I don't understand why the planet would quickly have a population of zero in all timelines though.
— Luke

well, if everybody had one and knew it worked, I suppose they'd all use it and exit any particular timeline. It's sort of like heaven: The sales pitch is great, but if it's such a better place, why does nobody voluntarily hit the button and go there? It's because from the perspective of the original timeline, it just looks like you vanish, never to be seen again. There is zero evidence that it is safe, let alone works. — noAxioms

In that case everyone would remain on the same timeline, so it does not follow that every timeline would quickly have a population of zero.

Bob would continue to exist on any timeline he travelled to (at least, until he dies).
— Luke

And Bob is missing from every timeline except one. Of course on the other timelines, there may be many people that attest to having traveled, and the evidence is there that it works. Those timelines would empty out faster than the original, if only from people going back to times when there were still people to meet.

Nobody on these worlds knows who the actual time traveler is (the one that created this world), not even Bob. — noAxioms

This still doesn't explain how every timeline would quickly have a population of zero.

But how could I already be there before I time travel?

With a time machine of course. — noAxioms

What I meant was: how could I already be in the past before I have ever time travelled? I could already be in the past (on a single timeline) if I had time travelled before but, given causality, there must have been a first time that I ever used the time machine to time travel. How could I already be in the past prior to that?

That sort of logic only holds water because there are no time machines possible. — noAxioms

Why are no time machines possible? That's not something I've said.

They'd not be a loop if they were caused. That it doesn't fit in with your notion of singular causality is irrelevant since all those rules must be discarded with reverse causality. — noAxioms

We can just discard causality and assume that time machines don't need to have had a first ever use, and we can conveniently disregard whatever history led up to that first ever use?

P1: I said I would accept this for this purpose, but there is no such requirement. If time travel was possible, somebody might be able to do it just by willing it. If a machine can do it, why can't a creature evolve a way to do it. The premise is something like saying you cannot get to grandma's house without a car. Well, that's false since evolution has given us a means of machineless locomotion. — noAxioms

That's fine, but if a creature evolved a way to do it, then there must have been a first time that they ever time travelled. Evolution actually works in favour of my argument because it cannot disregard the history that precedes the first ever time travel event.

P2 is unacceptable. It's like trying to prove God by asserting that the universe needs creating at some point (which is itself a self-refuting argument). An un-created time machine does not violate any rules in a universe where time travel is possible. — noAxioms

If you accept that one cannot travel without a time machine (P1) - at least, for the sake of argument - then it follows that a time machine (or the means for time travel) must be created or have evolved or somehow brought into existence in some manner. This is not trying to prove God in order to prove the existence of the universe; it's merely assuming the universe must have been brought into existence (which is quite self-evident). Hopefuly we can agree to the standard scientific view that the universe's existence began with the big bang, but even if we might assume that the universe has always existed, this is not very much like a time machine. To say that time machines have always existed is more like saying that waffle irons have always existed.

An un-created time machine does not violate any rules in a universe where time travel is possible. — noAxioms

Sure, but it would imply no time travel.

P3 seems false. I might make a time machine but never use it. We presume you mean the machine in the loop, so yes, it just happens to get used (the 'first time' say) in the story you are creating. I put 'first time' in scare quotes because there can't be a first time in a loop: — noAxioms

Why can't there be a first time in a loop? Loops are immune to causality?

P4 is OK, but seemingly irrelevant since your story involves only a single time travel event, no loop at all. — noAxioms

It's not irrelevant. I'm saying that if a loop involves time travel (as the examples in the SEP article do), then we can consider the first ever time travel event in that loop and what preceded it. Unless you are arguing that there is no causality in a loop or that time travel loops and time machines in loops have always existed? Why should causal loops be immune from causality; from having been caused? It seems like a bit of magic.

You're not reading my comment. I said that by your rules, a person can be in the presence of at most one actual time traveler. We could have a factory that made them like bags of cheetos, and everybody used them to get to appointments and catch the traffic light that just went yellow. If they were used like that, the planet would quickly have a population of zero in not just the original, but all the timelines. Despite that prediction, no person would ever be in the presence of more than one actual time traveler, which is the one and only person that created the specific timeline the person finds himself in (if he's still in it and hasn't left already). — noAxioms

Unless more than one person used the same time machine to time travel together. I don't understand why the planet would quickly have a population of zero in all timelines though.

Actually, nobody would use the machines, due to the overwhelming evidence of it being nothing more than a self-annihilation machine. So good thing Bob is the only person that has one, and only Bob fails to exist in pretty much any of the timelines. — noAxioms

Bob would continue to exist on any timeline he travelled to (at least, until he dies).

But the way you describe it, it isn't really the past, just a different timeline which maybe looks like 'the' past, but is actually just another line, 'a' past at best, one of many. There is only one 'the' past, and you didn't go there. — noAxioms

Right, but it would be logically impossible to travel to 'the past' (i.e. on a single timeline) unless I was somehow already there before I time travelled. But how could I already be there before I time travel?

But if he's there at all, history is gone. If I go back 250 million years to see the early evolution of mammals, I'm sorry, but humans will never evolve from that timeline. Your very presence destroys that... — noAxioms

How?

Loops don't have a start. — noAxioms

Maybe it all boils down to this. I'm arguing that causal loops require a start; that there must be an initial time travel event which causes the loop in the first place, and that what causes the initial time travel event and subsequent causal loop cannot be an effect of that initial time travel event. I don't believe that a time machine can just magically exist uncaused.

Try to state the logic of your statement formally. What are the premises? How does your conclusion (that the closed-loop machine must have been built) follow? One of your premises is perhaps that all things need creating at some point, but that premise begs a universe with no closed time curves. — noAxioms

I've been trying to state my premises. I will try again to be more clear.

My premises would be that:
- one cannot time travel without a time machine
- time machines need creating at some point
- there must be an initial time travel event following the creation of the time machine, when the time machine is first used to time travel
- the initial time travel event cannot cause itself (e.g. by a prior time travel event using the newly-invented time machine).

he should not be surprised by his sudden appearance at an earlier time, unlike everyone else on the new timeline (who we would assume have never encountered a time traveller before).
— Luke

By your rules, a person can only be in the presence of but one actual time traveler, even if other people on the timeline also remember time traveling. — noAxioms

I'm considering Bob to be the first ever time traveller. This is because I find the logic of single timelines and closed loops to be problematic at the first time travel event. Given that Bob creates the first time travel event, nobody has ever encountered a time traveller before.

Also, instead of considering the new timeline as a copy, you could consider it as a re-writing of history, but one which does not eliminate the original timeline.
— Luke

That would be a different convention. The new timeline is a rewrote-history according to traveled-Bob, and the old timeline becomes the copy from which he originated. — noAxioms

I don't disagree with this, except for your insistence that we call it a copy. In a sense it is, but it is also the only way that time travel can work; the only way a time traveller can visit a past they've never visited before without causing a contradiction, If you change the past from being history A to being history B, then you can't eliminate the fact that it was history A before you made it history B..The different histories are the different timelines. Otherwise, on a single timeline, there would be a contradiction: the past both does and does not contain a time traveller.

if they were to travel to the past then that would be altering something about the past
— Luke

There are stories/scenarios in which nothing is altered. It's more like watching the past on TV since nothing there can detect you. — noAxioms

Well, I wouldn't call that time travel or "travelling to the past", That is just somehow viewing the past at the present time of the viewers.

