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I am working on a story and came up with a doubt regarding an explanation given for time travel using wormholes as portals.

The below given is taken from Wikipedia link: https://en.m.wikipedia.org/wiki/Wormhole

"For example, consider two clocks at both mouths both showing the date as 2000. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2004 while the stationary mouth's clock read 2012. A traveler who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2004, in the same region but now eight years in the past. Such a configuration of wormholes would allow for a particle's world line to form a closed loop in spacetime, known as a closed timelike curve. An object traveling through a wormhole could carry energy or charge from one time to another, but this would not violate conservation of energy or charge in each time, because the energy/charge of the wormhole mouth itself would change to compensate for the object that fell into it or emerged from it."

Well let's consider A and B be two portals kept next to each other and let there be two clocks with those portals A and B respectively and those clock shows the synchronized year 2000. Let A be on earth and portal B taken along with the clock is taken on a journey somewhere in high velocity or speed. After journey let these two portals be kept next to each other as before. For an observer it could be seen that the clock kept with portal A which remained on earth shows year 2012. And the clock kept near portal B shows 2004 (due to time dilation). Like the time dilation happened to Copper in movie Interstellar. Both the portals are brought back and kept side by side. Well, when comparing their ages we could understand that only their ages differs. I mean portal A is 8 years older than portal B.

But if I were to go through portal B I will be exiting portal A the year I entered and not in the past or future. Then how could we use it as a time travel machine or how time travel works with wormholes?

a similar thought experiment is quoted below,

Kip Thorne who is the Feynman Professor of Theoretical Physics, Emeritus, at the California institute of Technology proposes a thought experiment in his 1994 book "Black Holes and Time Warps" (W.W. Norton & Co. 1994)

Say he obtains a small wormhole, which connects two points in space as if they were not separated by any distance at all. [What's New in Black Holes? A conversation with Kip Thorne]

Thorne takes his wormhole and puts one end in his living room, and the other aboard a spaceship parked in his front yard. Thorne's wife, Carolee, hops aboard the spaceship to prepare for a trip. The two don't have to say goodbye, though, because no matter how far away Coralee travels, they can see each other through the wormhole. They can even hold hands, as if through an open doorway.

Carolee starts up the spaceship, heads into space and travels for six hours at the speed of light. She then turns around and comes back home traveling at the same speed — a round trip of 12 hours. Thorne watches through the wormhole and sees this trip occur. He sees Coralee return from her trip, land on the front lawn, get out of the spaceship and head into the house.

But when Thorne looks out the window in his own world, his front lawn is empty. Coralee has not returned. Because she traveled at the speed of light, time slowed down for her: What was 12 hours for her was 10 years for Thorne back on Earth.

Now, as Thorne and Coralee hold hands through the wormhole, they are each traveling in time. Coralee has landed on Earth 10 years after she left, and there she will meet Thorne, 10 years older. But she can still reach through the wormhole and find Thorne, who is only 12 hours older. Thorne can step through the wormhole and find himself 10 years in the future, or his future self can step back 10 years into the past.

If I am wrong on my explanation please explain it in detail. THANKS FOR YOUR VALUABLE ANSWERS.

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    $\begingroup$ I do not have facts and sources and I'm not sure this is the right answer, but I believe this theory will only work if the wormwhole is dividing two identical alternate universes. Portal A is on Earth-A and portal B is on Earth-B. By entering Portal B, you leave the year 2012 in Earth-B and exit on year 2004 in Earth-A, which was, until now, exactly identical to what Earth-B's 2004 was. This bears the problem that the changes you make to Earth-A's timeline will not necessarily affect Earth-B, unless you make up a force connecting the timelines. $\endgroup$ – FFN Jan 6 '18 at 11:56
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    $\begingroup$ I know you're asking for the purposes of story writing but if this question is purely about the uses of real world physics it fits better in physics SE $\endgroup$ – Lio Elbammalf Jan 6 '18 at 12:54
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    $\begingroup$ Given that the earth is moving through space at thousands of miles per hour, time travel would require a space ship. Given the amount of radiation that would be generated by a wormhole, it's unlikely that a traveler would survive the transit. $\endgroup$ – pojo-guy Jan 6 '18 at 13:56
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    $\begingroup$ You can turn (almost) any form of FTL travel into time-travel. Which I take as strong evidence for the impossibility of FTL. $\endgroup$ – CodesInChaos Jan 6 '18 at 17:39
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    $\begingroup$ Cross-posted from Physics: physics.stackexchange.com/q/378293/44126 $\endgroup$ – rob Jan 6 '18 at 17:52
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You're asking us to explain whether or not something no one has yet proven to exist can do something that only some physicists think it can do.

