# Time Travel and Temporal Paradoxes **URL:** \ **Category:** In My Humble Opinion\ **Created:** [August 14, 2020, 5:05am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053 "2020-08-14T05:05:54Z")\ **Posts on this page:** 20\ **Page:** 4
**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:** [August 14, 2020, 11:08pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/61 "2020-08-14T23:08:44Z")
> [@begbert2](#): > > I’m here because the past happened the way it did. If the past was fluid, the present would not exist in a fixed state. Not to put too fine a point on it, but the present by definition does not exist in a fixed state. It is in a constant state of flux, chewing up the future to spit out the past. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 14, 2020, 11:36pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/62 "2020-08-14T23:36:47Z")
> [@eschereal](#): > > Not to put too fine a point on it, but the present by definition does not exist in a fixed state. It is in a constant state of flux, chewing up the future to spit out the past. Yup. This explains it well. Worth a watch (short): [![](https://img.youtube.com/vi/SrNVsfkGW-0/maxresdefault.jpg "Relativity of Simultaneity | Special Relativity Ch. 4") ](https://www.youtube.com/watch?v=SrNVsfkGW-0) ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 14, 2020, 11:52pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/63 "2020-08-14T23:52:36Z")
I missed the edit but also remember that, inside the event horizon of a black hole, space and time switch places. Granted we cannot see in there and it cannot affect the outside universe but physics lets things get that extreme. > **[How Time Becomes Space Inside a Black Hole](https://www.wgbh.org/program/pbs-space-time/how-time-becomes-space-inside-a-black-hole)** > > Find out how time and space switch roles when we move beyond the event horizon of the blac ---
**Author:** ![phs3](https://avatars.discourse-cdn.com/v4/letter/p/8491ac/32.png) [@phs3](https://boards.straightdope.com/u/phs3)\ **Post date:** [August 15, 2020, 12:03am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/64 "2020-08-15T00:03:41Z")
A friend contends that the continued existence of Facebook is proof that time travel will never be possible. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [August 15, 2020, 12:14am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/65 "2020-08-15T00:14:17Z")
I always figured that we know time travel is not possible otherwise we’d see a few million or billion people appear in the 1900’s trying to kill Hitler. ---
**Author:** ![E-DUB](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/e-dub/32/403_2.png) [@E-DUB](https://boards.straightdope.com/u/E-DUB)\ **Post date:** [August 15, 2020, 12:34am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/66 "2020-08-15T00:34:40Z")
Robert Charles Wilson’s novel “Last Year” postulated a time travel tech that established a bridge between the present era and the past, but the past was that of an alternate reality created at the moment the bridge was established. The other side of the bridge came out in nineteenth century Ohio and one crosser decided to travel to Germany to kill Hitler’s _father_. [https://www.amazon.com/Last-Year-Robert-Charles-Wilson/dp/0765332639](https://www.amazon.com/Last-Year-Robert-Charles-Wilson/dp/0765332639) ---
**Author:** ![Elendil\_s\_Heir](https://avatars.discourse-cdn.com/v4/letter/e/7cd45c/32.png) [@Elendil\_s\_Heir](https://boards.straightdope.com/u/Elendil_s_Heir)\ **Post date:** [August 15, 2020, 4:34am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/67 "2020-08-15T04:34:20Z")
> [@Whack-a-Mole](#): > > I always figured that we know time travel is not possible otherwise we’d see a few million or billion people appear in the 1900’s trying to kill Hitler. Yeah, about that…: [Wikihistory - Reactor](https://www.tor.com/2011/08/31/wikihistory/) ---
**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:** [August 15, 2020, 4:59am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/68 "2020-08-15T04:59:49Z")
By the time (heh) we invent time travel, Hitler will be distant history, and there will likely be more recent and more vicious figures to thwart. ---
**Author:** ![Folly](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/folly/32/3219_2.png) [@Folly](https://boards.straightdope.com/u/Folly)\ **Post date:** [August 15, 2020, 5:02am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/69 "2020-08-15T05:02:32Z")
Also, most theoretically possible time travel devices can’t take you back before the device was created as far as I know. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 15, 2020, 7:01am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/70 "2020-08-15T07:01:32Z")
All this discussion of paradoxes and violations of common sense feels too far divorced from actual physics. Could we start with something like a solution of Schrödinger’s equation where a particle travels backwards through time and interferes with itself, or explain why such a system is not possible? ---
**Author:** ![eburacum\_45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum_45/32/3385_2.png) [@eburacum\_45](https://boards.straightdope.com/u/eburacum_45)\ **Post date:** [August 15, 2020, 8:05am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/71 "2020-08-15T08:05:01Z")
> [@begbert2](#): > > Because the many worlds model of physics is one where the various universes are the outcome of random particle actions. Random particle reactions are not going to be make large metal rectangular prisms (police boxes, phone booths) appear spontaneously out of nowhere. And with time travel, what you have is entire time machines (and entire people with memories of a whole other life in a different time in their brains) appearing out of nowhere. If you have a universe that includes time travel, then the many-worlds concept can help explain the inconsistencies. Here’s the scenario. You live in a universe with no time travel, but you know how to do it. You then successfully create a wormhole loop that allows travel back in time, and you start to interact with the past. You are now busily creating new many-worlds which didn’t exist before, and which are all deterministic, even if they are affected by events that happened later in the timeline. But the history of these many-worlds is different to the one you started in, before you built the wormhole; that timeline did not include a wormhole _because you hadn’t built it yet_. How can this be? There was a different timeline before you built the wormhole, with no wormhole it it, and you know it exists because you have cellphone footage of it. In a many-worlds universe, this original universe