The scenario wants us to imagine that this is a logically-sealed causal loop. However, the time machine must have been built by someone else in order for it to have been stolen and then donated to the museum.
— Luke

Does not follow. That sort of reasoning is only valid if time travel is not possible. The whole point is that it was never built. — noAxioms

I understand the intended point of the example. You will need to explain why my objection does not follow. The "logical order of things" I described included time travel, so I don't see why my reasoning is valid only if time travel is not possible.

It wasn't the time traveller that built it, so it cannot be the donation by the time traveller that causes the existence of the time machine.
— Luke

The existence is caused by its own time travel to the past. Such is the nature of closed loops. — noAxioms

I understand that is the intended point of the scenario. However, I raised an objection to its logic, which was:

"If you don't have a time machine then you can't time travel, so you can't then obtain that time machine (or its technology) via time travel."

The examples provided rely on the time traveller having obtained their time machine or its technology via time travel before they ever had the means to time travel.

Since you can't time travel without a time machine, it cannot be a closed loop. The chain of causes cannot begin after the time travel event, because there can be no time travel without a time machine. Being in possession of a time machine must be the initial cause of everything that follows, so the initial possession of a time machine cannot be the effect of its time travel event.

Still, in my prior post I pointed out a hole in that story. — noAxioms

Which hole are you referring to? Entropy?

Infinite age since it's stuck in a loop. Somebody has to do one excellent refurbishment effort somewhere during each 10 years. — noAxioms

The loop could have started only 10 years ago. Also, the advanced technology of a time machine could give it thousands of years of repair-free use, but I see your point.

It's not a copy of Old Bob, since he time travels from the original timeline to the new timeline. — Luke

That part is just you saying it. It could be just as easily said that everybody (including old Bob) in the copy timeline is a copy. The machine could split Bob just like it splits everything else. The story doesn't go like that, but the story could go like that. It would still be time travel of a sort, especially from the PoV of the Bob on the created timeline. — noAxioms

That could be, but the existence of Bob and his time machine ends on the original timeline with the time travel event. This does not happen to anything else on the original timeline.

Also, Bob has built the time machine with the intention of returning to the past, so he should not be surprised by his sudden appearance at an earlier time, unlike everyone else on the new timeline (who we would assume have never encountered a time traveller before).

Also, instead of considering the new timeline as a copy, you could consider it as a re-writing of history, but one which does not eliminate the original timeline.

What's the point (or possibility) of time travelling to the past if it is to leave the past completely unchanged?

I take it you're not a historian. Those guys would love a machine that lets them go back, even in a way that cannot alter anything, just watch. — noAxioms

Ah, but if they were to travel to the past then that would be altering something about the past; namely, that the historians were not originally in the past (as time travellers). Their additional existence in the past, even as mere spectators, would change the past. This is the most basic contradiction in relation to time travel on a single timeline.

I think we're doing considerable damage to causality if any of this were plausible. OK, the Einstein time travel doesn't violate causality, but I personally don't think that one counts even if it meets the SEP definition. — noAxioms

It's not the sort of time travel I had in mind, either.

Incidentally, based on my very amateur understanding, I had thought that once the Einsteinian "time traveller" had returned to Earth, the same amount of time must have elapsed on Earth as it has for the traveller, given the time dilation effects of turning their ship around in order to return. When I read about the twin paradox long ago, I figured that although one twin can be in the future of the other, there is no way to transmit information to the Earthbound twin which could give them advanced knowledge about the future and that they must both return to the same proper time when they meet again. However, I admit that I don't fully understand these things and I'm probably way off. Besides, those sorts of time travel scenarios involving that type of "time travel" are not what I had in mind here.

The SEP article gives several examples of a single timeline without paradox, Some of the best are the loop ones, including a case where you don't even need to invent/build the machine. You just give it to your younger self when you're done with it. — noAxioms

There are three scenarios described in section 3.2 on causal loops. I reject them on the same basis as the arguments I have given here. I will respond to each of them below.

...imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists.

The scenario wants us to imagine that this is a logically-sealed causal loop. However, the time machine must have been built by someone else in order for it to have been stolen and then donated to the museum. It wasn't the time traveller that built it, so it cannot be the donation by the time traveller that causes the existence of the time machine.

The logical order of things is that:
(i) someone else built the time machine;
(ii) the time machine ends up in the museum (somehow other than via donation by the time traveller);
(iii) the time traveller steals the time machine from the museum;
(iv) the time traveller use the time machine to return to the past;
(v) the time traveller donates the time machine to the museum.

The causal loop (steps (iii)-(v)) begins only after the time travel event, but it cannot logically eliminate steps (i) and (ii). Logically, someone must have built the time machine to begin with; such things cannot "simply exist" uncaused. This is much like what I am suggesting with the original timeline of Bob. Young Bob first has to grow up through a period without time travellers in order to build the time machine and return to the past as Old Bob.

The time traveller must have possession of their time machine before the causal loop begins, in a way which does not rely on the causal loop. Otherwise, the time traveller cannot time travel in order to create the causal loop. If you don't have a time machine then you can't time travel, so you can't then obtain that time machine (that you don't have) via time travel (that you can't do).

...imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self.

The time travelling older self must have had this knowledge before the first time travel event, and must have obtained this knowledge in some way other than via the time travel event. Without the knowledge to begin with, they could not have time travelled and therefore could not have given this information to their younger self. They must have obtained this knowledge by some other means prior to first time travelling. This is another attempt to eliminate the start/cause of the causal loop.

Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it.

I don't buy the fact that the time traveller could not have done something else. Their only constraint is the presumption of a single timeline in this scenario and the avoidance of contradiction.

There is a sort of paradox with that scenario which is how the machine experiences no entropy: It stays perfectly new at all times, which isn't plausible for something that is thousands of years old. — noAxioms

I don't follow why it must be thousands of years old. I imagine the causal loop in these scenarios to be a much shorter period than thousands of years.

I am arguing that Old Bob cannot have been in the past originally, because Young Bob had not yet grown up to build a time machine or to time travel.
— Luke

You don't seem to understand my point, which is that there is not obvious convention as to if the old-Bob in the copy timeline is the same old-Bob from the original timeline. The usual conventions for saying this person is the same person that looked like him yesterday. "I bought a can of beans yesterday": True? By convention, yes, the person who bought the can of beans is the same person that submitted this post. We know that because we know the convention. There is no convention for crossing timelines. To me it looks like old-Bob commits suicide, but builds a copy of himself (and the machine) in a timeline with a copy of everything else. The convention could just as easily say that. — noAxioms

If we don't conventionally say that we are mere copies of ourselves in our normal passage through time from moment to moment, then I see no reason to apply a different convention to time travellers.

What was this alternate timeline doing before Bob traveled to it? — noAxioms

It did not exist before Bob travelled to it; the new timeline is created by Bob's time travel. It is the same as the original timeline, except now with the addition of a time traveller: Bob.

Did it have a 'present' 2024 that was altered by Bob's appearance in what was considered to be 1990 at the time? — noAxioms

The new timeline did not exist before Bob arrived, so there was no "original" version of the new timeline to alter. The original version is the original timeline, from which Bob departs. The original timeline does not have a time traveller arriving in its 1990. 1990 on the original timeline contains only young Bob (sans time traveller) who grows up to be time-travelling Old Bob. It is only on the new timeline that young Bob meets time-travelling Old Bob; when Young Bob and Old Bob exist together at the same time. The new timeline is created when Bob travels from the future (of the original timeline). To illustrate:

Original timeline:
1990: Young Bob (only) who grows up to become Old Bob the time traveller
2024: Old Bob time travels to 1990 in order to kill Young Bob.
2024 (post-time travel): Bob is no longer on this (original) timeline from this time onwards.

