There exists a time machine that, within the machine, allows travel through space to be equivalent to traveling through time outside the machine. You can only travel as far back in time as when the machine was first activated (assuming you've found a way to live as long as it takes to get there--time still exists within the machine).

Questions:

Does this set-up prevent the Grandfather paradox, and if not, 1) could it be altered to and 2) what would be the reprocussions of "triggering" this paradox?

If a technology is created at one time, and brought to a previous one, where it exists to the day it is supposed to be created, who is the creator?

Basically, if I understand correctly from Survivor's email to me, the Grandfather paradox is impossible. You would never be able do anything to your grandfather or father to keep you from being born. For if you tried, something would stop you before you could do it. I'm fuzzy on the details, but that's the gist of it.

That's not to say you can't change the past in small ways. Because you can. But the important events will always propogate through each change regardless of your actions. (I could be wrong here...).

OSC's Pastwatch, The Redemption of Christopher Columbus [SPOILER ALERT] had people from a future time watching and then meddling with the past, which resulted in complete obliteration of the future time, as if it never existed. The moment they went back, the future ceased to exist as they knew it. Those meddlers were doing what they believed was a very noble cause.

Now, I just wrote a time travel-like story where an old-ish man with heart trouble gets a phone call from his four-year-old son who had died twenty years ago at sixteen. With this improbable connection to the past, the protagonist instructs his 4-year-old son to avoid doing something, thus changing the future and avoiding his son's death. It's a minor change to the past, and would not have prevented any major events from occuring, say for instance Sept. 11th. Nor did this change to the past prevent the protagonist's heart trouble. He was destined to have a bad heart, always.

Anyway, the grandfather paradox is presumably impossible if we are take current thinking of time travel at face value. I'm certain Survivor or someone else will be along any minute to clarify or contradict anything I've said. And like I said, I could be wrong.

What you're describing is somewhat like _Quantum Leap_: Sam Beckett could only travel within his own lifetime. Yours makes more sense.

I suggest reading a lot of time travel stories, to familiarize yourself with what's been done, and also for the fun of it! Here are some I thought were good:

The Man Who Folded Himself, David Gerrold. He tries a lot of quirks of time travel (causality loops like your time-machine-invented-by-seeing-finished-product; meeting oneself).

Time and Punishment, a short story in Asimov's.

Timescape, Gregory Benford. Dealt with grandfather paradox in a new way.

To Say Nothing of the Dog, Connie Willis. Time won't let you mess it up, and sometimes the ways it stops you are creative.

"On Rotating Cylinders and the Possibility of Global Causality Violation," Larry Niven, a story despite its quirky title.

But as to what happens if you kill your own grandfather, I am familiar with three possible options and it will be difficult to convince mt that one is more valid than any other based on any current real scientific understanding because so far as I know, aside from noting that time slows down as we approach the speed of light (and that theoretically it is impossible to travel faster than that), we have no real information about time travel. If someone can tell me different (with sources) I'd be fascinated to read about it.

As a completely unnecessary aside, MIT recently held a time traveller's convention that no time travellers showed up to.

Oh yeah, three possibilities: 1. You can't kill your grandfather. 2. You can and the universe comes to an end. 3. You can and it creates an alternate universe in which you don't exist.

On a personal note, I tend to like three better but will read any book that is, at least, consistent about what you can and cannot do through time.

Oops...does your setup PREVENT the grandfather paradox? I don't think that it does. I mean, the creator of the machine couldn't go back and kill his own grandfather because he couldn't go back far enough but the creator's grandchildren could, if the mahcine stays around that long.

Hogwash. We all travel in time just forwards. There is no test whatsoever to determine if something has come back from the future.

If something seems to pop into existence (such as a subatomic particle popping in and out) we have no way of telling if they first existed millions of years in the future and just popped back. Or if they cease to exist or move into the past or into the future. From our point of view they either exist or they don't. It is already theorized that some subatomic particles are capable of instantaneous travel.

