# Temporal Paradoxes and the Possibility of Time Travel. **URL:** \ **Category:** Factual Questions\ **Created:** [November 21, 2017, 7:14pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307 "2017-11-21T19:14:30Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 23, 2017, 6:35pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/61 "2017-11-23T18:35:21Z")
I wanted to ask, not interested in fictional answers: if one can travel to them, then in what sense are there “alternate” or “parallel” universes? Aren’t these simply accessible regions of the normal universe? You can walk down the street, but do not usually describe it as travelling to another world. As far as extra dimensions, AFAIK there is no experimental evidence for the existence of these, but if there were, would it necessarily have to do with time travel? ETA what do the laws of thermodynamics look like in the presence of time travel? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [November 23, 2017, 6:39pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/62 "2017-11-23T18:39:52Z")
> [@Andy\_L](#): > > for the past and future follow deterministically from the now. It does? Could someone please expand on this? ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 23, 2017, 6:47pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/63 "2017-11-23T18:47:26Z")
> [@Exapno\_Mapcase](#): > > It does? Could someone please expand on this? It is false as far as elementary quantum mechanics goes: you cannot predict which slit the particle goes through. ETA or consider a Schrödinger’s-cat type of measurement of a superposition: the result is random. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [November 23, 2017, 6:52pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/64 "2017-11-23T18:52:29Z")
> [@Exapno\_Mapcase](#): > > It does? Could someone please expand on this? Rucker does say “Suppose” - he’s creating a toy model of a universe with a para-time to explore the idea, not suggesting he has a grand unified theory in his pocket (and he’s citing John Cramer, whose interpretation of QM is unconventional but not considered quacky ([Transactional interpretation - Wikipedia](https://en.wikipedia.org/wiki/Transactional_interpretation)) - and I think it’s deterministic, though non-local (but I’m not sure)). ---
**Author:** ![Voyager](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/voyager/32/133_2.png) [@Voyager](https://boards.straightdope.com/u/Voyager)\ **Post date:** [November 23, 2017, 11:14pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/65 "2017-11-23T23:14:37Z")
> [@DPRK](#): > > I wanted to ask, not interested in fictional answers: if one can travel to them, then in what sense are there “alternate” or “parallel” universes? Aren’t these simply accessible regions of the normal universe? You can walk down the street, but do not usually describe it as travelling to another world. > > As far as extra dimensions, AFAIK there is no experimental evidence for the existence of these, but if there were, would it necessarily have to do with time travel? > > ETA what do the laws of thermodynamics look like in the presence of time travel? Brian Greene covers this in detail in his latest book, **The Hidden Reality**. He covers lots of types of parallel universes. One is beyond our event horizon, so if there is an infinite universe there might be near duplicates of our world, but always inaccessible to us. This is a different type from the one where new universes diverge based on quantum events. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [November 24, 2017, 12:11am UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/66 "2017-11-24T00:11:04Z")
> [@jz78817](#): > > the 2002 film version of _The Time Machine_ kind of addressed it that way: > > [spoiler]Alexander invents the time machine after his fiancee was shot and killed by a mugger. He goes back and prevents the murder, but then she’s struck and killed by a carriage shortly thereafter. realizing no matter what he does she will still die at the same point in time, he travels around time to see if he can find a solution. accidentally traveling thousands of years into the future. the “Uber-Morlock” explains why there’s nothing he can do to save Emma: > > “You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you.”[/spoiler] I’ve never really understood that. Imagine that I have a time machine. Imagine as well that someone I love dies – to coin a phrase – an untimely death, late in the afternoon on December 1st. Imagine, too, that I’d be really broken up by this; and so, on December 2nd, I’d of course step into my time machine after setting it for ‘yesterday morning’. So if, on the morning of December 1st, I meet a one-day-older version of myself who looks like he’s been crying – and he tells me, with what sounds like genuine emotion in his voice, to make a certain phone call at noon, or have lunch at a particular diner, or leave a specific door unlocked, or whatever – then (a) I’ll do that, of course; but then, (b) on December 2nd, I’ll wipe a big grin off my face for a minute and pretend to be real sad when I fire up my time machine and playact the message the way I remember it: like an actor who got told _“as if you’d been grieving.”_ I mean, I’m minimally competent to do that, right? If I’m smart enough to build a damn time machine, wouldn’t I be smart enough to act out the show I saw? ---
**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:** [November 27, 2017, 3:23am UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/67 "2017-11-27T03:23:54Z")
In this case, you are going back in time on Dec 2nd to prevent a tragedy that you know isn’t going to happen, and never has; why should you bother? ---