What answer are you expecting?

Physicists can show you how the mathematics supports the possibility of time travel. However, it depends on cutting-edge concepts like the Casimir effect that we're only just beginning to understand. Just because the math supports it, doesn't make it so. Just because we've found a bit of evidence that suggests the possibility, doesn't make it so.

As Stephen Hawking said in a lecture, "If this were the case, it would raise a whole host of questions and problems. One of these is, if sometime in the future, we learn to travel in time, why hasn't someone come back from the future, to tell us how to do it."

While Professor Hawking admits to the mathematical plausibility of time travel, he suggests in that a reason we might not be overrun by tourists is that, despite early evidence such as the Casimir Effect, you can only travel into the future. From that same lecture:

A possible way to reconcile time travel, with the fact that we don't seem to have had any visitors from the future, would be to say that it can occur only in the future. In this view, one would say space-time in our past was fixed, because we have observed it, and seen that it is not warped enough, to allow travel into the past. On the other hand, the future is open. So we might be able to warp it enough, to allow time travel. But because we can warp space-time only in the future, we wouldn't be able to travel back to the present time, or earlier.

But, despite these observations from a man who knows more about physics than I ever expect to know about anything and everything in my entire life, there is a question that is really important...

Why do you want to know?

If you're writing fiction, then allow it or not. There's enough mathematics and enough opinions in both directions that you can use the concept (as a great many already have) in any way you wish.

The simple reality here is that nobody knows. Nobody! Wormholes, higher dimensions, time travel, are all extrapolations from theoretical physics that are described in mathematics but not actually proven to exist in any way, shape, or form. Your guess is as good as ours — there's simply no actual proof. So, tell yourself yes or tell yourself no, either is as good as the other.

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  • $\begingroup$ This doesn't seem to answer the question. The OP postulated a scenario with the two portals A and B and what happens when someone through one of them. Since this is worldbuilding, we can take wormholes as a given part of its reality. Admittedly that involves handwaving away numerous conceptual problems. This is fiction. Just do whatever you want answers are not very helpful IMHO. $\endgroup$ – a4android Jan 7 '18 at 3:45
  • $\begingroup$ @a4android, the OP is quoting from factual sources. He's not postulating anything. Once again, the reality-check tag is in play. $\endgroup$ – JBH Jan 7 '18 at 6:33
  • $\begingroup$ I realized that the reality-check also applies inworld -- whether a scenario or situation as described makes sense or is plausible. It's not merely applied to reality. The OP may be quoting from sources. However, go back and re-read the question to find what is actually being asked. The word being built assumes wormholes are real as per the quotations. $\endgroup$ – a4android Jan 7 '18 at 12:26
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Is time travel ... possible logically?

TL;DR: No.

Time travel is clearly impossible, as it would cause paradoxes.

Time travel is a staple of science fiction because of its impossibility in reality. But it is really more like magic in that it bypasses logic and requires a suspension of disbelief. So the answer to "Can we use this process to achieve time travel?" is going to be no, regardless of the process.

The immediate flaw with this particular suggestion is that relativity does not work that way. There is no "clock". Two different points have no shared time. One of the major points of relativity is that there cannot be simultaneity in time without simultaneity in space.

Wormholes would not work that way either. A "wormhole" is two points in three dimensional space that are far away from each other in that three dimensional space. But the two points are close together in a higher dimension space.

Not clear? Take a sheet of paper. Its surface defines part of a two dimensional space (which we call a plane). Pick two spots on the paper that are not close to each other (but are on the same side of the paper). Now bend the paper such that the two dots are touching. So now you have a wormhole in the two dimensional plane. This works because the two dimensional plane of the paper is in our three dimensional reality.

Now try moving one of the dots without moving the other. If you do this, you break your wormhole. They are no longer adjacent in the three dimensional space. To maintain the wormhole, you have to move both at the same time.

