Trying to understand how FTL would violate causality....

[PAllen]

I am not arguing who wrote copy 313 ( I do not mean the play itself ). It goes without saying that Shakespeare did. And surely he must have had more time than 6 days if he were to print 313 copies. Now, suppose, as you say, that copy 313 co-existed side by side with an older copy of itself, for as long as Shakespeare was on the task. What is the possibility of making a copy exactly similar to itself? Same amount of ink in each printed letter, same fabric and grain of pages down to molecular level, everything should be the same. Almost nil.
But suppose that this is not the same copy (as the one presented by traveller), and we are back to the original scenario you intended. How do you explain the fact that, right before our traveller begins his backward journey ( if the book he presents Shakespeare with survives through time ) there must co-exist two facsimile copies down to the minutest grain: one old and one new somewhere? As above, what are the possibilities of such a terrible coincidence? Almost nil.
Please allow me to be very skeptical on this, and stick to the impossibility of such a preposterous scenario and to my old aging copy paradox. But I have to admit that your scenario was a very successful one and got me to thinking.

There is no coincidence needed if an aged version of an object is identical except for age to its younger self. CTCs simply allow these to be brought together. Of course I find CTCs implausible, but with a block universe there are no possible contradictions of changing the past, e.g. killing your grandfather. I actually find the information paradox, which is allowed, far more profound an issue. It is also not addressed by chronology protection hypotheses.

 

[puzzled fish]

There is no coincidence needed if an aged version of an object is identical except for age to its younger self. CTCs simply allow these to be brought together.

Yes, we both agree on this. The trouble with this example is that a tremendous amount of very complex information gets recycled ( it's not just one or two bits we are talking about) and moreover it must be made to agree with a printed version of itself. It's the carriers of the information that are impossible ( if they are to be made to agree with each other ) and not the information itself.

 

[PAllen]

Yes, we both agree on this. The trouble with this example is that a tremendous amount of very complex information gets recycled ( it's not just one or two bits we are talking about) and moreover it must be made to agree with a printed version of itself. It's the carriers of the information that are impossible ( if they are to be made to agree with each other ) and not the information itself.

Nothing is recycled or duplicated. The aging of an object is a standard process. The aged state has greater proper time along a world line. A CTC simply allows the future of an oject to end up adjacent to a past state - but it is the actual future state, further along the object world line.

 

[PeterDonis]

right before our traveller begins his backward journey ( if the book he presents Shakespeare with survives through time ) there must co-exist two facsimile copies down to the minutest grain

This is not required at all. The only requirement is that the two copies contain the same play. There is no requirement that they are the same at the microphysical level; that condition is way, way, way too strong.

If you want to see this in a more startling way, imagine that you step into your time machine, not with a printed copy of the play, but with it stored as an ebook on your smartphone. Then, once you get back to 1590, you convince Shakespeare to copy out what's on your smartphone in his own handwriting and that becomes his manuscript of the play, which then has copies printed. Then you bury your smartphone somewhere and leave it to be found again 400 years later. The phone still contains the same information as a printed copy of the play (assuming that its flash drive's storage of the data is stable over that time period--which might not actually be true of today's smartphones, but we can presume it is for the smartphones in the time travel era), but obviously that information is stored in a very, very different microphysical state.

The trouble with this example is that a tremendous amount of very complex information gets recycled ( it's not just one or two bits we are talking about) and moreover it must be made to agree with a printed version of itself.

But we already know this is possible: it's just ordinary copying of a book.

 

[OCR]

A brutal follow on possibility is to kill the traveler so there is no record of the play's absence of authorship.

If the time traveler is killed in the past, there is no record of the play's absence of authorship... or of the time traveler. The time traveler never existed to "get in the time machine"... at any time !

That's pure and simply ... the grandfather paradox ...[COLOR=#black].[/COLOR]

 

[PAllen]

If the time traveler is killed in the past, there is no record of the play's absence of authorship... or of the time traveler. The time traveler never existed to "get in the time machine"... at any time !

That's pure and simply ... the grandfather paradox ...[COLOR=#black].[/COLOR]

No, it is not. Traveler was born, lived, and died as an adult. It just happens that the death date as globally assigned is earlier than the birth date. But it is a normal forward in time aging process for the traveler. Grandfather paradox is to kill your grandfather so you couldn't be born. It requires two different states of the past to exist. The scenario I describe entails only one version of the past. You should actually read the link you provide because it clearly explains this.

Maybe you just don,t understand the scenario. Traveler born as one continuous history, without any changing of the past. They are born, e.g. 2090, live till 2121, then follow ctc to the past, where they die. If recorded, they were always recorded as dying in 1590 at 31, then being born in 2090, etc.

 

[puzzled fish]

If you want to see this in a more startling way, imagine that you step into your time machine, not with a printed copy of the play, but with it stored as an ebook on your smartphone. Then, once you get back to 1590, you convince Shakespeare to copy out what's on your smartphone in his own handwriting and that becomes his manuscript of the play, which then has copies printed. Then you bury your smartphone somewhere and leave it to be found again 400 years later. The phone still contains the same information as a printed copy of the play (assuming that its flash drive's storage of the data is stable over that time period--which might not actually be true of today's smartphones, but we can presume it is for the smartphones in the time travel era), but obviously that information is stored in a very, very different microphysical state.

And assuming that your smartphone was 0 years old when you presented to Shakespeare, you find it now to begin your journey with a 400 year old smartphone. Back to post #64. You are just replacing the aging copy with an aging smartphone. Or you don't find it again and they both exist as duplicates of one another in the present? This is back to post #70.

 

[OCR]

You should actually read the link you provide because it clearly explains this.

Lol... I have, many times...[COLOR=#black].[/COLOR]
This one also...

The consistency paradox or grandfather paradox occurs when the past is changed in any way, thus creating a contradiction. A time traveler can do anything that did happen, but can't do anything that didn't happen. Doing something that didn't happen results in a contradiction. Consistency paradoxes occur whenever changing the past is possible.

Your error is...

A brutal follow on possibility is to kill the traveler so there is no record of the play's absence of authorship.

 

[PeterDonis]

And assuming that your smartphone was 0 years old when you presented to Shakespeare, you find it now to begin your journey with a 400 year old smartphone.

No, you don't. If the buried smartphone survives for 400 years, then it exists alongside its earlier self. The earlier self is the one that you take back in time.

Or you don't find it again and they both exist as duplicates of one another in the present?

They both exist in the present, but they are not "duplicates". See below.

This is back to post #70.

Which, as I've already pointed out, is wrong, at least as regards any claim of paradox or extreme unlikeliness. The smartphone that was buried for 400 years does not have to be microphysically identical (or even almost identical) to the smartphone you take back in time with you. It only has to contain the same information (although even that is not required since you are not relying on it to learn the play from--you're using printed copies based on Shakespeare's manuscript for that). And the processes by which that information is copied--from some printed version to the ebook on the smartphone, from the smartphone's screen to Shakespeare's manuscript, and from Shakespeare's manuscript to the printed version--are all perfectly ordinary, mundane copying processes that involve nothing unlikely at all.

 

[puzzled fish]

Nothing is recycled or duplicated. The aging of an object is a standard process. The aged state has greater proper time along a world line. A CTC simply allows the future of an oject to end up adjacent to a past state - but it is the actual future state, further along the object world line.

In the CTC there is no interaction between the two states past and present, or so I think. What if the present state (older guy) killed the past (young)?
In the example with the books, we are referring to, one book has to be created exactly similar to its older state, or in the second case one copy has to be sprung out of the press exactly similar to a 400 year old book. (Like a painter to draw an exact replica of Mona Liza on the same canvas with the same colors, and the only difference is their age). I stress, if the two objects are to be made similar with each other, this is almost impossible.

