The simplest logical way to look at it is that in fact all histories are part of the big history of the universe, that is they happen in the same physical time, one after the other. At a backwards time jump, what actually happens is that time flows as usual, but the whole universe reverts to the exact state in which it was at the destination moment of the jump. The whole universe except the time machine and everything that's in it, that is. When the time jump to the moment D has ended, the universe is again 100% in the exact same state as it was at moment D, except for the time machine that's additionally in it. The universe then resumes natural evolution starting from this state.

So actually, time hasn't reverted back to moment D, but the state of the universe has. If one jumps say from 2500 back to 2000, after the jump the physical time isn't 2000, it's still 2500 (or maybe more, depending on how long the jump has actually lasted, see below), but the whole universe is back at its state from 2000 (plus the time machine). Which means, even if the physical time is 2500 or later, practically it's 2000. Everybody in the universe will think it's 2000, it's all they will know and remember. Everything will be as if it was 2000. You look at the stars, they look like in 2000. You make archeological excavations, they won't show anything after 2000, and things from 1500 will look like 500 years old. Buildings, cars, fashion, weather, politics, music, they'll all be exactly as in 2000. Practically it's 2000 again.

So, there's a distinction to be made between physical time and practical time. For the people after the jump, the only time there is is 2000. They don't know and have no way to know about the time that passed between the "first" and the "second" 2000. Except for the time machine and its contents. So from their point of view, the real time line is the one in which there was nothing between the 2 moments, they're one and the same for them, and if they find out about the time that's passed in between (from the travelers, and by studying the contents of the time machine), to them it will be like some sort of supertime.

From this moment on, by "time" I'll mean practical time. When I'll talk about physical time, I'll indicate it as such.

An alternate, possibly better name for this model might therefore be the state reversal model. But this underlying mechanism is not necessarily part of the model. It is a variant of the model, the simplest one. Other variants might exist, which could enable features that in this variant aren't possible. The essence of the model are the multiple histories and the way they're created and ended at time jumps.

The universe might revert to its state either instantly, or it might take some time. If it does, it could be from very little (maybe a few seconds or even less) to very much, most likely it would revert from D to S in the same duration as it took to evolve from S to D. Maybe even longer.

If that's so, the people in the time machine have to have a way not to age while they're waiting for the process to end; or else the jump to the past would be itself a job for generations. Most likely though they'll do it in a reasonable time frame.

Also in this case, the physical time of the end of the time jump can actually be much later than that of the start. That is, in the above example, if they jump from 2500 to 2000 and the duration of the reversal is the same as that of the direct evolution, that is 500 years, the actual physical time at which they arrive to "year 2000" is 3000 AD. But it doesn't matter, because regardless of the physical time, the universe will be in the state of year 2000, and this is all that counts.

Anyway, be that as it may, this is of no big concern to us. What matters is that, be it instantly or over a period of time, the universe reverts to the state of the destination moment of the jump, except for the additional presence of the time machine and its contents.

This is probably the simplest logical mechanism for complete, unconstrained, paradox-free time travel. Which includes the possibility of going to the past and changing it.

Temporal cycling

Essential: after a jump to the past, in the new history it has to be ensured that the jump isn't repeated again. Otherwise, the natural case that would happen by itself if it's not avoided: Let's say Adam makes a 1000-year backwards jump. The old time line ends there. In the new one, assume Adam dies suddenly, for whatever reason, immediately after landing in the past. His body and the time machine decompose after a few years, there's no evidence of the jump and of their presence any more. As such, it's to be expected that history will repeat itself essentially the same way. So in 1000 years a new version of Adam is born who will repeat the jump at the exact same moment and with the exact same characteristics. The second time branch ends, in the third one Adam immediately dies, since everything is the same as before. This then goes on forever. It's the death of the universe. There will be no evolution any more, just history repeating itself once every 1000 years.

