Time paradox

Related:
* Time theme
* Time travel
* Time shifting

This most often unintentionally slips into a story, so this should be only considered when the story itself acknowledges the paradox.

Examples:
* Time travel causes the reason for the time travel in the first place (effect becomes the cause)
* Time travel makes the reason for time travel disappear (effect removes the cause)
... most other cases are simply more specific variations of the two above.

A paradox of either kind can be assumed to have one of the following effects:
* Cause is removed: effect no longer can exist, timeline restores itself. Becomes a time loop (time travel never occured and so the cause was never removed).
* Effect is the cause: no perceivable effect, wasted effort. Time continues as "normal", barring any farther tampering by the person trying to fix it.

Obviously the loop kind of paradox would "end" all existence.

Quantum universe "eliminates" the problems of timeloop, but it also makes other things less spectacular. Time travel has less effect, if any in such because any time travel would create a yet another branch in the ever increasing expanse of different timelines (any change is matched with a timeline where the change did not occur, and so forth, so "damage" to a timeline is impossible to repair though you might conceivably travel to a timeline where the damage never occurred).


Specific examples:
The Grandfather Paradox.
Being able to travel back in time opens the possibility that one can do something in the past to endanger, change, or eliminate their own existence. The Grandfather Paradox specifically is this. You travel to the past and kill you grandfather. Because of this, you don't exist. So, how are you, as non-existent, able to do anything at all; let alone travel back in time to kill you grandfather? Several possibilities about this have been debated. There's one simple explanation. Time travel is not possible. That solves a lot of issues. Multiple universes. If you succeed in killing you grandfather, well you didn't. You're grandfather was undisturbed. Though you have now created an alternate universe where you don't exits because you're grandfather was killed in that universe. Another possibility is, you can't kill your grandfather. Assume that time travel is possible, but attempts to kill your grandfather will always fail. Either the past is immutable or only allows changes that can lead to the situation that produced you and lead you to travel in time. Kinda puts a damper on free will.


Is the past real?
"Does the past exist concretely, in space? Is there somewhere or other a place, a world of solid objects, where the past is still happening?" -from George Orwell's Nineteen Eighty-Four
To accept Einstien's theories, accepts that time flows at different rates in various areas of space. So, it is quite literally true that there are places where the past is happening. Also, within the frame of reference of 'the past' mentioned here, there is a place where the future is happening. This is highly indicative that time travel in either direction is possible.
Btw, the above question was not asked in the context of literal time travel but rather in the context of control of people and thought.

Everything moves, so traveling in time is prohibitively dangerous.
You are on a spinning earth, revolving around the sun, with both revolving around the center of the galaxy, and the galaxy is moving away from (almost) all of the others. The spot you time travel from will be somewhere else in relation to the earth, sun, and galaxy when you arrive at a different time. There are mathematics to suggest that your destination will/must follow the movement of everything so that you can arrive at a different time yet in the same frame of reference you left from. Or rather, certain math would be proven wrong if you didn't. For example, the existence of a Closed Timelike Curve depends on the 'end' of a curve returning to it's starting point in time as well as space. If this is not possible (and everything known suggests that it is possible), Einstein's theories are fundamentally flawed. The answer to "everything moves" in space paradox could be, "every moves" in space and time.

The Butterfly Effect.
"A Time Machine is finicky business. Not knowing it, we might kill an important animal, a small bird, a roach, a flower even, thus destroying an important link in a growing species. The stomp of your foot, on one mouse, could start an earthquake, the effects of which could shake our earth and destinies down through Time, to their very foundations. With the death of that one caveman [who would have survived only by eating that mouse], a billion others yet unborn are throttled in the womb. Perhaps Rome never rises on its seven hills. Perhaps Europe is forever a dark forest, and only Asia waxes healthy and teeming. Step on a mouse and you crush the Pyramids. Step on a mouse and you leave your print, like a Grand Canyon, across Eternity"
This is the grandfather paradox on a massive scale.
The Quantum Eraser Experiment might suggest that a time traveler from the present stepping on a butterfly in our past could truly leave a canyon across Eternity and nothing at all about our present (or his) would change. Yah, QEE sucks.

Motivation Paradox.
A man's wife is crossing the street when she is struck by a car and dies. So he invents a time mach to go back and save her. He arranges for her to be in a slightly different place so the car doesn't hit her. But another car hits her and kills her instead. He successfully changed the past. He could make many other changes. But no matter what changes he makes, his wife will die. Because, he is unable to make any changes that prevent him from having the motivation to invent a time machine. Spike Lee's See You Yesterday this paradox possible more than any other fiction of its type. Personally, I think this paradox is easily defeated (sort of). The trick is to add another time machine inventor. Inventor A prevents Inventor B's tragedy even though they would prefer Inventor B's tragedy to occur; while Inventor B prevents Inventor A's tragedy under the same demotivate. A mutually dissatisfying compromise that prevents each of their own tragedies.

Information Passage Paradox.
This is variant of the Motivation Paradox. Sending information to the past to change it removes the motivation to send the information.