A temporal paradox (or time paradox) is the result of time travel causing an event to "uncause" that event. Temporal paradoxes affect the "time continuum," giving it a sudden continuum interruptus.
Temporal paradoxes not only figure prominently in the Most overused 100 Star Trek episode plots but are the well most dipped-into when script-writers need some techno-babble that claims to explain something without actually meaning anything at all. Them, or gravimetric phase-shifts.
After viewing a photo on the internet that one wishes to "unsee," time travel and the use of the temporal paradox is a promising strategy, as everyone else will have no memory of a photo that was never made in the first place. Unfortunately, time travelers seem to retain their memory of events even that they keep from occurring, so they cannot unsee anything.
The Butterfly effect was a full-length movie about a temporal paradox, though mostly about being a teenage fuck-up whose every effort to patch things up makes things worse, which he could have done without the muss and bother of time travel.
As an example of a temporal paradox you can see without buying a movie ticket, this article was written by someone who read it, went back in time, and wrote it to be identical to the article that he had read (or would go on to read).
As numerous additional examples, there is the remainder of this article, until someone goes back in time and mercifully prevents it from being written, a gentler alternative than either Revert, Undo, or Murder.
You travel to the past to kill a chick, and succeed, so that, in the future (your present), she ceases to exist. But this means you don't have to go back in time to kill her. But if you didn't go back, then she would still be alive. Then you'd have to send yourself back in time to kill her, but, if you succeed, she wouldn't be alive when you first planned to kill her. And, as it is pointless to kill a dead person, you give up the time travel. But look! She is still alive, because you didn't go back in time to kill her! And if you decide to kill that person again in her past, then you'd travel back in time, and if you succeed, that person would be dead in the future again.
Therefore: If Romulans travel back through time and kill your father just after your birth, you will grow up to be a brilliant but aimless nasty little punk who steals 250-year-old cars and picks fights, until you get the crap kicked out of you in a bar and a military officer straightens you out and gives you your own starship. Only, if you are that screwed up, he doesn't. And if you don't get your own command, there is no reason for the Romulans to go back through time. So you don't grow up that way, and you do get your own command after all. But, then, the Romulans travel back through time and cause the death of your father...and so on. This is known as the "Abrams-Orci-Kurtzman effect." If you should time-travel back to ancient Rome, it is instead known as deus ex machina.
Say you are trans-sexual, and you are taken back in time when you were a female and tricked into impregnating yourself. Your younger self then gives birth to yourself, which an older you then kidnaps and drops off in an orphanage several years in the past in order to ensure your own existence. This means that you are your own mother, father, son, and daughter, a result that Gilbert and Sullivan could only hope to have dreamed up.
If you travel back in time and accidentally kill your grandfather, you would not exist in the present, and thus you'd be in a condition totally inappropriate for travelling, to-wit: nonexistent. But, if you don't exist, then you cannot kill your grandfather, so your grandfather would live a normal life — well, except for being killed by an ungrateful grandchild. Unless the way you prevented your own conception is by having already impregnated your own grandmother while in the past when she was young. But that's just gross.
From a time of war between man and machines, you are sent to the past by your best friend, the leader of the human resistance, to keep the machines from killing his mother, thus allowing him to exist and continue to resist the machines. You have sex with his hot mom and become his father, securing his existence. However, when the machine hunting your friend's mother is destroyed, pieces are left behind. This allows scientists of the past to study and develop the machines that start to rise against humanity. This raises the question of whether it was such a great idea to go back in the first place, because if your friend (or son) had not existed, the machines would not have to kill his mother or him and would not have been made in the first place.
- Note. In the sequel, however, it turns out that the parts allow the humans to progress faster than they had been, and end up causing someone's death, undoing all his achievements and leaving his son and wife traumatized for life, which would stop Judgement Day.
- Note. But in the sequel to the sequel, Judgement Day is actually inevitable and the technology is developed anyway, because it seems that a robot from the future wreaking mass carnage and mayhem doesn't actually go unoticed. Also, Skynet has to be developed because the U.S. government cannot control its own army, what with sending it to vast areas of sand searching for WMDs. So they have to build a computer to control it all. Which can fly.
You find a time machine in perfect working order. Immediately, you are visited by yourself, only older. His time machine is broken, so he wants yours. However, he is a complete cock-sucker, so you try to kill him. Sadly, he kills you. But if he kills you, how could he kill you? And why did you go on to become such a cock-sucker, anyway?
Recycling ( or pileups )
You have a wristwatch your father gave you when you were young. You travel to the past to that happy day, where you meet your younger self with his new watch. You give him the watch that you have, so he now has two watches: the watch his father gave him and the watch you gave him (which is actually the future version of that same watch). When you go back to the future, there are infinite wristwatches and the world has become a gigantic Refuse Transfer Station clogged with discarded wristwatches, because the number of watches you have had since you were young instantly doubled an infinite number of times (or perhaps merely incremented by 1 an infinite number of times).