When writing about time travel the key is consistency. For me I like to define how functions in fantasy worlds:

Is time a type of gravity?
Is time a streaming element like a river?
Is time the program line numbers of the great program of existence?
Is time nothing more than memory of an observer?
Is time a predetermined step in the machinations of the universe?
Is time like thought?
Is time merely the distances travelled by steller bodies?

etc...

Once I determine how time in my fantasy world functions I design my time travel accordingly. One thing about time travel that I have always found irritating: is that authors never consider that going back for example five years and six days you would end up floating in space because the planet would not be in the same spot. I never see anyone take the movement of the planets into consideration. Gravity must work perfectly irregardless of the fact that during time travel you don't exist for the planet to act upon. Air, heat, or other needed items to live.

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One thing about time travel that I have always found irritating: is that authors never consider that going back for example five years and six days you would end up floating in space because the planet would not be in the same spot. I never see anyone take the movement of the planets into consideration.

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If your world has deterministic free will, then all sorts of time travel are possible.

Say a man from next Tuesday comes back in time to visit you today. Using deterministic free will, when next Tuesday comes along that man has no choice; he must fire up his time machine and come back to today. In fact, if he is enthusiastic about his time trip at the moment he arrives today, he must be enthusiastic about his trip at the moment he leaves next Tuesday. He cannot possibly have any other mood.

If you use deterministic free will, then the grandfather paradox is impossible. Because your grandfather was not murdered before you were born you may rest assured that no one, not you, nor The Terminator, nor rabid ninja squirrels from the future, can possibly go back in time to kill him.

With deterministic free will time travellers can come and go, no paradoxes are possible, the past is always safe. Too bad it's so hard to write an exciting story this way. How do you build suspense when your character's actions are predetermined?

If your world has non-deterministic free will then you can build lots of suspense, but every time trip creates some sort of paradox. This is the thing about time travel stories that pisses off readers. Once you open your world up to paradox you have told your reader "the rules in my world are flexible, I violate them whenever I like and make stuff up as I go along."

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Does this set-up prevent the Grandfather paradox, and if not, 1) could it be altered to and 2) what would be the reprocussions of "triggering" this paradox?

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No, it doesn't prevent the Grandfather Paradox, assuming your father was conceived after the time travel device was first activated. However, other things may still operate to prevent a paradox. For instance, if making a change in the past splits off a new universe, then there is no paradox if you kill your grandfather. Or, if changes that cause a paradox cannot happen at all, something will always prevent you from killing your grandfather.

However, if paradoxes are possible, the reprecussions are up to you.

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If a technology is created at one time, and brought to a previous one, where it exists to the day it is supposed to be created, who is the creator?

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A paradox happens only if the technology was never invented originally by anyone, and only exists because it was brought back in time from a future in which it only exists because it was brought back in time, and so on ad infinitum.

I believe it's possible to have deterministic free will and still have a time paradox. Take "All You Zombies" (a different version of the grandfather paradox) by Heinlein. The paradox will fall apart if the main character acts any differently at any of the key points in time. In fact, just about any unbreaking time-loop paradox depends on deterministic free will.

In fact, it's possible to implement the traditional grandfather paradox in a deterministic free will universe. Because of the type of person I am, I feel compelled to go back in time and kill my grandfather before my father was conceived. So I get in my time machine, go back 80 years, and kill him. As long as that change to the timeline doesn't propagate instantaneously 80 years into the future, I continue to exist, and my grandfather is dead. At some point, the effects of his death reach 80 years into the future. If the fact that there is no me to go back in time is a change to the timeline capable of propagating backwards, then I do not go back in time, my grandfather is no longer killed, and the original course of the timeline begins to propagate forward. Of course, that means once I'm old enough, I'll go back in time and keep the alternating cycle going.

If, however, changes in the timeline only propagate forward, then I will not be born in the changed timeline, but that does not erase my existence or the fact that I killed my grandfather.

Here's the set-up:

-I've got a time machine that exchanges one direction of space within the machine for the direction of time outside of the machine, and vice-versa.

