PHILOSOPHY - Metaphysics: The Grandfather Paradox [HD]

In this Wireless Philosophy video, Agustín Rayo (MIT) explains the grandfather paradox, one of the classic paradoxes of time travel.

Help us caption & translate this video!

[daru_klas]

Bruno couldn't hit his grandfather because his "sniper rifle" was a Thompson submachine gun and Bruno has no fingers :p

[MrYutoob]

i don't study philosophy, but i think the very existance of bruno is what stops him from killing his grandfather. if his grandfather died before his father was born, than bruno would not exist to kill his grandfather. its like how one has to exist before three in a number line.

[zyguli]

Assuming that time travel is possible, Bruno getting distracted, missing or changing his mind on shooting his grandfather is exactly option 2 - universes paradox preventing force.

[davewilliam]

How To Travel Back In Time
1. Get 2007 Calendar.
2. Buy 2000s technology.
3. Wear 2000s clothes.

[zachm]

My opinion is that a never ending loop would be created. Follow me here. So Bruno shoots grandfather and kills him. The instant grandfather dies Bruno's one parent (it could be his father or mother) never exist and in turn Bruno never exists. So in that instant right after grandfathers death the man (Bruno) who had just killed him now never existed meaning grandfather was never killed because the man who killed him does not exist due to grandfathers early death. But as soon as grandfather was not killed Bruno will exist again therefore he is able to kill grandfather. And thus the loop continues. Bruno becomes stuck in a never ending cycle of not existing and then existing. Stuck forever in that moment shooting grandfather over and over again.

[ChristopherHartbooks]

Brian Greene, in one of his books on cosmology, I believe, also alluded to the premise that to travel back in time and avoid the Grandfather Paradox, a new universe would have to emerge. It has shades of the multiverse.

[AMC]

you don't just disappear--you exist in the past via time travel.  You can go back to the present and no one knows who you are because "you" were never born, but that's all.

[gimbuman]

So...this guy's view is that there can't be different outcomes based on different events?

Saying "Back to the Future is inconsistent because it says a different thing happened at the same time because there was a different input" ignores that time travel happened. At that to the creepy rape-whisper, and the animations? I want my 9 minutes back!

[LifeGlitch]

If there's an infinite number of universes, then when someone travels back in time and changes the past, then they'll go to an alternate timeline. Thus, in one universe, grandfather doesn't exist and bruno still exist. However, in the second universe, bruno is gone because the time machine didn't go back to the future. Bruno is stuck in the second timeline forever. Changing the past again will only create a third timeline. Don't overthink the multiverse! Just know that you're creating alternate timelines that lean to other universes instead of changing your own timeline in the past. Thus, giant lizard people will be possible after a hundred screw ups because of the 101th timeline.

[rydude]

I was pretty excited about this video because generally, these formats are pretty informative and entertaining. That said, It was extremely uncomfortable to have you sensually whispering into the mic along side really weird visuals that weren't overly relevant. Its worth noting the main issue in this paradox as well; time isn't a singular plane, its dimensional. Every action has its own plane of existence, therefore traveling back in time would be on a separate dimension. One could theoretically travel backwards in time, on a separate dimension, kill his father and still exist there because he is of a different plane. If he were to travel back to the present he left, he could easily fill in for the existence he removed. Regardless, time travel has far too many hypotheticals that make it possible or not possible so to build a video off of the idea that "killing your grandfather means you won't exist" is absurd anyway. When talking about theory and hypothesis, its irresponsible to only focus on pieces of the whole. Hoping for better.

[mikekeller]

If we conclude that time is a human construct, which is simply a measure of motion - one possible, consistent way, for Bruno to kill grandpa could be a time machine that operates by reversing the motion of everything in the universe EXCEPT Bruno himself, and the machine. In this way, Bruno is not really travelling to the past, but is actually recreating the past in the future.

Think of it like undoing a move in a Chess game. The game could be 20 moves in, with white about to win, when all of a sudden, one player starts undoing all of the moves on all of the pieces, except for the Knight. From the Knights perspective, they've traveled back in time, and could even change the outcome of the game.

In a similar fashion, if we "undo" all changes in the universe, except for Bruno himself, then he could kill grandpa, with no consequence to himself.

It's worth noting this idea of time travel does not apply to Back to the Future, since at times, Marty see's his past self, meaning matter would have to be created during the process, and necessitating that something other than Marty is not as it was; but this too could be explained via localized time travel - if we rewound only all the particles on Earth (or those that were on Earth during the destination "time", and we replace particles that are missing from the rewind (because they are inside of Marty) with similar particles that were not present, we could wind up with a consistent story that contains two or more Marty's.

[looncraz]

The video has the parallel universe thing backwards. When you travel back in time, you generally do so in your own timeline/universe. However, sufficiently significant enough events will cause the timeline to "fracture." In reality, though, you are just following a different wavefront of probabilistic outcomes.

If you imagine time and the universe as being everything that was, will be, or COULD have been or could be, all in a superposition of each other (as physics would seem to indicate), then we are really only tuning ourselves to experience a different potential version of reality.

The universe doesn't change, only our state within it changes.

