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Old 16th May 2022, 07:07 PM   #801
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Hang on a moment. The point of the Tipler cylinder is that it creates closed timelike loop in spacetime. The goal is the loop. Tipler just came up with particular arrangement of mass and motion that satisfies the requirement.

That doesn't necessarily mean that it's the only way to bend spacetime all the way around on itself like that. Tipler tells us such bending is permitted by the theory. It's an open question if there are other mass-energy shenanigans that could be arranged to make it happen.
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Old 16th May 2022, 07:12 PM   #802
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Originally Posted by RecoveringYuppy View Post
Why is it we can rule out infinite length? I see how we can rule it out for a time travel mechanism for our own personal use. But don't see that we can rule out such a region of spacetime existing somewhere.
We can rule out infinite length because we're not talking about just very very long, we're talking about infinite. Things of infinite length cannot be constructed because they will never be completed. You will literally never get to the end of it. That's what "infinite" means. Infinite objects cannot exist in the universe.
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Old 16th May 2022, 07:12 PM   #803
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Originally Posted by Hellbound View Post
Then I misunderstood your previous objection; you seemed to be asking why such a space time region couldn’t exist without the physical cylinder. It can’t; it’s a consequence of the gravitational force and rotation of that cylinder.

And such a cylinder is a physical impossibility; it can’t exist. Naturally occurring or not. That was the point I was trying to make; there’s no possibility, within the bounds of we currently know, of a Tipler cylinder existing in this universe. And if we’re going to posit unknown physics, then by that argument anything is possible.
My question was:
Quote:
Why is it we can rule out infinite length? I see how we can rule it out for a time travel mechanism for our own personal use. But don't see that we can rule out such a region of spacetime existing somewhere.
And the Tipler Cylinder is allowed (but not demanded) by known physics.

And BTW I think I can agree it's ruled out in the observable universe but not in the universe.
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Old 16th May 2022, 07:39 PM   #804
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Originally Posted by theprestige View Post
Hang on a moment. The point of the Tipler cylinder is that it creates closed timelike loop in spacetime. The goal is the loop. Tipler just came up with particular arrangement of mass and motion that satisfies the requirement.

That doesn't necessarily mean that it's the only way to bend spacetime all the way around on itself like that. Tipler tells us such bending is permitted by the theory. It's an open question if there are other mass-energy shenanigans that could be arranged to make it happen.
Sure. The Tipler Cylinder (afaik) is merely the first such solution to be discovered. I'm certain that there are some very smart people looking for others.
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Old 16th May 2022, 08:04 PM   #805
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We know there's still a big gap in our model of reality, covering the mass-energy regime where both quantum effects and gravity dominate. The pre-Big Bang regime, for example.

It's possible that causality violation is trivial in such regimes. It's possible that a Kardashev Type IV civilization could harness enough mass-energy to synthesize such a regime on a small scale, and violate causality in some small way.

Or it's possible the whole thing is a literal pipe dream.

But I don't see any way Thorkil2 can possibly put it to rest once and for all.
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Old 17th May 2022, 11:56 AM   #806
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Let's put the question of time travel to rest once and for all.

Originally Posted by theprestige View Post
Hang on a moment. The point of the Tipler cylinder is that it creates closed timelike loop in spacetime. The goal is the loop. Tipler just came up with particular arrangement of mass and motion that satisfies the requirement.

That doesn't necessarily mean that it's the only way to bend spacetime all the way around on itself like that. Tipler tells us such bending is permitted by the theory. It's an open question if there are other mass-energy shenanigans that could be arranged to make it happen.

I agree, but I am pretty confident in ruling out a Tipler cylinder . I was speaking to that specific case, not of closed timelike loops in general.

I rate them unlikely, though*. So far all of them require physical impossibilities (infinite cylinders or torus singularities) or highly theoretical materials (exotic matter, AKA Cavorite, AKA Unobtanium). Even the wormhole solution requires exotic matter to make it transversable at anything approaching macro scales.

* outside some possible quantum effects at very small scales of space and time

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Old 17th May 2022, 12:44 PM   #807
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I think that causality violation even at the smallest scale would be a game changer for any civilization that figured it out.
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Old 17th May 2022, 03:44 PM   #808
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Originally Posted by theprestige View Post
I think that causality violation even at the smallest scale would be a game changer for any civilization that figured it out.
It depends. I suspect it'll be sort of like non-locality or quantum teleportation; interesting, and definitely an effect, but limited in usability.

