What you describe is called http://www.examiner.com/x-15701-Time-Travel-Movies-Examiner~y2009m7d27-Temporal-Theory-101nbsp-What-is-fixed-time-theory Fixed Time Theory, and there are many stories which are said to be Fixed Time, including http://www.mjyoung.net/time/monkeys.html 12 Monkeys, http://www.examiner.com/x-15701-Time-Travel-Movies-Examiner~topic655762-Terminator the original Terminator films, and http://www.mjyoung.net/time/somewher.html Somewhere in Time. (I disagree in each instance, holding that http://www.examiner.com/examiner/x-15701-Time-Travel-Movies-Examiner~y2009m8d6-Temporal-Theory-101nbsp-What-is-replacement-theory Replacement Theory better explains the events in each case.) It is one of the leading theories concerning time travel, but it’s difficult to tell interesting stories without incorporating something like http://www.examiner.com/examiner/x-15701-Time-Travel-Movies-Examiner~y2009m8d27-Temporal-Theory-101nbsp-What-is-a-predestination-paradox a Predestination Paradox, in which something in the past happens only because it causes the events in the future which cause it. (All three films mentioned contain such a paradox.)

I find the theory implausible because of causality and its rejection of free will. To accept that the past cannot be changed demands that the future also cannot be changed, that all events are predetermined and our lives are simply exercises in discovering what we did. I also find its failure of the causal chain irrational. That is, Tom is born in 1980, and in 2010 travels to 2000. The Tom who arrives in 2000 already experienced the events between 2000 and 2010, and therefore they already happened before he could arrive in 2000; yet from the perspective of 2000 they have not happened yet. Thus to my mind there must be a version of history between 2000 and 2010 which does not include the arrival of Tom who has, from his perspective, not yet departed from 2010; and that means that that version of history is altered the instant Tom arrives, from a history in which no one arrived from the future to a history in which he did.

Absent that, life is not a series of choices nor a chain of causes and effects, but simply a mosaic pieced together either by a brilliant divine artist or by a completely random scattering of pieces which only give the illusion of scientific order. I prefer to believe that we are actors in our lives, not puzzle pieces. The alternative is, of course, that you and I are not really “having a conversation” in the sense that we mean, but that you were destined to write your post and I to write mine, quite independently of each other. That’s what fixed time theory really means.

–M. J. Young

Re: quantum non-locality: and yet it was on an a Institute of Electrical and Electronics Engineers (I-triple-E) list of technologies likely to have practical application in the twenty-first century not so long ago, so it is more than a metaphor. What is possible has yet to be determined. (Teleporter technology has been suggested.)

–M. J. Young

There is a passage of time from the object’s perspective (or any other), namely the time needed to traverse the wormhole. Instantaneous wormholes can not exist, any wormhole has a finite length that can not be traveled faster than the speed of light. It may still be much faster than the light would take to travel through the “apparent” space, but this is not moving “faster than light”. It simply means that the observer missed the wormhole and the actual distance between its mouths is much shorter than she thought.

This concerns wormholes that already exist in the universe (none was ever seen). But transmission of information to prescribed locations would require creating wormholes at will. There are doubts that this is even theoretically possible, and at any rate it would take a long time to create one. Replacement theory on the other hand, needs creating wormholes to every other point in the universe instantaneously.

Quantum non-locality:

Quantum non-locality is a metaphor used to explain certain counter-intuitive features of quantum mechanics. Since we are used to classical objects it may be didactically helpful to say something like: if quantum particles were classical (which they are not) then they would behave non-locally. But no, it is not possible for a particle to exist in two places at once, or instantaneously feel a spacially distant impact. In fact, it can be proved that no transfer of energy or information is possible through ‘quantum non-locality’.

However, I have identified two situations in which this appears to be falsified under relativity.

The one of which I am more certain is the one that might prove less. It is established (theoretically) that an object passing through a wormhole emerges from the other end of that wormhole without passage of time from any perspective, “instantaneously”, without regard for the apparent distance. This suggests a sort of spacial folding, in most minds, that two points in space which appear to be at vast distances from each other are actually touching. The relative time of these two points is irrelevant to the question; using a wormhole it is possible for an object to impact a distant point in space sooner than would be possible at light speed.

The other is a quantum concept of which I admit having only a very weak grasp but which intrigues me greatly. According to quantum non-locality, it is possible for a particle to exist in two places at once. As it does so, whatever happens to it at either location happens at both locations. I am given to understand that distance is not a factor in this, and it follows logically that since it is the same object existing at different points, the impact on either would be instantaneously felt at the other.

These two concepts demonstrate the possibility that “instantaneous” transmission of information or effect is possible, escaping the limitation of the speed of light. I do not propose a relativistic or quantum mechanism by which time could be so universally affected; I only note that it is not a priori limited, as particularly Mr. Koshkin insists.

