# What's the status of time travel? **URL:** \ **Category:** Factual Questions\ **Created:** [March 3, 2003, 1:23pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746 "2003-03-03T13:23:46Z")\ **Posts on this page:** 20\ **Page:** 5
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 1, 2018, 10:11pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/81 "2018-09-01T22:11:34Z")
> [@](#): > > Quoth **Exapno Mapcase** : > > That assumes, as most time travel stories do, that a return to the present with knowledge of the trip occurs or the present catches up with the “future”. Right, so it’s the backwards trip that’s problematic, not the forwards one. And that backwards trip remains problematic regardless of whether there’s any forward trip or not. ---
**Author:** ![Jim\_Peebles](https://avatars.discourse-cdn.com/v4/letter/j/5f9b8f/32.png) [@Jim\_Peebles](https://boards.straightdope.com/u/Jim_Peebles)\ **Post date:** [September 2, 2018, 12:01am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/82 "2018-09-02T00:01:56Z")
> [@SamuelA](#): > > That’s not what statement c says. In more formal terms, it says that some function f(x) exists that can take as input \ and output \. That’s what intelligence is fundamentally, every single action taken by any creature on earth, from bacteria on up, that uses a control system is doing some variant on this. At our level we are finding more optimum solutions, given more sophisticated brains and much more data, both present and past, to work with. > > And statement c says we can discover such functions, and if you have converted your local environment state to actual numbers in a computer, and have a reliable mechanism for scoring the consequences\*, you can **objectively measure** the **quality** of a candidate algorithm. > > This is how we can make very rapid advances in AI research - and **we are** - because it’s not an opinion as to whether or not a candidate algorithm is better than a previous algorithm. It’s measurable, and we are also creating meta-algorithms that can programmatically generate other algorithms and thus we can search the space of possibilities to find optimal functions. I can link a couple of recent deepmind papers on this if you are interested. > > \*I can talk more about scoring if you want. It’s not an unsolvable problem either, currently existing limitations with this are why we can’t _yet_ build algorithms that run in the human domain and say, speak in a way intended to make friends, but there is a series of steps that can get there and I can explain why. What if inspiration is not the result of such a process? Do you think this is how people write good songs? ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 2, 2018, 2:01am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/83 "2018-09-02T02:01:13Z")
> [@Jim\_Peebles](#): > > What if inspiration is not the result of such a process? Do you think this is how people write good songs? Yes, I do. And there’s a bunch of AI research that has demoed this. The reason why AI created art is _not yet_ as good as human art is the scoring system lacks the sophistication of the internal systems that humans use for this purpose. So even though you can generate thousands of pieces of “candidate, might be good, might not be” art, the AI can’t tell what’s good and what isn’t and thus can’t get better as an artist. Demoed method instead use a hack. Since we don’t yet have the sophisticated systems that can actually evaluate art in a way similar to humans, we instead go by “does this art resemble art that humans already agree is good”. So you get…interesting…results but the AI agent is arguably not yet an artist. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [September 2, 2018, 2:52am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/84 "2018-09-02T02:52:46Z")
> [@Chronos](#): > > Right, so it’s the backwards trip that’s problematic, not the forwards one. And that backwards trip remains problematic regardless of whether there’s any forward trip or not. For the forward trip, you effectively cease to exist for a specified interval, so you cannot have an autoparadox. For the trip backward, you must, I think, precede your own existence, so that there is only one of you in the universe, and the interval you visit must end before it overlaps with your priorsubsequent existence, and you can return to your erstwhile present-ish, as long as the moment you return to does not overlap with the priorsubsequent interval bounded by your departure into the past. Maybe. ---
**Author:** ![mandala](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mandala/32/95_2.png) [@mandala](https://boards.straightdope.com/u/mandala)\ **Post date:** [September 2, 2018, 6:36am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/85 "2018-09-02T06:36:54Z")
Maybe science says physical things cannot be transported back and forth in time, but maybe a non-physical thing, such a the mind, can travel across time, and bring knowledge with it. Maybe through sheer willpower it may be possible to influence other minds in the vicinity and thus alter events. If the flow of information is all that matters in the end, we may not need to try to toss physical objects across the gulf. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2018, 7:38am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/86 "2018-09-02T07:38:15Z")
