The Grandfather Paradox

Metaphysics & Epistemology
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Introduction.

What is The Grandfather paradox?

The Grandfather Paradox is a paradox of time travel. Thus, you go back in time and murder your grandfather (or ancestor) before he has any children. Then, one of your parents never would have been born; and you never would have been born. But if you never had been born, then you would not exist to go back in time and murder your grandfather. The paradox regards any action that alters the past, since there is a contradiction whenever the past becomes different from the way it was.

Philosophical Analysis.

The eventual murder of your grandfather simply cannot happen. It is not logical possible since there is a contradiction of killing your grandfather to prevent your own birth.

However, if it doesn’t happen, it means that you exist and therefore, you can go back in time and kill your grandfather.

What this paradox creates is a loop. Anything a time traveller does in the past must have been part of history all along, and the time traveller can never do anything to prevent the trip back in time from happening, since this would represent an inconsistency.

We could then conclude that a time traveller would not be able to change the past as it is; they would only act in a way that is already consistent with what necessarily happened.

Questions.

Is the grandfather paradox – among others – a reliable example which proves that time travels are not logical and therefore impossible?

Do you know a solution to this paradox?

Note that I want to learn and discuss this with you. I’m not attempting to make you do my homework. :face_with_tongue:

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If time travel were a logical activity, then the logic we use would have led us to time travel a long time ago.

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Thinking is a logical activity.

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Exactly.

If time travel is ever possible some day, it would already be happening - there would be today some event caused by some person from the future.

But it can’t happen.

The very notion of time travel is a category error, or a metaphorical use of “travel”. We travel from place to place. The past isn’t a place. It’s a category of time (built by our construction of a stopwatch that marks start, duration and finish, showing the finish as happening after the start, to us as measured by the watch.)

The past is not a place, and it’s not even a being. The past cannot exist as a thing or a place or a present moment. It did once exist, but no longer does exist, or we wouldn’t call it past. If there was a past to go to, existing in waiting for us to go there, then it would be presently existing in waiting. But then it wouldn’t be the past.

So all references to the past that treat it as an object, a place, another world from another time - these are for poets to discuss.

We might be able to see light waves that reflected the past, but these would be present light waves, just waves that have traveled very far since they last bounced off the moment that existed in the past.

We might be able to travel so fast that time would reveal its relativity and we could return to a present earth that aged 100 years while you aged a few hours or days - but this still isn’t traveling into the future (other than as metaphor). This would be what it is - an experience of the relativity of time.

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Now that’s interesting. Can you justify it? Or is it just poetry?

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The relativity of simultaneity suggests otherwise. Events in one observer’s past can be events in another observer’s present or future. Current scientific theory supports eternalism over presentism.

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That’s interesting. Can you justify it or is it just poetry.

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It’s hard to say better. I would only add that the approach in which time (past, present, future) is “essence” is rather overrated. This is where all these logic games arise.

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Thank you all for your contributions. I am currently a bit busy to think deeply. I will come back and post if I have something interesting to add to your points and thoughts.

Cheers.

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I accept time as irreversible, directed, and acyclic—not a single linear, mutable track causally continuous with the present you can leave.

So the moment you decide to go back in time, another timeline branches.

You can kill your grandfather in that branching sense, as he was never your grandfather but the grandfather of whoever gets to be born in that branch.

Therefore, you cannot change your past because going back is just another forward branch.

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@Javi: I’m curious how much science fiction you’ve read. There are two basic strands:

  1. the blockuniverse: whatever you do in the past has already happened. (So you won’t kill your grandfather.)
  2. alternate timestreams: if you go back in time to a time-space co-ordinate you weren’t originally part of you kick off a new timeline. (So you can kill your grandfather; your origin simply doesn’t lie in this particular timeline.)

There are more complex and less linear concepts around. I can’t really think of much right now, since it’s been quite a while (I think I’ve come across probabilistic universes where nothing happens with certainty - kind of like a single time-stream that’s a pick’n’mix of what could be conceptualised as alternate timelines, and your existance is conscience-dependent and you can fade out if no-one can make sense of you…). There’s the game and anime Steins;Gate which has “big events” that must happen within a cord of alternate timelines and if you manage to prevent the big events you skip to some other time-cord… It’s complicated and the game goes into much more detail than the anime.

Basically, the big two concepts assume time is linear, but there are unique concepts out there that deal with non-linear time.

Also, I remember a different time paradox (bootstrap paradox): You go back in time with a watch given to you by your grandfather. You meet your grandfather as a young boy and give him the watch, which you will later receive again from your grandfather.

Questions: Since the watch is caught in a time-loop, where does the watch come from, and does it age with each loop? What’s aging like within a time-loop?

Let’s say such a non-origin loop is possible: a time machine would almost certainly end up like this, since crooks are almost certain to go back in time to sell the knowledge until time machines have always been known. This would require a non-linear time-cord theory, though, to work. That is causality warps in on itself until the origin has expelled from time and all that remains is a loop. (That means that time-streams themselves occur in some form of meta-temporality, no?)