Fortunately, this pessimist variant is not very likely. The second time line probably won't be exactly the same, either because the world is not deterministic, or because of the tiny difference given by Adam's body and the wreck of the machine. This tiny difference could in time lead to big differences, if we accept the butterfly effect, or maybe just little ones, if we don't, but they'll be there. In 1000 years these differences will either prevent a repeat of the jump altogether, or at least it will be done somewhat differently, at a different time moment and under different conditions. So either Adam doesn't die any more, or there will be differences that will continue to accumulate so that after a number of jumps (which can be anything from very small to very big), the circle is broken and Adam can go on doing what he came there for. Or in another way history stops from repeating itself.

Anyway, this situation would have to be prevented, either for avoiding infinite cycling or the occurrence of many intermediate branches that would make it difficult to predict how things will look in the end, when the cycle is broken. Of course in a "real" observation mission to the past there will be more than one person on board, so it's highly unlikely that they all die (except if the whole mission encounters problems). Other safety precautions would also probably be taken.

Consider the details of such a scenario. Adam (let's assume for simplicity that it's still just one person, which is not very likely but makes no difference to what I'm saying here) does the jump, makes his observations in the past he came for, then of course he'll jump back to the future, his present, to report the findings. Only now he's in a different reality. In his time now there is another Adam preparing to take more or less the same jump, with the same objectives. So Adam1 will have to time his comeback jump a few days earlier, or to be on the safe side a few months, possibly years earlier, to find Adam2 and tell him the jump has already been done and give him the results. These 2 Adams will both continue to exist in the new time line, like cloning. It would be a little frustrating for A2. But then, it would of course be expected, the scientific leader of the project at least will be aware that he might be in the second history and at some point Adam from the first history might appear. Ruling out the possibility of the mission being unable to return, he'd do his job normally and assume that if A1 hasn't come back until the time of the launch, he's in the first history and things must to go as planned. It would be Adam1's responsibility to make sure he arrives in the new history before the jump is done again and prevents it.

More about cloning below.

This is how things might happen in reality after time travel becomes possible.

We established above that there has to be a unique control, otherwise extremely unpleasant situations could occur. We're working in this assumption.

Some way or the other the unique control is realized. There is a small group of people who know about the possibility of time travel. It's a strictly guarded secret, other than them nobody knows about it. They have the means to find out when somebody else is about to discover it independently (for instance, some experiments need to be done in the early stages that they can detect, plus of course they are well connected in academical and government environments, have spies etc.). They're of course in some way affiliated to the american government, and have strong ties with at least one major university. Maybe also with a few occult organizations, since they're one themselves (there's much room for imagination here, freemasons, jews running the world and the like).

It has to be secret because of its immense military potential. They could use it in case of a major conflict they lost or are about to loose; going back a few years in time, armed with the necessary knowledge, might mean a whole world of difference. At first at least, only the american government knows how to do it, so they'll obviously go to great lengths that this stays this way.

When a potential re-discoverer appears (which should happen fairly rarely), he's either briefed in and co-opted into the Group, or if it's ruled that they're already too many or it's inconvenient for some other reason, he's eliminated one way or the other (either by luring him towards other research interests, or if this doesn't work killing or capturing him, or some other way).

What happens if a backwards jump has to be done ? Informative trips to the past, ones that don't change reality significantly, would of course be very tempting, but it turns out they're not an option. First, it's not clear if they're possible in the first place, as detailed above. Second, even if they were possible, opening a new history that's very similar to the past one would mean that a whole lot of people would have to go through a whole lot of suffering once again. That would probably not be worth it regardless of what info could be obtained by it. Except maybe if they're done over just a few decades or maybe centuries, assuming the world is relatively peaceful during this time; but then the informative value wouldn't be too big anyway. Then there's the issue of temporal deletion, any trip to the past would have to take care not to delete the people who did the first informative trip; this is probably not very important and could likely be solved somehow anyway.