 

[PeterDonis]

In the example with the books, we are referring to, one book has to be created exactly similar to its older state

No, it doesn't. It only has to contain the same information. Please consider carefully what that means. Copying information from one book to another does not require any sort of extremely unlikely microphysical duplication. It happens all the time in our ordinary world.

 

[OCR]

Yeah, you have that right...

Maybe you just don,t understand the scenario.

They are born, e.g. 2090, live till 2121, then follow ctc to the past, where they die. If recorded, they were always recorded as dying in 1590 at 31, then being born in 2090, etc

...

 

[PeterDonis]

or in the second case one copy has to be sprung out of the press exactly similar to a 400 year old book

Just to go into detail on why this is wrong, here is the full history of the book (I'll use the original version where you take a book back in time, instead of the smartphone variant I proposed). I'll give years in the book's proper time, then global coordinate time (Gregorian calendar), separated by a slash.

0 / 1620: The printed book containing Shakespeare's play is created, using his manuscript of the play as a source.
380 / 2000: You find the printed book in a used book store and buy it.
400 / 2020 - 1590: You step into a time machine, carrying the printed book, and are taken back to Shakespeare's time.
401 / 1591: You show Shakespeare the printed book, and he copies out his manuscript of the play from it.
402 / 1592: You bury the printed book in a vault.
430 / 1620: The printed book is in the vault.
810 / 2000: The printed book is in the vault.
830 / 2020: The printed book is in the vault.
831 / 2021: The printed book is in the vault.

Nowhere in any of this is any process required for which we don't already have abundant evidence to show that it is possible, except for the time travel itself.

[Edit: Fixed some of the proper time year numbers.]

 

[puzzled fish]

Just to go into detail on why this is wrong, here is the full history of the book (I'll use the original version where you take a book back in time, instead of the smartphone variant I proposed). I'll give years in the book's proper time, then global coordinate time (Gregorian calendar), separated by a slash.

0 / 1620: The printed book containing Shakespeare's play is created, using his manuscript of the play as a source.
380 / 2000: You find the printed book in a used book store and buy it.
400 / 2020 - 1590: You step into a time machine, carrying the printed book, and are taken back to Shakespeare's time.
401 / 1591: You show Shakespeare the printed book, and he copies out his manuscript of the play from it.
402 / 1592: You bury the printed book in a vault.
430 / 1620: The printed book is in the vault.
809 / 2000: The printed book is in the vault.
829 / 2020: The printed book is in the vault.
830 / 2021: The printed book is in the vault.

Nowhere in any of this is any process required for which we don't already have abundant evidence to show that it is possible, except for the time travel itself.

What you are saying is that I didn't take copy 313 with me when I traveled because it is still in the vault and never got out. Ok, then which copy is it, because remember I bought it from a used bookstore, 380 / 2000. Which then is it? Is it 312? Or is it in the vault, too? 311? Or maybe none of the original Shakespeare's copies? A new copy? Back to post #70.
Sorry about my writing, is only intended to be in good humor.

 

[Stephen Tashi]

I don't understand whether the assumption of a "block universe" is being made in these time transportations of documents. If a copy of the play exists today and we were to travel "back in time" then would we not pass through the states the involve how the copy of the play was created in reverse order? So when we arrived in the remote past, our copy of the play would no longer exist - nor would we, for that matter.

By that line of thinking, a time traveler cannot arbitrarily decide to create an information paradox because when he "goes back in time", he may go back in a path that "un-creates" the information he wishes to transmit as well as un-creates himself. That still leaves open the possibility that a "lucky" time traveler might find a way to go back to a state where he exists - e.g. go back to a day in Shakespear's life that never existed in any other way except being a day when the time traveller was present. However, this only shows that time travel does not rule-out certain paradoxes. It doesn't show show such paradoxes would definitely exist. Such paradoxes might be prevented by other physical laws.

 

[PAllen]

Lol... I have, many times...[COLOR=#black].[/COLOR]
This one also...

Your error is...

No it's your error. There was never a 1590 in which the travler did not die. Always born in 2090, always traveled in 2121, always died in 1590. Try to understand the word you quoted. They agree with me, not you, that this is NOT the grandfather paradox.

 

[puzzled fish]

No, you don't. If the buried smartphone survives for 400 years, then it exists alongside its earlier self. The earlier self is the one that you take back in time.
They both exist in the present, but they are not "duplicates". See below.
Which, as I've already pointed out, is wrong, at least as regards any claim of paradox or extreme unlikeliness. The smartphone that was buried for 400 years does not have to be microphysically identical (or even almost identical) to the smartphone you take back in time with you. It only has to contain the same information (although even that is not required since you are not relying on it to learn the play from--you're using printed copies based on Shakespeare's manuscript for that). And the processes by which that information is copied--from some printed version to the ebook on the smartphone, from the smartphone's screen to Shakespeare's manuscript, and from Shakespeare's manuscript to the printed version--are all perfectly ordinary, mundane copying processes that involve nothing unlikely at all.

Pardon me, but if this is not the same exact smartphone I took back in time with me, then what is it? You know smartphones had not been invented 400 years ago, Peter...
Paradox

 

[PAllen]

Yeah, you have that right...... View attachment 131648

CTCs are very bizarre, but they don't involve any locally unlikely physics, nor do they lead to grandfather type paradoxes in GR as a theory of manifold plus metric. They do lead to information paradoxes, though. Emojis don't strengthen your argument.

 

[PeterDonis]

What you are saying is that I didn't take copy 313 with me when I traveled because it is still in the vault and never got out.

No, that's not what I'm saying. Look at the timeline again. That is the timeline of copy #313 (or whichever copy you took with you). Look at the calendar years: in every year from 1620 through 2020, copy #313's timeline intersects that year twice, not once. There is only one timeline, but that timeline crosses those years two times. Each of those crossings is part of copy #313's timeline. From the standpoint of any of those years, considered as a spacelike surface, there are two copy #313's. One of them is the one labeled by some proper time year from 0 to 400, in other words, the "original" one that was printed and which you are going to take back in time with you. The other is the one labeled by some proper time year from 430 to 830 (I just realized I originally labeled those years wrong, I've gone back and fixed them), which is in the vault and stays in the vault indefinitely.

 

[PeterDonis]

if this is not the same exact smartphone I took back in time with me, then what is it?

See post #89.

 

[OCR]

They agree with me, not you...

Perfectly fine... but I agree with me.

Lol... they seem to have an affect, though...

Emojis don't strengthen your argument.

 

[PAllen]

I don't understand whether the assumption of a "block universe" is being made in these time transportations of documents. If a copy of the play exists today and we were to travel "back in time" then would we not pass through the states the involve how the copy of the play was created in reverse order? So when we arrived in the remote past, our copy of the play would no longer exist - nor would we, for that matter.

By that line of thinking, a time traveler cannot arbitrarily decide to create an information paradox because when he "goes back in time", he may go back in a path that "un-creates" the information he wishes to transmit as well as un-creates himself. That still leaves open the possibility that a "lucky" time traveler might find a way to go back to a state where he exists - e.g. go back to a day in Shakespear's life that never existed in any other way except being a day when the time traveller was present. However, this only shows that time travel does not rule-out certain paradoxes. It doesn't show show such paradoxes would definitely exist. Such paradoxes might be prevented by other physical laws.

I am making the assumption of block universe. This means, ipso facto, that there is only one version of any spatial hypersurface. However, it does not mean there is any undoing if CTCs are present. They are simply paths in the manifold the are timelike everywhere but can end up at or near an earlier event on the same world line. There is forward aging all along such world line. In this sense you don't really decide to create an information paradox; instead the universe simply contains one. At some level, information paradoxes are unavoidable if CTCs really exist.