I'm open to being wrong, though.
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Old 17th May 2022, 05:35 PM   #809
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Originally Posted by Hellbound View Post
It depends. I suspect it'll be sort of like non-locality or quantum teleportation; interesting, and definitely an effect, but limited in usability.

I'm open to being wrong, though.
A time loop computer would be way, way more useful than a quantum computer. That would require only just macro scale time loops.

The fact that nobody is seriously looking into it suggests that nobody really thinks it is a serious possibility.
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Old 18th May 2022, 04:16 AM   #810
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Originally Posted by arthwollipot View Post
We can rule out infinite length because we're not talking about just very very long, we're talking about infinite. Things of infinite length cannot be constructed because they will never be completed. You will literally never get to the end of it. That's what "infinite" means. Infinite objects cannot exist in the universe.
For all we know, the universe itself is infinite and has been so from its beginning, so the sentence I highlighted has the status of a bare assertion.

Originally Posted by arthwollipot View Post
Sure. The Tipler Cylinder (afaik) is merely the first such solution to be discovered. I'm certain that there are some very smart people looking for others.
Although the Tipler cylinder was discovered before the Gödel universe, the Gödel universe was the first solution discovered to have closed timelike curves, because that property of the Tipler cylinder was discovered decades later. Several other solutions with loops in time are now known.
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Old 18th May 2022, 06:53 AM   #811
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Originally Posted by Hellbound View Post
So far all of them require physical impossibilities (infinite cylinders or torus singularities)
The Tipler Cylinder we're discussing is a long standing solution of General Relativity, the most accurate theory we have on the subject.
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Old 18th May 2022, 01:06 PM   #812
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Page 21, I guess it haven't been put to rest yet?
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Old 18th May 2022, 08:01 PM   #813
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Originally Posted by W.D.Clinger View Post
For all we know, the universe itself is infinite and has been so from its beginning, so the sentence I highlighted has the status of a bare assertion.
Even if it is, it is still impossible to build an infinite object within it.
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Old 18th May 2022, 08:49 PM   #814
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Originally Posted by arthwollipot View Post
Even if it is, it is still impossible to build an infinite object within it.
Is it impossible for one to exist naturally?
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Old 18th May 2022, 08:53 PM   #815
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Originally Posted by RecoveringYuppy View Post
Is it impossible for one to exist naturally?
My gut says no, but if I'm honest I really don't know. Why? Do you think it's likely that a naturally occurring Tipler Cylinder might be out there somewhere?
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Old 18th May 2022, 08:55 PM   #816
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I think it's possible since I have no reason to reject General Relativity.
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Old 18th May 2022, 08:55 PM   #817
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I also want to say this, which is something that just occurred to me.

A Tipler Cylinder also has to rotate. Even if it were possible to build it, you would need to set it in motion. Infinite length means infinite mass means infinite inertia which means infinite energy required to get it moving.
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Old 18th May 2022, 08:58 PM   #818
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Originally Posted by arthwollipot View Post
I also want to say this, which is something that just occurred to me.

A Tipler Cylinder also has to rotate. Even if it were possible to build it, you would need to set it in motion. Infinite length means infinite mass means infinite inertia which means infinite energy required to get it moving.
And why couldn't that exist in the infinite universe that contains it?
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Old 18th May 2022, 08:58 PM   #819
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Originally Posted by RecoveringYuppy View Post
I think it's possible since I have no reason to reject General Relativity.
Just because it doesn't break General Relativity is no reason to think that one might actually exist. My opinion is that the likelihood of a naturally-occurring Tipler Cylinder actually existing is so close to zero as to be negligible.
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Old 18th May 2022, 09:01 PM   #820
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How did you calculate that likelihood?
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Old 18th May 2022, 09:26 PM   #821
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Originally Posted by arthwollipot View Post
My opinion is that the likelihood of a naturally-occurring Tipler Cylinder actually existing is so close to zero as to be negligible.
So close, or is it exactly zero? I think you're reluctant to go out on a limb and say precisely zero because you know this is an allowed solution of GR.

You didn't say much about how you are thinking of this likelihood. Is it, for example, one TC intersects one cubic foot in every 10^14000 cubic megaparsecs? Doesn't really matter how you answer. Just think about it for the next paragraph.