[Battossai]

The question is: what happens in universe 1? Does Marty dissapear? because for Marty's father to be cool in universe 2, Marty from universe 1 has to fly to a different universe. So after he travels to universe 2 in the delorean, does he cease to exist in universe 1? Also what happens to the Marty in universe 2. This question only works if Marty's father from that universe had him though or if he Marty from universe 2 didn't travel to universe 1. What I mean is, if Marty's father was indeed his father, wouldn't there be two Martys? Does Marty from universe 1 replace Marty from universe 2 or will there be two of them? Meaning the one from universe 2 and the one who traveled to that same universe.

[jettpix]

So if I use a hammer to drive a nail in, and then destroy the hammer, the nail is no longer driven in?

If the above doesn't make sense to you, imagine how this video sounds. The fact that any person has existed means that at one point at least they were in existence. Even if they were to somehow cease to exist, there was still a point in which they existed that you cannot erase because it is an independent record. For instance, if I know Bruno is a valid existing person, and then his grandfather is killed, supposedly killing himself in the process, I would still have the knowledge of Bruno existing up until the point of the assassination. It would exist in my memory as a record. Even if you were to argue that those memories could be erased, they still existed before they were erased, just as Bruno existed before killing himself. You cannot undo the existence because you're only changing instances and because of permutations, each instance is independent. When you think of this dimensionally, it is easier to follow. I was produced by my father, however the fact that my father can die while I continue to exist shows the independence. Once a new creation exists, it contains a "proof" of its creator, simply by the fact that it exists, and therefore it follows that its creator existed. If its creator existed, then it follows that there is no possibility that it's creator did not ever exist, otherwise it could not have created it's offspring.

The problem with most people is they make assumptions about the concept of time. Time is a construct of finite beings which is only used because finite minds require an order of events and because they don't think dimensionally. If you consider a movie, it is made from many still images in a specific order. The only reason the order is necessary is to imply time (sequence) itself. Theoretically, you could devise an indefinite amount of permutations to the movie and they would all be valid, but not all would make sense to us because of our human concepts of physics and time, so we would reject that movie as being sensical.

Confused? Try reading another language you are not familiar with. It's not a false language, you just can't make sense of it because your conception of it isn't presently in existence. Maybe ten years down the road though you have learned to read and write it and now it makes sense.

In summary, a human cannot actually travel through time in the way they think. You cannot change events because they are fixed in existence dimensionally. There are an infinite quantity of dimensions but humans are restricted from accessing them inherently. One could write an entire book on this subject, but the basis is determining fallacy. Once you understand that building truths on top of fallacies is still building fallacies (even if partial truths are used), then it will become easier to reason.

[nebularwinter]

The final message seems to be that to "really win" in the "fight against causality" is impossible, just by definition. In fact this last thing is what seems to actually be the true "paradox", here.

[juancarlosespinoaparicio]

I think time travel is possible, and it's best explained in continuum. There are multiple timelines in Continuum. Whenever time travel occurs, a new timeline is created. To help with this explanation, they used the metaphor of a tree. When someone travels back in time, a new branch of the tree grows and events can play out in drastically different ways. Timelines, or branches, can also collapse if too much damage occurs to them.

[Drestanto]

At the crucial time,
"I was buying cigarettes at the store"

and

"I was at home watching TV"

is still Consistent if....

The store is his home and he is buying cigarettes from his own store while still watching TV in the process of buying the cigars

[goatjimmiejohnson]

Q: What would happen if you went back in time and _did not_ kill your grandfather, but instead, you stayed on some remote island and eliminated all possibility to change anything - what then? 
A: Everything unfolds as it did the first time, and eventually your mother is giving birth to a baby which would later grow up to be a second you. This is proof that the grandfather paradox is actually a faulty concept. You cannot go back and prevent your own existence; You can only go back and prevent a second you from existing. 
From the observer's point of view, which is assumed to be the same as your own perspective, your existence proceeded forward the entire time, as would be indicated by any watch you happened to be wearing. Your iPod would still contain the same songs, and you'd still be wearing the same apparel even if you go back in time to a point before these items were invented. What are people thinking - if you prevent Levi Strauss from being being born that their jeans are going to suddenly disappear? lol  No, you can't prevent creation once it's done, you can only prevent it from being created a second time.

[theodoreklopman]

Some people say that if someone time traveled back in time, and did something major that would have changed history’s course, then the course that was “supposed to have happened” would still happen. Logically, however, this is paradoxical and impossible. Basically, if there was one timeline that would be changed, then the time traveling will always happen. The person who time traveled goes back to the past and changes the past, so only that version of history exists, and is all that has ever existed. But if that version of history has always existed, then the universe where the person went back in time is nonexistent, leading to an endless loop, where in a specific version of this phenomenon, it is dubbed the “Grandfather Paradox.” This is my view on the paradox, and I see it as the only logical way that this paradox could happen, even if it loops forever.

[PoochPatrolers]

If Bruno kills his grandfather he would just become a John Doe when he returned back since he wasn't born. I don't believe he would disappear. He'd be what I call an "orphaned time traveler". And if you don't like that then time would split n create a new timeline