# Why the “grandfather” paradox - time travel related **URL:** \ **Category:** Factual Questions\ **Created:** [August 29, 2018, 5:22am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452 "2018-08-29T05:22:48Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [August 29, 2018, 4:01pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/21 "2018-08-29T16:01:03Z")
I’m my own grandpa… ---
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [August 29, 2018, 4:05pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/22 "2018-08-29T16:05:47Z")
> [@Machine\_Elf](#): > > It’s worth noting that [Stephen Hawking once held a party for time travelers from the future,](https://www.washingtonpost.com/news/speaking-of-science/wp/2018/03/14/the-time-travelers-party/?utm_term=.bc0db8f2f56f) but nobody showed up. He publicized the reception only _after_ it happened. 🆒 If you have a time machine and can go to any party in history, are you really going to go to one that’s so lame that no one else showed up? So awkward. ---
**Author:** ![Tom\_Tildrum](https://avatars.discourse-cdn.com/v4/letter/t/e95f7d/32.png) [@Tom\_Tildrum](https://boards.straightdope.com/u/Tom_Tildrum)\ **Post date:** [August 29, 2018, 4:07pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/23 "2018-08-29T16:07:29Z")
> [@Arkcon](#): > > So, checking Wikipedia, the trope has existed since the 1930’s… It may have been conceived of by many people before that, but we don’t know about it because each time, the person’s grandchild went back to sometime before they’d articulated it and killed them. ---
**Author:** ![lonesome\_crow](https://avatars.discourse-cdn.com/v4/letter/l/a587f6/32.png) [@lonesome\_crow](https://boards.straightdope.com/u/lonesome_crow)\ **Post date:** [August 29, 2018, 4:48pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/24 "2018-08-29T16:48:33Z")
> [@Lucas\_Jackson](#): > > 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: [Temporal paradox - Wikipedia](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. I think you are correct Lucas Jackson. It works if it states ‘…go back in time to before i was born. I then kill one or both parents. Now, this means that i was not born’. That is still a paradox. I think the reason it uses grandparent is to highlight the **‘cascading effect’** that can then be applied to an infinite number of actions. If it just uses ‘parent’ it makes it seem a more limited and narrow scoped example, that is, something more specific to do with a one-dimensional existence of ‘the self’. ---
**Author:** ![gdave](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gdave/32/3683_2.png) [@gdave](https://boards.straightdope.com/u/gdave)\ **Post date:** [August 29, 2018, 4:50pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/25 "2018-08-29T16:50:16Z")
My understanding is that the specific example in the grandfather paradox is that you go back in time and kill your grandfather without knowing at the time who he is. That is, you go back in time, run into some random guy, get into a heated altercation that results in his death, and you discover you’ve actually killed your own grandfather. It’s specifically “_grand_father” in this case because you’d be a lot less likely to recognize a young version of your grandfather than a young version of your father, and thus be a lot more likely get into an deadly altercation with him. As to why it’s “grand_father_”, there’s probably a sexist assumption of who the audience for the hypothetical is, and of their understanding of human behavior. The audience being asked to consider the hypothetical is assumed to be men. An audience of early 20th Century men would probably find it at least plausible that they might get into a deadly altercation with a strange man in a strange land, but would probably outright reject the idea that they might kill a woman, accidentally or otherwise. In short, phrasing it as “grandfather” just made the scenario a lot more plausible for the intended audience. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 29, 2018, 4:57pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/26 "2018-08-29T16:57:56Z")
> [@Jasmine](#): > > 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**](https://journals.aps.org/prd/abstract/10.1103/PhysRevD.46.603) > > 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 yeah, that’s kind of how it was approached in _The Time Machine,_ but even simpler. Boiled down to “you can’t use your time machine to change the past, because if you stop the event that caused you to build the time machine, you never would have built the machine.” ---
**Author:** ![lonesome\_crow](https://avatars.discourse-cdn.com/v4/letter/l/a587f6/32.png) [@lonesome\_crow](https://boards.straightdope.com/u/lonesome_crow)\ **Post date:** [August 29, 2018, 5:07pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/27 "2018-08-29T17:07:20Z")
> [@lonesome\_crow](#): > > I think you are correct Lucas Jackson. It works if it states ‘…go back in time to before i was born. I then kill one or both parents. Now, this means that i was not born’. That is still a paradox. > > I think the reason it uses grandparent is to highlight the **‘cascading effect’** that can then be applied to an infinite number of actions. If it just uses ‘parent’ it makes it seem a more limited and narrow scoped example, that is, something more specific to do with a one-dimensional existence of ‘the self’. \ ***‘cascading effect’** …sorry i’m new posting here. ---