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  • $\begingroup$ your answer sounds clear but i added another thought experiment in my question. this thought experiment was given by a well known professor. well if your statement "To maintain the wormhole, you have to move both at the same time." is correct then i couldnt understand why he gave such an explanation. $\endgroup$ – Peaceman Jan 6 '18 at 13:54
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    $\begingroup$ Your understanding of a wormhole is wrong. A wormhole doesn't need a higher dimension. You're probably thinking about visualizations of wormholes, which certainly need that higher dimension just to visualize a non-trivial topology. But those visualizations don't capture everything; in particular, the time dimension is missing. Don't confuse the visualization with the “real” thing (“real” in scare quotes because as far as we know, traversable wormholes are not possible within the known physics). $\endgroup$ – celtschk Jan 6 '18 at 21:19
  • $\begingroup$ -1. The physics in this answer is incorrect. $\endgroup$ – Charles Staats Jan 7 '18 at 3:50
  • $\begingroup$ Alas, this does not answer the OP's question. You're not alone. Almost everybody seems to want argue about the possibility of time-travel instead of actually answering the question. $\endgroup$ – a4android Jan 7 '18 at 3:56
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    $\begingroup$ By your reasoning you would always immediatly break the wormhole. Everything in the universe is moving with respect to each other. Moving them both at the same time (presumably in the same direction) assumes some kind of intergalactic reference point. $\endgroup$ – D.J. Klomp Jan 9 '18 at 1:51
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You are building a world and in your world wormholes, as we understand them, are real. Therefore, according to your scenario portal A has remained on Earth while portal was transported at relativistic velocity and returned to Earth. A clock at the mouth of portal A would show 2004, while portal B's clock shows 2012. This means there is a time differential of eight years between the two portals.

Let's move forward in time to year 2048 and Bob decides to step through portal B. When he emerges from portal A he will have arrived in 2040. This is due to the eight year time difference between to the two portals. Unfortunately, Bob doesn't have the correct visa for 2040 and is sent back through portal A. he re-emerge from portal B back in 2048.

Alice, on the other hand, steps into portal A in 2048 and when she exits portal B she will find herself in the year 2056.

Two portals consisting of two wormhole mouths with a time difference of eight years will enable people to travel in time back and forth between intervals of eight years. Specifically, in this instance, time travel can be only accomplished in eight years intervals.

For example, if Alice having arrived in 2056 decides she wants to see more of the future steps through portal A again. She emerge from portal B in 2062. If she wanted to go further forward into future, say, the year 2848, then she have to pass through the pair of portals one hundred times to eventually get there (if starting from 2048). Naturally it will take one hundred passages through the pair of portals, in reverse order, to travel back to 2048. To be clear, Alice will have step into portal B to exit from portal A and keep doing this until she is back in 2048 again.

Wormholes, based on the concepts we know and that emerge from our current understanding of science, although there are major caveats as to whether this could be a viable technology in the real world, however, in a fictional world using wormholes for time travel is scientifically plausible. This time travel will be limited to "jumps" equivalent to the time difference between portals.

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    $\begingroup$ That limitation could be used to add to the drama of the novel--especially if the plot involves the separation of people who want to be together. $\endgroup$ – James Grossmann Jan 7 '18 at 7:12
  • $\begingroup$ @JamesGrossmann Limitations often make better plots. Challenging to both the writer and the reader, in the nicest possible way. $\endgroup$ – a4android Jan 7 '18 at 12:28
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The mathematics that we currently use to model wormholes permit such time travel. However, that is a far cry from declaring that they are actually possible. To this date, we have not yet observed a wormhole, much less one which connects different times, so we cannot say whether they actually exist. What we can say is simply that the math permits it. It's similar in nature to how you can have a problem which ends up having a negative answer and a positive answer, and we discard the negative one because it's "not physical." The mathematics of the model claims the second solution is there, but practically speaking, we never see it.

As for your experiment involving traveling wormholes, it's worth remember that a wormhole is not actually a thing. It doesn't behave in any way like the matter you and I are used to. Its an aspect of the topology of space. Space itself gets a hole. Thus, you should expect the math for calculating time dilation to get exponentially more difficult. Wormholes make garden variety general relativity problems look like kindergarten assignments.

