1. Jul 7, 2014

### TaCoCaT

An immortal woman who is about to go back in time is mailed a book from an unknown address. Inside the book reveals explicit instructions on how not to die while in the past leading up to the present day. The book is written by her future self and mailed to herself by her future self who is now in the present too.

Would this be possible? Is it a paradox? Would she be able to write this book and mail it to herself in the present? It would only be written if she had already gone back in time but it also would not have been written if she died in the past having never been given the book to begin with.

I am vaguely aware that this is like the age old chicken or the egg conundrum, so I guess I'm just asking a bunch of physics minded people a ridiculous version of that.

2. Jul 8, 2014

### Staff: Mentor

In most sci-fi stories I've read, communication was done by placing a want-ad in a well-known newspaper for someone in the future to read.

In the Back to the Future movie Marty McFly gives the 1950's Doc a note to watch out in the future for the terrorists that attack them in the present day mall parking lot. Also there was a Western Union telegram that was deliverd to Marty in 1950 sent by Doc in the 1880's

http://en.wikipedia.org/wiki/Back_to_the_future

http://en.wikipedia.org/wiki/Back_to_the_Future_Part_III

In one Star Trek spinoff story, Spock goes back in time to see who it was who rescued him when he was a little boy attacked by some vulcan creature. It turned out it was himself.

http://memory-beta.wikia.com/wiki/Spock

There was a more complicated story I read where a guy makes a time machine goes back to tell his former self what to do to become richer which changes his future self to be greeder until finally his past self rejects his future selfs extreme impatience and aggresiveness insuring that he won't make the time at all.

Last edited: Jul 8, 2014
3. Jul 8, 2014

### Staff: Mentor

It sure looks like a paradox to me. In order to know the exact circumstances of how not to die, that would imply that she had to die, thus a paradox forms.

Remember, when time gives you a paradox, just sit back, relax, and do a little fishing. (Paradox sounds like Pair of Docks)

4. Jul 9, 2014

### HallsofIvy

There was one novel (I think "The Man Who Folded Himself" by David Gerrold) in which a young man finds a time machine in the woods behind his house. Years later (in "his own time line"), when he is very elderly, he take the time machine and leaves it in the woods for himself to find. Yes, that's a paradox. Where did the time machine come from "originally"? Or does the question make sense?

"Star Trek" for making mistakes with time travel. In "Start Trek: The Voyage Home", Scotty shows an engineer in "present day" San Francisco how to make "transparent aluminum". When McCoy objects to his "changing the past", Scotty responds "How do we know he didn't invent it", as if as long as you don't know that you are changing the past, it doesn't count! And, of course, Scotty, as a well educated engineer would know who had invented "transparent aluminum". He should have told McCoy that he had recognized the engineer as being the man who had invented it. Of course that still leaves the question- if Scotty learned of "transparent aluminum" from text books based on this man's work, and this man learned it from Scotty, where did the idea for "transparent aluminum" really come from?[/B]

Last edited by a moderator: Mar 25, 2016
5. Jul 9, 2014

### Staff: Mentor

There's a cute rom-com called About Time starring Domhal Gleason and Rachel McAdams that plays with the notion of going back and changing things in the pas tin a limited way with the attendent paradoxes and problems in the present. The travel is limited to your life and you learn not to change events before some life event like a birth because then it might happen differently.

6. Jul 11, 2014

### alteredmind

I suppose it would be possible if time travel into the past were possible. if she died then she wouldn't be imortal obviously. I don't think it is a peradox. she time traveled into the past so that is how she was able to tell her past self how to not die. in the future that she came from time travel into the past could have been her scientific achievement that she one a nobel prise for and somehow her scientific and technological achievement got into the wrong hands bye some nut job. According to my theory that nut job could of gotten a hold of her somehow and he was so powerfull that there was nothing she could do to stop him from killing her so she decided to give instructions to her past self on how not to die and knowledge to build a time machine into the past as well and to keep it a secret until the woman were to possibly avoid getting killed, and having authorities put him away in prison for life and stripping him of his power. that might have changed the past but at least it saved her life and a important scientific achievment for human kind

7. Jul 11, 2014

### Algr

The examples in this thread are causality loops, which I believe are distinct from paradoxes. A paradox example would be if you go back in time and prevent yourself from inventing a time machine.