If the backwards jump has world-changing potential, everybody who knows about it will have to be taken on board or they risk dying (again, except if the jump is done over a little time period so all or some of the people involved are already born at the destination moment). If that's not possible for some reason (technical or economical limitations), it gets ugly, they start eating each other; or there's an accepted way of deciding who will make the jump and who won't. Anyway, in such a scenario things get tense. Most likely though, it's not the case, everybody knowledgeable can be taken on board (that's one of the reasons why as few as possible must know the details).

Since it would probably kill everybody else living in that time, a reality-changing backwards jump (RCJ) would be a major enterprise which would require careful planning. Also because everything else from the erased history is going to be lost (except for what's in the time machine), such as the possibility of archeological findings from it.

In these conditions, the responsible strategy would be to only do RCJs as little as possible, only go back to correct truly catastrophic situations. They probably would have no reason to want to go before 1945 (while other nations such as Russia, China, Germany, France and practically everybody else might want to try to challenge american domination this way). Things like 9/11, the Iraq war, hurricane Katrina or the economic crisis would obviously not be enough to justify such a jump; it would take a major catastrophe such as nuclear holocaust or worldwide ecological disaster.

Actually, they might want to try to go back before 1945 to stop the invention of atomic weapons, or at least control them better. More generally, they might try a serious action for controlling science to be applied just for peaceful reasons. Or maybe not. This falls under the general notion of trying to improve history, and it would probably be attempted in catastrophic circumstances only.

(Actually, this idea of the americans inventing time travel assumes it would be found in the next few hundred years. If, as it looks more likely, it will take more than that, there's no saying how the world will then look like, aside from the fact that the more likely outcome is that self-destruction happens long before it can be prevented this way. For local color though I'm going to stick to the american variant, as improbable as it is.)

When such a catastrophe would occur, the first problem would be to act quickly enough, or the jump should be done from a safe location (subterranean bunker or something like that, maybe even from outer space). They do the jump and correct things one way or the other, assuming it's possible; the details are uninteresting at this point.

So, obvious first use - recovery from a major catastrophe.

Second probable use: jump to the future to avoid death. As the people involved are getting old, or maybe get serious diseases that can't be treated in their time, they might be tempted to try to go a few hundred or thousand years into the future, hoping that by then there's a solution to their problem. Such actions could be done periodically, with everybody in this situation at the given moment. Obviously, there are risks, one cannot know how things will look like in 1000 years; besides, after a jump to the future you risk being wiped out by even informative backwards time jumps. But maybe they'll want to take their chances.

Also tempting would be informative trips to the future, but it seems they're also not very easy to do. Besides the risks involved and the possibility of time deletion, which are the same as for death-avoidance trips, the biggest problem would be coming back to report the findings. This would have big potential for changing the future history, so the future Group won't allow it. That is, if the Group still exists, which would mean things are largely OK. If the mission finds total chaos and post-catastrophe conditions, they could come back and open a new time line, making the people in their time aware there's a potential big problem. It might work this way. They set a return date a few days after the takeoff; if by that date the mission is not back, they know they're in the first time line and things are either OK or the mission is there to open a second time line if necessary. If the mission is back, they are in that new time line; they now know they have a big problem so they're better prepared to deal with it.

Such a jump would probably be made over a large period of time, a few thousand years at least. Because of the possible risks, it might be done with an unmanned mission. It would be less of an informative mission and more of one for additional safeguarding of the Earth.

This might make the people in the first time line feel relaxed, which might in turn lead to the exact catastrophe they were trying to prevent. A somewhat paradoxical situation. It's just relative though, they cannot be 100% sure that the mission will be back if they find disaster, especially because in such conditions unforeseen difficulties might arise. They'll have reason to feel somewhat protected but not to the point of becoming sloppy.