New timeline:
1990: Old Bob arrives in 1990. The new timeline now contains Young Bob and Old Bob together at the same time (this is not the case on the original timeline). Old Bob kills Young Bob.
1990 (post Young Bob's murder): Old Bob lives out his life.

If Old Bob was at 1990 on the original timeline (i.e. on a single timeline), then the time travel event must have occurred before Bob built the time machine.

You seem to regard them as copies yourself, as evidenced by several comments (my bold): — noAxioms

You can consider them to be copies, I suppose, but it's the only way I can make sense of the occurrence of time travel. It's not a copy of Old Bob, since he time travels from the original timeline to the new timeline. But the new timeline might be considered to contain a copy of everything/everyone else. Alternatively, the new timeline could be considered as how the (original) past gets changed by the presence of a time traveller from the future (following the time of their arrival). I believe that is what we should expect from a time travel event. If you are going to time travel back in time to kill Hitler as a baby, then Hitler can't have already been killed as a baby in the past. What's the point (or possibility) of time travelling to the past if it is to leave the past completely unchanged?

There is no point or possibility of travelling to a destination if you are already there.
— Luke

So by this wording, the young Bob that gets killed is not Bob. He is not already there, but is rather killing a copy, somebody else, having left the young Bob that is actually himself back in the original timeline unkilled. — noAxioms

It is (Young) Bob that gets killed, but not (Young) Bob from the original timeline, that's true. Young Bob from the original timeline is the one that grows up to be Old Bob the time traveller/murderer. This is the only way to avoid contradictions, paradox and violations of causality.

The younger self does not time travel; the older self does.
— Luke

If the two of them were the same person, this would be a direct contradiction. But you seem to regard them as not the same person. — noAxioms

I do regard them as the same person at different times, but the logic of time travel allows for an older version of a person to meet their younger self. That is the scenario of the Grandfather paradox.

So if (actual) Bob goes to some parallel world in 1990, and waits several years for the perfect opportunity to take out the young-Bob copy1 that is there. The moment comes, and he fires his gun only to find it wasn't loaded. Opportunity lost, and there won't be another one. But he has a time machine, so he goes back a day and loads the gun that yesterday-Bob (also a copy) can use to complete his task (of killing young-Bob copy2, leaving young-Bob copy1 un-shot back in the first alternate timeline). — noAxioms

You could consider them to be copies or you could consider it to be Bob actually changing the past from what it was originally. Obviously, Bob cannot change the past on the original timeline, because he was not in the past (as a time traveller) on the original timeline. If Bob were in the past (as a time traveller) on the original timeline, then his time travel would not change the past. In fact, in that case, Old Bob would be in the past before his time travel event (or without needing to time travel). It is only if Bob were not in the past (as a time traveller) on the original timeline that he could change the past from what it was (albeit on a new timeline) by the mere insertion of himself into the past as a time traveller.

Although Bob cannot change the past of the original timeline, he can create a new and different past to that of the original timeline, which I think is as much as can be expected while also avoiding paradox or contradiction. In order to change the past, the past needs to have been a particular way previously. Similarly, if I want to change my hair colour from brown to blue, I can't change the fact that it was brown originally.

All the examples of 'is time travel' at the top of the SEP article are single-timeline examples. I'm not saying that traveling 'sideways' to a different line is or is not time travel, but I'm saying that those examples cannot all be senseless. Yes, they all have potential paradoxical consequences, all discussed in the article. — noAxioms

Right, but I'm attempting to point out why I think single-timeline examples of time travel are senseless, and why I believe that a second timeline is necessary to avoid contradiction or paradox. However, I may have done a very poor job of that so far.

My answer was that it is Old Bob from the original timeline who time travels and kills his younger self (on the new timeline). — Luke

My reason for asking was to figure out justification of that assertion. I'm not saying it's wrong, just an arbitrary designation. Most designations of identity have pragmatic reasoning and are thus not arbitrary. This doesn't, so the question needs asking, and the answer needs justification. — noAxioms

I am arguing that Old Bob cannot have been in the past originally, because Young Bob had not yet grown up to build a time machine or to time travel.

You wanted to explore the implications. I'm trying to do that. — noAxioms

I appreciate it. Hopefully I've helped clarify my position a little better with this post.

Apparently what I am doing right now does count as time travel, so long as I move. — noAxioms

As long as you move at a relatively different speed to the rest of us, I suppose. Anyhow, that's not quite what you said earlier, which was that you are time travelling by staying on your own timeline. This could be achieved by something like 'waiting', which the SEP article categorises as not time travel.

My point was that it is senseless for Bob to travel to the past if he is already there
— Luke

Him already being there was the point: To alter what he (younger self) would have otherwise done. — noAxioms

I agree that the point of the time travel is for him to be at the earlier time in order to alter what he would have otherwise done. But this is what necessitates his time travel to the earlier time destination. There is no point or possibility of travelling to a destination if you are already there. That is my point.

I see no reason why the younger self cannot have already time travelled before. — noAxioms

The younger self does not time travel; the older self does. Old Bob travels back to the time when he was younger. In the context of the Grandfather paradox (or one of its variants), the purpose of the time travel is for Old Bob (the time traveller) to murder his younger self. The younger self has not yet grown up to build a time machine, so he cannot have already time travelled before. Obviously, there may be scenarios - unrelated to the Grandfather paradox - in which Young Bob jumps into Old Bob's time machine after he arrives and then Young Bob uses the time machine to time travel to another time, but I see no point in complicating matters further.

Another mistake could be made, 'necessitating' a second correction. I put it in quotes because the mistake cannot be corrected on the more original (more real?) timeline. — noAxioms

I haven't suggested that the original timeline is "more real" than others. However, you have suggested that the inhabitants of the second timeline are merely "copies".

I don't think so. I'm assuming that Bob returns to the same past that he lived through when he was younger
— Luke

Poor assumption. If I'm to 'kill grandfather', I'd have to go back at least a century. Maybe I want to witness the asteroid taking out the dinosaurs. You can't put in a rule that says you can only travel a short ways to some past with you in it somewhere. — noAxioms

I don't rule out that Bob can travel to some time before his birth. I was referring to the context of the (Grandfather paradox) scenario I've described, where Old Bob time travels in order to murder his younger self.

You also contradict yourself. You say on one hand that it is senseless to go to a time when you exist, and on the other hand you're presuming Bob does this 'senseless' thing. — noAxioms

I'm saying time travel is senseless on a single timeline, and that it only makes sense if there is more than one timeline; where a second (or more) timeline is created as a result of the time travel event.

Any travel to the distant past will destroy the history you know. Everyone talks about critical events that make a change, but just appearing and stepping on a bug is enough. That said, killing grandpa isn't necessarily paradoxical. Maybe you're not actually related to him, but rather the mailman. I know my grandfather was a cheater. Why can't grandma be? — noAxioms

For the sake of argument, let's presume that he really is your grandfather. Then it would be paradoxical; at least, on a single timeline.

