[LeeE]

I think that just about all of the paradoxes that seem to accompany going back in time are due to requiring simultaneous movement in both directions i.e. the time frame for the time traveler continues moving forwards, or stops, while he is traveling backwards in time.  Of course you're going to end up with paradoxes concerning where you are if you're trying to move in two opposing directions at the same time.

In the scenario where events are sequentially unrolled, you end up back in the past but the future no longer exists.  Because nothing is isolated in the universe, such a rolling back would require the entire universe to be rolled back.  When the time pointer then starts moving forward again, events may indeed occur differently but because the previous future has been rolled back and no longer exists there would be no paradox.

This type of time reversal isn't much use for time travel as we want it to work though.

For a time traveler to be able to remember the future he came from he can't travel within the set of dimensions that contain our universe.  He would, therefore, need to entirely exit our set of dimensions and continue moving forward in a different spacetime set that was oriented so that his future end point in that set aligned with his target date in the past in our set.  When he then reached that point he would need to exit the set that he was traveling through and renter our set.

The problems to be overcome include a) finding and proving the existence other spacetime sets b) establishing their possible orientation and c) finding a mechanism and method of moving from one spacetime set to another.

This sort of time travel would seem to allow paradoxes but even then I'm not sure that they really are paradoxes, or at least simple absolute paradoxes.  For example, if a time traveler went back in time and killed their grandparent, so that their parent never existed, it's not a paradox for the time traveler because in his time line, the killing of his grandparent occurred after his parents' and his own birth.  For everyone else's time line, everything that subsequently happens is part of their future, including the presence of the time traveling killer, who is not now from the future but is part of the present.

All just speculation, of course, and it doesn't address all paradoxes, but I do think you have to be careful about what actually is a paradox and what is really just an unrecognised contradiction in terms.

[graham.d]

Lee, what you descrobe is what I referred to as parallel universes. There is EITHER paradoxes OR sets (probably infinite sets) of parallel universes. A traveller who changes something in the past can only return to a universe where that change was made (by him) and not to the universe from which he started. This is logically correct but, although attractive from a Schrodinger cat point of view, I believe is not sustainable in many (if any) of the Quantum Theories that study such matters more deeply - I think there is a problem with the total energy. It would also be a pain if you trod on a blade of grass that somehow changed the world to something unrecognisable when you return to the future. This concept has been explored by many SciFi books.

[Karen W.]

Could it be something to do with ' how do you get back ?'...unless you can travel with your time machine how can you activate it in the past (before it was invented) to bring you back ?

Wouldn't you have to make sure the time machine was equiped with all the knowledge to inform you as to how to replicate its use and design along with any future information they would have needed to get the materials and the knowledge that was used for that.. etc like the process would have to be detailed in every way every step  so that it could be replicated again in the time you land yourself in... quite complicated it seems to me... imformation like the processing of fuels etc the type of power used where to find fuel what it comes from how to process it same with plastics or anything really that the past had not encountered it would be complicated to say the least and depending on how far you were able to go back would be the extent of explanation and usage of tools materials supplies and information etc....

[LeeE]

Lee, what you descrobe is what I referred to as parallel universes. There is EITHER paradoxes OR sets (probably infinite sets) of parallel universes. A traveller who changes something in the past can only return to a universe where that change was made (by him) and not to the universe from which he started. This is logically correct but, although attractive from a Schrodinger cat point of view, I believe is not sustainable in many (if any) of the Quantum Theories that study such matters more deeply - I think there is a problem with the total energy. It would also be a pain if you trod on a blade of grass that somehow changed the world to something unrecognisable when you return to the future. This concept has been explored by many SciFi books.

Yeah, I think there's an energy issue with bifurcating sets.  If a set splits so that there are now two parallel sets instead of just one, either energy is needed to form the new set or the energy of the original single set must be divided between the two resulting sets, leaving the original set with only half its original energy.  Note that I'm using the term 'energy' very loosely here, but the energy and mass present in the original set has to be duplicated in the bifurcated set, so the total energy and mass of the two sets would have appeared to have doubled from nowhere.  That is unless we posit a reservoir of unlimited energy/mass that can be drawn upon to power the bifurcation.

