According to trusty Wikipedia: — Luke

Trusty yes.

I generally agree. Time travel exists, but only to the future, never the past (since, as stated) there is no "past" to travel to.

If you insist that you can travel into the past or future in your imagination — Corvus

I never said anything about imagining. The comment to which you are replying was a reference to your pressumption of presentism. LuckyR seems to presume it as well:

Time travel exists, but only to the future, never the past (since, as stated) there is no "past" to travel to. — LuckyR

I personally don't insist that there is no "past' to travel to. I give equal ontology to all of spacetime, not just one 3D slice of it. Reverse time travel (as typically envisioned) is not possible because it would constitute transfer of information outside of somebody's future light cone, something which relativity forbids, and something which has never been demonstrated .

Strictly speaking there is no tomorrow in reality.
...
There is only "Now" for the whole universe and its members.

Your opinion, not mine. "Tomorrow" is a relative reference, sort of like (one km to the east). There is no objective location that is 'one km to the east', but relative to any given reference location in a place where 'east' is meaningful, there is.

What you call tomorrow is in your imagination as a concept or idea.

I am not speaking as an idealist when I made the comment. To me, 'tomorrow' is just as real as 'one km east of here'. All of Einsteins theories presume the same, but it is admittedly a presumption. There are alternatives to his theories that don't make this presumption, but they came almost a century later.

It's fine to presume such things, but a topic about time travel seems to require that the 'destination' exists in order for you do deny your ability to get there. You can't argue that it can't be done because only the present exists, because there's no way to prove that opinion.

:ok:

This short video summaries my points on time travel.

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).

Thanks for posting the video. — Luke

You are welcome.

However, I will argue, there must be two (or more) parallel timelines in order for time travel to make sense. — Luke

Sure. If you say, you are allowing the parallel timelines running in possible worlds, then the argument becomes logically tenable. But one might still demand to prove the existence of the parallel time lines, before progressing further.

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

It doesn't lead to apparent contradictions, but it doesn't make it true claims either. :)

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.

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. — Luke

From my own perspective, time doesn't exist. It is a mental concept. There is only "Now", no past and no future. What you call the past is your memory of the past now, and what you call future is your imagination.

Therefore I don't need to prove a single timeline or indeed anything at all. I would say, there is no such a thing as time. If you don't agree, prove that time exist, prove the past and future exist. If you cannot prove them, then all your points were just imagination. This would be my points to you. :D

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?

This seems inconsistent with the video you posted which describes time as “a measurement of the progression of events”. — Luke

Of course there are changes in the physical world and bodies. But that is not time. Time is measured quantities. There are durations and intervals, which is different from time. The claim that time is a mental concept, doesn't mean there is no physical and bodily changes.

Time is a conceptual product which is a mental product from observing the changes and intervals in the physical world such as the rising Sun every morning, the movement of stars, change of the seasons ... etc etc. It is not some physical entity.

You say year 2024 1990 ... but this is just some contingent contract of the human civilisations. It doesn't exist in the real world. It could be year 0 tomorrow if we all agreed.

Year AD1 was the year Christ was born. Do you believe in Christ's life? If not, then Year AD1 could have been any number. No one knows what year it should have been. We have no such thing as Time. It is an illusion.

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.

Your concluding statements would only be true, if and only if you proved the "If~" parts as Truths which complies with the objective facts. So far, I don't see any evidence or proof for your "If" statements having been proved either True or False, therefore they remain as groundless conjectures.

You say year 2024 1990 ... but this is just some contingent contract of the human civilisations. It doesn't exist in the real world. It could be year 0 tomorrow if we all agreed. — Corvus

This is true, and a serious problem for discussions of time as currently being explored.

But this doesn't mean 'time doesn't exist'. It means are symbols for it are arbitrary. I'm not trying to say it does or doesn't exist - just that this doesn't go to that question i don't think.

It may be that it's actually the year 14,564,335,235 AT (all time).

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

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

1. Time is not a physical object that can be moved or manipulated. It's simply a measurement of the progression of events.

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 alone and just the traveler going somewhere.
In non-presentist terms, it would require a discontinuous, or non-timelike worldline. Well, the worldline is just an abstraction, so it being discontinuous is not in itself a problem Several people have proposed valid methods to do (forward) time travel utilizing discontinuous worldlines.
Anyway, I have a problem with number 1.