Then recall that we haven't currently ruled out that the universe is infinite. So what non-zero number and answer to the above paragraph do you come up with that leads to anything other than concluding there are an infinite number of Tipler Cylinders in the universe?

Then think about the fact that zero times infinity is an ill defined concept. So even if you go out on a limb and say the likelihood is precisely zero you still haven't actually ruled them out.
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Old 18th May 2022, 10:56 PM   #822
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I understand what you're trying to get at, RecoveringYuppy, but I just think your line of argumentation is absurd.

For a start, there is no evidence that the universe is infinite, and the evidence that we do have strongly suggests that it is not. If the big bang model of cosmology is correct, which it seems to be, then the universe has been expanding for a finite amount of time. Being neither a professional physicist, nor (at this time) a science fiction author, I see no reason to expend my limited brainpower on contemplating what it might be like if the universe were infinite.

You are however correct when you say that a Tipler Cylinder can only exist in an infinite universe.
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Old 18th May 2022, 11:13 PM   #823
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Originally Posted by arthwollipot View Post
I understand what you're trying to get at, RecoveringYuppy, but I just think your line of argumentation is absurd.
Do you have a good reason for that or is it an argument from incredulity?

Originally Posted by arthwollipot View Post
For a start, there is no evidence that the universe is infinite, and the evidence that we do have strongly suggests that it is not.
Citation? As far as I know there are only lower bounds to the size of the universe. Infinite in extent is still on the table.

https://en.wikipedia.org/wiki/Universe#Size_and_regions

Originally Posted by arthwollipot View Post
If the big bang model of cosmology is correct, which it seems to be, then the universe has been expanding for a finite amount of time. Being neither a professional physicist, nor (at this time) a science fiction author, I see no reason to expend my limited brainpower on contemplating what it might be like if the universe were infinite.
That's time, not extent. Our theories can't solve for the size. When combined with observational evidence infinite extent is not ruled out.
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Old 18th May 2022, 11:28 PM   #824
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Okay, okay, fine, whatever. A naturally occurring Tipler Cylinder might exist. I grant your premise. What next? What follows from that?
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Old 18th May 2022, 11:37 PM   #825
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That's it. I think that Tipler Cylinders being possible and the universe possibly being infinite are accurate statements of our current understanding. That's enough.
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Old 19th May 2022, 12:07 AM   #826
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Originally Posted by RecoveringYuppy View Post
That's it. I think that Tipler Cylinders being possible and the universe possibly being infinite are accurate statements of our current understanding. That's enough.
Uh... okay then. I still think it's overwhelmingly unlikely that naturally occurring Tipler Cylinders exist. I'm prepared to say that I am 100% sure that they don't. It's such a massively unlikely configuration of matter that I think you'd have to wait trillions of times the age of the universe for it to spontaneously occur. And this is an opinion, which does not require a citation. The only way I can be proved wrong is by humans actually discovering a naturally occurring Tipler Cylinder in space and if that happens I will be utterly gobsmacked. After retrieving my jaw from the floor I will come here and apologise.

Until then I will spend no more brainpower on contemplating whether naturally occurring Tipler Cylinders exist.
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Old 19th May 2022, 02:25 AM   #827
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But we are not talking about putting the idea of Tipler Cylinders to rest.
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Old 19th May 2022, 05:40 AM   #828
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Originally Posted by arthwollipot View Post
I still think it's overwhelmingly unlikely that naturally occurring Tipler Cylinders exist.
Most astrophysicists share that opinion.

Originally Posted by arthwollipot View Post
For a start, there is no evidence that the universe is infinite, and the evidence that we do have strongly suggests that it is not.
That is not an accurate summary of the evidence. The best evidence we have suggests the universe is either infinite or amazingly large (much larger than the observable universe).

Originally Posted by arthwollipot View Post
If the big bang model of cosmology is correct, which it seems to be, then the universe has been expanding for a finite amount of time.
Popular accounts of the Big Bang are often written as though the universe was unimaginably small at the Big Bang and then expanded, but that is not an accurate account of Big Bang cosmology.

The FLRW models of a Big Bang universe that are most compatible with astronomical observations are of two kinds: (1) the universe is infinite and has always been so, or (2) the universe is spatially closed and has always been so.

Originally Posted by arthwollipot View Post
Being neither a professional physicist, nor (at this time) a science fiction author, I see no reason to expend my limited brainpower on contemplating what it might be like if the universe were infinite.
Paraphrasing that argument, arthwollipot thinks the universe is likely to be finite because arthwollipot doesn't want to contemplate what an infinite universe would be like.