-So, if you walk in the right direction inside the machine, you are travelling through time, not space, outside of the machine. And the time it takes you to move, moves you through space, not time, outside the machine.

-This does present a problem if you have to walk a distance that would take, using time within the machine, longer than your lifetime. Assume I've found a way to manage that. (This is actually, in my story, the most obvious instance of technology being around since before it was invented, and an issue I need to work with if I'm to keep not only causality in check, but capitalism as well. )

I hope that clears things up.

Thanks to everyone who's replied so far! Sometimes my brainstorm machine needs this kind of oil to go another round.

As for the time travellers convention at MIT, I went, and was greatly disappointed not to see any future (or past) party-goers show up at the little arrival pad they'd set up (complete with police tape and a smoke machine, for effect). Of course, I've heard that the coordinates they asked everyone to pass around actually take you to Ames Street, about a block away, so perhaps the travellers arrived, didn't see any party-goers, and left.

At any rate, the lecturers were excellent, and it was awesome of them to speak on such short notice. As a matter of fact, it was an anecdote by one of them that led me to look into "time-like loops" in the first place--talk about causality! Perhaps, due to the preservation of this post, the travellers knew this and decided not to show up--their presence would shoot down this type of time travel, unless MIT is hiding something. Perhaps they wanted to preserve my classic, award-winning and best-selling novel.

Perhaps I've finally let my imagination go TOO far...

The alternative is that whatever Marty does "in the past", he already exists and is part of that past, whether it is in an alternate universe from his own "timeline" or whether his "timeline" has been obliterated. In this case nothing he does needs to be consistent with his own past, that past has "already happened". Either it still exists unchanged in some parallel universe or the past, having happened, is overwritten all the time.

The technology paradox is, quite simply, not a paradox. Most of the order that exists in the universe is not invented by humans, it came to exist as a result of the creation of the universe, Humans don't invent it, they only discover it. A technology that always existed in a time loop is a good example of this. While it might seem like something that man would have had to create, rather than who or what is responsible for creating the universe, it isn't. Humans can create nothing that is not part of the universe, that is absolute.

(Note that we're departing from the theory of time travel as shown in the movie, which doesn't really hold water.)

If one assumes that the effects of events can travel forward within a timeline only with the normal flow of time, then there is a "causality lag." Even if Marty kills both his parents by running them over with the DeLorean, since his birth was not until the late 1960s, it will be more than a decade before Marty is not born.

So what happens to the Marty we're following? Nothing. Assuming he never went back to the future, a thirty-year-old Marty will observe the passing of his birthday with no ill effects. The effects of there not being a Marty born on that day continue to propagate forward through time. But on a fateful day in 1985, there will be no teenage Marty going back in time to 1955.

Now the effects of the interference have caught up with the time travel event, and since the time travel event does not happen, there is no teenage Marty showing up in 1955. The timeline in 1955 begins reverting to how it was before the interference, but again that change propagates forward only with the normal flow of time. Which means our forty-something Marty in 1985 is still unaffected.

But what if Marty kills his parents and then travels back to 1985? He will find his world as he left it, because the change he made by killing his parents has not had time to propagate to 1985.

However, if he waits until 2005 (1985 + 20 years) and then travels back to 1974 (1955 + 19 years), he will find a world in which his parents were killed and he was never born.

The plot of Back to the Future cannot exist in a strictly deterministic world. The information in Marty's brain the moment he leaves 1985 would need to be exactly the same as the information when he arrives in 1955. That means Twin Pines Mall cannot become Lone Pine Mall, Marty cannot prevent his grandfater from hitting his father with the car, his father cannot become a science fiction writer, etc. These events would all violate strict determinism.

Like most science fiction stories, BTTF was written using a non-deterministic model.

Since all actual "ripple effects" are already known and observd to travel at this speed, nobody will ever notice any "time lag". I, living however many decades after my parents meeting, instantaneously experience every effect of that past event which materially affects me. It doesn't matter that the initial event was something very small, by the point in time at which it affects me, it has "rippled" itself into the effects which I notice.