Well, besides the fact that it isn't possible in the first place, there are valid scenarios discussed in SEP that allow travel to the original timeline. CTCs are one example. — noAxioms

Again, I was speaking within the context and logic of the Grandfather paradox, which is presumably a 'doctor' or 'leap' type of time travel rather than the others described in the article. I wasn't really considering those other sorts of time travel scenarios.

I assume that it is the Bob (or Luke) from the original timeline
— Luke

I don't make that assumption. I try to work it out. — noAxioms

You asked whether it was Old Bob or only a copy of Old Bob who killed his younger self. My answer was that it is Old Bob from the original timeline who time travels and kills his younger self (on the new timeline). When Old Bob time travels, he disappears from the original timeline (at his time of departure) and appears on the second timeline (at his time of arrival).

As I replied earlier, I wouldn't call this recreation of another time in the present time to be time travel.
— Luke

I'm not recreating a time. I'm just moving a Studebaker forward in time by a century. That's pretty much exactly what you're describing, except in the forward direction. So tell me why that's not what I did. How do you back the assertion that the car didn't travel through time, but Bob (also going forward say) did. Was it the lack of a fancy time machine looking device with blinking lights and stuff? — noAxioms

Going by your earlier description:

Recreating a piece of some past state. Indeed, this isn't time travel being described.
I can build a new 1928 Studebaker, even giving it the same serial number as one made in that year. Has that car time traveled or is it just a new thing? I satisfied the conditions of the OP by doing so. Is it even a Studebaker if I built it instead of the defunct company? — noAxioms

I've said that I'm not (or that Bob is not) recreating a past time (e.g. 1990) in the present, and that Bob actually time travels to 1990. You say that recreating the 1928 Studebaker now is not time travel and I say that recreating the year 1990 now is not time travel. If you were to recreate it today, then you haven't really moved a Studebaker forward in time by a century. As you note, "this isn't time travel being described". I thought we were on the same page here?

The time traveller was either never at the destination time and cannot return there without contradiction (having two conflicting histories on a single timeline), or else they were always there and therefore cannot "return" there.
— Luke

How is the 2nd clause different from the first? — noAxioms

According to the second clause, the time traveller always appears alongside their younger self in the past. The problem with this view is that the effects of time travel precede the causes of time travel. Given the normal temporal order of things from past to future, Bob appears in the past alongside his younger self in this scenario before he has built a time machine or time travelled. Therefore, there is no longer any need for Bob to continue on to build a time machine. We may ask where is the starting point of this causation chain? There exists a paradoxical temporal loop in which causes are effects and effects causes. Presumably, Bob built the time machine (in his later years) for the purpose of returning to the past, so did he forget that he was already in the past? Where do time traveller Bob's memories start and end in this scenario? If time traveller Bob is always in the past, then how can he return to the past via time travel?

The first clause presumes that time traveller Bob is not already in the past ("the first time around") before he has time travelled. However, if Bob were to subsequently time travel to the past, then this would create two different histories on the same timeline; one in which time traveller Bob is not in the past and another in which time traveller Bob is in the past.

Both just seem to say that you can't travel to your own timeline, which is partly silly because I am doing it now. — noAxioms

I wouldn't call that time travel in the relevant sense. The SEP article attempts to draw the relevant distinction: https://plato.stanford.edu/entries/time-travel/#WhaTimTra

IOW, does forward time travel necessitate a branch in timelines? — noAxioms

I think forward time travel is a bit trickier, but I suspect that it does also require a branch in timelines. I would prefer to consider the "simpler" backwards time travel for now.

Bob must travel to, and insert himself into, a past time at which he didn't always already exist as a time traveller.
— Luke

Why this restriction? I go back to 1955 (standard destination). Hang around until 1970, and go back to 1960 this time, where "I" already am as a time traveler. What's wrong with that? Can he also make a 3rd branch off the original timeline? — noAxioms

My point was that it is senseless for Bob to travel to the past if he is already there (as is the case with a single eternalist timeline). I don't foresee issues with additional branching. I was only considering a singular time travel event.

Can I, having just made the machine, branch a new line off some other timeline where I never existed in the first place, say some version of 1980 where my parents didn't survive WWII? — noAxioms

I don't think so. I'm assuming that Bob returns to the same past that he lived through when he was younger. It's just that he cannot have been there as a time traveller the first time around, before he built his time machine.

Meanwhile, why do you want to kill anybody? — noAxioms

I don't; that's the scenario of the Grandfather paradox. For example, see here: https://plato.stanford.edu/entries/time-travel/#GraPar

The young-Luke you find back there is not you since 'you' is presumably on the original timeline. You've no reason to kill this other person or for that matter, anybody. If you kill yourself, have you killed Luke, or did a copy kill himself? — noAxioms

Call it a copy if you will but this is the only way that time travel is possible. It is not possible for Bob to return to the past on the original timeline due to the resulting violations of causality and contradictions. The second timeline differs from the original only in that there is no time traveller on the original timeline until after Bob has constructed and used his time machine. The only way for Bob to return to the past to meet his younger self is on a second timeline, so the only way for Bob to return to the past and kill his younger self is on a second timeline.

If you kill yourself, have you killed Luke, or did a copy kill himself? — noAxioms

I assume that it is the Bob (or Luke) from the original timeline who time travels to the past and creates a second timeline in the process. Bob therefore remains on the original timeline until his time travel event when he relocates to the second timeline.

I think you need to consider the question I asked about the Studebaker in my first post in this thread (about post 57). Is that time travel? If not, why not? What is your machine doing that my example with the Studebaker did not? — noAxioms

As I replied earlier, I wouldn't call this recreation of another time in the present time to be time travel. I am assuming that Bob's time machine actually works and transports him to a period of time other than the (present) time that we are currently living through together.

Sounds like a copy to me. — noAxioms

Having given this some further consideration, although some may find it unsatisfactory that Bob does not kill himself on the original timeline, I believe that the "branching solution" or multiple timelines view of time travel is the only way any sense can be made of time travel; and it is the meaning of time travel portrayed in most fictional accounts (AFAIK).

It should be noted that the singular timeline view of time travel, such as that depicted in the Grandfather paradox, does not permit the time traveller to kill themselves on the original timeline, either, because this is what produces paradoxical results. The time traveller cannot visit an earlier time on the original timeline without falling into contradiction, because there was either no time traveller at the destination time "the first time around" or because, otherwise, the time traveller was always at the destination time (i.e. the first time around) and time travel is therefore impossible (or, at least, unnecessary). The desire to time travel in order to alter the history of the original timeline that leads to the point at which the time traveller departs in their time machine is therefore futile on a singular timeline view. The time traveller was either never at the destination time and cannot return there without contradiction (having two conflicting histories on a single timeline), or else they were always there and therefore cannot "return" there.

On the multiple timeline view, although Bob creates a second timeline as a result of his time travel, at least from his perspective, it is possible for him to return to the past to do whatever he likes, including killing his younger self. Moreover, for time travel to make any sense, Bob must travel to, and insert himself into, a past time at which he didn't always already exist as a time traveller.

I believe the multiple timeline view is what is usually understood to be time travel and is the only way time travel makes any sense. Understandably, however, it may not satisfy those who want to alter the original timeline or murder any of its past inhabitants.

there must be two (or more) parallel timelines in order for time travel to make sense. The timelines branch off into two or more timelines following the first time travel event.
— Luke

That says a parallel timeline [world] is needed, created since it doesn't otherwise exist. — noAxioms

It does exist. It's the same 1990 that was there before (on the original timeline), except that before it did not contain any time travelling Bob. The only difference is that now it does contain a time travelling Bob, because Bob has just time travelled back from 2024 to 1990.

