, reason is easily defeated by emotion so they'd have to try.

For whatever reasons, people would try and it would lead to a highly improbable set of coincidences that they'd all fail for one reason or another.

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Life is like a box of chocolates. All of your choices are bad for you.

Exactly, maybe THATS why we don't see time traveller tourists, even if it has/will be'en invented, is that it sucks for your family. And most people care about their family. Another thing that makes it pointless, is that you never change your own universe, you just make a differant one. So all those people suffering under the eternal bondage of the lizard people of omeaga-9 'up stream' are still suffering, you just created a branch where OTHER people don't. And you can't really enter to this promiced land, because the changes you made, made it very likly you were never born.
Of course, it would be nice if you could move from strand to strand like a dancing spider. At least then you can go home.

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Is human memory perfectly infallible? Signs point to no. Does history record every detail of everything that has ever happened with 100% accuracy? Again, not so much.

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort

Exactly, history, (whether there is a past or not) is a man made construct. History is written by the victors.
And that is why "single universe time travel" is goofy. Lets say you go to kill your own grandfather, you don't succeed. But then lets say you decided to REPLACE your grandfather, kill him, and take his place. According to single universe time travel, you should be able to do that. but the only difference, is what you decide to do after you have killed him. Why should the universe care about an event that makes no differance in history?
And again, with the anti-matter. Maybe I am anthromorphising, but if extremely unlikely events are occurring, just because that if the 'normal' version happened, it would change history, I call that the work of a aware agent. Many of the tactics to keep the universe consistent violate, or restrain in ways worth of Maxwell's demon, the laws of the universe.

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The single-line universe doesn't "conspire" to do anything. That's just how it happened. You are here, so you must have failed.

Huh? I totally fail to get that. How did God sneak into this?

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"If this were play'd upon a stage now, I could condemn it as an improbable fiction."
Shakespeare, Twelfth Night
Illuminati's Razor-The most complicatedly evil answer is usually the most correct answer. - Fazor
"Every book is a children's book if the kid can read." - Mitch Hedberg
"Distance doesn’t matter much in space, where if you just start a thing off with the right kind of shove, sooner or later it will get where you want it to go." -Frederik Pohl, Mining the Oort