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:If you insist that you can travel into the past or future in your imagination — Corvus
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 .Time travel exists, but only to the future, never the past (since, as stated) there is no "past" to travel to. — LuckyR
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.Strictly speaking there is no tomorrow in reality.
...
There is only "Now" for the whole universe and its members.
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.What you call tomorrow is in your imagination as a concept or idea.
You are welcome.Thanks for posting the video. — 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, I will argue, there must be two (or more) parallel timelines in order for time travel to make sense. — Luke
It doesn't lead to apparent contradictions, but it doesn't make it true claims either. :)However far-fetched this may seem, it does not violate causality and leads to no apparent contradictions. — Luke
But one might still demand to prove the existence of the parallel time lines, before progressing further. — Corvus
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.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. — Corvus
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.This seems inconsistent with the video you posted which describes time as “a measurement of the progression of events”. — Luke
If I travel to 1776, then that was a time when Kant was alive. — 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.It's possible that someone could invent the technology for time travel. — Luke
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
Great summary, thanks. All packed into less than a minute to boot.The problems associated with time travel cited in the video are as follows — Luke
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.1. Time is not a physical object that can be moved or manipulated. It's simply a measurement of the progression of events.
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.2. The laws of physics, including the laws of thermodynamics, make it impossible to go back in time.
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.3. The idea of travelling back in time would violate the laws of causality, meaning that an effect cannot occur before its cause.
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.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.
Mostly point 3 that actually says that, seemingly the only point that isn't straight up unbacked conjecture.Point 2 says that time travel is impossible due to the laws of physics
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.Late in 2024, Bob enters his time machine for the first time and sets course for the year 1990.
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.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).
And apparently Bob fails in his effort to destroy the bad thing resulting from his technology.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)
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.On the second timeline (B):
1990(B) - Bob(B) arrives in his time machine.
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However far-fetched this may seem, it does not violate causality and leads to no apparent contradictions.
Great summary, thanks. All packed into less than a minute to boot. — noAxioms
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
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
Time travel isn't itself paradoxical. — noAxioms
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
And apparently Bob fails in his effort to destroy the bad thing resulting from his technology. — noAxioms
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
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
Time exists, but not in the way the would-be time travellers think. :DBut 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
If you established your own country or created your own world, then you could run it with that, suppose.It may be that it's actually the year 14,564,335,235 AT (all time). — AmadeusD
They seemed to be the concluding statements from your arguments.Only one of these two statements is "in "if" form". — 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.Anyhow, do you deny that Kant was alive in 1776? — Luke
Time exists, but not in the way the would-be time travellers think. :D — Corvus
If you established your own country or created your own world, then you could run it with that, suppose. — 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.But one might still demand to prove the existence of the parallel time lines, before progressing further. — Corvus
A physical clock measures something. Hard to deny the existence of something that can be measured.From my own perspective, time doesn't exist. It is a mental concept. — Corvus
Saved me from typing it. Most of the thanks was for that.I just transcribed most of the very short video. — Luke
— 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.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
Wasn't wasted. Your Bob example showed how that paradox can be easily avoided.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
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.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
No, the antecedent state would be 1990 minus 1 second. That cannot produce an old-Bob.The antecedent state would be old Bob's time machine transporting him from 2024 to 1990 — Luke
How about a growing block model then? — noAxioms
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
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
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
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
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
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
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.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
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.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
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.What precedes old Bob's appearance in 1990 is the use of the time machine in 2024 — Luke
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.Not sure if there was anyone witnessing the Big Bang — 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.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
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.I am not sure what the physical clock measures. — Corvus
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
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.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
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
You said you're creating a new world, not altering the original, in effort to avoid the paradox. — noAxioms
You said that 2024 is the antecedent state, so that means the alternate (copy) 1990 state was created at that time. — noAxioms
The original 1990 doesn't exist anymore. — noAxioms
You can't travel to somewhere that doesn't exist. — noAxioms
There's no contradictions with it because killing the copy young-Bob isn't killing old-Bob's actual ancestor. — noAxioms
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