The 2nd sentence implies the 1990 new timeline branches off the 2024 'travel' event, which means no actual travel, just a universe creation event at 2024. — noAxioms

I wouldn't consider it time travel if a new 1990 must be invented at/in 2024. I consider that Bob's time machine actually works and transports him back to 1990 from 2024. He can't already be in 1990 (at his 2024 age, as Old Bob) without having time travelled.

The 1990 new timeline does branch off the 2024 travel event, but it creates a new timeline - following the 2024 travel event - which begins at 1990, in which Old Bob has now time travelled back and inserted himself into 1990. He wasn't at 1990 on the original timeline (as Old Bob) because he had not yet built the time machine. Everything else is exactly the same as it was in 1990 upon Old Bob's arrival, except that now it includes Old Bob. Isn't that just what we usually mean by time travel? If I wanted to travel back in time to kill Hitler, presumably I'm not already there/then.

How is this Bob in the new timeline the same Bob as the old timeline? — noAxioms

Because he was Young Bob on the original timeline and then he grew up (into Old Bob) to build the time machine and used it to time travel back to 1990 on the new timeline to kill Young Bob. It's a logical sequence of events: Young Bob grows into Old Bob who time travels back to 1990 and kills his younger self. Old Bob continues living after this, and Young Bob no longer continues on to build a time machine. That's how the new timeline differs from the old timeline, which is a result of Old Bob's time travel and subsequent actions.

They are different timelines, but they started out the same way, and it's the same Young Bob in both up until the arrival of Old Bob from the future, which is when the two timelines diverge. However, the original timeline must precede the new, second timeline. Bob can't time travel or return to the past before he has built the time machine.

Sounds like a copy to me. Old Bob is a continuation of the not-murdered original young Bob, not the Bob that gets murdered. — noAxioms

Right, Old Bob is not the Bob that gets murdered, because Old Bob is the murderer of Young Bob. You could say that Old Bob only murders a copy of Young Bob on the new timeline and that he does not murder the original Young Bob. However, murdering the original Young Bob is not possible according to the logic of the scenario, because the time machine needs to be built before Old Bob can travel to the past in order to kill Young Bob. Bob effectively changes the past from what it was on the original timeline (from 1990 onwards), which was his intent.

I don't have a single-timeline scenario. Heck, I don't have a scenario at all. — noAxioms

Sorry, I just assumed this from past discussions, since I thought you were firmly on the side of eternalism.

Is it better now? — noAxioms

Yes, thanks for taking the time.

I've long since expressed that the branching solution resolves the grandfather paradox. — noAxioms

I assume by "branching solution" you mean something similar to what I've described here. Otherwise, I'm not sure what that means.

I only just found out yesterday that the idea of parallel worlds is not a new idea as a solution to the grandfather paradox.

You said you're creating a new world, not altering the original, in effort to avoid the paradox. — noAxioms

I have never said this.

Logically, it is necessary for Bob to build and use the time machine in 2024 before he time travels to 1990. This is not creating a new world, it is altering the original. It's Old Bob time travelling to 1990 from 2024. There is no point or possibility for Old Bob to time travel to 1990 if he is always at 1990.

And, on your view, there is no "original" 1990 to alter.

You said that 2024 is the antecedent state, so that means the alternate (copy) 1990 state was created at that time. — noAxioms

Of course. If time travel is to make any sense then the time traveller can't always have been at the time travel destination, otherwise there is no point or possibility of time travel.

The original 1990 doesn't exist anymore. — noAxioms

That's not my position. I claim that both timelines exist in parallel, pre- and post-time travel, each with different histories following the insertion of the time traveller into 1990.

You can't travel to somewhere that doesn't exist. — noAxioms

You also can't travel to a destination if you are already at that destination. Incidentally, 1990 does exist and it's Old Bob's time travel destination. It is only after he time travels to 1990, inserting himself into a time that he wasn't before, that the two timelines diverge.

There's no contradictions with it because killing the copy young-Bob isn't killing old-Bob's actual ancestor. — noAxioms

You can call it a "copy" if you like. There are two parallel timelines, after all: one timeline in which Young Bob grows up to build a time machine in 2024 and another in which Young Bob gets killed by Old Bob. However, what supposedly happens to Old Bob in your single timeline scenario after he murders Young Bob? He just vanishes into thin air?

It sounds like your machine doesn't travel at all then. It manufactures a new world in 2024 that looks like how things were in 1990. It's a new thing, a copy. The time is still 2024, but the calendar hung on the wall is set to 1990. Rather than going through the bother of putting a copy of old-Luke (and the machine) in this newly created world, it would save effort by just creating the world like it was but without young-Luke.

The original 2024 timeline marches on, without you and the machine if the universe-creation process involves the destruction of the machine and its occupant, and still with you if it doesn't involve that and only places a copy of you and it in the new world created.

Anyway, if you hand-wave away all the physical reasons why this cannot be done, I have no problem envisioning time-travel scenarios that are free of paradoxes. — noAxioms

Could you explain why it must be a "new thing, a copy" of 1990 recreated in 2024 and why Old Bob cannot actually travel back to 1990?

It seems logical to me: Old Bob cannot time travel back to 1990 until he has built the time machine, and he does not build the time machine until 2024. This entails that there is a 1990 (on the original timeline) in which Old Bob did not appear and in which only Young Bob exists. It is not until 2024 (on the original timeline) that Bob first builds the time machine and uses it to travel back to 1990. It is only after this time travel event in 2024 that both Young Bob and Old Bob appear in 1990 together.

It seems like you're saying that my depiction of 1990 (on the second timeline) is incorrect because the real 1990 must always contain both Bobs, but you would need to explain the logic of that scenario. How do you account for the fact that both Bobs appear in 1990 prior to Old Bob's invention of his time machine? Does Old Bob still go on to invent his time machine in 2024? If he always appears in 1990, it seems unnecessary for him to build or use a time machine in 2024, so where is the time travel here? Maybe the original timeline gets "written over" (or "saved over") after the time travel event?

These sorts of contradictions and causal violations do not exist in the parallel worlds case.

If time travel is not using a time machine to travel back and insert yourself into an earlier time (at your time travelling age), even to perhaps a time before your birth, then I don't know what time travel is.

How about a growing block model then? — noAxioms

I don't recall why but I never fully endorsed the growing block theory. However, let's leave those ontological theories aside for now. They will probably come up again later.

Your Bob example showed how that paradox can be easily avoided.
Another way is to scratch the parallel world and let Bob simply destroy his younger self, and the time machine appears in 1990 uncaused. It's going to do that anyway (in violation of physics), but we're supposed to be ignoring known physics for this exercise. — noAxioms

Doing that would not remove the paradox, unlike the parallel world scenario.

OK, I said it wasn't paradoxical, but it's still a violation of the physics that we're ignoring. If sending information outside of the cause's future light cone constitutes a paradox, then its still a paradox. — noAxioms

I don't follow why it would be a paradox, only that it might be a violation of the physics. However, we need to assume that time travel is possible for the sake of argument. The typical grandfather paradox scenario also assumes that time travel is possible.

The old timeline still has the bad technology. It just doesn't have Bob anymore. If it's just Bob that's the problem, he could fix that quick without bothering to build the machine. — noAxioms

In the scenario I sketched, Bob's motivation for wanting to kill his younger self - because he thought the technology was too dangerous - is something I only added for the sake of giving him a motive. The important factor, which is relevant to the paradox, is not that the old timeline has bad technology, but that young Bob is murdered by old Bob on the new timeline.

