Science a GoGo's Home Page Forums General Discussion Physics Forum Is a closed time-like loop an inescapable trap?

J. Richard Gott (Time Travel in Einstein's Universe) has this to say on the subject of time travel to the past:
“A time traveller who visits the past is just someone whose world line somehow loops back in time, where it could even intersect with itself. This would allow the time traveller to shake hands with himself. The older man could meet up with his younger self and say, “Hi! I’m your future self! I’ve travelled back in time to say hello!”…..The surprised younger man would reply, “Really?” He would then continue his life, becoming old and eventually looping back to that same event - where he would recognize his younger self, shake hands, and say, “Hi! I’m your future self! I’ve travelled back in time to say hello!”

One slightly worrying thought that springs to mind regarding the above scenario is that, once the first time traveller has succeeded in looping back in time, and has completed the loop by meeting himself, he seems to have set in motion what looks very much like a potentially indefinite succession of loops, in which he continually returns to his past and meets himself, after which his younger self continues with his life; but once the loop has been formed, can he ever progress beyond the point from which he first went back?

OK, so nobody wants to talk about closed time-like loops, past-directed time travel or retrocausality.

Have a look at the current New Scientist, see if that makes you think.

I have spent considerable time on this scenario as you presented it and gave it due diligence. Unfortunately when I got through with the exhaustive analysis, I no longer felt like talking about it. (In other words, it did its job.)

However, I didn’t want you to think that no one was interested or that it was ignored.

You only asked one question: “… once the loop has been formed, can he ever progress beyond the point from which he first went back?” I don’t think that one loop around is enough to guarantee (condemn) that the loop must continue indefinitely. The reason is communication. Once the older self meets the younger self, it stands to reason that the younger self would ask the older self how many times he’s done this. When the younger self reaches the decision point, he’s “armed“ with that knowledge. He may eventually choose differently…something like “oh, a hundred times is enough let’s break the loop.” The real danger would occur the first time that an estimate is used. For instance; “How many times have you done this?” “Oh, about a thousand.” Now, despite communication, the information won’t increment beyond this point and there won’t be any change in impetus…no alteration of choice.

Now, for the paradox (impasse):

When you run through the loop a couple of times, at first glance it seems that circumstances result in a continuous parade of older selves marching beyond the decision point at an interval equal to loopback time… but after some diligence, I realized that it wouldn’t work that way. The first aspect (of course) is that there would be an infinite amount of clones generated. The second is that it’s a parallel function… not a serial function. The clones would not march out at periodic intervals because the decision point and (loopback) arrival times are fixed. All clones (infinite in number) would arrive at once. This would constitute a maximum violation of the laws of conservation. This is a prime example of why I believe that time travel to the past is impossible.

Of course the only way around this would be alternate timeline/ alternate universe scenario… but I don’t buy it. I’m a cause and effects type of person. The cause of the alternate timeline is the duplicate material. Duplication would have to occur for at least one moment to have an effect…it’s that moment that is impossible.

I think one loop would be enough to cause serious problems, because in order to change the original decision to go back there would have to be an "immutable" spacetime event in which two different decisions were made, which must be impossible.

When you say that all the clones "would arrive at once", do you mean they would all have to be at both the decision point, and the meeting point and, in fact, at every point between the two. That is the conclusion I came to, which is one reason why I share your doubts about the viability of past directed time travel. However, when someone with the professional background of Gott claims it is possible, it deserves some attention.

Yes, J.R. Gott is a likable chap.

In all honesty, I’m guilty of being too picky…perhaps too detailed. The difference between the first time through the loop and the second time is that the former younger self (now the older self knows that it’s his second time through.

The young Bill S. is traveling though life and out of nowhere appears an older Bill S. Just think of all the new information that the younger Bill S. is going to acquire…one of those bits of info is the count. Of course there would be others like; how many kids will I have?...or, Did I have any regrets? Determinism wouldn’t apply.

However, in keeping with the spirit or intent of the problem; I’ll concede that Bill S. goes through every time. Then the paradox is realized. In theory there would be an infinite amount of older Bill S. at the loopback arrival time and not only all points in between the arrival and decision points but beyond. Of course, the universe would actually be broken at the arrival point instantly.

“…I share your doubts about the viability of past directed time travel.” We have something that is rare in these here parts…consensus.

“…when someone with the professional background of Gott claims it is possible, it deserves some attention.” Agreed; However, I’d like to extend that respect to all participants of the universe. I think that some value can be culled from everything…everyone included.

I’m reminded of Louis Savain’s initial diatribe that prefaces his GUT presentation. (I know that you are familiar with this.) I think that he’s trying to make an appeal to fairness… trying to preempt bias. To paraphrase; Just because someone has impeccable credentials or an extreme acumen doesn’t mean that their work or ideas have automatic merit. Likewise, lack of the same should not be an automatic dismissal. Then he goes on trying to support this “statement” by citing that; so far, the established scientific community has actually failed to derive a working GUT so why would we place so much credence on a “bunch of losers”. Emotional delivery aside; there is some wisdom to this. One thing’s for sure…They can’t all be right. In conclusion; I respect Gott, but I don't think that he's got it right. (Just my opinion.)

As far as Savain’s theories…that’s another subject for another day.

All this circuitous time travelling needs some thought, and perhaps I should dig out some of my old notes, but for the moment the exigencies of Christmas must take priority.