**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:** [November 27, 2017, 3:34am UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/68 "2017-11-27T03:34:14Z")
And it’s not just “Eh, go back for the heck of it”. Before he could go back in time, he had to invent the time machine, which was an extraordinarily difficult task, one which could only be accomplished by someone monomaniacally devoted to it. Which he wouldn’t be, if it weren’t for her death. ---
**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:** [November 27, 2017, 10:13pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/69 "2017-11-27T22:13:31Z")
FWIW: I forget which eminent physicist said it but one of them considered “Bill & Ted’s Excellent Adventure” to be the most realistic time travel movie ever set to film. He was not saying it was a realistic movie, just if you had time travel then you’d get something like that. ---
**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:** [November 28, 2017, 12:20am UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/70 "2017-11-28T00:20:36Z")
Eh, it was better than most, but there were still some holes there. For instance, they were under a big deadline crunch to finish their assignment. Why? If it’ll take you three days to finish, then set the time machine to three days before the due date, and work on it. Come back via the scenic route, not via the time machine. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [November 28, 2017, 3:26am UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/71 "2017-11-28T03:26:29Z")
> [@Chronos](#): > > Eh, it was better than most, but there were still some holes there. For instance, they were under a big deadline crunch to finish their assignment. Why? If it’ll take you three days to finish, then set the time machine to three days before the due date, and work on it. Come back via the scenic route, not via the time machine. I can’t recall much about it, but if you take the ‘Way Back Machine’ to my childhood, in Boy’s Life, there was a serial about some kids with a time machine. On at least one occasion they went to some deserted place/time with a beach for a week (after their adventure) to rest up and catch up on their homework. The only other thing I gotta say is: Wylde Stallion Rules! ---
**Author:** ![Czarcasm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/czarcasm/32/4050_2.png) [@Czarcasm](https://boards.straightdope.com/u/Czarcasm)\ **Post date:** [November 28, 2017, 3:39am UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/72 "2017-11-28T03:39:39Z")
> [@Blue\_Blistering\_Barnacle](#): > > I can’t recall much about it, but if you take the ‘Way Back Machine’ to my childhood, in Boy’s Life, there was a serial about some kids with a time machine. > > On at least one occasion they went to some deserted place/time with a beach for a week (after their adventure) to rest up and catch up on their homework. > > The only other thing I gotta say is: > > Wylde Stallion Rules! [Apparently those “Time Machine” stories ran for about 30 years,](https://en.wikipedia.org/wiki/Time_Machine_series) from 1959 to 1989. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [November 28, 2017, 12:14pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/73 "2017-11-28T12:14:25Z")
> [@eburacum45](#): > > In this case, you are going back in time on Dec 2nd to prevent a tragedy that you know isn’t going to happen, and never has; why should you bother? But that’s my whole point: _your_ whole point, here, is that I’ll ask myself why I should bother – with the implication that I’ll therefore decide _not_ to bother, such that the tragedy would ensue. You’re asking the “why should you bother” question in hopes of establishing that I’ll say “oh, yes, that’s true; I wouldn’t bother.” But, instead, here I am, saying “nope, that’s false; I would bother.” And your reasoning is _why_ I’m saying “nope, that’s false; I would bother.” You seem to think I’d say “I wouldn’t bother” – _and_ that I can reason well enough to answer an implied follow-up question with “and so the tragedy would ensue.” But if you grant that I can reason out how the first answer would lead to the second, then you should grant that I can reason out a sensible “I would bother.” Since – again – here I stand, saying “I would bother.” ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [November 28, 2017, 1:35pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/74 "2017-11-28T13:35:02Z")
You can only bother (or know to bother) \*if \*you’re aware of both potential paths from pre- the tragedy to “now” post- the tragedy. If you haven’t lived both paths, how can you know of both? In fact, what does it even mean to know in this case? In _Ground Hog Day_ the protagonist relives a loop remembering each iteration. So practice makes perfect for him. But why? When you rewind to an earlier time, how do you bring back the info from that time’s future? If “you” travel back, then if you are the person who was there the first time, you must not be aware of the tragedy that awaits in the near future. If you’re not that “you”, then there should be two of “you” wandering around; the original one ignorant of the future and the visiting you who knows the future. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [November 28, 2017, 4:03pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/75 "2017-11-28T16:03:16Z")
