Why the “grandfather” paradox - time travel related

In the zombie thread just bumped, Chronos says, “As for why it’s generally believed to be impossible, the classic example is the “grandfather paradox”. Suppose that I invent a time machine, and go back in time to before my parents were born. I then kill one or more of my grandparents. Now, this means that my parents were never born, and therefore neither was I.”

I’ve read this not only numerous times here on the dope, but other places as well. Wiki for example: https://en.m.wikipedia.org/wiki/Grandfather_paradox

But why grandfather? Why not just father? I’m sure I’m missing something basic but I just can’t seem to figure it out.

Or your younger self, if time travel works the way it does in the movies.

I once read a “short-short” story in which a guy invented a time machine to commit suicide. All the other ways were messy and/or painful. He decided to kill his father, as his father had been very cold to him and his mother. so he shoots his dad out in the barn, but to his surprise he finds he’s still in existence. What he didn’t know is that his father was sterile due to a war wound and his mother was telling the hired hand to pull out soon so she wouldn’t get pregnant.

It’s basically arbitrary. I don’t think there’s any reason it could not have been the Father paradox.

I think it’s just a narrative device. People tend to think of their grandfather as a figure from the past while they think of their father as somebody from the present.

The counter to the GP is simply you will create an entirely new timeline for yourself, independent of any prior events. That doesn’t sound too far fetched, does it?

Well we don’t know if time travel is possible, nor how/if the universe would resolve the inconsistency. So it’s a sort of counter but we don’t know if it’s correct.

It’s a little like the Fermi Paradox, in that the paradox “solved” by thinking of a potential resolution – there are many possible solutions. The point is, we don’t know which, if any, of the ones we’ve thought of are correct.

Well we don’t know if time travel is possible, nor how/if the universe would resolve the inconsistency. So it’s a sort of counter but we don’t know if it’s correct.

It’s a little like the Fermi Paradox, in that the paradox “solved” by thinking of a potential resolution – there are many possible solutions. The point is, we don’t know which, if any, of the ones we’ve thought of are correct.

And here Mijin demonstrates time travel, going back a little more than an hour to Ninja himself! :wink:

My guess is, it’s to prevent people from fighting the hypothetical by saying “yeah but what if your father had already impregnated your mom?”

Yep, like “Last Thursdayism” or “Russell’s Tea Pot”. They are just concrete examples that illustrate the point. There are certainly many other examples that could also illustrate the same point, but it gives us a short-hand term that has gained enough momentum to be the name of the concept.

Time travel shouldbe seen as the old style cassette tape. If it jumbles up you pull the tape out, cut the jumbled portion and retape it. That portion of music is removed but the rest of the tape plays as it should. Horn section and all.

Same with the time travel question. Take grandpa out and he will be out for the duration you killed him. When you leave that era he comes back as if nothing happened and can play the horn section.

It’s worth noting that Stephen Hawking once held a party for time travelers from the future, but nobody showed up. He publicized the reception only after it happened. :cool:

This one (non)incident isn’t absolute proof that time travel doesn’t exist, but it’s one piece of evidence. And with many such pieces of evidence - that is, the utter lack of any time travelers from the future being credibly witnessed here in the present - it seems very likely that time travel just isn’t possible, and never will be

To sum it up in the most general way possible, you simply can’t “violate causality”. It would create a time paradox.

You develop a time machine and decide that you would love to see how and where your mom grew up. You go back in time, procure a vehicle, and eagerly drive to the address you derived from your research. As you drive down the street, you eagerly check house numbers, knowing you are very close. Suddenly, a little girl emerges from in front of a parked car immediately in front of you. You hit and kill her. That little girl was your mom.

You couldn’t have killed her, or you wouldn’t have been born. But, if you weren’t born, you couldn’t have gone back in time and killed her. But, then you would have born and gone back and killed her. Etc.

To change the past you require a closed timelike curve. Stephen Hawking proved that closed time-like curves cannot be created in a finite system without using exotic matter. His attempt at a proof was in his paper on the Chronology Protection Conjecture

The “Chronology Protection Conjecture” mentioned above is the closest we have to a mathematical approach to constraining causality violations, but it’s just a conjecture and has not been proven - though it hasn’t been disproven either. LOL

You could still have a “grandfather” paradox in this situation.

Suppose his father was cold to him because he knew he was not the father. With him out of the picture, Mom might be free to marry the hired hand, and maybe protagonist might grow up in a happy household, live a fulfilling life, and never embark on a time-traveling suicidal quest.

I enjoy time travel movies as much as the next dude, but the paradoxes are often stupid or silly. You just have to turn your brain off a little bit (on enough to get the cool side of the story, off enough to suspend disbelief). Which is why my favorite time travel movie is “Bill & Ted”, where they lull you into turning your brain completely off, and then present interesting non-paradoxical time travel plot points. (“Trash can… remember a trash can!”)

There is a short story wherein each guy who builds a time machine goes back to kill Hitler, and other time travelers go back to prevent them from killing Hitler. Apparently killing an historical figure really screws things up.

Your father might be more likely to recognize you as an adult and prepare to fight you off, knowing what you’re probably up to.

So, checking Wikipedia, the trope has existed since the 1930’s, selecting the grandfather. Never the infant self, which should be the easiest to do, if suicide is the choice. So, clearly, it isn’t. Killing one’s infant self has lower impact on the timeline, so the paradox is less, even if its real and significant, so we don’t write the trope that way.

The Wikipedia page also conflates it with the “Killing infant Hitler” paradox. Again, you want a big effect. Not a “Stop Kristalnight” or “Block Wannsee Conference.” There doesn’t seem to be a neo-nazi authored “Block Potsdamn Conference.” These are things that would help, but that’s not what the trope is about, its about being so over the top, the timeline has to ripple and reset.

Many years ago, on the internet, probably the USENET, there was a discussion of time travel effects of second Terminator film. With the destruction of all Terminators, parts, raw data, the major scientist, and the last T-1000 in the molten steel, people argued the timeline had to (if successful, which it wasn’t because there were more films to make, but play along, please,) reset with a ripple, causing John Conner to disappear, leaving a very confused Sarah Connor with no memories of why she got herself so buffed, given that there was no reason or mechanism for John Conner to have been born.

The important thing is, however the paradox resolves itself, it can’t resolve itself in that way. Humans don’t just vanish. Their impact – food they ate in their lifetime, oxygen they breathed, CO2 they released, memories they formed, etc can’t simply ripple and replace. The explanation went that would be a violation of thermodynamics. Granted, this is intense physics happening, and you might say it supersedes thermodynamics. But really, if you don’t know thermodynamics, you don’t know anything. You can’t have no chance of knowing whats going on in reality, because then, its not reality. Even if its difficult to know reality, it has to at least be possible. Disappearing matter and energy just can’t be a thing.

The exact same possibility applies to your grandfather.

It would be a strange side effect of time-travel if your grandfather impregnated your mom.