The antecedent state would be old Bob's time machine transporting him from 2024 to 1990
— Luke

No, the antecedent state would be 1990 minus 1 second. — noAxioms

I disagree. What precedes old Bob's appearance in 1990 is the use of the time machine in 2024. That's how old Bob comes to be in 1990. You seem to keep wanting to deny the possibility that Bob can time travel.

Physics doesn't allow a vehicle to just materialize from nothing. But I'm told to ignore this inconvenient problem. Hollywood depicts it frequently, and they can't be wrong, right? — noAxioms

If you don't allow for Bob to be able to time travel then we will be unable to discuss the grandfather paradox.

Thanks @AmadeusD. Your comments are appreciated here. I would say Kant was alive in 1776 too!

If I travel to 1776, then that was a time when Kant was alive.
— Luke

It's possible that someone could invent the technology for time travel.
— Luke

And another thing, forgot to add. Your concluding claims are all in "If" form. They are not propositions. They are hypothesises and conjectures themselves in "If~" form. — Corvus

Only one of these two statements is "in "if" form".

Anyhow, do you deny that Kant was alive in 1776?

Great summary, thanks. All packed into less than a minute to boot. — noAxioms

Thanks, but it would be a stretch to call it a summary. I just transcribed most of the very short video.

The video author seems also to presume presentism, implying that time itself would have to be re-wound (and the entire universe with it) in order to 'go back', rather than time being left along and just the traveler going somewhere. — noAxioms

I propose that we avoid bringing presentism and eternalism into the discussion, that we simply assume time travel is possible and see what the consequences are for causality and contradiction.

As you may recall from previous discussions on time, my ontology of time involves a blend of presentism and eternalism (in short, that without presentism there is no 'progression of events', and without eternalism there is no timeline(s) of events). If eternalism solves a problem for time travel, that's great.

Causality doesn't say an effect cannot occur before its cause, it says that the effect (information travel) cannot occur outside the future light cone of the cause. The future light cone is physical and objective (not frame dependent). The plane of simultaneity (referenced by the word 'before') is frame dependent and an abstraction, at least it is under Einstein's theory. — noAxioms

Right, given our current knowledge and technology. But let's assume that time travel is possible and see whether we can avoid a paradox.

Time travel isn't itself paradoxical. — noAxioms

Oh, then we are in agreement and I've wasted my keystrokes. I thought the grandfather paradox indicated that time travel itself is paradoxical?

OK, the 'spawn a new timeline' explanation. Yes, that avoids the grandfather thing, but doesn't resolve the physics violation of the machine in the first place, in particular, what caused the 1990 state with two Bob's in it. — noAxioms

Bob's time machine is the cause of the 1990 state with two Bobs in it. But if you want to know how a time machine works, I have no idea.

And apparently Bob fails in his effort to destroy the bad thing resulting from his technology. — noAxioms

Not with the spawning of a new, second timeline (once old Bob time travels back from 2024).

Um, that's a blatant violation. 'Old Bob' in 1990 is not the result of an antecedent state. If 2024 is the antecedent state, then the rest of this new timeline is not the result of that other antecedent state. — noAxioms

Isn't this simply denying the possibility of time travel because of, well, time travel? The antecedent state would be old Bob's time machine transporting him from 2024 to 1990 (that is, 1990 on the new timeline, which now includes old Bob. Note that 1990 on the original timeline does not include old Bob).

From my own perspective, time doesn't exist. It is a mental concept. — Corvus

This seems inconsistent with the video you posted which describes time as “a measurement of the progression of events”. You appear to deny that there is any progression of events. Nevertheless, I have little interest in trying to convince you otherwise, but I wonder how you account for the fact that we are all aging and that children become adults? Is that all in your mind?

But one might still demand to prove the existence of the parallel time lines, before progressing further. — Corvus

One might equally demand to prove the existence of a single timeline before progressing further. I don’t see how this might work either way. I’m merely showing that time travel is hypothetically possible with a way to avoid the contradictions of the grandfather paradox and violations of the laws of causality.

Thanks for posting the video.

The problems associated with time travel cited in the video are as follows:

1. Time is not a physical object that can be moved or manipulated. It's simply a measurement of the progression of events.
2. The laws of physics, including the laws of thermodynamics, make it impossible to go back in time.
3. The idea of travelling back in time would violate the laws of causality, meaning that an effect cannot occur before its cause.
4. Time travel raises numerous paradoxes, such as the grandfather paradox, in which travelling back in time and changing a past event would alter the present and create a contradiction.
5. Even if time travel were possible, it would require immense energy and advanced technology beyond our current capabilities.

I have no issue with Point 1. I might add that time includes the progression of events, and is not merely its measurement, but this is not my concern here.

Points 2 and 5 make virtually the same point as each other. Point 2 says that time travel is impossible due to the laws of physics (however, it should be noted that this is only according to our current knowledge). Point 5 subsequently acknowledges the limitation of our current knowledge and technology and allows for the possibility of time travel.

Points 3 and 4 I believe are incorrect. This is what I was trying to point out with my earlier sketch of parallel timelines, which does not violate the laws of causality. I will attempt to develop this further below.

Imagine Bob was born in 1980. In 1990, at the age of 10, Bob has an idea for how to create the technology for time travel. Bob continues his studies and in 2024 he creates the technology which will allow him to time travel. Bob foresees the dangers of this technology, however, and realises that he must use his new time machine to go back and stop himself from ever creating it. Bob must kill his younger self.

Late in 2024, Bob enters his time machine for the first time and sets course for the year 1990. Bob arrives in 1990 and seeks out his younger self. He will probably be in school, Bob thinks. Bob finds his younger self, sneaks up behind him and takes his life as quickly and painlessly as possible. Old Bob is satisfied that his younger self will now never grow up to create the time machine and humanity can continue on more peacefully (at least for a little longer) without it. Bob is never discovered to be the murderer of his younger self and lives out the rest of his life quietly, until he dies in a fatal car accident in 2008.

Given that Bob has somehow worked out the technology for time travel and assuming that time travel technology or time travel itself does not somehow violate the laws of causality, what other laws of causality have been violated in this scenario? Bob is born, invents a time machine in 2024, travels to 1990, kills his younger self, and then old Bob dies in 2008. That is one event after another of cause and effect. Unless one can specify how time travel or its technology violates the laws of causality, then I don't see how else they have been violated.

As for Point 4, there is no contradiction. According to the standard "paradox", there is only a single timeline. Given a single timeline in this scenario, old Bob first arrives in 1990 to kill his younger self. Since he was murdered, young Bob cannot grow up to become old Bob, so he cannot build his time machine, so he cannot travel back in time to kill his younger self. Therefore, Bob could not have travelled back in time to appear in 1990. Hence, a contradiction. Or, per the grandfather paradox, if Bob were to travel back in time and kill his grandfather, then one of his parents could not have been born and then neither could Bob. Hence, a contradiction.

However, I will argue, there must be two (or more) parallel timelines in order for time travel to make sense. The timelines branch off into two or more timelines following the first time travel event. Let's call them timeline A and timeline B. Timeline B differs from timeline A only by the addition of the time traveller (and all that causally follows).