> [@LSLGuy](#): > > You can only bother (or know to bother) \*if \*you’re aware of both potential paths from pre- the tragedy to “now” post- the tragedy. > > If you haven’t lived both paths, how can you know of both? In fact, what does it even mean to know in this case? In _Ground Hog Day_ the protagonist relives a loop remembering each iteration. So practice makes perfect for him. But why? When you rewind to an earlier time, how do you bring back the info from that time’s future? > > If “you” travel back, then if you are the person who was there the first time, you must not be aware of the tragedy that awaits in the near future. If you’re not that “you”, then there should be two of “you” wandering around; the original one ignorant of the future and the visiting you who knows the future. Well, yeah. But I’m hereby telling you that, if a visiting ‘me’ from the future relates something he knows first-hand, then I’d presumably (a) take the relevant action; and then, at a later date, (b) go through the motions of being a visiting ‘me’ who _acts_ like he has first-hand knowledge while ignorantly reciting stuff word-for-word. You say I wouldn’t bother. I say I _would_. Who should I believe, you or me? 🙂 ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [November 28, 2017, 4:39pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/76 "2017-11-28T16:39:53Z")
> [@The\_Other\_Waldo\_Pepper](#): > > I’ve never really understood that. > > Imagine that I have a time machine. Imagine as well that someone I love dies – to coin a phrase – an untimely death, late in the afternoon on December 1st. Imagine, too, that I’d be really broken up by this; and so, on December 2nd, I’d of course step into my time machine after setting it for ‘yesterday morning’. > > So if, on the morning of December 1st, I meet a one-day-older version of myself who looks like he’s been crying – and he tells me, with what sounds like genuine emotion in his voice, to make a certain phone call at noon, or have lunch at a particular diner, or leave a specific door unlocked, or whatever – then (a) I’ll do that, of course; but then, (b) on December 2nd, I’ll wipe a big grin off my face for a minute and pretend to be real sad when I fire up my time machine and playact the message the way I remember it: like an actor who got told _“as if you’d been grieving.”_ > > I mean, I’m minimally competent to do that, right? If I’m smart enough to build a damn time machine, wouldn’t I be smart enough to act out the show I saw? the difference is you already had the time machine. you didn’t build it for the express purpose of going back and preventing your loved one’s death. that’s the key part of the quote: “You built your time machine because of Emma’s death.” if she hadn’t been killed, he never would have built a time machine. Her death is crucial to its existence. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [November 28, 2017, 4:46pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/77 "2017-11-28T16:46:23Z")
> [@jz78817](#): > > the difference is you already had the time machine. you didn’t build it for the express purpose of going back and preventing your loved one’s death. that’s the key part of the quote: “You built your time machine because of Emma’s death.” > > if she hadn’t been killed, he never would have built a time machine. Her death is crucial to its existence. Yes, in that specific example, that’s true. But I was leapfrogging from there to the general case: if I already have a time machine, and I get visited one morning by a haggard-looking and sad-eyed me who says to keep my wife from checking the mailbox at noon – and I then keep her from checking the mailbox at noon, and a drunk driver smashes his car into my mailbox at noon – well, look, we can all agree that I’d then be capable of (a) being really happy, but (b) pretending to be really sad when popping back to give myself that same message, right? Because, I assure you, I have no problem doing that. ---
**Author:** ![Slash1972](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slash1972/32/6461_2.png) [@Slash1972](https://boards.straightdope.com/u/Slash1972)\ **Post date:** [November 28, 2017, 4:49pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/78 "2017-11-28T16:49:04Z")
The presence of your future self fractures your time-line into a new one where you don’t need to go back in time to warn yourself. ---
**Author:** ![That\_Don\_Guy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/that_don_guy/32/538_2.png) [@That\_Don\_Guy](https://boards.straightdope.com/u/That_Don_Guy)\ **Post date:** [November 28, 2017, 7:27pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/79 "2017-11-28T19:27:39Z")
Wasn’t it Douglas Adams who said, if (backward) time travel was possible at all, then it would have been invented at every point in time, as when somebody went back, somebody else would “invent” it there? My theory as to why backward time travel “within your own universe” is impossible: 1. Normal motion from one point to another without going through a connected set of points between the two is impossible. 2. Similarly, you cannot go back in time to a particular point in time without traveling through all of the points in time between the present and that point. 3. However, when you try to go back even the smallest fraction of time, you are blocked by something - namely, yourself at that point in time. ---
**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:** [November 28, 2017, 7:58pm UTC](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307/80 "2017-11-28T19:58:25Z")
Who said you had to go straight back? If I’m in a car eastbound on a road and want to get to a point west of where I am, I don’t need to put the car into reverse. I can just make a turn, drive down a different road, and turn back onto the original road west of where I started. [Previous page](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307.md?page=3) [Next page](https://boards.straightdope.com/t/temporal-paradoxes-and-the-possibility-of-time-travel/802307.md?page=5)