8. Jul 17, 2014

### MFPunch

This is only my personal opinion but I don't think causality loops are possible. In the case of the OP's question the book is merely a chronicle of what happened to that person between the point they arrived in the past and the point they departed in the future. It doesn't matter what the book says because the point is they survived to write it. The book plays a part in their survival only because the decisions contained in it just happened to be the correct ones. If the person had died in the "past" then the book wouldn't exist. You can substitute the book for a coin that will be flipped to make all major decisions and you'd end up with the same thing. At some point the book will be lost or destroyed forcing them to create a new one in the future to give to their past self.

As for the thing with Scotty and the transparent aluminum the easy answer is some series of events would cause the person at the factory to die before they could tell anyone else about it and in some way or another the data would be lost. Then at some point in the future someone else would independently create transparent aluminum.

But honestly time travel in to the past just seems completely ridiculous. Possibly a device could be created that would allow travel backwards no earlier than its creation but even that is a huge maybe. Just willy nilly jumping 10,000 years into the past just seems out of the realm of probability and maybe even possibility.

9. Jul 30, 2014

### Paintjunkie

look at the universe as a fractal with multiple presents and futures?

10. Jul 30, 2014

### dkotschessaa

Sci-fi always entails 1 of 3 time travel philosophies:

1) Paradoxes are a problem, and will cause some sort of catastrophic "hole in the continuum" blow up thing, so must be avoided. (The Back to the Future Model)

2) Paradoxes are impossible, so evens will always unfold in such a way that they cannot happen. (i.e. if you go back and try to shoot yourself or you grandparents, you'll miss every time, or the gun will misfire, or you'll never see them.

3) More recently, the "multiple timelines" philosophy. You can do anything you want, and it's no problem, because you create a new timeline that runs parallel to the existing one.

11. Jul 30, 2014

### Spock2230

Seems to me these causality loops form a predestination paradox. The premise for the woman to live so long is that she received information from her future self who has lived long enough because she had received information that has allowed her to live this long, ad infinitum.

Whether or not this is possible is empirically indeterminate and so I guess there won't be much value debating it. If I'm correctly updated, backwards time travel has not been proven possible.

If the future walks into the past, it would be really tense.

12. Sep 30, 2014

### andrewkirk

It's neither possible nor necessary, as described.

The future self has no need to post the book to the past self, because futureself already knows that pastself doesn't die, and that futureself can't do anything to change that.

Futureself may post the book to pastself anyway, because that's what she was always going to do (just like I was always going to type this post - cue discussion on free will). Which raises the unanswerable question of 'what does one mean by necessary?'.

The reason that the story is impossible is that the list of things not to do because you might die is enormous, possibly infinite. Did she for instance include in the book: 'Don't stab yourself the instant you get there; don't stab yourself one second later; don't stab... etc etc etc'. A microscopic difference in how one moves or where one sits could determine whether a random cosmic ray strikes the body and starts a terminal cancer. Futureself would have no knowledge of such things, and could not include them in the book.

What futureself can do is something limited and simple like in the Back to the Future example cited. It would be one or a small number of instructions, such as 'take the blue pill, not the red pill'. That doesn't guarantee not dying, for the reasons just stated. What guarantees not dying is that futureself already knows that, whatever futureself does, pastself does not die.

13. Sep 30, 2014

### Khashishi

There could be causality loops which essentially cause themselves, with no reason for being outside of the loop. Essentially, someone went back in time because of events that happened because they went back in time. It's all consistent. Since there's no external cause, one can wonder why the timeline isn't completely full of these random uncaused time loops.

14. Oct 4, 2014

### moriheru

Have you thought about the many world interpretation of Schrodingerscat. There is a world in which Schrodingerscat is dead and there is a world in which Schrodingerscat is alive. So say you where to travell back in time and kill yourself resulting in a paradox, there would be a world in which you are dead and alive. Most paradoxi(I believe this is the plural of paradox) are the product of representing time as a one dimensional line.

15. Oct 7, 2014

### BobG

A more relevant question to "a bunch of physics minded people" would be about whether sending someone or something back into the past violates any laws about conservation of energy, momentum, mass, entropy, etc.

For example, one could imagine that a time machine would require impossible amounts of energy to run. No problem. Just set up a time machine that transports energy from a future sun back to the present in order to supplement the power of the current sun (Being that we invented the time machine in the present and the people in the future don't exist yet, what can they do to stop us from stealing the energy of their sun?)

It would take some serious rewriting of physics to expand laws of conservation to encompass moving across times. In essence, energy, momentum, etc would appear to pop up and disappear at random to an observer observing from normal time scales.