> [@Chronos](#): > > Right, so it’s the backwards trip that’s problematic, not the forwards one. And that backwards trip remains problematic regardless of whether there’s any forward trip or not. If you are actually traveling forward in time, no gimmicks, no alternate universes, no monkey’s paws, wouldn’t that fix the future up to that point you travelled to? ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 2, 2018, 9:56am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/87 "2018-09-02T09:56:11Z")
> [@TriPolar](#): > > If you are actually traveling forward in time, no gimmicks, no alternate universes, no monkey’s paws, wouldn’t that fix the future up to that point you travelled to? Why would it? In our universe, the regions near black holes where gravity is high are thought to experience time dilation. One can at least posit the possibility of a gradient so steep that time freezes for the entity inside it. Doesn’t mean that entity will know the outcome of the outside universe, deterministic or not. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2018, 10:05am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/88 "2018-09-02T10:05:03Z")
> [@SamuelA](#): > > Why would it? In our universe, the regions near black holes where gravity is high are thought to experience time dilation. One can at least posit the possibility of a gradient so steep that time freezes for the entity inside it. Doesn’t mean that entity will know the outcome of the outside universe, deterministic or not. What does that have to do with anything? If you can actually travel into the future, the future of this universe, then that future becomes fixed. If it isn’t then you may have traveled in time but not to the future of the universe you started out in. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2018, 10:18am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/89 "2018-09-02T10:18:09Z")
You could say that not all of the future has become fixed, that only the future you can observe is invariant. Or that the future as we normally think of it does not exist, but in that case there is no actual future to travel to. ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 2, 2018, 10:22am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/90 "2018-09-02T10:22:30Z")
> [@TriPolar](#): > > What does that have to do with anything? If you can actually travel into the future, the future of this universe, then that future becomes fixed. If it isn’t then you may have traveled in time but not to the future of the universe you started out in. What I’m not understanding is that implicitly when you say “the future is fixed” you are implying that you have a chance to travel to the future more than once. You won’t know if something is fixed or not if you can’t repeat it. And you can’t travel to the future more than once without the ability to travel backwards in time. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2018, 10:42am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/91 "2018-09-02T10:42:18Z")
> [@SamuelA](#): > > What I’m not understanding is that implicitly when you say “the future is fixed” you are implying that you have a chance to travel to the future more than once. You won’t know if something is fixed or not if you can’t repeat it. And you can’t travel to the future more than once without the ability to travel backwards in time. It doesn’t matter whether or not I travel back and forth multiple times. If I instantly jump 1 hour into the future right now and you don’t observe the universe as I did 1 hour later then we aren’t in the same universe. ---
**Author:** ![SamuelA](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@SamuelA](https://boards.straightdope.com/u/SamuelA)\ **Post date:** [September 2, 2018, 10:43am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/92 "2018-09-02T10:43:37Z")
> [@TriPolar](#): > > It doesn’t matter whether or not I travel back and forth multiple times. If I instantly jump 1 hour into the future right now and you don’t observe the universe as I did 1 hour later then we aren’t in the same universe. That’s fine, but how are we going to compare notes? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 2, 2018, 10:53am UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/93 "2018-09-02T10:53:15Z")
> [@SamuelA](#): > > That’s fine, but how are we going to compare notes? Why do we have to? But if you need that then I’ll just slow my time travel down to 0 seconds per second for one hour and then we can compare notes. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 2, 2018, 12:28pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/94 "2018-09-02T12:28:21Z")