**Author:** ![Pantastic](https://avatars.discourse-cdn.com/v4/letter/p/3bc359/32.png) [@Pantastic](https://boards.straightdope.com/u/Pantastic)\ **Post date:** [August 29, 2018, 5:49pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/28 "2018-08-29T17:49:09Z")
Like someone else pointed out, the original construction wasn’t that you go back in time to kill your grandfather, but that you fail to recognize him and end up killing him or causing an accident that kills him. It removes the question of the time traveler wanting to create a problem since the death is an accident, and it uses a grandfather instead of a father because you’re much more likely to recognize a young version of your own parents than your grandparents. The example used now tends to be a guy just looping back in time to shoot his own grandfather, which may as well use himself or one of his parents as the target, but in the original form choosing ‘grandfather’ made a lot of sense. > [@carnivorousplant](#): > > 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. I’ve always found the ‘we’ve invented a time machine, lets kill Hitler’ idea in time travel stories perplexing. There’s no guarantee that you even get a better result. It’s pretty easy to come up with ‘no Hitler’ timelines that result in a massive nuclear exchange wiping out civilization or that have some other similarly huge war that still kills a huge number of people. This is especially true when one considers that China actually lost around the same number of people as were killed in the holocaust, and that the Pacific part of the war wasn’t driven by Hitler at all. Plus you’re pretty much guaranteed that, even if the world you end up with is better, it’s going to be different enough to disrupt the time traveler inventor’s personal history rather dramatically. And there’s no real reason to think that people in the far future would really see Hitler as a wrong to be righted in the same way that US-EU-Russia people did in the past few decades; it’s already becoming distant history for us, and people in Africa and non-commonwealth southeast Asia never had the same ire for him as people with a more western background. Also two relevant comics: > **[Saturday Morning Breakfast Cereal - 2013-03-19](https://www.smbc-comics.com/comic/2013-03-19)** > > Saturday Morning Breakfast Cereal - 2013-03-19 > **[Saturday Morning Breakfast Cereal - 2014-02-12](https://www.smbc-comics.com/?id=3266)** > > Saturday Morning Breakfast Cereal - 2014-02-12 ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [August 29, 2018, 6:11pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/29 "2018-08-29T18:11:37Z")
> [@Pantastic](#): > > …I’ve always found the ‘we’ve invented a time machine, lets kill Hitler’ idea in time travel stories perplexing… Whoever runs Germany in WWII may not invade Russia. Might be tougher for the Allies -1 to win. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 29, 2018, 9:02pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/30 "2018-08-29T21:02:13Z")
[WikiHistory](https://www.tor.com/2011/08/31/wikihistory/) ---
**Author:** ![cmkeller](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmkeller/32/118_2.png) [@cmkeller](https://boards.straightdope.com/u/cmkeller)\ **Post date:** [August 29, 2018, 9:14pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/31 "2018-08-29T21:14:37Z")
Maybe this is overthinking it, but I’d say that if one becomes his own father, then he necessary generated half of his own genetic material, which is therefore impossible, because where did it originate from? A grandfather could theoretically share zero genetic material with his grandson. If he’s the maternal grandfather, and the entirety of his mother’s contribution to his genes came from her mother, then his genetic material can exist without necessarily having had to pre-exist. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [August 29, 2018, 10:59pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/32 "2018-08-29T22:59:32Z")
> [@cmkeller](#): > > Maybe this is overthinking it, but I’d say that if one becomes his own father, then he necessary generated half of his own genetic material, which is therefore impossible, because where did it originate from? A grandfather could theoretically share zero genetic material with his grandson. If he’s the maternal grandfather, and the entirety of his mother’s contribution to his genes came from her mother, then his genetic material can exist without necessarily having had to pre-exist. If a T-800 Terminator goes back in time and gets squashed in a press by an unruly human and then other humans use innovations gleaned from its broken body to design the computer that eventually designs the entire Terminator series (including T-800’s), where does the “genetic material” (i.e. design) for the T-800 come from? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 29, 2018, 11:44pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/33 "2018-08-29T23:44:05Z")
**cmkeller** , I think you’re the first one in this thread to talk about someone becoming their own ancestor. ---
**Author:** ![RTFirefly](https://avatars.discourse-cdn.com/v4/letter/r/c77e96/32.png) [@RTFirefly](https://boards.straightdope.com/u/RTFirefly)\ **Post date:** [August 29, 2018, 11:47pm UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/34 "2018-08-29T23:47:55Z")
> [@Pantastic](#): > > I’ve always found the ‘we’ve invented a time machine, lets kill Hitler’ idea in time travel stories perplexing. There’s no guarantee that you even get a better result. It’s pretty easy to come up with ‘no Hitler’ timelines that result in a massive nuclear exchange wiping out civilization or that have some other similarly huge war that still kills a huge number of people. This is especially true when one considers that China actually lost around the same number of people as were killed in the holocaust, and that the Pacific part of the war wasn’t driven by Hitler at all. Plus you’re pretty much guaranteed that, even if the world you end up with is better, it’s going to be different enough to disrupt the time traveler inventor’s personal history rather dramatically. And there’s no real reason to think that people in the far future would really see Hitler as a wrong to be righted in the same way that US-EU-Russia people did in the past few decades; it’s already becoming distant history for us, and people in Africa and non-commonwealth southeast Asia never had the same ire for him as people with a more western background. My take on it is, if you’re changing the entire history of the world, you’re also changing most of the details slightly as well. Let’s say the night you were conceived, assuming it happened after Hitler came to power, (a) your father orgasms a fraction of a second earlier or later, and another sperm cell wins the race, (b) your parents have sex but no conception occurs, or (c) they don’t have sex at all. In any of these scenarios, there is no ‘you.’ And the same is true for pretty much everyone else conceived after that time. Think Hitler’s holocaust was big? This one’s 100x bigger! > [@](#): > > Also two relevant comics: > [Saturday Morning Breakfast Cereal - 2013-03-19](https://www.smbc-comics.com/comic/2013-03-19) > [Saturday Morning Breakfast Cereal - 2014-02-12](http://www.smbc-comics.com/?id=3266) And gotta love [xkcd: Kill Hitler](https://xkcd.com/1063/) ---
