Ron Barnette's Zeno's Coffeehouse Challenge #45 Result

Dear Zeno's patrons: Here are the results....very creative suggestions...over 350!

First the challenge, as written, then some replies.....

Maggie and Time Travel


As dear Maggie talked to Susan one night, a faithful Coffeehouse patron, she expressed her interest in the topic of time travel, and what such a possiblility might be like. (Both, who appreciate motion, time, and possibilites, in keeping with Zeno's challenges, are quite serious about this extended topic). Susan, a skeptic on such a notion, raised the following objection and argument, based on a hotly debated philosophical issue:

"Look, Maggie, if it were possible to travel in time to the past, then you could travel in time to a time where you meet, and kill, your grandfather, before he even met your grandmother. As such, you couldn't have been born, which contradicts the fact that YOU could have traveled to such a time in the past. Thus, time travel in the past is impossible."

Maggie's mind is Susan correct in her argument? Is time travel in the past impossible?

Help!!! As good logical patrons, can you shed some light on this?????....Ron Barnette, Zeno's Proprietor

I have selected five respondents, who represent well the diverse ideas brought to bear on this challenge and debate. These ideas include topics such as multiple universes, space/time loops, alternative pasts, fixity and change in the past, nature of time; some claim that time travel into the past is possible, while others maintain that this is! Diverse thoughts indeed. Here are some samples from Sue Chetwynd, Leon, Mike, John Brooks, and Daniel Cristofani...thanks to these folks and others!! Enjoy....Ron Barnette

*an exchange with Sue C: Sue.Thanks again for your continued interest in this issue...a fun one, filled with many presuppositions and explicit assumptions, I least that is true about me! I still wonder, though, if traveling and BEING in the past, where and when one wasn't previously there, doesn't entail that the past would be changed...if only through the facts and relationships under the time travel scenario, if even passsive. Follow? If so, then the assumed 'fixity' of the past is, in fact, incompatible with the mere traveling to it. Btw, I have received some highly creative responses to this one, in addition to your wonderful thoughts. I think I touched a nerve:) Best, Ron

Hi Ron, If the past isn't unchanging, then I suggest the contradiction doesn't exist.  Unless we think that the future doesn't develop out of the past, a different past creates a different future, and then, however we think of it, we are surely on a different time track - one in which I apparently appear from nowhere and bring about a future that doesn't contain a new me.  Surely the contradiction only arises if I break a loop, i.e. get born, travel back in time, and make it impossible for me to be born.  If I am not born, but just arrive in a time track, then the fact that I make it impossible for me to be born in that time track isn't a contradiction.  Of course this presupposes a view of reality that branches at each decision point (or something similar).  I guess I actually think there is only one reality, the one that has happened, in which I didn't kill my grandfather.  In that case either time travel isn't possible, or if it is travellers will either be able only to observe, or they will find that they were already there and cannot (or perhaps just do not) change what happened.  And this wouldn't mean a different past.  Travelling to the past doesn't make it the case that our ability to act there changes the past.  We might have always been there in the past, although we didn't know that before time travel was discovered, and therefore whatever we did was/is part of history. I think I prefer this 'do not change the past' alternative, since that seems to pose fewer problems with freedom of the will.  I think Robert Heinlein uses ideas such as this in some of his Lazarus Long books.   Fun discussion, tho'.


Thanks, Sue, for your Zeno's response. Very thoughtful indeed. I wonder a
bit about the view that the past is unchanging in the context of traveling
back in time...for wouldn't one's simply being there and engaging in any
activity alter the past? If so, then it would seem to be the case that time
travel is inconsistent with the notion of an unchanging past. No?
Ron Barnette

Sue in city: Warwick

country: UK

comments: If you have really travelled into the past, then you were there
in 'your past'.  If we view the past as unchanging, which we seem to do,
then the answer seems to be that since your grandfather didn't die before
the state of affairs that resulted in your conception, birth etc., you won't
be able to kill him.  If you do kill him, what you have travelled into isn't
your past, but some alternative reality.  In that case, the fact that you,
or your analogue, won't exist isn't paradoxical.

