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iamthemob's avatar

If time travel in both directions were possible, is the idea of a paradox a paradox?

Asked by iamthemob (17147points) September 24th, 2010

If time travel in both directions is possible, it seems to be that there is an almost necessary implication that the timeline as a whole has “already occurred.” In other words, traveling to what is the future from our perspective would be traveling to the past from all points ahead of that, ad infinitum, so that at one point (potentially) all points in time would be in the past – i.e., the end of time.

Therefore, is a paradox possible? For instance, the idea of going into the past and killing your grandfather would mean you never existed to do it. Therefore, you couldn’t have done it. Also, in the future, your grandson could travel to his past and kill your son, and therefore would never have existed to do it.

So, if time travel is possible in both directions, does that require that the timeline be of a particular nature? (static or malleable, single or parallel, etc.) If so, what?

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15 Answers

lillycoyote's avatar

Are you talking about some specific type of paradox? I don’t think anything you’ve said would preclude the existence of one or any number of paradoxes. Maybe I’ve missed something. As to the rest of it, maybe time isn’t linear maybe it’s a Mobius strip or something. Perhaps if time travel was possible or is possible now in somebody’s present maybe everything is in constant flux, or each change creates a new, separate time line that exist right next to it, in another similar universe or maybe time travel is just highly regulated with extremely harsh penalties for screwing stuff or maybe someone created some kind of fail that keeps people from committing antepatricide or whatever you might call it. Who the hell knows? Anyway, somebody had to be the first one to answer this question. A shame there isn’t time travel because I have premonition that I will have something more interesting or more coherent to say about 1 minute after my 15 minute editing window has expired.

iamthemob's avatar


I am taking the most general definition of paradox here – but basically, where something is done which negates the possibility of it being done.

And I think that time travel, were it possible, would reveal a lot of how the timeline could not possibly behave…I think that the ability to travel into the past from an “objective” perspective negates the possibility of paradoxes, and I’m wondering if that’s accurate assumption.

For instance, if time travel is possible only in one direction, and that direction is the future, and we know that it is impossible (somehow) to travel back in time, we can assume a static timeline which either (1) continues to form from an objective perspective or (2) is complete as of the time time began. ;-)

If time travel is possible only to the past, and we know that’s the only possibility, we can assume that the timeline continues to form, and either (1) it is fluid and changes in the past will erase the future so that time will continue anew from that point or (2) it can branch off so that parallel timelines will continue on without touching.

Rarebear's avatar

Many science fiction books were written on this.
Here is my favorite story in its entirety. A Sound of Thunder by Ray Bradbury

downtide's avatar

You should read Changewar by Fritz Lieber It’s a collection of short science fiction stories at least partly on the subject of time travel and the paradoxes it creates. One of my favourite collections of classic science fiction.

iamthemob's avatar


Which was adapted into the WORST movie based on a story by anyone, particularly Ray Bradbury.

I didn’t think about this – but there’s a difference then between opening a time portal and time travel, conceptually. A portal would allow travel to the past and return to your own timeline regardless of the changes to the past. Ray Bradbury’s example would actually be more along the lines of traveling. For instance, if there’s a portal that opens, and then closes, your presence in the past already changes it. Therefore, if you change it too much you need to hope that you don’t end up creating a future where the technology differed significantly enough that you cannot call the portal back. If it’s traveling in the machine, you don’t have to worry about getting back to the present…you just better hope it’s not too different from the one you left.

A time machine that allows you to go both back into the past and return to your future, but not into your future, would demonstrate I think a requirement that the timeline continues from the present into the future, and that it branches off.

All of this presupposes that all possibilities don’t already exist and we aren’t just traveling between parallel “what if” realities.

Rarebear's avatar

Never saw the move. I heard it was awful so I didn’t bother.

iamthemob's avatar

@Rarebear: god in heaven…the bad sci-fi movie of the “weak” style CGI, oh god

whothei0's avatar

i would have to day yes but we would never realize it. because there would be a constant time line but as soon as time travel happens it would create parallel time lines. and a paradox would be the constant shift between the two time lines as one acts on the other and vise versa.

iamthemob's avatar


But what about the “future paradox” issue?

whothei0's avatar

@iamthemob how do you mean?

iamthemob's avatar


I’m wondering if you think that in such a situation, the timeline is fixed generally (e.g., the future has already happened) or that the past is an objective thing in the timeline, and is fixed.

whothei0's avatar

yes but there are many splits in the time lines as to where we go into the future.

Jabe73's avatar

According to what little I know about M-Theory there may be different dimensions of time for everything that surrounds us. If you would could travel to the past and kill your grandfather then you would alter the future events for the dimension this pertains to. However your “current” self would still exist in this dimension of time but you would be altering the events for another dimension. That will probally be the next thing I order, books about the Multiverse Theory. I can’t find it on the internet but I remember reading about an experiment several scientists did with an extremely thin metal string where they caused the string to vibrate the electrons seemed to be in several places at the same time. It was some scientific magazine I remembered reading about this experiment in. Perhaps the sub-atomic particles (energy) were vibrating back and forth between time.

JustmeAman's avatar

Here is a little different take on the matter. For us the physical there is no time travel but there is a dimension where time has no limitations or does not exist. In that dimension everything is at once in the now which is not time. It would be like watching a movie and that you could not alter what took place or what is going to take place. Distance also would not be a concern because the constraint is not present in that dimension. Again this is from what I have viewed and discovered and is my opinion.

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