So what’s the nature of time? Is it linear or not? Is there one stream of events, or do multiples branch off all the time? Is there a meta-temporality that keeps track of and organises multiple passings? Is the apparent linearity of time a function of consciousness rather than the world as such? And so on.

There are many absurd things you can do with time travel: For example: you place a diamond on your desk. You wait ten minutes than go back one minute, take the diamond, go back one more minute take the diamond again, and then again until you reach the time when it wasn’t there. You then place all the diamonds you have on your desk, wait ten minutes… You see where this is going? What about energy conversation in this scenario?

Time travel is funky.

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In other words, it might be possible to view the past, “visiting” it but only as an observer. Not “traveling” as such, but much more fun, since we get to watch whatever we want to (sitting in our Marvelous Past-Light-Waves Reflector) without the danger of screwing it up with a paradox.

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This is a fun one. I personally don’t think there is a solution unless we can figure out how time travel would actually work. I think the paradox shows us how complex it is but doesn’t necessarily eliminate it logically.

For example, when you go back in time and kill your grandfather, are you still in the same timeline? or is it possible that timeline has now branched and the you who is not born would have been in this new branch you created?

Another idea I’ve had was what if you are stuck in a kind of loop. Let’s say during your first timeline, you go back and kill your grandfather. The next timeline that follows doesn’t have you in it, so you can’t go back and do it again. So another timeline emerges where your grandfather is not killed and therefore you are born. Almost like in alternating loops you kill your grandfather and don’t kill your grandfather.

I hope that made some kind of sense lol.

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I’m curious about your thoughts on the block universe idea. Do you think that a block universe would mean everything is predetermined and we have no free will? Or do you think we still have free will or possibly a hybrid of predeterminism and free will?

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“pre-determined” seems to me an in-universe concept. I’m not sure it makes sense when looked at everything-that-happens from outside. Different temporal structures might render the “pre” in predetermined meaningless. Not sure. I haven’t thought of this much. Partly, because I find the free-will problem somewhere between uninteresting and meaningless. Inside a block universe we have a position in time; looked at from outside, there are positions that contain us and positions that don’t and at any point there either is “free will” or not (whatever that means).

To see what I mean about perspectives here: if we look at a block universe from outside, we’re “always” there, occupying the same time-space co-ordinates. Does this mean we’re immortal? No. The outside perspective is not the inside perspective. See what I mean?

I don’t see the blockuniverse having much relevance to the free-will quesiton (though I might be missing stuff).

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You make a good point about “pre” being an in-universe concept. I think that makes the question all the more relevant though since free will is an in-universe concept. Could a structure that is already written, shape our choices?

I’ve considered that in the block universe model, the outside perspective might not be us. It might just be a consciousness field that lights us up. So, we may have no real access to the outside perspective, only the inner one. So, while the field might not move, we might.

I had this idea that what if the block universe was correct and the structure exists, but hasn’t become fully formed until the conscious experiencer makes a choice. Like all possible branches exist but none become real until we choose. Kind of like superposition in a way.

And thanks for engaging with this even if free‑will debates aren’t really your thing.

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That again is the in-universe perspective. See here, too:

The outside perspective is a theoretical construct born of abstraction. Time travel creates a meta-temporal level, not necessarily because of any feature of time travel, but because temporality is an inherent feature of our perception. That’s why think in terms of “already written” or “outside perspective”.

When you make a decision, you’ll always make it at a time-space co-ordinate. And when you imagine everything that’s ever been going on as a set of time-space co-ordinates, you imagine them being there simultaneously, but that’s a kind of meta-temporality. To me, a block universe means little more than that events happen linearly from the past to the future, so that if someone were to travel through time, all that would mean is a discontinuous time co-ordinate; something that usually wouldn’t happen. But any decision has to take place in a time-space co-ordinate - whether free or not.

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Sorry, I’m not a big fan of science fiction. So, I haven’t read too much.

These are very intriguing questions.

First, I think it is important to understand what the loop does. A loop is a curved shape where a process repeats itself almost continuously.

Since that is what a loop is about, I think that the watch doesn’t get old with each loop.

However, it is more complex to conclude where the watch comes from. Does it come from my grandfather or me? If we say, “It comes from my grandfather,” folks would say that this happened because I previously gave it to him. Then, if we say “it comes from me”, folks would say that this is because our grandfather will give it to me in the future, etc.

A funny situation that ends up in a big loop.

I guess the same event happens as it already did from the beginning to the end, or from the past to the present and future.

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This photo shows snowman meat kebabs.

Now the question: is this enough for a yeti to fill up for lunch?

IMG20260522110315159
IMG20260522110315159960Ă—1280 179 KB

@Javi @FireOlogist

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No, because I have already eaten it? :thinking: