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

[PAllen]

Physical systems are conventionally described by specifications that give their "state". Is a point in "spacetime" supposed to represent the (complete) state of a physical system? What system would that be? A point particle? The entire universe?

I don't understand how the notion of a physical system being completely described by a state is compatible with the notion of "going back in time". In particular, if you "went back in time" to alter the "state" of a physical system in the remote past, your presence in the description of that physical system would change the description of its state. So you would not actually have arrived in the physical system that lacks your presence, you would have arrived at a physical system with a different state description (because you are present).

It seems to me that to discuss "going back in time" , one must use incomplete descriptions of physical systems. So if a person were to go back in time to kill his grandfather and we wish to say this is a paradox then we must count a past day when the time traveller wasn't present as being the "same" physical system as a past day when the time traveller was present.

If spacetime is a static representation of a state space then one can draw curve in that state space that goes backwards in time, but that does not change the states that are represented by the points on that curve. So how can anything like a kill-you-grandfather paradox be formulated if we use a consistent concept of the state of a physical system?

If you take a block view of spacetime, there are no paradoxes like killing your grandfather. Either you were or weren't present before you were born, you did what you did if you were present in your past, etc. But what you have instead are information paradoxes. For example you have accept possibilities like plays without authors: I go back in time hand Shakespeare a copy of Julius Caesar, he then writes it out, it becomes famous, I go back in time with a copy, etc. There is only one version of any spatial slice of spacetime, but the play has no author.

 

[Stephen Tashi]

But what you have instead are information paradoxes. For example you have accept possibilities like plays without authors: I go back in time hand Shakespeare a copy of Julius Caesar, he then writes it out, it becomes famous, I go back in time with a copy, etc. There is only one version of any spatial slice of spacetime, but the play has no author.

I can see that such a scenario is an information paradox if we assume that there are deterministic physical laws that determine how states change into other states - in such a manner that a given state must have only one set of states that preceede it. If the state of the universe whose description includes "I have a copy of Shakespear's Julius Caesar" can only follow from the state whose description includes "Shakespear was given a copy of Julius Caesar by a time traveller from the future" then the play indeed has no author. However, if the state of "I have a copy of Shakepspear's Julius Caesar" could have followed from several different prior states, one of which includes "Shakespear writes Julius Caesar" and one of which includes "Francis Bacon writes Julius Caesar" then the authorship of the play becomes ambiguous, but do we have an information paradox?

 

[PAllen]

I can see that such a scenario is an information paradox if we assume that there are deterministic physical laws that determine how states change into other states - in such a manner that a given state must have only one set of states that preceede it. If the state of the universe whose description includes "I have a copy of Shakespear's Julius Caesar" can only follow from the state whose description includes "Shakespear was given a copy of Julius Caesar by a time traveller from the future" then the play indeed has no author. However, if the state of "I have a copy of Shakepspear's Julius Caesar" could have followed from several different prior states, one of which includes "Shakespear writes Julius Caesar" and one of which includes "Francis Bacon writes Julius Caesar" then the authorship of the play becomes ambiguous, but do we have an information paradox?

Determinism is irrelevant. All you need assume for an information paradox is that closed timelike curves are possible and that there is only one version of any interaction.You need not assume that any future state can be predicted from past states. That is, a quantum block universe model simply assumes all history is played out. Whatever interactions that are unpredictable from their past have already happened. Add to this closed timelike curves, and you have the possibility of information paradoxes.

 

[puzzled fish]

If you take a block view of spacetime, there are no paradoxes like killing your grandfather. Either you were or weren't present before you were born, you did what you did if you were present in your past, etc. But what you have instead are information paradoxes. For example you have accept possibilities like plays without authors: I go back in time hand Shakespeare a copy of Julius Caesar, he then writes it out, it becomes famous, I go back in time with a copy, etc. There is only one version of any spatial slice of spacetime, but the play has no author.

Is the aging copy of Julius Caesar a member of the same set of information paradoxes? For suppose you have in your hands a 400 year old copy. You travel back in time and hand it to Shakespeare. For the time Shakespeare reads and copies the book, time passes as normal. Suppose this procedure ( for Shakespeare to copy the book) takes more than a year. In the meantime, the copy ages more. Now, regardless if you take your book back to the time you began your trip at, or you leave it with Shakespeare for another 400 years, in both cases you end up ( or begin with) with a book > 400 years. Paradox.