Basically, I can make the argument that, because it takes time for a bullet to travel to a target, the target will not actually be hit by the bullet until a certain length of time after the shot. This is all well and good. But suppose I decided that the gun went back in time ten years and shot the target. Now we have two different definitions of when the shot occured. One the one hand, the shot took place a five years after the gun was made. Because the gun will be made five years in the future, therefore the shot will not hit the target until five years after the gun is manufactured, so the shot will only actually hit the target just a bit after the point in time from which we sent the gun back.

On the other hand, you could regard it as being the case that the shot was fired on that day ten years before we sent the gun back. Then the target would be hit on that day.

The creation of "time lag" usually rests on using both of these ideas as convenient. On the one hand, Marty pushes George out of the way of Lorraine's family car when Marty is...seventeenish? So, it will take years before that event can "ripple" into George and Lorraine not getting married or having any kids, including one who will travel back in time. Therefore, Marty will be much older when those effects occur.

But the proper age to give for Marty in that scenario isn't seventeen, it's negative thirteen. If the event only happens when Marty is seventeen, then it won't be happening in his past, pretty much by definition. If it doesn't happen in his past, it cannot affect him.

However you chose to construct the argument that it doesn't happen when Marty is negative thirteen, it is the same as saying it doesn't happen in his past. Whether it is not the past at all, or whether it is a past but not his past, or if it simply doesn't happen at all, they all mean the same thing. If Marty is seventeen when the event happens, and not negative thirteen, then the event is in his present rather than his past.

Now, whether or not the event is in his present affects whether he can interfere. But whether the event can affect his past depends solely on whether it is in his past.

I do seem to recall that you held a deterministic philosophy, but it wasn't anything to do with old-fashioned notions of physics (or maybe it was...). You seemed more interested in behavioral determinism, the idea that humans are just so much clockwork. I suppose that a strict interpretation of old-fashioned ideas about physics along with a purist materialism could logically lead to such a position, so maybe you were an old school physical determinist in the first place. But I didn't know that then (and really, I don't know it now).

Anyway, whatever you meant, I don't see what it has to do with time travel. You'll note that I used a machine rather than a human to illustrate my version of the paradox. It doesn't matter whether or not the agent of intervention in the past has "free will" or is "complex/chaotic" or anything like that, except that it seems more plausible to state that a simple clockwork could be forced into a binary solution set where either solution would imply the opposite solution.

I wasn't referring to deterministic physics in particular. I was referring to a deterministic model of free will, in which the universe is seen as a clockwork. People make decisions, but they cannot help making the decisions they make. Brains are made of atoms and molecules, finite particles which behave in a clockwork manner. Whatever pattern exists in your neirons the moment you make a decision determines what that decision will be.

When this is invoked in a time travel story paradoxes are impossible. Events that happened in the past are simply part of the pattern that leads to future decisions.
Good stroies with deterministic free will are very, very hard to write. That is why I am partial to them.

I'll give an example of what makes it difficult. Like most people I am incapable of discussing time travel without resorting to examples.

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Always in motion is the future. Difficult to see, it is.

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Spec fic stories often dabble with information from the future reaching the past. As long as characters have nondeterministic free will it is easy to write. But imagine how you would write The Empire Strikes Back if Yoda could clearly see any future event he chose and was never wrong. Suspense would be difficult, since Yoda can tell everyone everything that will happen to them, and there is nothing they can do to avoid it. If Yoda foresees that Luke will stumble into a trap, then Luke will stumble into a trap. If Yoda forsees that he will warn Luke about the trap then he must warn Luke. But Luke will still fall into the trap.

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The information in Marty's brain the moment he leaves 1985 would need to be exactly the same as the information when he arrives in 1955.