On the original timeline (A):
1980(A) - Bob(A) is born
1990(A) - Bob(A) has the inspirational idea for time travel technology
2024(A) - Bob(A) builds his time machine and travels to 1990
2025(A) onwards - the world continues on its course of the original timeline (A)

On the second timeline (B):
1990(B) - Bob(B) arrives in his time machine. This timeline differs from the original timeline only due to the fact that it now includes a time traveller, Bob(B). Bob(B) murders Bob(A) and then lives out the rest of his life on the second timeline.
1991(B) onwards - the world continues on its course of the second timeline (B)
2008(B) - Bob(B) meets his unforunate demise in a car accident

However far-fetched this may seem, it does not violate causality and leads to no apparent contradictions.

Maintaining the assumption that there can only be one timeline leads to the contradictions and violations of the laws of causality. If one dispenses with this assumption, then time travel is logical and causal.

The original timeline is required because the time machine needs to be invented in 2024 before Bob can appear in 1990. There must be a first, original 1990 in which no time travel event takes place before Bob can time travel back to 1990 (from 2024).

I knew the negation of the statement was clearer, and it gave the ground for the truth, which entailed the falsity of your statement. — Corvus

Absolute nonsense. Goodbye.

It is not true, because its negation is true. — Corvus

You said it wasn't clear what the statement or its terms mean. How do you know it's not true if you don't know what it means?

So, you statement is made up with the terms which doesn't have clear meanings. Therefore your statement is not true, and the negation of the statement is true. — Corvus

Non sequitur. If it's an unclear statement, how does it follow that it's not true?

How is that? — Corvus

Very poor. You've offered zero support for your assertion that time travel "is an impossibility from the reality of 2024".

In what sense is it possible, or under what ground is it possible? — Corvus

I already answered that. It's possible that someone could invent the technology for time travel.

You said it was impossible and that my premise was false. The onus is on you to explain why it's impossible.

Your premise "If I travel to 1776" is an impossibility from the reality of 2024, and therefore it is false. — Corvus

Why do you say it's impossible? It is possible that someone will invent the technology for time travel in 2024.

Someone has to wake him up from the grave, and reinstate him as the professor of the university, and make the universe as it was in 1776. — Corvus

There is no need to "make the universe as it was in 1776" or to raise Kant from the dead. If I travel to 1776, then that was a time when Kant was alive.

Your argument is like saying that in order to travel to Greece, you need to recreate Greece at your current location. What really happens is that you travel to Greece, you don't bring Greece to you at your current location. Similiarly, with time travel, you travel to another time, you don't bring the other time to you. Therefore, there is no need to "recreate" the other time in our time, or to "make the universe as it was in 1776" in our time in 2024. You are travelling to another time, not recreating it here and now. Kant was alive in 1776 so if you were to travel to 1776 then it would not be possible to raise Kant from the dead, because he would not be dead.

Well, Kant has been dead for over 200 years. How else could you meet him, if you are going back to his time. Someone has to wake him up from the grave, and reinstate him as the professor of the university, and make the universe as it was in 1776. :nerd: — Corvus

Kant is alive in his time. I'd be going back to his time.

The whole point of time travel is about going to the place at the time of the past or future with the historical or futuristic people in real flesh in the reality at the time. — Corvus

I agree. I don't understand this argument:

It is just physical, metaphysical, logical and QM impossibility to wake up all the deads from the graves, and rebuild all the castles which had been demolished, and reinstate all the past monarchies and governments into the power .... etc. — Corvus

Time travel has nothing to do with waking the dead or rebuilding demolished castles.

When you say "I am here." in the each different locations, you are not saying anything about the locations themselves, but you are stating that YOU are in a location. — Corvus

Likewise when I say "I am at the present time" or when you talk about the present: you are not specifying anything about the times themselves, but you are stating that YOU (or we) are at a particular time.

And no matter how far back or forward, you imagine to have gone to, it would be always the present, because everything happens in present. You cannot escape from it. — Corvus

Right, I agree. But how does this help your argument against time travel? I'm happy to refer to earlier and later times instead of past and future times if you'd prefer.

Another problem with time travel would be, that you might have gone to the past or future, but the rest of the universe will still stay at the present. There is no point of you going back to 100 years back, if the rest of the universe stays at the present. — Corvus

Why not? That's what time travel is.

It is just physical, metaphysical, logical and QM impossibility to wake up all the deads from the graves, and rebuild all the castles which had been demolished, and reinstate all the past monarchies and governments into the power .... etc. — Corvus

This is not what time travel is. A time traveller does not bring (objects and events from) other times into our present time. Rather, a time traveller leaves our present time to arrive at other times.

If one looks at a kind of causation chain that has taken one from birth to the present, at each temporal step a host of causes converges to form the next step, not an easy thing to grasp. Stanislaw Lem had an Ergodic theory of history in which going back in time and performing an act wouldn't necessarily cause a radically different present. The fact that so many aspects of causation go into effect for a moment might mean that they "average out" and any one might have very little effect compared to the others.

The Grandfather paradox might not be completely binding. Give it some thought. — jgill

On my view, there is simply no paradox. The timeline branches off into two parallel timelines once time travel first occurs. The Grandfather "paradox", supposedly, is one of contradiction.

The Wikipedia article on time travel states:

If one were able to go back in time, inconsistencies and contradictions would ensue if the time traveler were to change anything; there is a contradiction if the past becomes different from the way it is. The paradox is commonly described with a person who travels to the past and kills their own grandfather, prevents the existence of their father or mother, and therefore their own existence.

The SEP article on time travel states:

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible.

Let's say Bob travels back in time to 1990 with a plan to kill his younger self. When he arrives, there will be two versions of Bob in 1990: young Bob who is not a time-traveller and old Bob who is a time-traveller. According to the Grandfather paradox, if Bob were to kill his younger self, then he would no longer exist. But why? There were two version of Bob in 1990 and only one of them was killed. Why would older, time-traveller Bob suddenly vanish into thin air?

I understand that young Bob can now no longer grow up to build a time machine, but so what? He already did that. And now he continues to live out his existence as old Bob from 1990 onwards because that is the logical sequence of events following the time travel event. The time travel event creates a second timeline (from 1990 onwards) in which young Bob no longer goes on to build a time machine, but on the first timeline he did build a time machine and went back and killed young Bob.

According to what I've read, there cannot even be two Bobs in 1990 because that causes a contradiction (e.g. Bob is in two places at once). But of course there must be two Bobs in 1990 if Bob is to time travel at all. They're not both the same version of Bob obviously (one is a time traveller), so it seems wrong to say that there is any contradiction.

There are geographical places such as the countries, cities and towns, not the past or future. You cannot escape the present. It is a universal law, which the whole universe and its contents must abide by. — Corvus

I could say that I am "here" at my current location (or "geographical place"), whereas earlier I was at a different location and later I might be at a location different to both of these. While I am at each location, I can sensibly say that I am "here" at each location. This is no different to being at the "present" at different times. I always find myself "here" no matter the place and at the "present" no matter the time. Although you can neither escape "here" nor "the present", this does not entail that you cannot depart from or arrive at different places or times.

It seems the whole imagination has been based on the wrong assumption that the past and future are some sort of geographical destinations such as Tokyo, NY, Paris ... etc, which is not.

Another wrong assumption is that time is some type of physical distance laid out like a road or highway.

The reality is that time is an illusion, and there are only Durations (already proved and declared by Newton), and the past and future are concepts, not geographical places you can arrive at or depart from. — Corvus

What makes these assumptions wrong? What argument supports your assertion that times are not "geographical places you can arrive at or depart from"?