16. Oct 7, 2014

### Khashishi

I would stay away from any ideas that are too crazy, since you won't be able to predict how people will act in those circumstances, so you won't be able to write a story.

One thing that bothers me about Terminator, is what keeps the robots from continually sending robots back in time until successful? Where is the limitation? If you have a time machine, you have infinite resources. Well, unless you invent something in your universe to limit that.

17. Oct 7, 2014

### BobG

There were limitations. For one, you couldn't send tools, weapons, or even clothes - yet, somehow, a terminator with a metallic skeleton covered by human skin could go through the machine (they really don't explain that part very well at all). Plus, Kyle went through just before the time machine was destroyed. They had to build yet another time machine to send their second, improved terminator back in time, and yet another in order to send a third, even more improved terminator back in time. Presumably, there were limitations on the precise times you could send a human back to? For example, the Earth would be at different locations in physical space depending on the particular time you sent a person back. Sending a person back in time doesn't mean just sending a person back in time - it also means sending them to the physical location of the Earth at the particular time you're sending a person back to. Because the future location of the Earth has changed during the time they built the new time machine, the physical locations they can send a person to back in time has also changed.

They don't go into the details of time travel very well, but I could imagine lots of limitations. And the difficulty in separating time and space would be an obvious one.

Interesting trivia: OJ Simpson was originally considered for the role of the Terminator, with Arnold Schwarzenegger considered for the role of Kyle. OJ Simpson was rejected because the movie makers finally decided the public could never perceive of OJ Simpson as a killer. And then Schwarzenegger was switched to the role of the Terminator because if he were Kyle Reese, then you'd need to find a heck of a big person to be the Terminator (OJ Simpson would have fit the bill pretty well, but there aren't too many other actors that would).

18. Oct 7, 2014

### andrewkirk

Good point!

I think the answer is different depending on which mode of time travel is used. The two I understand are (1) timelike loops and (2) travel through additional dimensions.

If one 'time travels' by following a closed timelike loop, there is no problem with conservation laws because the traveller does not suddenly 'pop into existence' in a region of spacetime where she wasn't before. She will always enter the region along a timelike curve, and the conditions needed to preserve the conservation laws will be maintained.

That is not the case in travel through additional dimensions. That is where our 4D spacetime manifold is embedded in a higher-dimensional manifold and time travel is accomplished by 'taking off' from our 4D manifold by travelling orthogonally to it, then travelling through the embedding space until 'landing' on another part of our 4D manifold that has an earlier cosmic time coordinate (eg in the FLRW frame) than the point from which we took off.
The equation for conservation of momentum and energy, which is $T^{\alpha\beta}_{;\beta}=0$, relies on mass-energy only entering a spacetime region along a path that is in the manifold. In this case the mass-energy of the traveller enters the region along a path that is not in the manifold, so those preconditions do not hold and the conservation law will be violated.

One imagines there may however be an analogous conservation law that applies to the embedding space, so that even though conservation is not maintained in our manifold, it is maintained for the embedding space.

19. Oct 7, 2014

### Northman

I would think that the book only works if once she arrives in the past she proceeds step by step, moment by moment, in the exact manner she had in her "previous" journey, uninterrupted by any disturbances. The book, however, would be such a disturbance, altering her journey completely, therefor rendering the book useless to her.

(First post by the way - hello everyone!)

20. Oct 8, 2014

### Simon Bridge

The idea sounds similar to Pratchett's Good Omens - in this book information is retrieved, rather than sent, from the future.
The retriever gets a garbled account of the future which is recorded with the hope of benefiting future generations.

Greg Benford's Timescape involved deliberately sending information back in time in a hard-sf context - though this is an "avert disaster" type story... so it involves the grandfather paradox.

There are other examples, like Butterfly Effect and the Foundation Series where the results of acting on otherwise very accurate information about the future in the face of changing circumstances are discussed. What's always in common is that the physical time traveller retains history. The book may have a coffee stain, for example, which remains even though the original event that stained the book no longer happens. (This is the usual paradox that appears in such stories.)

If the idea is that the book's instructions, if not followed, will result in death or misfortune (the author is stipulated to be immortal) in the past - then the story is like the above. From the writer's POV, they are just putting down their own history - it's a diary. But the writer recalls receiving the diary was instrumental in getting where they are today (see Timescape - where the researcher posts a letter to the future that has already has a reply.)

These are variations of the grandfather paradox - and certainly involve causality violations.