 

[PeterDonis]

Now, regardless if you take your book back to the time you began your trip at, or you leave it with Shakespeare for another 400 years, in both cases you end up ( or begin with) with a book > 400 years.

No. The 400 year old book you take back to hand to Shakespeare is the one that was printed by Shakespeare himself (or someone who took his manuscript and printed it) 400 years before you stepped into your time machine. Shakespeare takes the book you hand him and writes his own manuscript, which then gets printed, and 400 years later you take it and step into your time machine...

As for what happens to the book you handed Shakespeare after he uses it to write his manuscript, that's not specified in the scenario.

 

[PAllen]

No. The 400 year old book you take back to hand to Shakespeare is the one that was printed by Shakespeare himself (or someone who took his manuscript and printed it) 400 years before you stepped into your time machine. Shakespeare takes the book you hand him and writes his own manuscript, which then gets printed, and 400 years later you take it and step into your time machine...

As for what happens to the book you handed Shakespeare after he uses it to write his manuscript, that's not specified in the scenario.

Actually, I was thinking of the simple possibility the time traveler hands Shakespeare a modern book, which he then copies in his own hand and destroys. A brutal follow on possibility is to kill the traveler so there is no record of the play's absence of authorship.

 

[puzzled fish]

No. The 400 year old book you take back to hand to Shakespeare is the one that was printed by Shakespeare himself (or someone who took his manuscript and printed it) 400 years before you stepped into your time machine. Shakespeare takes the book you hand him and writes his own manuscript, which then gets printed, and 400 years later you take it and step into your time machine...

As for what happens to the book you handed Shakespeare after he uses it to write his manuscript, that's not specified in the scenario.

Ok. Suppose that the book the traveller handed to Shakespeare, gets destroyed. And further suppose that Shakespeare was in the habit of carefully numbering the copies that were printed by himself. What you are saying is that I began my journey back in time with one of those copies that somehow was found in my possession.
Let's say copy 313. But history records show that copy 313 has been destroyed. ( Suppose that the book the traveller handed to Shakespeare, gets destroyed: is what we began with.)
Paradox.

 

[PAllen]

Ok. Suppose that the book the traveller handed to Shakespeare, gets destroyed. And further suppose that Shakespeare was in the habit of carefully numbering the copies that were printed by himself. What you are saying is that I began my journey back in time with one of those copies that somehow was found in my possession.
Let's say copy 313. But history records show that copy 313 has been destroyed. ( Suppose that the book the traveller handed to Shakespeare, gets destroyed: is what we began with.)
Paradox.

No paradox at all, though not the scenario I intended. Copy 313 was written by Shakespeare say January 10, 1590. Its world line continues to 2121 when it travels to January 5, 1590. Shakespeare copies it and destroys the duplicate on January 11, 1590 (soon after he finishes copying it). There are 6 days when the very old state and the developing new state of the book coexist; but that feature goes without saying if closed timelike curves are possible (in this case, we rely on an 'almost closed' timeilike curve). It is no paradox at all, just weird. What is paradoxical is the information generation issue: who wrote the play? No one - it just exists without authorship.

 

[puzzled fish]

Actually, I was thinking of the simple possibility the time traveler hands Shakespeare a modern book, which he then copies in his own hand and destroys. A brutal follow on possibility is to kill the traveler so there is no record of the play's absence of authorship.

Which leaves us with the strange possibility a 400 years old copy of Julius Caesar to co-exist somewhere together with the brand new modern facsimile copies of itself, in the unfortunate circumstance that it doesn't get destroyed. A most curious find indeed!

 

[puzzled fish]

No paradox at all, though not the scenario I intended. Copy 313 was written by Shakespeare say January 10, 1590. Its world line continues to 2121 when it travels to January 5, 1590. Shakespeare copies it and destroys the duplicate on January 11, 1590 (soon after he finishes copying it). There are 6 days when the very old state and the developing new state of the book coexist; but that feature goes without saying if closed timelike curves are possible (in this case, we rely on an 'almost closed' timeilike curve). It is no paradox at all, just weird. What is paradoxical is the information generation issue: who wrote the play? No one - it just exists without authorship.

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.

 

[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 ....

 

[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 ....

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....
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....
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.