Right now, we observe the same Universe, and nobody thinks that’s odd or problematic. If, right now, you jumped ahead an hour, then an hour from now, we would again observe the same Universe. Why would that be any more problematic? > [@](#): > > Quoth **eschereal** : > > For the forward trip, you effectively cease to exist for a specified interval, so you cannot have an autoparadox. For the trip backward, you must, I think, precede your own existence, so that there is only one of you in the universe, and the interval you visit must end before it overlaps with your priorsubsequent existence, and you can return to your erstwhile present-ish, as long as the moment you return to does not overlap with the priorsubsequent interval bounded by your departure into the past. So you’re saying that, for instance, I couldn’t go back to wish 18-year-old me a happy birthday? Why not? 18-year-old me isn’t the same person as 2018-me. Heck, yesterday-me isn’t the same person as today-me, nor even one-second-ago-me as this-second-me (though those are really close). ---
**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:** [September 2, 2018, 7:12pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/95 "2018-09-02T19:12:21Z")
> [@Chronos](#): > > So you’re saying that, for instance, I couldn’t go back to wish 18-year-old me a happy birthday? Why not? 18-year-old me isn’t the same person as 2018-me. Heck, yesterday-me isn’t the same person as today-me, nor even one-second-ago-me as this-second-me (though those are really close). The problem is if you try to pass useful information to your past self (stock exchange prices, exam answers, race winners etc). This gives your past self an advantage you didn’t have (unless you do happen to remember this stranger turning up at your party and giving you a load of mysterious information). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 2, 2018, 8:32pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/96 "2018-09-02T20:32:41Z")
Yeah, but the same issues would arise if I passed those stock tips to _anyone_. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [September 2, 2018, 9:03pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/97 "2018-09-02T21:03:08Z")
> [@Chronos](#): > > Yeah, but the same issues would arise if I passed those stock tips to _anyone_. Lets try it out. Give them to me, and we’ll see if the universe implodes. But yeah, when people say that they are not sending matter, but only information back, as if that is some sort of loophole breaker, it really is information itself that cannot violate causality, the matter is just the information of where a bunch of particles are hanging out for a bit. ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [September 2, 2018, 9:42pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/98 "2018-09-02T21:42:04Z")
> [@Chronos](#): > > So you’re saying that, for instance, I couldn’t go back to wish 18-year-old me a happy birthday? Why not? Going back some years to wish 18 Year Old You a HB means 18YOY will relive the timeline since that date, as you have effectively restarted the timeline. Since we do not live in a clockwork universe, there will be innumerable changes from that date to the date you traveled from. Not the least of which would be “which sperm out of 20+ million successfully impregnated that egg?” repeated millions of times. Thus, anytime someone invents time travel and goes back in time, they restart the timeline, putting the invention of time travel itself at risk. Ultimately, the only stable timeline would be one where time travel is never invented. If anyone does invent it, they’ll go back in time over and over again until a stable timeline is found. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [September 2, 2018, 9:47pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/99 "2018-09-02T21:47:56Z")
> [@Cheesesteak](#): > > Going back some years to wish 18 Year Old You a HB means 18YOY will relive the timeline since that date, as you have effectively restarted the timeline. Since we do not live in a clockwork universe, there will be innumerable changes from that date to the date you traveled from. Not the least of which would be “which sperm out of 20+ million successfully impregnated that egg?” repeated millions of times. > > Thus, anytime someone invents time travel and goes back in time, they restart the timeline, putting the invention of time travel itself at risk. Ultimately, the only stable timeline would be one where time travel is never invented. If anyone does invent it, they’ll go back in time over and over again until a stable timeline is found. In which case, the only stable timeline is one in which no species ever evolves that is capable of developing time travel. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 2, 2018, 9:49pm UTC](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746/100 "2018-09-02T21:49:32Z")
That only applies if the past is mutable. [Previous page](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746.md?page=4) [Next page](https://boards.straightdope.com/t/whats-the-status-of-time-travel/158746.md?page=6)