with no wormholes is easy to explain. Because of some random quantum event, like a radioactive element decaying at the wrong time, there exists a whole sheaf of universes where you failed to create the wormhole in the first place. You were _in_ one of those universes when you successfully created the wormhole. So by attempting to create a wormhole back into the past, you created a whole new set of universes with backwards time travel, but in some of those universes you failed, thereby allowing the existence of the original timeline with no wormhole. Backwards timetravel in a universe with many-worlds is a nightmare - I think it is possible you may never visit exactly the same timeline twice. [Novikov consistency](https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiw6P_o35zrAhXEoXEKHfVzB7AQFjAFegQIBBAB&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FNovikov_self-consistency_principle&usg=AOvVaw0rf6lE3gQOiFWYEIKMBzqd) is a lot simpler, but also a lot more boring. Not as boring perhaps as the universe with no time travel at all, which seems to be the one we are in. For now. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 15, 2020, 8:30am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/72 "2020-08-15T08:30:05Z")
‘Many worlds’ is merely a name and/or interpretation of a linear combination of states in basic quantum mechanics. So I still say we cannot do without a quantitatively worked out example here. By the way, it is probably best to discount “sci-fi” interpretations— for instance, consider the recent [article](https://phys.org/news/2020-07-simulating-quantum-butterfly-effect-realm.html) where a simulated quantum system was evolved backwards in time, then disrupted, then run forwards again. Surprise, there was no “butterfly effect”. ---
**Author:** ![eburacum\_45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum_45/32/3385_2.png) [@eburacum\_45](https://boards.straightdope.com/u/eburacum_45)\ **Post date:** [August 15, 2020, 10:46am UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/73 "2020-08-15T10:46:27Z")
Yes, I read that. But the model they were using may have been incomplete, as so many are. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [August 15, 2020, 6:48pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/74 "2020-08-15T18:48:15Z")
With a precision of how many centimeters? How would you like to apparate? On your sofa, _in_ your sofa (with a spring though your brain), halfway through the floor or stuck on a wall? And what speed do you wish to have at that moment, and in what direction? Because that varies on the surface of the Earth depending on your latitude: being smacked at your nearest wall or by your nearest wall at 10 mph is an inconvenience, but at 300 mph it becomes a hassle. And the Earth moves much faster than that any given moment relative to the Sun, but in the opposite direction six months later. The rounding errors keep on adding up and I still do not know wich is the right frame of reference to calculate the coordinates of the apparition. ---
**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:** [August 15, 2020, 6:56pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/75 "2020-08-15T18:56:14Z")
If you need to take all that into account, then you probably need to also take into account the Sun’s orbit around the galaxy, as well as the galaxy’s proper motion. ---
**Author:** ![Ann\_Hedonia](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ann_hedonia/32/319_2.png) [@Ann\_Hedonia](https://boards.straightdope.com/u/Ann_Hedonia)\ **Post date:** [August 15, 2020, 7:10pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/76 "2020-08-15T19:10:29Z")
My theory is boring. I think some sort of time travel might be possible but not in the way we generally think or write about it. I think most “timelines” are REALLY short and that would be a serious limitation on time travel. I could see a technology for fast and lightweight commuting equipment that achieved speedy answers to complicated problems by sending the answers back in time. But that technology is never going to work in a way that gives you the answer before you ask the question. The information is traveling backwards on a timeline that started when you asked the questions and that’s the limit to how far back it can travel. Boring, but it avoids the paradoxes. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [August 15, 2020, 7:23pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/77 "2020-08-15T19:23:59Z")
It seems misleading to think of time travel as “apparition.” The classical picture is that you make a seemingly normal trip, except space-time was distorted in some weird way so that you end up at the same time and place where you started. ---
**Author:** ![running\_coach](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/running_coach/32/15836_2.png) [@running\_coach](https://boards.straightdope.com/u/running_coach)\ **Post date:** [August 15, 2020, 7:36pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/78 "2020-08-15T19:36:40Z")
> [@k9bfriender](#): > > If you need to take all that into account, then you probably need to also take into account the Sun’s orbit around the galaxy, as well as the galaxy’s proper motion. There was a short, short sci-fi story involving a very stubborn boy named Stevie who swore “he would stand here and not move!” The story then listed all the various velocities involved and ended with “Needless to say, little Stevie got what he deserved.” ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [August 15, 2020, 7:50pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/79 "2020-08-15T19:50:14Z")
Indeed. But which one is the correct one to take into account? You must make a choice, and only one (perhaps?) is correct. ---
**Author:** ![Pardel-Lux](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pardel-lux/32/20182_2.png) [@Pardel-Lux](https://boards.straightdope.com/u/Pardel-Lux)\ **Post date:** [August 15, 2020, 7:55pm UTC](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053/80 "2020-08-15T19:55:57Z")
Misleading? Not for me: I picture time travel as beaming (of Star Trek fame) in time. Because if it is not like that, how does it work? Do you take the time travel machine with you? Or does it precede you, and then you apparate inside it? Or do you move through time, like if walking? But then the space in which your goal is moves in most sensible frames of reference faster than you (and your machine) can move with chemical rockets, how are you going to keep up? [Previous page](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053.md?page=3) [Next page](https://boards.straightdope.com/t/time-travel-and-temporal-paradoxes/918053.md?page=5)