On the other hand though, if the total energy/mass of all sets is constant, and the energy/mass total for any particular system is halved every time a bifurcation occurs, then the energy/mass total for our 'current' set will be an incredibly tiny fraction of what it once was.  Such a reduction of a set's energy/mass total over time could parallel other time-related phenomenon in the universe though, like the Big-Bang and the expansion of the universe.

Another alternative is that the bifurcated sets are not wholly new additional sets but just different 'views' of the single original set, so the total energy/mass is common to all sets.  This would mean that everything in the one 'real' set would have to be able to occupy vast super-states of every possible outcome, with each specific 'view' only showing one specific state.

[DoctorBeaver]

I'm a great believer in the simplest explanation being the most likely. The simplest explanation in this case being what I said. It takes but 1 premise - that if you travel back from time B to time A, then from time B's perspective A->B happened the way it did & cannot be altered. Bifurcation & conservation of energy are irrelevant.

[graham.d]

But, DB, it doesn't hold together and leads to insurmountable logical difficulties. It throws away the concept of causality for example. No backward time travel ideas can be "simply" explained (or even reasonably postulated) because it is probably not possible, at least not to any significant ealier time. It makes for amusing SciFi stories where the physics does not have to be strictly complied with.

[DoctorBeaver]

But we don't know the physics of it.

[graham.d]

IMHO, to avoid causality and logical issues it has to be a parallel universe scenario. I'm afraid I have not studied the details, but, as I understand it, this does not fit with any of the models we have for the universe because of energy considerations. It is true we could try to postulate new physics to encompass what we would like to believe, and there have been arguments to put forward for such a view independent from the idea of time travel, but so far I don't think there have been any that are wholly self consistant.

As a SciFi fan I enjoy all the ideas that come up and can live with suspension of reality, including Dr Who's Tardis, Star Trek's matter tranportation, Faster than light travel, all aliens speaking English and hosts of other extrapolations. But interestingly, and being very nerdish, it annoyed me when Star Trek had the ship in a "geostationary orbit over the North Pole" or when in 2001 (and even in a recent movie) they didn't have low gravity on the moon once inside buildings. I guess its because I dont like something that is even inconsistant with the assumptions made within the self contained fictional scenario. Sad, isn't it :-)

[LeeE]

I'm a great believer in the simplest explanation being the most likely. The simplest explanation in this case being what I said. It takes but 1 premise - that if you travel back from time B to time A, then from time B's perspective A->B happened the way it did & cannot be altered. Bifurcation & conservation of energy are irrelevant.

The premise that once something has happened it can't be changed doesn't work if time travel is allowable.  The fact that the time traveler didn't exist when time 'A' occurred, but then subsequently must exist there if he is able to travel back to arrive there changes the past and violates the premise.

Conversely, if the past cannot be changed then there is no scope for someone who wasn't already there to arrive there, meaning that time travel is impossible and the premise is redundant.

Actually, there is one way out of this, but it invokes infinite loops, which are even more problematic as you can't establish any start points and end up disappearing up your own fundament; the traveler cannot ever perform the journey for the first time.  If there was a first-time journey, then on that journey there would be no memory of having done so previously, but if the traveler's memory is to be the same on each trip through the loop, because the premise says that nothing must change, he must always remember doing the previous journey, thus precluding a first journey where he didn't.

[DoctorBeaver]

But he did exist at time A because he went back to it. He just hadn't been born yet.

[lyner]

? []

[Nizzle]

Doctor Beaver,
IMO: Your thoughts that the past is written in stone and is unchangeable can only be true if the future is also already written in stone and unchangeable.
This is still within the laws of causality, however, mankind does sacrifice free will for it.

[LeeE]

But he did exist at time A because he went back to it. He just hadn't been born yet.

But this doesn't address the first-time issue.  Just as he can't ever make the journey for the first time, he can never be born for the first time, as the sequence of events that occur between time 'A' and time 'B' include his birth and cannot be altered.

Looking at it another way, it's impossible to count the number of times the journey is made because the total would increase with each journey through the loop.  The premise says that nothing can be different though, so the total can't change.