So here's a thread about time travel in fiction.  

I have been reading Timescape by Gregory Benford.  That is the subject of the spoiler warning, as his picture of time travel takes a long **** time to be presented verbally; personally, however, I could see him building up to it, although the exact nature was a bit surprising.

But first, I want to start with a description of how time travel and changing the past works in most fiction:



As you can see, two dimensions of time are necessary for this to make sense.  To have a timeline that exists leading up the the main character who changes the past, the timeline itself also must be subject to aging.  In other words, not only do you have a sequence of events within the timeline, you also have a sequence of timelines.  For all we know, this is exactly the case, a fact that Benford hangs a nice floral lampshade on early in his book.

It's a nice idea; it certainly allows us to imagine that maybe the reason we don't meet time travellers is because there aren't any "yet."  Though we are 15 billion years along the T1 axis, we could easily be at the very beginning of the T2 axis.  (Unless of course you watch Doctor Who, in which case we're all over the damn place.  Moving on...)

At least on TV, no matter how many times a character says so, it is not necessary to create what they'll inevitably call a "self-fulfilling prophesy."  Benford calls the same thing a "causal loop," a term which sounds more like physics and less like sociology.  You can go back and kill your grandfather, and unless you run him over in a Delorean you won't disappear.

Benford's fiction is for an audience with a longer memory and/or attention span, so he solves the same problem by allowing one universe to have causal loops, but the creation of a grandfather paradox is forbidden.  This is nice of him, and he deserves our thanks.

Here´s what he does

The event at x causes the blue causal loop (in blue).  The event at y creates a whole new universe.  In Benford's timegrid, universes splitting off from one another is a routine fact of life, so this is no big deal.  This splitting is caused in the natural world by gravity, and can be counteracted by using tachyons; basically, you can communicate across universes that are otherwise closed off from one another using tachyons, in addition to talking to the past.  BUT if you create a grandfather paradox by talking to the past, there is neither a loop, nor a change in your past, but a new timeline is created which can't be reached by any means.  In other words, an artificial copy of your universe (up to the point where you "changed the past") is artificially created merely by the attempt to cause a grandfather paradox.  

Here's the problem: if the two universe already existed "before" the change was made, then you don't need time passing in two directions.  You could still have the same plot.  The parallel timelines already differ before T1=y, because y sends an event into the past of the other timeline.  No paradox, no subuniverses, or oberuniverses.  No closed or open geometry.  You don't even have to drag quantum mechanics into it.

Benford, however, explicitly states that there is a "split" between the two universes.  A whole new universe!  Ageing in both directions.  Wow.  He does admit, at the end, that he threw in some metaphysical something or other (I forget the exact words) but it seems that he can build a serviceable plot around a single temporal dimension.

This post was last modified: 05-20-2012 03:39 AM by Lang.

i would like to reply. i'll take time to think about it though

Okay, thanks

These theories about timelines are based on the premise that time travel is possible.

Which, as far as we know, it isn't.

The reason you can't go back in time and kill your own grandfather is that travelling to the past is impossible. But it does make great fiction.

I've had a lot of fun in my published stories with time.  I particularly like the idea of an ouroboros, or a loop in time, so that you keep ending up back at the same time.  And because it's an ouroboros, you then can't change anything, because to do so would be to change the pattern, which is impossible in such a time loop.  
Alison

This post was last modified: 05-20-2012 11:58 PM by Alison.

Triangle

A good film

http://www.youtube.com/watch?v=17XqBdCiHOI

Time travel to the past is impossible. Time travel to the future can theoretically be done. Either get moving really, really fast or get close to something really, really big. Time will actually pass more slowly for you than for anything else. Of course, this is also very impractical, and will probably never be done just because of that.

Let’s say that time is more of an energy instead of an event. In this case, there is not one sequence of events that could be altered and break off into a new universe, but rather given that time is energy, there exists infinite possibilities for how sequences of events come together.

You could think of time as the gravitational pull of a star (like the sun). Time pulls together events (which are like molecules, or dust particles) which form into universes, which orbit the field of time (like a solar-system, except it’s a time-system). And just as the gravitational pull and presence of other planets impacts earth (tides, avoidance of asteroids, observational phenomena), the presence of other universes existing within the same time-system impacts our universe in subtle ways, and causes interesting phenomena to occur which we can observe and not explain. Because time is a subtle energy (like gravity) it cannot be explained properly, only experienced.

Keep in mind that there can exist multiple time-systems.



Now, also imagine that thoughts are like subtle energy- in fact depending on the state of our mind while we are doing an activity, time can seem to pass very quickly or slowly. It is an illusion, seemingly, but it impacts how we feel about the activity, the day/month/year, and the effort we put into whatever we are doing, and how quickly we get tired. In a way we can time travel with our thoughts...it would seem to explain dejavu.It would also explain why we can be asleep for 5 minutes and swear we were sleeping for hours; waking up with very intricate and seemingly lengthy dream tales.

I believe we can travel to other universes (or other events) within our particular time-system during sleep, and with intense concentration. This would account for imagination and "insight" which we have when we realize the answer to something spontaneously. Also, because there are other time-systems, experienced travelers can journey to systems of the past and the future, and access information from there. When you time travel mentally, you go places, but you don’t need your body to get there.

That would explain why we don't "see" time travelers. It would also explain "ghosts" and other phenomena.

We are all on a one way journey through time. It is possible to get there quicker using the methods you describe. The effects are measurable even between Earth and man made satellites, which need to be adjusted for tiny differences in how quickly time moves.

I meant that time travel to the past is impossible according to our current scientific knowledge.

I like the idea of a multiverse which has an infinite number of sub-universes, so anything that could have happened has happened somewhere, and where time can only progress forward (but can vary in speed!). Say if you wanted to travel back in time 50 years, you would actually transfer to a different subuniverse that was exactly 50 years behind your own. If you were to kill somebody there it would not have any influence on your original universe. You could then either go back to your own universe, where the killed person still exists, or go back to another universe which is a sub-universe that has progressed 50 years from the moment when you killed the person.

Time paradoxes cannot exist, but as far as fiction goes, difficulties in travelling to the exact universe you want to go to could make for an interesting story line.

Why is it that anyone who travels backwards in time automatically becomes a murderer of old-age pensioners?  I mean, couldn't they kill their grandmas as well, or instead of?  Or maybe not kill anybody, just observe, record and come back (forward), a bit like a quantum-leaping David Attenborough?
Alison

This post was last modified: 05-21-2012 10:52 PM by Alison.

The very act of being in the past would change events. Killing your grandfather is just an example of the problems that would be caused by time travel to the past.

I disagree.  If there was a way to go into the past and you did it, then you must have "always" done it.  Unless time has a second dimension, changing the past is not a requirement, or even a possible result, of travelling to the past.  

I recognize that it's all speculation, I'm just looking at some of the implications that don't seem to come up.

Yes. This was how it worked in "Bill and Ted's Excellent Adventure".

if you could mentally time travel then you would truly go back as an observer, and changing the past would not be an option.