One of the less obvious problems with that argument is that most people find the idea of a curved or flat but spatially closed finite universe even harder to contemplate than an infinite universe.

As explained by Paul M Sutter:
Quote:
Cosmologists aren't sure if the universe is infinitely big or just extremely large. To measure the universe, astronomers instead look at its curvature. The geometric curve on large scales of the universe tells us about its overall shape. If the universe is perfectly geometrically flat, then it can be infinite. If it's curved, like Earth's surface, then it has finite volume.

Current observations and measurements of the curvature of the universe indicate that it is almost perfectly flat. You might think this means the universe is infinite. But it's not that simple. Even in the case of a flat universe, the cosmos doesn't have to be infinitely big. Take, for example, the surface of a cylinder. It is geometrically flat, because parallel lines drawn on the surface remain parallel (that's one of the definitions of "flatness"), and yet it has a finite size. The same could be true of the universe: It could be completely flat yet closed in on itself.
In the recent context of this thread, it's somewhat ironic that a finite universe seems to imply a cylindrical universe or something even stranger (such as a (flat) toroidal universe, or a (curved) hyperspherical universe in which, upon setting out in any direction and following a geodesic, you would eventually end up where you started).
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Old 19th May 2022, 07:58 AM   #829
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Originally Posted by arthwollipot View Post
Uh... okay then. I still think it's overwhelmingly unlikely that naturally occurring Tipler Cylinders exist. I'm prepared to say that I am 100% sure that they don't. It's such a massively unlikely configuration of matter that I think you'd have to wait trillions of times the age of the universe for it to spontaneously occur.
Fine, it's your opinion and you don't have to change it but my point is simply that phrases like "massively unlikely" don't lead to zero occurrences in an infinite universe.

Also I'm not sure that thinking of a natural Tipler Cylinder as "forming" frames the potential issue well. It could exist from the beginning.
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Old 19th May 2022, 07:24 PM   #830
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First, thank you, W. D. Clinger, for correcting a couple of misunderstandings I had. I know I can always rely on your knowledge, and I readily admit that I sometimes think I know more than I do. It's always a good day when I learn something new.

Originally Posted by RecoveringYuppy View Post
Fine, it's your opinion and you don't have to change it but my point is simply that phrases like "massively unlikely" don't lead to zero occurrences in an infinite universe.
I feel the same way about not being able to prove that God doesn't exist. I think it's so unlikely that I feel comfortable saying that it's zero for the purposes of normal discourse while acknowledging that technically, I can't absolutely rule it out.

Originally Posted by RecoveringYuppy View Post
Also I'm not sure that thinking of a natural Tipler Cylinder as "forming" frames the potential issue well. It could exist from the beginning.
Again, this is technically correct, which as we all know is the best kind of correct, but for the purposes of normal discourse I'm effectively ruling it out.

Again, find one and show it to me, and I will acknowledge my error. And quite frankly I find this line of argumentation - that you always have to acknowledge that you can't prove something doesn't exist - to be particularly lame and annoying. Notice how most of of the other contributors to this thread have long since lost interest.
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Old 19th May 2022, 07:31 PM   #831
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Originally Posted by arthwollipot View Post
Notice how most of of the other contributors to this thread have long since lost interest.
Sigh. Good point. I should lose interest in this forum.
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Old 20th May 2022, 07:42 AM   #832
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Originally Posted by arthwollipot View Post
And quite frankly I find this line of argumentation - that you always have to acknowledge that you can't prove something doesn't exist - to be particularly lame and annoying.
Not a single person in this thread has used that line of argumentation. Certainly I haven't, I have been quite explicit that this is not what I am saying.

To be frank myself, what I am finding increasingly lame and annoying are the constant implications, without any valid reasons given, that there is something irrational about my conclusion that I can't rule every version of backwards time travel out.

Others are welcome to rule it out. I am not.

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Old 20th May 2022, 09:07 AM   #833
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Originally Posted by Robin View Post

To be frank myself, what I am finding increasingly lame and annoying are the constant implications, without any valid reasons given, that there is something irrational about my conclusion that I can't rule every version of backwards time travel out.