**Author:** ![Civil\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/d78d45/32.png) [@Civil\_Guy](https://boards.straightdope.com/u/Civil_Guy)\ **Post date:** [August 30, 2018, 12:19am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/35 "2018-08-30T00:19:04Z")
I thought the short story “By His Bootstraps” by Robert Heinlein was an excellent treatment of how time-travel can really mess with cause and effect. [spoiler]On arriving in the future, the protagonist finds a notebook in his own handwriting. Pre-word processing, don’t you know. It has notes on how to deal with the society he now finds himself in. After spending several years in the future, he decides to make a fresh copy of the notebook, and he then destroys the old copy to remove any chance that it could fall into the wrong hands. Suddenly, he looks at the time portal just as his younger self is being shoved through it (details skipped here) – just as he remembers it happening. The older self thinks fast, and leaves the notebook where his younger self is sure to find it. So who authored the contents of the notebook to begin with?[/spoiler] ---
**Author:** ![j666](https://avatars.discourse-cdn.com/v4/letter/j/9de0a6/32.png) [@j666](https://boards.straightdope.com/u/j666)\ **Post date:** [August 30, 2018, 12:58am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/36 "2018-08-30T00:58:41Z")
> [@Sloe\_Moe](#): > > 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? That was how Gibson did in in _The Peripheral_. Me, I like the argument that you can’t because you didn’t; time is not linear, that is only how we experience it. ---
**Author:** ![E-DUB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/e-dub/32/403_2.png) [@E-DUB](https://boards.straightdope.com/u/E-DUB)\ **Post date:** [August 30, 2018, 1:04am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/37 "2018-08-30T01:04:46Z")
> [@jz78817](#): > > yeah, that’s kind of how it was approached in _The Time Machine,_ but even simpler. Boiled down to “you can’t use your time machine to change the past, because if you stop the event that caused you to build the time machine, you never would have built the machine.” I think that it was a little less clear than that. You might have been able to change the past, as long as none of those changes prevented the construction of the time machine, you were good. But just remember, there’s nothing to prevent your grandfather from coming forward and killing _you._ ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [August 30, 2018, 1:22am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/38 "2018-08-30T01:22:56Z")
> [@E-DUB](#): > > I think that it was a little less clear than that. You might have been able to change the past, as long as none of those changes prevented the construction of the time machine, you were good. But just remember, there’s nothing to prevent your grandfather from coming forward and killing _you._ “ **Über-Morlock:** You built your time machine because of Emma’s death. If she had lived, it would never have existed. So how could you use your machine to go back in time and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you. You have your answer. Now go.” ---
**Author:** ![Atamasama](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/atamasama/32/12961_2.png) [@Atamasama](https://boards.straightdope.com/u/Atamasama)\ **Post date:** [August 30, 2018, 6:43am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/39 "2018-08-30T06:43:52Z")
> [@Chronos](#): > > **cmkeller** , I think you’re the first one in this thread to talk about someone becoming their own ancestor. _All You Zombies_ was a Heinlein story I was obsessed with as a nerdy kid. [Wikipedia entry](https://en.wikipedia.org/wiki/All_You_Zombies) [Entire text online](https://gist.github.com/defunkt/759182/ad44c6135d168ae54503a281bb7e1a24c6c2ea0c) In that story the main character manages to impregnate a younger version of himself and then give birth to himself. And it was all a plan that he carried out himself later in life using a time machine. It was accomplished because he was intersex and began life as a fully-functioning female before becoming a fully-functioning male later in life (against her consent) after sex reassignment surgery and hormone treatment allowed his/her male genitalia to become functional. It’s really convoluted but I loved the concept when I was younger. ---
**Author:** ![Smapti](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/smapti/32/17938_2.png) [@Smapti](https://boards.straightdope.com/u/Smapti)\ **Post date:** [August 30, 2018, 6:52am UTC](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452/40 "2018-08-30T06:52:07Z")
> [@Chronos](#): > > **cmkeller** , I think you’re the first one in this thread to talk about someone becoming their own ancestor. I refer you to the case of Philip J. Fry, who managed to both kill and _become_ his own grandfather. 😃 [Previous page](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452.md?page=1) [Next page](https://boards.straightdope.com/t/why-the-grandfather-paradox-time-travel-related/820452.md?page=3)