For safety reasons, this time jump would start on the earth (probably a safe underground location) but might end in the future somewhere in outer space. It sounds technically challenging but once time travel is possible it shouldn't be much of a problem. This might even be the standard way of doing time jumps, to avoid the collision with the matter at the destination point. It seems likely that in nearly 100% vacuum, material superimposing would have neglectable consequences.

The US will protect the secret of time travel to the best of their abilities, but other forces, especially China, who is expected to become the main challenger in the near future, might find out about it. In case of a conflict, they might try to destroy the time travel system, because of the immense military advantage it gives to their enemy. And they might even succeed, even though it's more likely that the americans would be able to thwart such attempts. In case the chinese do succeed, a safety mission to the future could prove Earth's last salvation.

Generally, backwards jumps for post-catastrophe recovery (including, from the american point of view, defeat in a major war) would be done over as little time as possible, probably around 100 years maximally.

Another possible use could be doing backwards jumps regularly starting from some moment on, say every 10 years jump 10 years back, loading the machine with a huge amount of information about the scientific and technological progresses done in this time. This would practically freeze the general development of the world to a certain point in time, thus also freezing the major problems like population growth, nuclear proliferation, ecological degradation. It might be a good way to keep them under control until solutions are devised to properly deal with them. Even though it would still mean causing some people to re-live pain, it would be on a far lesser scale than for a big jump backwards, so it might be considered worth doing.

Any jump backwards into a time when the Group exists would have to be coordinated with the Group of that time. It would most likely mean a major catastrophe has occurred in the future, so the mission coming from the future would join forces with the present Group to try to solve the problem. Or it would be a short jump like the ones mentioned above; in this case, again, extensive coordinated work would have to be done.

As long as the Group works properly and is the only force doing time jumps, things can be kept under control. If there are others who can independently do it, or if the Group's mechanisms deteriorate, it can get messy. These could also be very interesting scenarios.

The Ark

Another tempting variant would be a substantial jump back to a well-chosen time in history, in order to create a better, controlled variant of history.

Obviously, the machine would then have to be loaded with a lot of information, as well as personnel qualified in many fields. Kind of a backwards Noah's Ark.

That moment, as said, could be 1945, in order to get rid of atomic weapons, or maybe before, maybe even far before that. Such a major enterprise might be done either after a total catastrophe, or a little before it, when it's considered imminent, in order to spare a lot of people a lot of pain. Also, it might be done even in the absence of such immediate prospects; at some time it could be decided that the general situation has deteriorated to the point where there's little chance to correct it through normal means, so again, to avoid it this is the best way to go.

Of course, they'd also take with them as many as possible of the world's major artworks, some in physical form, the most virtually. This of course would include all important literature, music, theater, movies and any other important art. It would require immense storage space, but obviously this wouldn't be much of a problem.

It would be likely done as a succession of backwards jumps of 20 - 30 years each. This, of course, in order to take this once-in-history opportunity to study the time line that is going to be abandoned in as great detail as possible. After all, it is the only history of humanity up to this point, and with all its shortcomings it has at least one thing to show for itself - the Ark. Since the Ark will for a long time in the new history be the main source of knowledge for humankind, it will be of course of greatest importance to also know the history that lead to its creation. Also, to better be able to avoid the same missteps in future histories.

So they'd repeatedly jump back for 20 - 30 years and stay for a few months, maybe a year or 2 at each stop. During this time, the people of the Ark, obviously extremely qualified in their field of work, would talk to the important personalities of the moment. They'd go to libraries, visit places and take pictures and films. They'd participate covertly to the major historical events, filming moments such as the battles of Austerlitz and Stalingrad and the discovery of America by Columbus. They might even stop to watch the first landing on the moon, see if it really happened.

(Of course, the very act of filming a major battle would mean that battle has to happen all over again, so it's also possible that for humanitarian reasons this is not done, or it's restricted to only a few very important moments.)