Where did it come from,where did it go?
This paradox mainly inspired rednex and a bunch of other dr who wannabes. It's when an object appears out of nowhere, and when you wait for 5 minutes .You send it back in time 5 minutes. Well guess what, the object becomes it's past self. This might explain random objects appearing and disappearing in your house, like old 1900's stuff, expensive bills, and over-rated movie...discs...or something like that.
Electrical engineers will recognize the temporal paradox as just another feedback loop. Cases where you exist, if and only if you don't exist, are the same thing as taking a
NOT gate, whose output is 1 if the input is 0 and vice versa, and wiring its output back around to its input. In this case, the result is unstable. It is a "multivibrator," an effect that engineers overwhelmingly prefer even to anal plugs. (Two of these, cross-wired, are called "bi-stable," a bit of queerness that is the basis for all computer memory.)
So a person who has travelled (or will travel) into the past and has made changes that work against his own existence should pulsate between existence and nonexistence. The key question is: What is the frequency? How many times per second does the time traveller throb into and out of reality? There is no answer anywhere in the professional literature.
Separately, we mention above that the time traveller cannot unsee anything because he remembers even people and things that he prevented from coming into existence. Another unanswered question is whether the time traveller can take Polaroids of the nonexistent things, and bring the pix back to the present to show his friends.
Time-travel Best Practices
Don't alter the past
While time travelling, don't alter the past. Altering the past includes:
- Killing people. Those people include important historical figures and any of your ancestors.
- Killing yourself during a journey to the past. This immediately creates a "plot bifurcation" that will confuse most movie-goers.
- Killing certain snakes and ocelots that were just about to mutate into homo sapiens.
- Having unsafe sex with people from the past. This risks injecting modern viruses into the past and changing human history, which would otherwise have to wait for Africans to catch the diseases from farm animals and chimpanzees.
- Issuing checks or money orders. By definition, they will be back-dated, which is illegal.
- Entering a certain storeroom of the Dallas Book Depository on a sunny day in 1963 and pushing Kennedy's assassin out the window just before he can fire, even though this means the Vietnam War would have gotten even huger and you would have been drafted and shot dead in a rice paddy "before" you could do any of the above.
Exception. If you were supposed to do any of the above to ensure the normal unfolding of human history, then don't not do it.
Don't blab about their future
While time travelling, it is best not to say anything at all. Any utterance could give the citizens of the past dangerous information about the future that could alter the temporal continuum, which is the panties underneath the very fabric of space-time. Dangerous disclosures include:
- Any and all hot investment tips. If you find yourself in September 1929, don't tell anyone — not even a closest friend from the era — to pull all their money out of the stock market. Remember: Practice "tough love."
- If one of your parents ever disclosed that he slept around, there is no need, while in the past, to blab it to a young version of the other parent. Not even long enough in advance that they might "talk it out." You wouldn't want to drive them apart before you were even conceived.
- Leaving objects from the future behind in the past is a disclosure of sorts. This not only includes modern weapons that would fatefully alter the balance of power, but DeLorean cars and computers, especially those with WiFi. You learned as a Boy Scout to always police your own campsite. This was not because candy wrappers in the woods is really a problem — but to prepare you to travel time.
- Any and all innovations. Do not give an engineer from the past the secret of producing transparent aluminum. Not even if he is so impressed as to agree to embezzle large sheets of PlexiGlas® that will get you back to the present with the whales you stole from the local aquarium.
Exception. As above, if the person you are talking to was destined to know it or invent it anyway, then don't not disclose it.
Caution. Although Modern Man now knows exactly which species are "invasive" and which belong in a habitat, and what the ideal climate is and even the correct level of CO2 in the air, it is important for the time traveller to figure out whether the correct unfolding of history requires that a friend know/do something, or that he not know/do it. If Earth's future requires that your pal save his wife from being run over by a truck at the next corner, then tell him, even though none of you can see it coming. If, instead, she must die, then just keep it buttoned, tackle him when he tries to pull her back, and try to look sad and doleful afterward. Time travel is not always pretty. But someone has to do it.
How to tell which is the case? It always works to build a small computer, using wires, vacuum tubes, and small pieces of mica available on-site. If necessary, disassemble your cellphone and use some of the parts to complete the construction. After you finish, write a small operating system and a web browser, and go on-line and pull up the next day's newspaper. This will tell you exactly how to act.
Don't break the law
It is tempting for the time traveller to steal whatever resources he needs to complete his mission and return to the present; also, to kill whomever gets in the way. After all, you have a job to do, and everyone else only has themselves and their world, which is all over and done with anyway.
The risk is if the citizens of the past realize that the future will give rise to such a culture of theft and murder as to make their pursuit of morality senseless. With such a realization, a strange little man with a toothbrush mustache could take over an entire country, start a World War, and gas six million Jews. Erm — unless that, too, was destined to happen anyway. In which case: Why didn't you steal some for me too