2. The laws of physics, including the laws of thermodynamics, make it impossible to go back in time.

Not at all. But it presumes a self-contradictory version of dual-presentism, that the universe causality is made to go backwards (less entropy) but real time continues to go forwards.
Fact is, my abrupt appearance in 1955 would not violate entropy laws at all, nor would it violate thermodynamic law. It makes a hash of causality, but that's not brought up in this point.

3. The idea of travelling back in time would violate the laws of causality, meaning that an effect cannot occur before its cause.

This one has teeth, but is worded wrong. 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. It isn't under presentism of course, so that assumption yet again.
Anyway, yes, backwards (not forwards) time travel would violate causality laws, if they're valid laws.

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.

Closed time loops are allowed under relativity, but like several other things, that doesn't mean there are any at a classical scale. Time travel isn't itself paradoxical.
Also, what did grandfather ever do to deserve this abuse? If you want to illustrate the paradox, go back 5 seconds and kill yourself, or otherwise disable the machine, which would probably happen anyway with a 2nd machine materializing right in the same place.

5. Even if time travel were possible, it would require immense energy and advanced technology beyond our current capabilities.[/quote]This is nonsense. 'If impossible thing, then [arbitrary unfalsifiable conclusion]'. The energy requirements are meaningless unless a method to do it is proposed.

Point 2 says that time travel is impossible due to the laws of physics

Mostly point 3 that actually says that, seemingly the only point that isn't straight up unbacked conjecture.

Late in 2024, Bob enters his time machine for the first time and sets course for the year 1990.

A nit: He has to set his course for an event, which has 4 coordinates, not just one. Pretty much all the fiction (except xkcd) seems to forget that. Everything moves, but it is always assumed that the machine will reappear at the same map-location as it left despite the motion of stars, planets, etc. OK, Dr Who doesn't work that way. It's a car, and it travels in space as much as time.

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).

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.

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)

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

On the second timeline (B):
1990(B) - Bob(B) arrives in his time machine.
...
However far-fetched this may seem, it does not violate causality and leads to no apparent contradictions.

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.

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).

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?

To inject myself here, I'd say Kant IS alive in 1776.

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

But this doesn't mean 'time doesn't exist'. It means are symbols for it are arbitrary. I'm not trying to say it does or doesn't exist - just that this doesn't go to that question i don't think. — AmadeusD

Time exists, but not in the way the would-be time travellers think. :D

It may be that it's actually the year 14,564,335,235 AT (all time). — AmadeusD

If you established your own country or created your own world, then you could run it with that, suppose.

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

They seemed to be the concluding statements from your arguments.

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

No I didn't deny anything about Kant or 1776. My point was that you need to prove your "If" statements are true to the objective facts, to make them into true statements.

Your arguments might be valid, sound and not contradictory in modal logical forms, but that doesn't mean that they are true in the actual world. Bear in mind that, whatever happens, they all happen in the actual world. Before the proofs, they just remain as your conjectures and another hypothesises, which are not adding much more to the OP question.

Time exists, but not in the way the would-be time travellers think. :D — Corvus

Fair enough.

If you established your own country or created your own world, then you could run it with that, suppose. — Corvus

My point was more that if we counted from the Big Bang time might have some relevance beyond social time-keeping. If the year we're using is based on the very first change that ever occurred, its much more palatable, I think, to take it as 'something'.

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

Only if it is claimed that they necessarily must be. We're assuming them here to see if it makes time travel possible. It doesn't, but it does remove some of the issues and paradoxes.

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

A physical clock measures something. Hard to deny the existence of something that can be measured.
You seem to get around this by defining time differently than, well than how physics defines it, which boils down to 'what a clock measures'. I agree that the coordinates we assign to time is pure abstraction.

I just transcribed most of the very short video. — Luke

Saved me from typing it. Most of the thanks was for that.

— Luke

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. — Luke

How about a growing block model then? The past exists. You can go to it, but since it is 'the past', you cannot change it. So a new branch is created (MWI style, but with physics violations), very much like your Bob story. I think that would satisfy both of us. The video presumes (I think) one would have to recreate the entire past state of the universe, hence the excessive energy required.