They'd most likely be discrete towards the general public, would avoid publicity since this would harm the mission, on the other hand there's no reason for much secrecy either since this reality is immediately going to be discarded anyway. So if they find Washington after the american independence war and want to question him, there's no reason not to tell him who they are and where they come from, they can also prove it to him, of course without giving him too many details. Or, if it's deemed useful and they have enough resources, they could take him with them, to chat comfortably. Likewise also Newton, Leonardo, Aristotle, Archimedes and many others.

In case they go back until before the start of history, say around 5000 BC, obviously in the next time line Archimedes, Caesar and many others would have completely different lives, probably wouldn't even exist any more. In these conditions, even though now physically it's almost completely dead, the previous history branch is still part of the new history by that all knowledge brought by the Ark has been discovered in it. When an education system is established in the new reality, children would learn the basic theory of time travel, and that most of the existing knowledge (especially at begin of the new history) was found in the old history. They'd learn about Pythagoras, Aristotle and Archimedes, and all the others, even though they're not in their direct past, and as far as dates are concerned are even in the future.

Also, it would be interesting if the Ark stopped when Archimedes was a little boy, say around age 10, they took him with them and gave him a top-level education from year 2500. In this situation, Archimedes would learn how he himself invented the principle named after him, among many other things, in the old history, but now in the new history he didn't get to live enough in those same conditions to invent them again. The procedure would probably be like this: first they'd stop around 215 BC, shortly before Archimedes' death, they'd take him on the Ark and question him about everything he wrote, everything he invented and more generally all he knows; which would mean they learn more or less everything there is to know about mathematics and science in general at that moment, because he probably knows almost everything worth knowing. Then they'd do a stop about 60 years earlier, to take him as a boy, as said, give him an advanced education and with it the chance to make new discoveries. Of course, things could be pushed even further, he could be cloned into more instances, but maybe this wouldn't be necessary. But it would be very plausible for them to take both old and young Archimedes on the Ark, the first one for questioning, the second one to be educated and make new contributions to science. This could of course be done for other personalities too, who would then get together, each one of them in the old and young version. Or maybe for some just the old version; for instance, it would be less useful to take Washington or Caesar as children.

Still assuming the beginning of the new history at about 5000 BC, it doesn't necessarily mean their journey to the past would end there. They could decide to go further down, still for documentation purposes. Maybe even 100 - 200.000 years before, to study human development from pre-history to civilized society. They risk though to disturb this evolution, maybe it doesn't happen the same the second time, for better or for worse. Besides, again it would mean repeating death and suffering, so maybe they wouldn't make this last step. If it was still considered worth doing, they'd also make a few stops on the way, one every few hundred to thousand years. They'd collect data on the way down, and on their way back they'd be able to compare things. Returning from that distant past could be done either in real time or more likely as a series of jumps to the future. Anyway, it would still be an enterprise for many generations.

In the end, the Ark would stop at 5000 BC and begin ruling and forming the new world, based on the vast amount of knowledge they got with them.

One other thing to note is that all this happens in the assumption that time jumps are possible, but looking back in time from the distance, as described above, is not. Though it seems more plausible that looking back in time is easier than jumping back, so it would be discovered before or at most simultaneously. So stopping along the way wouldn't be necessary any more, everything there is to know from history could be learned this way, before doing the time jump to the begin of the new history. But for a good script, we can also assume that this is not the case, that backwards time jumps are possible but looking back in time is not. It's not even that unlikely, since for the future as well, jumping to it would be possible, but looking at it from the distance wouldn't be since it would contradict the causality principle.

Summary of the script

So there would be 6 main uses of time travel:

1. backwards jumps for recovery from a global catastrophe

2. the Ark, that is a huge backwards trip to repair history

(these 2 are the major ones)

3. additional Earth-safety precaution trips to the far future

4. freezing the world for quicker scientific and technological development

5. life-prolonging trips to the future

6. cloning people and objects

Maybe also others.

Informative jumps in the classical sense, to the past as well as to the future, would most likely be ruled out. But, as said, within the Ark project a major information action could be done along the way.

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