You cannot travel into a place where the destination doesn't exist. We are all nailed into the present until deaths under the universal law. — Corvus

Whatever time I am at is the present time for me. Therefore, if I were to build a time machine and use it to travel back 100 years, then it would be the present time for me when I depart today and still the present time for me when I arrive 100 years ago.

Why would you assume that? — flannel jesus

It's based on what I've read of time travel paradoxes such as the Grandfather paradox.

Why would you assume that? That's very abstract. How about something more simple: it's impossible, as far as we can tell, because there's no known physical phenomena that could allow us to do it. — flannel jesus

Isn't that just saying that we don't have the technology or knowledge to time travel? This doesn't mean that it is hypothetically impossible.

Impossibility of time travel seems to be one of the universally necessary truth. — Corvus

Let's suppose I build a time machine and use it to travel to a time before my birth. What makes it impossible? I take it your view is based on the immutability of a single timeline, but that's never been proven and I can find no good reason why it must be assumed.

If one believed in the multiverse which runs on different times, would it be then imaginable in one them? — Corvus

It has been imagined in numerous works of fiction, so it does not appear to be unimaginable.

But multiverse itself is a theoretical hypothetical idea, which is not proven to be existence yet. — Corvus

The question of the OP is whether time travel is hypothetically possible. I don't see why not.

According to trusty Wikipedia:

Time travel is the hypothetical activity of traveling into the past or future. Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction. In fiction, time travel is typically achieved through the use of a hypothetical device known as a time machine.

The idea of time travel is more easily understood in terms of time travel to a past or earlier time than the present time. Let's say that a time machine was first invented at the present time, tn, and is to be used to return to an earlier time, te. This means - prior to any use of the newly invented time machine - that there is an "original" version of time, te, which is pre-time-travel (and contains no time traveller). Once the time machine is used, there will then be a second version of that same time, te, which is post-time-travel and contains the addition of the time traveller. That is, the use of the time machine and the introduction of time travel means that a second timeline branches off and exists concurrently with (or as a parallel universe to) the first "original" timeline. This is required in order for the concept of time travel to make sense. If you want to use the time machine to go back and kill Hitler, then there needs to be an original timeline in which Hitler exists and will do, is doing or has done his evil deeds before anyone can go back in time to kill him.

The same holds for time travel to a future time, assuming that the events of the "original" future timeline would have played out a particular way prior to any time travel.

I don't really understand the argument that you must recreate the past exactly as it already did occur or that you cannot logically go back in time to kill your grandfather. These assume that there can only be one timeline, original or otherwise. But I don't think that assumption is consistent with the logical concept of time travel.

Again, I'm referring to "affectation" as defined by Merriam-Webster online as I said in the OP:

Affectation" according to the Merriam-Webster Dictionary Online, is:

"a. Speech or conduct not natural to oneself: an unnatural form of behavior meant especially to impress others; b. the act of taking on or displaying an attitude not natural to oneself or not genuinely felt."

I wouldn't consider it "natural to myself" to believe that someone across the street from me is 5 inches tall, but would consider it "natural to myself" to by surprised by, and to dispute, someone who did believe that. — Ciceronianus

What isn't natural for you to believe might be natural for someone else to believe (and vice versa). Again, by what criteria do you judge whether some belief or assumption or philosophy is an affectation? Is it simply when others are lying about their beliefs?

C: Look, there's Sulla across the street
X: I had no idea he's only 5 inches tall.
C: What the hell are you talking about?
X: Well, look at him. Look at my finger. He's only slightly bigger than it.
C: Are you serious?
X: Oh my God, he's growing!
C: He's just crossing the street towards us.
X: How do you know he's not growing? He looked small, now he looks bigger. If you're right, then we can't trust our own sense of sight.
C: Do you actually think he's growing?
X: Well, he might be. He might not. Why do you think differently? What's wrong with you? You're the crazy one. — Ciceronianus

I don't see how this addresses my previous post.

Your OP question presupposes that some philosophy (or philosophising) is affectation while other philosophy (or philosophising) isn't. I am questioning this presupposition. As I alluded to in my last post, I don't see what criteria you use to judge that some philosophy (or questions or assumptions) is or is not affectation.

Your exchange above appears to suppose that X's comments are affectation without explaining why they are affectation. Is it because they are counterintuitive or controvert common sense? Is the critieria for affectation any philosophising (or any questions or assumptions) which is abnormal or contrary to common sense or to accepted wisdom? If so, then should philosophy (and science, too, I suppose) be restricted only to questions or assumptions that remain within the bounds of currently accepted wisdom and common sense; only to what is currently accepted or understood to be true? That wouldn't seem to leave much ground for any new ideas.

I don't understand the part after the comma. Are you saying: Proposing that certain views are affectations...validates what we do all the time? — Luke


It's a play off of the definition of "affectation" appearing at the beginning of the thread. If I criticize the view that we cannot know what the "external world" is, or whether it is, as an affectation I'm claiming that view is unnatural because we act as if it is and know what it is all the time. So, the claim it is an affectation isn't unnatural or aberrant, because it reaffirms that we act as if the external world exists and that we know what it is. — Ciceronianus

Thanks, I see what you're saying now.

However, although I agree that we naturally act without any doubt about what the "external world" is or whether it is, and although I might agree that it is unnatural to question such things, I would also question how natural it is to criticise those who question such things. It is one thing to act without question regarding things such as the existence of the "external world", but another thing to engage those who do question its existence. I think there is a clear distinction between acting without doubt about such things and engaging those who do doubt such things, and I question whether engaging the doubters is as much an affectation as is the doubting itself.

And, if not all of philosophy (or all philosophising) is an affectation, then by what criteria do you determine which parts of philosophy are affectation and which parts are not?

Does the present discussion meet its own criteria? Is it only those philosophical discussions that are anti-philosophical which are relatively free of affectation?
— Luke

It would seem to me that proposing that certain views are affectations isn't itself an affectation, as it would be to validate what we do all the time. — Ciceronianus

I don't understand the part after the comma. Are you saying: Proposing that certain views are affectations...validates what we do all the time?

Any philosophical discussion which doesn't require us to disregard or consider of no real value how we live in determining the nature of what we interact with in the course of living will, in all likelihood, be relatively free of affectation.
— Ciceronianus

I'm wondering whether there is any such philosophical discussion. Can you give an example of the topic of such a discussion?
— Luke

Pragmatism? — Tom Storm

Pragmatism is a broad topic, so I doubt that all philosophical discussions involving Pragmatism meet Ciceronianus' criteria for avoiding affectation.

I take it that "how we live" includes the differing values, worldviews and/or philosophical positions of each of us, rather than assuming some universal common sense view. Further, that we each have the opportunity to consider and reflect on positions that may differ from our own or that we had never previously considered, as well as to question the views we hold at any particular time.

Does the present discussion meet its own criteria? Is it only those philosophical discussions that are anti-philosophical which are relatively free of affectation?

Any philosophical discussion which doesn't require us to disregard or consider of no real value how we live in determining the nature of what we interact with in the course of living will, in all likelihood, be relatively free of affectation. — Ciceronianus

I'm wondering whether there is any such philosophical discussion. Can you give an example of the topic of such a discussion?

I don't mean to claim all philosophy is affectation. — Ciceronianus

What (or whose or what topics in) philosophy is not affectation, in your view?