Basically the “predestination paradox”. Which closely resembles the “ontological paradox”, For example:
The problem with either (as someone asked up thread) is that you can essentially put objects in history with no origin. Sort of like Fry being his own grandfather. Or a much simpler example - Imagine you go back in time. You meet a small child and give them a custom lighter (or some other unique item) you’ve been carrying around since some stranger gave it to you when were little. Guess what? You just met younger self. So where and when was the lighter manufactured? And how old is the lighter?
I don’t want to talk about time travel because if we start talking about time travel, we’ll be here all day talking about it, making diagrams with straws.
You forgot about the part where you lost the lighter and were despondent, and your dad had to go and find another one just like it to get you to stop crying.
Larry Niven also once wrote an essay on time travel in which he made the following argument (paraphrased and simplified here):
Either time travel is possible, or it isn’t. If it isn’t, the argument ends here.
If time travel is possible, either it’s possible to change the past, or it’s not. If it’s not, the argument ends here.
If it is possible to change the past, then time travelers can and will do it… until one of them eventually makes a change that prevents the discovery of time travel. Ergo, no changing the past.
However, there is a form of time travel that happens so often people don’t even pay attention to it anymore. Walking from X to Y takes Z minutes, driving there takes 1/10 Z, flying there takes 1/100 Z.
…you forgot the part where my “dad” was really future me who was on his (my) way the to past for a sex change operation so he (I) could then have sex with myself and be my own parents…
The impotent observer (the past is set in stone) is one model. Here is another that was explored in a sci-fi story I read several decades ago. A girl is growing up, raised by a single mother. Instead of going to school, the girl is made to memorize all sorts of current events: World Series and Super Bowl results, companies whose stock shoots way up (this was pre-PC days, but there were still skyrocketing stocks) and so on. Meantime, the mother is engaged in some mysterious activities that puts food on the table. I am a bit hazy on the details, but I suppose the girl grows up, gets pregnant and then…there is a sudden discontinuity and the girl wakes up 20 years in the past, gives birth to a girl and…well you can all finish the story. Except it never really finishes, does it.
I think the only consistent answer to that is “There would be no such lighter.” You would not possess an object given to you as a child by your future self to be given to your past self.
It would be similar to envisioning an invention that, say, mows 200 acres of grass by snapping your fingers. I guess that such a device will be invented by the year 3000, so I travel there, take the device back with me, reverse engineer it, market it, and cause it to be in production 1000 years from now. Paradox? No. What would happen is simply that my device wouldn’t be there in the year 3000 for me to introduce into history.
It would be a virtual lighter, turned into a real lighter by being part of a Closed Timelike Curve. CTCs have different rules to the rest of the universe.
when Sheldon is interviewing Leonard as a prospective tenant, they agree that if either of them discovers time travel, their first stop will be at this interview
then they both look around, see nobody else and sigh regretfully
When I was in college, I did a paper for a “Mathematical Modeling” class on time travel, presuming a “multiverse” set-up where all possible alternate histories exist.
Under this model, there is a measured “distance” between any two universes, having to do with how recent was the last branching point the two have in common. For instance, we, here, are “closer” to a universe where the Nazis won WWII than we are to a universe where Napoleon conquered Britain.
In my model, you can’t go back in time and murder your “exact” grandfather…but you can murder someone who is very “close” to him. Your grandfather in a universe close to ours. In fact, as close as you want; however the effects start to propagate forward, so that paradoxes have “event horizons” around them.
Kind of pretty. Pointless, but, hey, I got an “A.”
There’s another model which is a lot of fun to play around with which is that there’s only a single timeline, and travelling to the past changes it permanently. So I go to the past, kill my grandfather, and travel back into the future, I’ll find myself in a future in which grampa vool was killed by a mysterious stranger who emerged from a time machine, and there’s no record of young Max ever having existed, and of course any ripple effects from said grandfather dying might well have altered the world entirely beyond my ability to recognize it.
This might seem like a model in which travelling to the past is so insanely dangerous that no one would ever do it… but I think if you’re careful you can still probably still come up with reasonable things to do, particularly involving very short trips.
Killing grandfathers doesn’t necessarily create paradoxes. There’s always the possibility you’ll find out that your biological grandfather was the milkman all along.
Maybe you can only time travel as far back as when the time machine was activated. That is to say someone has to make a door or portal or some sort of anchor in spacetime to go to.
Maybe it’s invented tomorrow but going back more than a couple hours requires more energy than the entire universe.