The problem of the lack of a universal “now” is a separate issue; but I am not addressing time travel as a physicist but as a metaphysician, trying to grapple with the issues of causality once temporal sequence is disrupted.

–M. J. Young

You wrote, “For example, you could have a wormhole with one opening in your office in 2030, and the other end in your office in 2000. Just because you walk into it once doesn’t compel you to walk into it every time you get back to 2030. And, even if you do walk into it again, it’s a 30-year-older you following the original you.”

My concern about the traveler entering it again is here: in 2000 I was 45, and in 2030 I will be 75. If I travel back to when I was 45, I am 75, but my 45-year-old self is there. Assuming I live to be 105, I will still be alive in 2030 when I departed for 2000, and my younger self will now be 75. What if he doesn’t enter the wormhole? What if he does? I expect that the outcomes must be different. Fixed time tells me that he cannot fail to do so, because I already arrived in the past; parallel dimension theory tells me that’s irrelevant because this is a different universe anyway. In replacement theory, it is possible for him not to enter the wormhole, and it has consequences to this universe.

You wrote, “As a digression, I’d also like to point out that determinism and free will are not necessarily incompatible, and you could even argue that free will doesn’t make sense without determinism.”

I am comfortable with that. I figure that whatever I choose is based on my free response to all the influences which impact me to that moment, and were I to live through that moment again with exactly the same history and influences, I would choose the same way. Not everyone is comfortable with that idea, but it is necessary for replacement theory to resolve.

You wrote, “If the time traveler remembers the original timeline (and aged, etc. along that timeline), but that timeline doesn’t exist, then there are effects without causes.”

It’s certainly complicated, and I’m not completely certain I am right on all this; but I do insist (see my Back to the Future analysis) that the only time traveler who exists after the point of departure is the one for whom through the final version of history is his only history, and thus unless there is some mechanism for transferring memories from one self to the next, there are no memories of the universe that no longer ever was.

I also agree that the concept of a history ceasing ever to have been is difficult to grasp in either temporal or atemporal terms, and is one of the most difficult aspects of replacement theory. I tend to analogize it to history being a recording on a video tape, in that as the new history is created the old is erased; but this does not explain the memories the time traveler might have if he came from the original history and created the second. That problem resolves once time stabilizes, though, since for time to stabilize the traveler who leaves from the future is identical to the one who arrives in the past, having lived through the same history, which is the final form.

I suspect that your objections are beyond what I can answer, but I hope this helps you understand a bit more clearly what I am saying.

–M. J. Young

As for which one is “right,” well, philosophers have been arguing that since the ancient Greeks (and, more usefully, since JME McTaggart’s “Unreality of Time” in the early 20th century clarified the issue).

In fact, many people studying quantum gravity argue that spacetime (and therefore time) can be derived (as Antiphon, Leibniz, etc. historically argued, but obviously couldn’t physically prove) from relations between events, and that both of these pictures are further-derived approximations.

And Julian Barbour in “The End of Time” (not the Doctor Who finale, BTW, although I did love that) argues that they are two different illusions that in a completely timeless mega-dimensional configuration space (as Parmenides and Buddha tried to articulate, but without the mathematical machinery).

So, don’t get too hung up on which one is “right”; just make sure you always know which view of time you’re using in a particular argument (not to mention for which observer).

But I think I’ve seen the deeper problem behind most of Koshkin’s objections. I think you need to be clearer about the ontological status of the idea of a timeline being replaced. His objections are all over the map and somewhat contradictory because the theory is all over the map and somewhat self-contradictory.

Forgive me in advance for what I’m sure will turn out to be a long, meandering, and poorly organized meander. But I think it’s worth following me–either I’m right, or you can figure out where I’m wrong and from there derive answers to a wide class of critiques.

I think that, however you resolve your theory, it’s going to have to be either Novikov (aka “Fixed Time”) in disguise, parallel universes in disguise, Novikov in disguise implemented on top of parallel universes in disguise, or something either inconsistent or incomplete. On top of that, you’re going to be stuck with a lot of unnecessary baggage that adds nothing to the theory.

Of course that’s not to say that it can’t be useful for pedagogical purposes (or RPG purposes), to describe how the universe looks from the reference point of a time traveler. (The fact that centripetal force is not an extra force doesn’t mean that it’s not sometimes a useful idea.)

I think that the central problem that’s keeping you from getting this straight is that you’re trying to imagine time travel without (general) relativity. GR inherently allows the possibility of closed timelike curves (CTCs). Someone traveling along such a curve can be moving locally into his own future all the time, and yet arrive at his own past. At that point, he could just go back around the curve again, but he doesn’t have to (at least not in 3 or more dimensions)–you can have a worldline that curves off to the side sharply enough that it comes back and intersects itself, and heads forward again normally from there.