There are reams written about causality ... so I cannot do it justice here. But I can try to give the idea:
Causality is, in a nutshell, "effects always follow causes" ... this is a little more complicated in relativity but amounts to an event only having causes that are in their past light-cone (communication cannot happen faster than the speed of light). Anything else is a "causality violation". But that is not the only way of thinking about "how things happen". It is certainly possible to make sure that a narrative has causality loops that are internally consistent and "them's the rules". A paradox is possible with causality violations - but a causality violation may not be paradoxical.

A paradox appears more forcefully when a future action effectively cancels itself out. This is the "grandfather paradox" of time travel. Like: you invent a time-travelling bullet and, later, realize how terrible that was and out of remorse decide to end your life from before you invented the bullet. Thereby the means of your death never gets built. You can see why this is a paradox.

... it is usually resolved, as previously mentioned, through the multi-universe approach; what Pratchett call's "the trousers of time". It's not all that new: see Brunner's, Times Without Number (1968) or Anderson's Time Patrol (1960). This approach is useful because it allows someone, the hero, to be able to remember the, now erased, history. The version of the grandfather paradox here would have to involve the invention of time travel itself. The time travelling bullet of the previous example then has the distinction of coming from another timeline - there would presumably be many artifacts from other timelines appearing in whatever one the story protagonists call home.

The reason for having time travel in a SF story is almost always about the grandfather paradox - though sometimes the purpose is to provide an SF mechanism for writing an alternate history (i.e. Turtledove's The Guns of the South). This, I think, is central - you need to decide what the role the book-from-the-future plays in the story. Then you will be able to see what sort of course to steer.

21. Nov 3, 2014

### Curious3141

Actually, some stories use something other than these 3 philosophies. Case in point: the (pretty darn good, IMHO) short story "Ripples in the Dirac Sea" by Geoffrey A. Landis.

You can read it here: http://diracsea.net/ripples-in-the-dirac-sea/ [Broken]

No paradoxes are possible because changes in the "past" are always erased when you "catch up" to the present. In a sense, that's a little like your scenario 2, but since you can actually do anything (that's physically possible) in the "past", it's not quite the same. I believe your scenario 2 actually prevents (via a deus ex machina if nothing more plausible presents itself) our intrepid tempo-naut from changing the event stream in his (personal) past.

There are glaring issues with that story - for example a very major inconsistency in the proposed mechanics of time travel. If physical damage is negated when the traveller is returned to the present, how are memories preserved? Even if one posits a non-materialistic view that the mind (and hence memory) is distinct from the brain, that still doesn't explain how the video camera sent to the "past" is able to retain its recording when merging into the present. A videographic/photographic record is still just a physical/chemical reaction, and it should've been undone when the camera "returned" to the present.

Last edited by a moderator: May 7, 2017
22. Nov 3, 2014

### Simon Bridge

... the central point in most of the story is that paradoxes are impossible because you cannot change the past - you always return to the present you left. It's repeated that way through the story.

23. Nov 3, 2014

### Curious3141

Well, yes, but I interpreted scenario 2 to refer to the "Universe" preventing you from changing anything in the past that would cause a paradox.

In other words, the time traveller wouldn't even be able to make a change in the past that he's able to observe the immediate consequences of, while still in the past. An obvious example would be: missing all shots/getting a jammed weapon/being arrested before attempting an assassination of Hitler/one's grandfather. I think this is how most people would interpret scenario 2.

In the short story I mentioned, the traveller is still able to make changes in (his version of) the past and observe the immediate consequences in that past, it's just that those changes are completely "reset" when he returns to the present.

But you're correct in that the closest fit for the story is scenario 2, if you allow a little leeway in how you interpret scenario 2.

The story does reference scenario 3 (the multiple timelines arising from changes caused by the traveller), but discards it as a wishful notion.

24. Nov 3, 2014

### Simon Bridge

... the central point in most of the story is that paradoxes are impossible because you cannot change the past - you always return to the present you left. It's repeated that way through the story.

25. Nov 4, 2014

### dragoneyes001

lets look at it in the same way cryogenics began. today the lady can die of many diseases which may be treatable in the future the future lady has access to future medical knowledge and gets it back the past lady who treats herself with currently unknown methods which make her live to become immortal.
this wold parallel the star trek transparent aluminum way of looking at it. the knowledge definitely exists in the future and can change the past yet the usage even the dissemination of that may very well be how it came to exist in the first place.