# Equation for probability of time traveler? **URL:** \ **Category:** Factual Questions\ **Created:** [July 20, 2015, 10:24pm UTC](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734 "2015-07-20T22:24:55Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [July 22, 2015, 4:33pm UTC](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734/21 "2015-07-22T16:33:34Z")
> [@scoots](#): > > Isn’t one of the major problems with time travel the Earth’s movement through space? Even if you could travel back to last week you would find yourself suspended in empty space because the Earth has moved on since then. Unless time travel involves rewinding the whole universe so that the Earth goes back in time (and space) as well as you. But rewinding the entire universe seems like a big ask when all you want to do is send one person back a couple of days. I don’t know if this one would be so hard to overcome (assuming, of course, that any backward time travel is possible at all). For example, if faster-than-light travel is possible, one implication might be that reverse time travel is also possible. Thus, an FTL space ship can travel to 100 years ago as easily as it can travel to 100 light-years away. Time is just another dimension to travel through, if this mechanism were possible. While maneuvering through time and space, your ship matches velocity with the target and then time travelers land. (Aha! Using flying saucers and anal probes, no doubt.) Or if you think wormhole travel is the way to do it, then presumably the other end of the wormhole is located where you want to go, and the energy cost of travel includes the cost of the necessary changes to your momentum to match the target reference frame. ---
**Author:** ![Habeed](https://avatars.discourse-cdn.com/v4/letter/h/a587f6/32.png) [@Habeed](https://boards.straightdope.com/u/Habeed)\ **Post date:** [July 22, 2015, 5:02pm UTC](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734/22 "2015-07-22T17:02:30Z")
You can’t speculate about backwards time travel without discussing paradoxes. Depending on the mechanism determines the “probability”. 1. The universe loads a “save game” when the time traveler travels With this mechanism, there is only exactly 1 event somewhere in the entire universe where the time traveler arrived. 1. The universe only permits paradox free travels through some kind of silly probability mechanism (like a famous movie where the time traveler tries to kill Hitler but is stymied by chance over and over again). In that case, a time traveler would be prevented from creating a paradox - so they could not simply announce themselves because this would change the future enough that they would no longer time travel into the past from the future. Etc. As you can see, the kind of time travel that would be obvious is the kind that would create paradoxes and therefore be impossible or only happen once. ---
**Author:** ![AHunter3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ahunter3/32/368_2.png) [@AHunter3](https://boards.straightdope.com/u/AHunter3)\ **Post date:** [July 22, 2015, 5:32pm UTC](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734/23 "2015-07-22T17:32:22Z")
I grew up with science fiction and love a good time travel story as much as anyone, and I have no difficulty suspending disbelief and immersing myself in the storyline. But I think time travel in real life is nowhere near as coherent a concept as it appears to our imaginations. To “travel” is to move from one point to another, where “move” is a motion trajectory x the passage of time. That makes no sense when applied to time itself: if there is one “point in time” and another different “point in time” we are conceptualizing time as a dimension; and in that framework one does not “move”; instead, the shape of one’s 4-dimensional location extends as a sort of person-shaped pipe through the time dimension, occupying this 3D space then and that 3D space later. Visualize a chunk of spacetime as a graph in four dimensions. You are defined as the set of coordinates you occupy and hence you’re a complicated but fully connected and uninterrupted shape within that graph. From this vantage point there’s no separate “you” that can “travel”. It’s like being asked about the characteristics of datapoint 3, 17, -19 when it is at 24, -203, -9. Datapoint 3, 17, -19 _isn’t_ located at 24, -203, -9, so the question makes no sense. The notion of time travel makes sense to our imaginations (with strong appeal) because of how our consciousness experiences time: past and future are not symmetrical poles of an axis to our consciousness, there’s a specific direction of flow; time ticks along and carries us along and we can neither get off the conveyor belt nor go back to a segment already elapsed. At the most, there might be meaning, of some sort, to the notion of rewinding the flow and reexperiencing it, — and indeed perhaps that happens all the (ahem) time — but to “go back” is to rewind and unravel back to one’s consciousness as it was then, meaning that one would not experience it as “going back” at all. ---
**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\ **Post date:** [July 22, 2015, 5:46pm UTC](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734/24 "2015-07-22T17:46:32Z")
I am not from the future, yet I am ahead of my time. Prove me wrong. 😛 ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [July 22, 2015, 10:28pm UTC](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734/25 "2015-07-22T22:28:16Z")
> [@Learjeff](#): > > An even bigger problem is the energy equation. If you manage to transpose yourself in time AND space to land on the right spot, you’ll be going the wrong direction (SPLAT!) That is, you would if you still have any kinetic energy relative to the environment. If you do, you must have removed it from the past. That kinda messes up conservation equations at that time, doesn’t it? Seems to me you’re screwed either way. If nothing else, you’ve messed up conservation of matter. > > I wonder how the wormhole models handle this. Bear in mind that I’m not a wormhole expert, but … I think that matter, energy and momentum are all _sort of_ conserved. If you move an object from Europe to America you conserve its mass/energy; you can, if you want to move it back. If you move an object through a wormhole to the past, the mass/energy is still conserved; you can, if necessary, take it back to the future. So mass, energy are conserved in a multiply-connected way. But in practice, not so much. You could get all of the mass in the future and bring it back to the past if you want to; you could always in theory take it back - but in effect you’d be adding extra mass to the past, and you might end up creating a black hole or something even nastier, like a collapsed universe. Not to mention a paradox. [Previous page](https://boards.straightdope.com/t/equation-for-probability-of-time-traveler/725734.md?page=1)