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I was referring to a deterministic model of free will, in which the universe is seen as a clockwork. People make decisions, but they cannot help making the decisions they make. Brains are made of atoms and molecules, finite particles which behave in a clockwork manner. Whatever pattern exists in your neirons the moment you make a decision determines what that decision will be.

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The theory of deterministic free will isn't that people's neurons will fire in a certain way to produce decisions no matter what happens in the world around them. It's that people's decisions are so tied to what happens in the world around them that they cannot truly choose to do anything other than what they choose.

If you run a portion of the clockwork universe a hundred times in a row, people will make the same decisions 100 out of 100 times. But if you introduce a change into the clockwork, decisions dependent on what was changed will be different.

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There is a definite lag in the propogation of a "ripple". But the ripple propagates through causality from the first moment at which it affects events, not from the moment it was originated. There may be a seventeen year old Marty around to push George, but that push begins to affect the life of negative thirteen year old Marty right then.

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I'm going to take the rest of this post to gussy up my previous hypothetical "mechanical grandfather paradox" a bit further.

Okay, let's say we take a gun with a timer attached to the trigger. We set up a conveyer belt leading to the time machine, which is set for 1 minute into the past. We place the gun on the conveyer belt such that it is constantly aimed at the exact position occupied by the firing mechanism one minute ago, and the timer is set to go off immediately after the gun enters the time machine. We leave the room.

When we enter the room (after we calculate the gun has reached the time machine, been sent to the past, and existed for one minute before getting back to the point in time from which it was sent), we won't find that the gun has shot off its own firing mechanism. We'll never find that (unless it's a thing where the bullet ricocheted off something and hit the gun after it was sent into the past).

Sometimes the shot will have come close to hitting the target, but usually (and this is to be expected because of the quantum nature of a temporal displacement event) it will miss completely. In some of the cases (a very few, even with a large number of tests) where it came close, we find that the gun would have hit it's firing mechanism if a shot fired at that gun a minute before it entered the time machine hadn't disturbed the position of the gun slightly.

Currently, our mathmatical models don't suggest that there is more than a single loop. If the timer also has a clock, the clock will only read one minute fast (the same is true for forensic methods of dating the gun).

Depending on the time machine used, the gun will be completely destroyed by the time displacement (or a failure of same) fairly often.

If you use a human to perform this experiment, the result is the same to an outside observer. Humans are more easily dagamged by temporal displacement, and sometimes the damage will be mostly psychological (sometimes this leads to physical damage, as in the case of those who shoot themselves in the conventional fashion after displacement--like the ricochet scenario mentioned above).

Does a human exposed to the grandfather paradox experience some kind of infinite time loop? Not usually. And those who considered it a likely outcome are more likely to remember something of that sort happening. I'd submit that their memories are unreliable, a result of false preconceptions and mental damage rather than a genuine experience. But the simple fact is that an outside observer simply can't tell for sure what happens in all possible universes, we can only tell what happened in this universe.

While it's interesting, in my opinion the theory as explained in the article makes no sense whatsoever. For example:

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"Quantum mechanics distinguishes between something that might happen and something that did happen," Professor Dan Greenberger, of the City University of New York, US, told the BBC News website.

"If we don't know your father is alive right now - if there is only a 90% chance that he is alive right now, then there is a chance that you can go back and kill him.

"But if you know he is alive, there is no chance you can kill him."

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Now, maybe Prof. Greenberger is referring to the possibility of going back and killing your father after you were conceived -- in which case, from a temporal paradox perspective your father is no different from anyone else, so why specify him as the target?

But I fail to see how uncertainty about your father's current viability status would allow you to kill him prior to your conception, while certainty that he is alive would not.

To me, it seems quite likely that the time-space continuum is a lot more complex and robust than we really think. Does that mean it determines our actions? No, but it does mean it is capable of confining our actions no matter what we do.

I have to say, the popular concept that observation actually has anything to do with the principles underlying quantum mechanics is one of my pet peeves. It is merely that quantum physics means that there are things humans really can't be certain about without observing them.

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I just want to know where the watch came from in SOMEWHERE IN TIME.

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