 

[puzzled fish]

No, that's not what I'm saying. Look at the timeline again. That is the timeline of copy #313 (or whichever copy you took with you). Look at the calendar years: in every year from 1620 through 2020, copy #313's timeline intersects that year twice, not once. There is only one timeline, but that timeline crosses those years two times. Each of those crossings is part of copy #313's timeline. From the standpoint of any of those years, considered as a spacelike surface, there are two copy #313's. One of them is the one labeled by some proper time year from 0 to 400, in other words, the "original" one that was printed and which you are going to take back in time with you. The other is the one labeled by some proper time year from 430 to 830 (I just realized I originally labeled those years wrong, I've gone back and fixed them), which is in the vault and stays in the vault indefinitely.

Please re-write your list. I do not understand. There is a contradiction in it. How do I travel back with a book labelled 0 years when I bought it from a bookstore with 380 years on it (second line)?
To cut a long story short, the only way out of this paradox that I am aware of, is only when there is not a considerable amount of infrormation to be transmitted. Eg. you find an equation in a book, learn it by heart, travel in the past, write it down on a blackboard, it's a wave equation, Maxwell is becoming aware of it, end of the story. No books, no media, no press. I think this is the essence of what PAllen is trying to say, but his example was very complicated.

 

[PeterDonis]

How do I travel back with a book labelled 0 years when I bought it from a bookstore with 380 years on it (second line)?

You aren't reading the timeline. Read it again. When you step into the time machine, the book is labeled 400 years, not 0.

 

[PeterDonis]

I think this is the essence of what PAllen is trying to say

No, it isn't. There is no need to limit the complexity of the information.

 

[Stephen Tashi]

They are simply paths in the manifold the are timelike everywhere but can end up at or near an earlier event on the same world line.

It's the "at" a earlier event that's the problem for perspective time travellers. They have to gamble that there is an earlier state of the universe where they were present.

At some level, information paradoxes are unavoidable if CTCs really exist.

I see that information paradoxes aren't prevented if CTCs exist. I don't see any proof that information paradoxes must exist. I don't see any demonstration that a paradox can be created at will by a time traveller. To suppose that a time traveller goes "back in time" is one thing. To suppose he goes back in time and takes things with him and continues to exist himself is assuming more.

 

[PAllen]

It's the "at" a earlier event that's the problem for perspective time travellers. They have to gamble that there is an earlier state of the universe where they were present.



I see that information paradoxes aren't prevented if CTCs exist. I don't see any proof that information paradoxes must exist. I don't see any demonstration that a paradox can be created at will by a time traveller. To suppose that a time traveller goes "back in time" is one thing. To suppose he goes back in time and takes things with him and continues to exist himself is assuming more.

They don't need to gamble. If there are CTCs they can access, and they feel like doing so, then they must 'have' done so.

If any substantial body goes back in time and interacts with things, you have events that are influenced by their future an also influence their future. This is tantamount to an information pradox.

 

[Stephen Tashi]

They don't need to gamble. If there are CTCs they can access, and they feel like doing so, then they must 'have' done so.

If any substantial body goes back in time an interacts with things, you have events that are influenced by their future an also influence their future. This is tantamount to an information pradox.

You resorted to "if" to give those examples. I agree that if a time traveller did find a path to go back in time to a day when he, Shakespeare, and the time traveller's copy of Julius Caesar did exist then an information paradox could exist. However, I don't see the proof that such a path must exist if FTL travel is possible.

I return to the question of what state space is being discussed in the notion of time travel. Does going back in time mean going to a previous state of the universe, where "state" means a complete physical description of the universe? - including the detail of whether the time traveller is present in that state?

 

[Ibix]

Why isn't this possible? I inherit a note saying: Dear Mr Shakespeare, Please duplicate this note and give the copy to Mr John Ibix (my great-great-whatever grandfather). However, in the copy please add 1 to this number: 1. Then please destroy this original. Then I put the note in a time machine and send it back to Shakespeare.