For example, you could have a wormhole with one opening in your office in 2030, and the other end in your office in 2000. Just because you walk into it once doesn’t compel you to walk into it every time you get back to 2030. And, even if you do walk into it again, it’s a 30-year-older you following the original you.

Given this picture, everything becomes a lot simpler to understand. An N-Jump is just a worldline with a loop in the middle of it. An infinite loop is a worldline that terminates in a loop (or figure-8, or any other closed curve). A infinite sawtooth is a worldline that goes around and around in an endless helix. And a finite sawtooth just involves going around the loop a few times before going off somewhere else.

In no case is there any need to drag the rest of the world along through some sort of instantaneous change. You might have to drag part of the world into the curve with you, depending on the scenario (and I suppose you could even drag in the entire observable universe if you were really clever), but this can be easily done in a relativistic way. The normal causality of the universe flows out from the future light cone of that intersection (including flowing into the loop, if you’ve gone into your own past light cone).

Before Novikov (or at least before Kip Thorne), physicists believed that any universe with such a loop was impossible. Novikov’s Conjecture is that, as long as history viewed from outside the loop is self-consistent (which the N-jump, infinite loop, and sawtooth are, if envisioned as I described), and normal physics apply within the loop, then the universe can accommodate it with no problems. (Novikov didn’t prove this; he just showed that the known arguments against CTCs don’t apply if his consistency requirement holds, so it’s at least possible for them to exist, given our current understanding.)

This doesn’t necessarily require determinism. You could have an actual choice, upon returning to 2000, between continuing on away from the loop vs. going around a figure-8.

If spacetime is fixed, this choice is determined; if it’s not, it may not be. But either way, there’s no need for mystical forces or cosmic coincidences to prevent you from making the “wrong choice.” As long as every path you can possibly choose (and there could be any number of these, probably even an infinite number) is consistent, everything works. This is the same as our everyday experience. If the universe is determined, then there’s only one path for your worldline to follow; if not, there are multiple possible paths. Adding loops to the possibilities doesn’t change that.

It’s true that the paths are restricted to those that generate consistent histories. But I don’t believe RT can accommodate inconsistent choices any better than Novikov. There are only 3 possibilities in RT, and all 3 of these can be explained in a Novikov universe.

As a digression, I’d also like to point out that determinism and free will are not necessarily incompatible, and you could even argue that free will doesn’t make sense without determinism. But I can’t explain that, short of pointing to Daniel Dannett’s books “Elbow Room” and “Freedom Evolves” and the various philosophical debates that have followed on from them.

The biggest difference between TR and Novikov is that the original timeline never existed. There may be multiple paths that you can choose the second time around in 2000, but there is no path that involves you not being in 2000 if you find yourself in 2000. (Your local 2000, or even two different local 2000s, could easily be in the middle of the CTC, which would be necessary for certain endless loop scenarios. Otherwise, all of the stuff you interact with wouldn’t be part of the infinite loop. But even those aren’t parallel universes; there are just two areas of spacetime that are both 1 year later than 1999, both existing in the same global spacetime.)

That eliminates the ontological problems with RT. What does it mean for a timeline to cease to exist? Ceasing is something that happens in time; there’s no before and after outside of time. A segment of time that doesn’t exist just never happened. Moreover, if both timelines are in any sense ontologically real, then what you’re describing is a multiverse, not a single universe. For RT as described, you really have to pick one or the other, but neither one is acceptable.

It also eliminates a serious causality problem. If the time traveler remembers the original timeline (and aged, etc. along that timeline), but that timeline doesn’t exist, then there are effects without causes. If that timeline does exist, not only do you have multiple universes, you have cross-universe causation. But if time travel all happens along a single (locally timelike) timeline with a loop in it, through a single universe, as described by GR, normal causation works around the loop just fine.

In summary, the concept of universe replacement is either inconsistent with physics, a convoluted way of disguising perfectly normal causation in a universe with CTC and subject to Novikov, or a convoluted way of disguising multiple universes. You need to make it clear which one you mean–and, once you do so, you won’t need it anymore (except maybe for pedagogical or story-telling purposes).

On a side note, the supertime idea doesn’t seem to add anything useful. The issue you’re trying to solve is that you want local (total) causal ordering even when you don’t have chronological ordering. But that problem is irrelevant. You already have a total chronological ordering for any worldline without an infinite loop. And if you want to order an infinite loop for some reason, you can trivially transform it into an infinite helix where each iteration is indistinguishable from the last. And if supertime is actually a second time dimension–well, either you’ve got 3+2 relativity, or a 5th dimension that doesn’t participate with the other 3+1 (which is going to require some really odd topology), but either way this just makes it harder to create a total order, not easier. Plus, it’s hard to see how that’s any different from taking an array of parallel universes and slapping a metric on them.

–M. J. Young