As Charles Staats linked, I highly recommend reading the Wikipedia article on the Novikov Self Consistency Principle. In particular, the Polchinski Paradox may be of great value for your understanding. In the Polchinski Paradox, a billiard ball is sent back in time using a wormhole on a collision course with itself. If the ball collides with its past self, it knocks it off course so that it does not go through the wormhole. This particular paradox was solved in 1991, by Fernando Echeverria and Gunnar Klinkhammer:

Upon considering the scenario, two students at Caltech (where Thorne taught), Fernando Echeverria and Gunnar Klinkhammer, arrived at a solution to the problem that managed to avoid any inconsistencies. In the revised scenario, the ball emerges from the future at a different angle than the one that generates the paradox, and delivers its younger self a glancing blow instead of knocking it completely away from the wormhole. This blow changes its trajectory by just the right degree, meaning it will travel back in time with the angle required to deliver its younger self the necessary glancing blow. Echeverria and Klinkhammer actually found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each situation. Later analysis by Thorne and Robert Forward illustrated that for certain initial trajectories of the billiard ball, there could actually be an infinite number of self-consistent solutions.[6]:511–513

Echeverria, Klinkhammer and Thorne published a paper discussing these results in 1991;[7] in addition, they reported that they had tried to see if they could find any initial conditions for the billiard ball for which there were no self-consistent extensions, but were unable to do so. Thus it is plausible that there exist self-consistent extensions for every possible initial trajectory, although this has not been proven.

(Emphasis mine)

Billiard balls are clearly far simpler than spaceships full of people, but they do form the foundation for how you tackle such a problem.

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As others have said, time travel into the past is, as far as anyone knows, impossible. The only logically possible retro-time travel scenario that I've heard of involves the existence of multiple timelines/universes, in which a person travels, not to the past in this universe, but to a similar past in another universe. (See The Fabric of Reality, a popularized explanation of the multiple-worlds interpretation of quantum mechanics, written by physicist David Deutsch.) Not all physicists believe that there are multiple universes.

But if you want to read some adroit hand-waving in a time-travel novel, check out Michael Crichton's novel, Timeline.

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  • $\begingroup$ Crichton does have one major cheat in his novel. Hurricane-strength handwaving concealed with a bit of cunning sleight of hand. Sleigh of hurricane-strength handwaving! Mmm. Not quite what I had in mind when I wrote that sentence. Be that as it may. Michael swanwick's The Bones of Earth is worth a look too. It has time -travel by wormhole. And dinosaurs too! $\endgroup$ – a4android Jan 7 '18 at 3:54
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Time travel is logically consistent, in the sense that the physics of relativity can describe a universe in which time travel happens. There is no evidence that our universe is such a universe and strong reasons to suspect it is not -- or at the very least, that any time travel within our universe must be extremely limited. But I'll skip over these for now.

Manipulating wormholes

One of many kinds of theoretical universe in which time travel would be possible is a universe in which intelligent life has the ability to create and control wormholes, and in particular can move one end of a wormhole at relativistic speed as in the thought experiment quoted in the OP. Here are some consequences:

  • So far as I know, it's not logically possible to use this method to travel back farther in time than the creation of the wormhole. [I've been surprised before by relativistic time travel thought experiments, so you might want to confirm this on Physics.SE.] This neatly deals with the question of why we don't see any time travelers from the future -- they can never travel farther back than the invention of time travel. But it may put unacceptable constraints on your story.
  • If you don't want to invent random new laws of physics (like someone vanishing into thin air when their intervention prevents their parents from meeting), you need to obey the Novikov self-consistency principle. In short, this means that time cannot be rewritten; when you go back in time you can't change anything. (You can try, but somehow it will turn out that your efforts were already part of the timeline you thought you were changing.)

Self-consistency can present a huge problem for storytellers. After all, what's the fun of going to the past if there's no possibility of changing anything? It can be done (see bottom of post), but it's definitely a challenge.

[Note: Self-consistency also creates problems that lead many/most physicists to believe that time travel is not possible in our universe. For instance, if a time traveler decides, as an experiment, to travel back in time by a year and cut down an insignificant tree that was grown and protected for this purpose, and was standing when they left, how is it that they just happen to not cut it down?]