*and Leon wrote: city: Melbourne

state: Vic

country: Australia

comments: I myself regard the grandfather paradox as decisive against time travel. However, one strategy for at least shaking this resolution (perhaps not quite toppling it), is it to invoke some kind of sceptical argument.

"Susan, but how do you KNOW that I wouldn't be born if my grandfather had never met my grandmother. Arn't you merely assuming that many of our commonsense notions of reality are true? Isn't it merely inductive inference that people are created by the coupling between men and women? Yours is an argument to the best explanation, one that coheres well with the rest of our common beliefs, but it is hardly fool proof."

I am not myself inclined to be persuaded by this, but it is one path that Maggie could take. Especially if she is fond of conspiracy theories. Maybe time travel had always existed, but then some evil neuroscientist come dictator implanted into our minds with a particular computational program that only allows us to make sense of regularities, and automatically looks for a way out when two premises contradict. This has allowed him to usurp time travel ability (since no one can work out a way of designing it without contradiction).

Desperate, but the best I could do.

*and Mike wrote: city: Wynnewood

state: PA

country: USA

comments: This would result in the looping paradox in which it would continuously occur.   But because its a hypothetical you can not accurately predict what exactly would happen, logically you fall into this looping paradox, but because this is time travel and most likely not logical the loop may or may not occur, because the past would then have to be rewritten up to the point where you decided to travel back in time.  You could be born from a different person or because you killed that person that universe may still exist but it is looped in a paradox, which would state the multiverse (multiple universes) theory true.  But to the topic of is time travel possible, yes, if there are multiple universes.  Only then would time travel be possible because your mere presence in the past alters the future and changes the conditions under which time travel has occured.

*and John Brooks wrote: city: Chicago

state: Illinois

country: USA

comments: A possible objection to Susan's argument might be that since Maggie has already been born, nothing she could possibly do in the past could prevent her birth. Thus, she could do nothing to prevent her grandparents (whoever they may turn out to be) from meeting one another. So, time travel would not be impossible for Maggie.

However, that counterargument misses the more fundamental objection that it would be logically contradictory, no matter what one might have done in the past, for one to exist before one is born.

A possible reply to that objection might be that time travel into the past within one's own lifetime still would not be logically precluded.

However, time travel into the past within one's own lifetime would imply the ability to exist in more than one place simultaneously; an ability that only a "supernatural" being could possess.

Thus, assuming that Maggie is not God, time travel into the past for her would be impossible.

*and Daniel C wrote and shared multiple scenarios: city: Portland

state: OR

country: USA

comments: Here are some different scenarios which could work. Let's say Maggie travels from 2004 to 1930.

Scenario One.

When Maggie arrives in 1930, the universe immediately divides into two parallel universes; the only difference between them, initially, is that Version B has Maggie in it, and Version A does not. (In fact, perhaps the separation does not take place all at once, but spreads outward from the point of her arrival at the speed of light.) Version A is the universe in which Maggie will be born, later, and from which she will eventually travel to 1930 of Verson B; it proceeds quite untroubled (apart from WW2 etc.), since Maggie is not there in 1930 to screw it up. In Version B, Maggie can kill her grandparents or whoever she likes, without endangering her own existence, since she comes from Version A, not Version B. (Even if she tries to live quietly from the time of her arrival in Version B, her presence will have subtle ripple effects, which will indirectly affect sensitive physical events, human reproduction among them; maybe her parents will never meet, maybe they'll have a boy!
 , maybe a girl but it'll be subtly different. Cf "butterfly effect".)

We could sum up Scenario One by saying Maggie travels not to her own past, but to an alternate past of her own creation--which is, however, just as real as the past that produced Maggie. We would say that in 1929 the universe had two real (not "possible") futures, Version A and Version B, and after 1930 there are two universes, which exist separately for billions of years, becoming more and more different. Maggie can never return to Version A; if she travels to the future, it will be to the future of Version B, and if she travels to the past again she will just create a version C.

Scenario Two.