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

Wasn't wasted. 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.

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.

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). — Luke

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.

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. That cannot produce an old-Bob.
Either that or he didn't actually go to 1990, but simply rearranged the entire state of the 2024 universe to correspond to what it looked like in 1990, which seemingly is what the video envisioned.

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?

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.

My point was more that if we counted from the Big Bang time might have some relevance beyond social time-keeping. If the year we're using is based on the very first change that ever occurred, its much more palatable, I think, to take it as 'something'. — AmadeusD

I heard that Japanese folks have their own time system counting from their monarch's coronation days. So they run their king's name and year 25 or 30 whatever, as their alternative year counting.

Not sure if there was anyone witnessing the Big Bang, and recorded the time and how it went through. Unless that is the case, many people would still take it as another mysticism.

In Chinese Lunar Calendar, it is still 2023 November or December at the moment. Their new year day would be sometime in February I think. The main point is, time is a civil contract.

Only if it is claimed that they necessarily must be. We're assuming them here to see if it makes time travel possible. It doesn't, but it does remove some of the issues and paradoxes. — noAxioms

Sure, I am not saying it is not allowed to have conjectures and hypothesis on time travel. My point was the claim that "If X, Y, Z, then time travel is possible." remains as a hypothesis until X, Y, Z had been proved as truths which complies to the objective facts in the actual world.

A physical clock measures something. Hard to deny the existence of something that can be measured.
You seem to get around this by defining time differently than, well than how physics defines it, which boils down to 'what a clock measures'. I agree that the coordinates we assign to time is pure abstraction. — noAxioms

I am not sure what the physical clock measures. But if it did, and if it is not something which is non conceptual time, then I would imagine it couldn't be time itself at all. It must have measured some particles going through changes into some other entities similar to the Nuclear fission process, which is the duration of the process. Would it be time itself? I believe not.

What precedes old Bob's appearance in 1990 is the use of the time machine in 2024 — Luke

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.

Not sure if there was anyone witnessing the Big Bang — Corvus

We did. It's not like it happened a finite distance away and the view of the bang has already passed us by. Of course the really early events are obscured by the opaque conditions back then. The window through which we look took a third of a million years or so to turn transparent. By that measure, nothing could 'see' the big bang since it was all obscured behind a blanket until then.

Sure, I am not saying it is not allowed to have conjectures and hypothesis on time travel. My point was the claim that "If X, Y, Z, then time travel is possible." remains as a hypothesis until X, Y, Z had been proved as truths which complies to the objective facts in the actual world. — Corvus

Nothing ever gets proved. I can go to grandma's house if I have a car, and the weather is acceptable, and if I draw breath. But technically I cannot prove any of those.
Point is, requiring 'proof' is going to far. Evidence of X,Y Z is probably enough for plausible time travel. Right now, that evidence is very negative.

I am not sure what the physical clock measures. — Corvus

It measures proper time, which is very defined in both interpretations of time. It doesn't measure the advancement of the present, or the rate of the flow of time. That sort of time is more abstract, and there is no empirical way to detect it, let alone measure it. So maybe we're talking past each other when I reference the sort of time that clocks measure, vs you referencing the latter.

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.

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? — Luke

No I can't. You won't let me discuss interpretations at all. You said you're creating a new world, not altering the original, in effort to avoid the paradox. That means an act of creation of a new world.
You said that 2024 is the antecedent state, so that means the alternate (copy) 1990 state was created at that time. It's all I have to work with. I see why the video says it needs a lot of energy.

The original 1990 doesn't exist anymore. You can't travel to somewhere that doesn't exist. You have to create it, a copy of it. I'm running with that assumption when trying to understand what you're suggesting.

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

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. — noAxioms

I would think, yes, but I think there's more to it. On this view, that 'copy' only began in 1990. It could be reversed to 1990, but no earlier. The original time-line could be reversed to Bob's birth. I'm sure this means something, but I can't grasp what. But, it certainly seems, prima facie, that the time machine in fact replicates the chosen moment (but it is 'real' as such) and then that moment runs forward as-if it were an alternate time-line.

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?