One potential danger with self-consistency: Multiple solutions. For instance, consider the following two scenarios:

  1. A character is attacked by robbers and dies.
  2. A character is attacked by robbers and is saved by a mysterious stranger, who later turns out to be that same character returned from the future.

Both 1 and 2 are completely consistent with your world's internal logic. The trouble is that a solution (or "bootstrap paradox") like 2 can be manufactured to get your character out of almost any situation; it's basically a "get out of jail free" card that you as author can use any time you write yourself into a corner. If you use this device once a few readers will be slightly bothered by it. If you use the device often your readers will know that any situation can be resolved that way, and they may lose interest in your story.

Other forms of time travel

The usual alternative to self-consistent time travel is alternate universes. In this approach, your characters discover or create a portal/wormhole to the past, where they are able to change the timeline. The problem (which many stories ignore) is that if they travel back to their own time, they will find that nothing has changed -- the past they were changing was not their own, but the past in an alternate universe. If you don't want to deal with this, you could find some excuse to make the trip to the past a one-way trip.

But note: if you create both ends of the wormhole in the same universe, they're going to stay in the same universe. So you can't use the technique described in the Thorne thought experiment. (Also, note that a discovered/created wormhole certainly could have both ends in the same universe if you're willing to deal with self-consistency.)

Fudge the laws of physics

The two scenarios above are quite constraining on a story, so many writers have decided to just fudge the laws of physics. If you take this route, your best bet is to come up with some reasonably self-consistent rules about who gets to remember what and how communication works between different "timezones", and stick to these rules. A couple key details that need to be worked out:

  1. What happens if someone kills their own grandfather, or tries? (Do they return to their own timeline and find that no one remembers them? If so, they should be able to remember how the timeline "used to be" even once they've changed it, and people who did not time travel will remember things differently.)
  2. Can someone in the past interact with someone in the present (e.g., by sending messages)? If so, what does it look like when the past is changed? Does a new dent suddenly appear in the car while the present person watches, or does the person in the present believe the dent has always been there? [Note that if you're not fudging the laws of physics, one of two things happens: either the past is never changed (self-consistency) or changing the past does not affect the present at all (alternate universes).]

Side note: Self-consistency story arc examples

Here are a few story arcs I've seen that did not violate self-consistency.

  • A character does not know/believe/accept that the past cannot be changed. They journey back in order to change it. But somehow or other they don't. You can be creative about why not, but there are a couple of "twists" that have proven especially poignant if done well:
    • They end up causing the very thing they were trying to stop. (This is a classic.)
    • They decide not to change the timeline after all. This can be due to personal growth, new knowledge (that volcano eruption was necessary to stop an alien species from invading earth), or some combination.
  • A character, having gone back in time, seems to change things, because they were mistaken about how things went the first time. Surprisingly, Harry Potter and the Prisoner of Azkaban handles this quite well in spite of ignoring just about every other aspect of physics. (But note that the Harry Potter book does suffer slightly from a bootstrap paradox.)
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  • $\begingroup$ actually.. i am working on a story in which wormhole act as a shortcut to a far away star and not act as time travel portals. what i couldnt understand is how do they(professors in OP) explain how it act as a time travelling machine. because i fell it as illogical. well, thanks for your answer. it would be nice if you could answer to this question too worldbuilding.stackexchange.com/questions/101636/… $\endgroup$ – Peaceman Jan 7 '18 at 9:02
  • $\begingroup$ -1. The physics in this answer is incorrect. $\endgroup$ – Brythan Jan 7 '18 at 16:44
  • $\begingroup$ The point about not being able to create a wormhole farther back than the invention of time travel is illogical. It would mean that the whole of reality and all laws of physics would suddenly change upon the invention of the wormhole by humans. $\endgroup$ – D.J. Klomp Jan 9 '18 at 1:41
  • $\begingroup$ @DJKlomp How is that any different from assuming that humans can create wormholes in the first place? (Although I agree that point does not necessarily work if we're just manipulating wormholes' end "points" without creating or destroying wormholes. Not sure what happens in that case -- most likely if you go back in time before the wormhole was relativistically manipulated, time travel is theoretically possible but wholly impractical. E.g., each wormhole transit takes you back a microsecond.) $\endgroup$ – Charles Staats Jan 9 '18 at 2:28

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