If Shakespeare follows the instructions the result seems genuinely paradoxical. What would an observer outside the CTC see on the note?

 

[Ibix]

Doesn't this also apply to say, waves on a rope? The rope isn't getting any closer even though the waves seem to be moving toward you.

I don't see the analogy. The flipbook with wrinkled pages represents spacetime and has a concept of time built into itself. We see the gravitational wave passing us because we see one page of the book[1] at a time and compare it to our memories of previous pages. GR just describes the whole book, with the wrinkle, in one go.

[1]Our past light cone, more precisely.

 

[PeterDonis]

I return to the question of what state space is being discussed in the notion of time travel.

Remember that we're discussing this in the context of GR, where spacetime is a single 4-dimensional manifold. We're discussing the case where this manifold happens to have timelike paths that loop back around so that they intersect the same spacelike hypersurface more than once. But each spacelike hypersurface, which is the closest thing to a "state of the universe" in this model, is what it is: a given timelike curve intersects it a well-defined number of times, and that is an invariant fact about the spacetime.

Does going back in time mean going to a previous state of the universe, where "state" means a complete physical description of the universe? - including the detail of whether the time traveller is present in that state?

It should be evident from the above that the answer to this is "yes" (reading "state" to mean "spacelike hypersurface").

 

[GTOM]

Well, not according to this one.

https://arxiv.org/abs/1407.2528

If that is crackpottery, at least i'd like to know why is it, because i only saw mathetmaics and implications.

Otherwise does relativity truly show that time is more than the changing of things? Yes time slows down at big speeds, near a black hole, but is it really more than electromagnetic based interactions slow down under theese conditions? Our measurement of time and everything else depends on electromagnetic based interactions.

 

[PAllen]

You resorted to "if" to give those examples. I agree that if a time traveller did find a path to go back in time to a day when he, Shakespeare, and the time traveller's copy of Julius Caesar did exist then an information paradox could exist. However, I don't see the proof that such a path must exist if FTL travel is possible.

"Must" is too strong a claim. Sorry if I implied that. However any material object occupying an exact CTC is itself an information paradox. You have a structured odbject without origin. You can, of course, add rules that say this can't happen. However most physicists believe it is CTCs themselves that are prevented in the real world. For example, if the dominant energy condition is mostly true, and the perturbatively stable variants of Kerr interior avoid CTCs, all cases are prevented. Personally, that is what I would bet on.

 

[PAllen]

Why isn't this possible? I inherit a note saying: Dear Mr Shakespeare, Please duplicate this note and give the copy to Mr John Ibix (my great-great-whatever grandfather). However, in the copy please add 1 to this number: 1. Then please destroy this original. Then I put the note in a time machine and send it back to Shakespeare.

If Shakespeare follows the instructions the result seems genuinely paradoxical. What would an observer outside the CTC see on the note?

This is where the block universe comes in. If there is one note whose world line begins in 1590, is then passed to your ancestor, then sent back in time, and destroyed in 1590, then it can have only one thing written on it. Shakespeare wrote one thing on it, and that is the only state it has. Whatever Shakespeare's internal perception is, he would effectively ignore the instruction, simply because that is what he did. Block universe erases true free will, which is certainly related to how it implements the chronology protection conjecture.

 

[puzzled fish]

You aren't reading the timeline. Read it again. When you step into the time machine, the book is labeled 400 years, not 0.

Yes, I've read the timeline again and it seems to me exactly identical to Pallen's answer.
I've explained myself before that a near CTC allows two objects to co-exist, I have no objection to that. My objections is to the making of the copy itself. Copying is a procedure that requires a tremendous amount of interactions with the original object. And suppose that a block-universe is constructed in such a way that past-present is inter-woven in a way as to account for those interactions. I find, macroscopically, the probability that I might be able to reproduce an exact identical copy of a complex object like a book, to be 0 (almost.. very).
By the way, I like post #99 very much. It is like my aging copy example. What do you have to say about this?