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Paradoxes in Time Travel

» Predestination paradox and the Ontological paradox

The predestination paradox is when a time traveller travels back in time to change something, only to find out that the change was meant to happen all along. An example of this is given on the Time Models page:

• You are told by a strange voice to walk into an elevator.
• You walk into the elevator.
• An anvil drops right where you were just standing.
• The elevator turns out to be a time machine.
• You travel back in time, to see your former self standing in front of an elevator, with an anvil hanging above your head.
• You tell your past self to walk into the elevator.
• Your past self walks into the elevator...

This situation repeats itself forever, with no known origin and no known end. The ontological paradox is like this, only as a result of time travel, information or objects have no origin. For example:

• Your grandfather gives you a piece of paper, telling you to time travel back in time.
• You take that piece of paper with you as you travel back in time.
• You meet a young boy and give him that piece of paper, asking him to return it later.
• That boy turns out to be your grandfather, who then returns it to you when he is old.

In the above situation, where did that piece of paper first originate from? No one knows.
Speaking of grandfathers though...

» Grandfather paradox

• A mad scientist (not you, of course), travels back to a time before he was born
• He kills his grandfather before his grandfather had met his grandmother
• Therefore, his parents couldn't have been born.
• Therefore, the mad scientist himself could not have been born.
• Therefore, the mad scientist could not have travelled back in time to kill his grandfather.
• Therefore, his parents were born.
• Therefore, the mad scientist was born.
• Not knowing of this loop, the mad scientist time travels back...

» Twin paradox

Remember time dilation? Instead of watches, let's put two identical twins in this hypothetical experiment:

• One identical twin stays on Earth, while the other goes on a space ship
• The twin on Earth continues living as usual, while the twin on the ship goes around the universe
• The twin on the space ship travels near the speed of light for a month, then goes back to Earth
• The twin on Earth has now aged considerably (and I'm talking decades here), while the twin on the space ship is only two months older than he was before his trip.

» Novikov self-consistency principle

The solution to these paradoxes? Say hello to the self-consistency principle.

Developed by Dr. Igor Novikov in the mid-1980's, the self-consistency problem simply states that the paradoxes are impossible. Easy as that. So if the mad scientist wanted to time travel back in time to kill his granddaddy, he couldn't. Why? Because it's impossible (according to this principle). No matter what, the mad scientist will be stopped in some way. He may get run over by a bus, or squashed by an anvil. There's no way of knowing.

Moral of the principle? Don't time travel to kill people. It's just sad, and you'll probably get hit by a truck.