Others are welcome to rule it out. I am not.
How do you deal with the paradoxes that would inevitably spring up? The usual “killing your father before he impregnated your mom” sort of thing. Short of a “many worlds” interpretation?
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Old 20th May 2022, 09:12 AM   #834
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Originally Posted by Robin View Post
Not a single person in this thread has used that line of argumentation. Certainly I haven't, I have been quite explicit that this is not what I am saying.

To be frank myself, what I am finding increasingly lame and annoying are the constant implications, without any valid reasons given, that there is something irrational about my conclusion that I can't rule every version of backwards time travel out.

Others are welcome to rule it out. I am not.

To be fair, I suspect arth was referencing the recent arguments about “natural” Tipler cylinders, rather than yours.

I reserve the right to be wrong, as always


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Old 20th May 2022, 09:19 AM   #835
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Originally Posted by Fast Eddie B View Post
How do you deal with the paradoxes that would inevitably spring up? The usual “killing your father before he impregnated your mom” sort of thing. Short of a “many worlds” interpretation?

The most common view I’ve seen is a holistic one, for lack of a better word.

People tend to think of time travel as going back and making changes; as if there is a first time, or some sort of individual timeline separate from the universe or unique to a person.

The holistic view is that what’s in the past already happened. Whatever you’re going to do in the past is already done before you go back. So asking a question like “What if you kill your grandfather?” Is functionally equivalent to asking (without time travel) “What if your grandfather had died?”. He didn’t, so he doesn’t.

Whatever you’re going to do after you travel back has already happened; it’s in your past. You can’t go back and change it because you didn’t; it’s not different.

And, as mentioned, this runs smack dab into the face of determinism.


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Old 20th May 2022, 10:00 AM   #836
Robin
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Originally Posted by Fast Eddie B View Post
How do you deal with the paradoxes that would inevitably spring up? The usual “killing your father before he impregnated your mom” sort of thing. Short of a “many worlds” interpretation?
Maybe just review the thread, I don't think I want to go over it yet again.
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Old 20th May 2022, 10:30 AM   #837
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A:"There are specific reasons that X can be ruled out."
B:"Here is why I think those specific reasons fail to rule X our"
A:"It is lame and annoying for you to keep suggesting I need specific reasons to rule X out"
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Old 20th May 2022, 10:37 AM   #838
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Originally Posted by Hellbound View Post
It depends. I suspect it'll be sort of like non-locality or quantum teleportation; interesting, and definitely an effect, but limited in usability.
The quantum effects that make a transistor work are very tiny and limited in usability. Until you start chaining hundreds of millions of them together in a CPU, and make hundreds of millions of CPUs. Then it completely changes the course of human civilization.
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Old 20th May 2022, 10:43 AM   #839
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Originally Posted by arthwollipot View Post
Just because it doesn't break General Relativity is no reason to think that one might actually exist. My opinion is that the likelihood of a naturally-occurring Tipler Cylinder actually existing is so close to zero as to be negligible.
The point is not the Cylinder. The point is the closed timelike curve.

We don't need the sun's massive gravity well to trigger fusion reactions. We just need to artificially create fusion-promoting conditions on a small scale. Turns out we can do that relatively easily nowadays.

Knowing that closed timelike curves are permitted by GR (and that there are solutions besides the Tipler Cylinder), all that remains is to figure out:

a) what kind of mass-energy expenditure is necessary to force such a curve on local spacetime (at any scale); and

b) whether that kind of mass-energy expenditure is likely to be accessible to us at some point.
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Old 20th May 2022, 07:38 PM   #840
Myriad
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Originally Posted by Hellbound View Post
The most common view I’ve seen is a holistic one, for lack of a better word.

People tend to think of time travel as going back and making changes; as if there is a first time, or some sort of individual timeline separate from the universe or unique to a person.

The holistic view is that what’s in the past already happened. Whatever you’re going to do in the past is already done before you go back. So asking a question like “What if you kill your grandfather?” Is functionally equivalent to asking (without time travel) “What if your grandfather had died?”. He didn’t, so he doesn’t.

Whatever you’re going to do after you travel back has already happened; it’s in your past. You can’t go back and change it because you didn’t; it’s not different.

And, as mentioned, this runs smack dab into the face of determinism.

To change the past or change "the timeline" you need for there to be more than one time dimension in the first place. (Or your time machine/method must add one, which seems a taller order.) Most time travel stories imply a second time dimension in which the alteration of history or timelines occurs. It's disguised as the equally fictional and equally strange (but far more familiar) narrative time.
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