When Maggie arrives in 1930, she somehow cannot kill her grandfather; maybe she changes her mind, maybe she gets hit by a truck. In fact, whether she wants to or not, she ends up doing exactly what is necessary to bring about the exact version of 2004 she came from. I want to be clear about this: in Scenario Two, there IS no version of 1930 without Maggie in it, and there never was a version of 1930 without Maggie in it. There is only one version of the universe; the events of 1930 cause the events of 2004, in the usual way, and in the process they shape the 2004 Maggie who will come to 1930 and help to cause those same events.

Now let's examine this. With this kind of circular causation, you would expect the ends not to fit together; there is no obvious reason why Maggie, on arriving in the past, would do exactly what was necessary to produce the exact future that produced that exact Maggie--or anything vaguely like it (e.g. refrain from killing ancestors). For the ends to fit and for the circle to be completed smoothly seems to require some outside agency.

There have been many books and screenplays where the circle was completed smoothly; in those cases it was done by the writers, who were an outside agency relative to that fictional universe. In those cases the circle only had to fit together in a coarse-grained way; there were only a few dozen events, at most, that needed to be reconciled, and they were large-scale physical events, e.g. the movement of objects and of human bodies.

For the circle to be closed smoothly at the microphysical level of detail, so that the state of the universe at the moment of Maggie's arrival (including Maggie herself) will, in the course of 74 years, by the operation of normal physical processes, produce the exact same Maggie down to the last electron, just in time for her departure in 2004, could not happen by accident. If such a thing were to happen, it would mean there was some power or law at work that we have no knowledge of at this time. A supreme being could do it, if he were interested in puzzle-art and not at all interested in human "freedom". Or there might be some principle of regularity, deeper than the known laws of physics. None of this seems very likely, and consequently Scenario Two, though possible, isn't a very good bet.

Other Scenarios.

Many other models have been used in works of fiction, but these have usually been muddled and ill-conceived. A sampling:

-Like Scenario One, only when Maggie arrives in 1930, Version A is destroyed and replaced with Version B...apparently, it's not just that Maggie is in Version A first and then in Version B, but rather there is some separate kind of "sideways time", never properly explained or acknowledged, in which the entire Version A timeline exists "first", and is "later" replaced with version B.

I suspect that this mistake comes from treating the timeline of a person as something more fundamental than the timeline or timelines of the universe. In Maggie's timeline, she first experiences the effects of a 1931 in which her grandfather is alive, then she experiences a 1931 in which her grandfather is dead. She has moved sideways into a different universe. But this does not imply that the entire timeline of the universe is going sideways through a separate kind of time, in which Version A of 1931, 2004, 198002, etc. all exist simultaneously for a while, and are then instantly replaced with version B of all those years.

-Like Scenario Two, only Maggie is struggling to complete the circle herself; if she fails, it will cause her destruction, or the destruction of the contents of the surrounding space, or of the entire universe. This too seems to involve the idea of "sideways time"--the idea that the entire history of the universe can "change" from being one way to being another way, and in particular from being internally consistent to being inconsistent, and then back to being consistent (or nonexistent). The connection between sideways time, personal time, and universe time in this version would be hard to explain sensibly. Besides which, the job of completing the circle at the necessary microphysical level is, as already mentioned, way beyond human capabilities.

-An especially messy version: when Maggie goes back, she makes some things different than they were, producing Version B which is different than Version A; then she tries to undo the "damage" by shaping Version B to make it match Version A again, in order to get them to rejoin. In some cases, she must finish this job by some ill-defined time--say, before the first event of Version A's history that she knows about.

(Notice that any of these scenarios requires that there be some physical mechanism that would allow people to travel back in time; no such mechanism is known to exist. Also, unless supplemented by further explanations, any of these scenarios appears to violate the law of conservation of energy...which is fine, since our evidence for that law is based only on observations of A. situations that don't feature the departure or arrival of a time-traveller, and B. situations that are big enough (i.e. planetary or greater scale) that the arrival or departure of several time-travellers would fit into the margin of error of our measurements.)