# Time travel quandry **URL:** \ **Category:** Great Debates\ **Created:** [December 29, 2010, 5:22pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717 "2010-12-29T17:22:20Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Nobody](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@Nobody](https://boards.straightdope.com/u/Nobody)\ **Post date:** [December 29, 2010, 8:00pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/21 "2010-12-29T20:00:47Z")
> [@Eonwe](#): > > Let’s say I have a time machine, and an orange. I place the orange on the time machine and send it back to five minutes ago. We’ll say the time is 10:00 (am or pm, it doesn’t matter). You send an orange back to 9:55. > [@](#): > > After this event occurs, and the orange is upon the time machine of five minutes ago, wouldn’t it no longer need placing on the machine? Wouldn’t it have been impossible for me to have put the orange on the time machine, as it was already there? At 9:55 there are now two oranges, the orange that was always there, orange A, and the orange that was sent back in time, orange B. From 9:55 until 10:00 there are two oranges. At 10:00 you place orange A into the machine and send it back and we go back to only having the orange again. Doesn’t sound paradoxical to me. ---
**Author:** ![Damuri\_Ajashi](https://avatars.discourse-cdn.com/v4/letter/d/898d66/32.png) [@Damuri\_Ajashi](https://boards.straightdope.com/u/Damuri_Ajashi)\ **Post date:** [December 29, 2010, 8:05pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/22 "2010-12-29T20:05:20Z")
> [@Skald\_the\_Rhymer](#): > > Another possible way to look at it is the Novikov consistency principle. Briefly, it means that, in a universe in which time travel exists, nothing can be achieved in a trip to the past that is inconsistent with the traveller’s history (which is, of course, part of the history of the universe of the universe “before” the trip in question; any seeming alteration is simply the revelation of new information, not the creation of new events. F’instance, I can travel back to 1865, shoot Lincoln, and frame John Wilkes Booth for the crime (including making him think he did it), but I cannot stop Lincoln from being shot. Any attempt to do so must inevitably fail. > > Incidentally, for fiction that is the time-travel setup I prefer. > > In the OP’s scenario, if he attempted to send the orange back to a place where he knew it was not, then either the time travel machine would fail in that instance, or something would intervene to remove the orange from the spot he sent it to before hte past him was able to see it. Who polices this consistency? God? ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [December 29, 2010, 8:05pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/23 "2010-12-29T20:05:21Z")
> [@Agent\_Foxtrot](#): > > It can, as long as you never open the cylinders. However, by opening them, you put an end to the cycle. It fits in my hypothesis. You just create a loop. It didn’t travel, so it does, which causes it to not, and so fourth. It can be summed up with: when A is true then A becomes false when A is false then A becomes true Which is a loop, not a paradox. Further it requires no outside intervention from the “universe”. ---
**Author:** ![Agent\_Foxtrot](https://avatars.discourse-cdn.com/v4/letter/a/9fc29f/32.png) [@Agent\_Foxtrot](https://boards.straightdope.com/u/Agent_Foxtrot)\ **Post date:** [December 29, 2010, 8:06pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/24 "2010-12-29T20:06:47Z")
> [@Nobody](#): > > At 10:00 you place orange A into the machine and send it back and we go back to only having the orange again. Doesn’t sound paradoxical to me. Yes, but what if you decide not to? That was part of my argument. ---
**Author:** ![Agent\_Foxtrot](https://avatars.discourse-cdn.com/v4/letter/a/9fc29f/32.png) [@Agent\_Foxtrot](https://boards.straightdope.com/u/Agent_Foxtrot)\ **Post date:** [December 29, 2010, 8:07pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/25 "2010-12-29T20:07:50Z")
> [@The\_Tao\_s\_Revenge](#): > > You just create a loop. It didn’t travel, so it does, which causes it to not, and so fourth. > > It can be summed up with: > when A is true then A becomes false > when A is false then A becomes true > > Which is a loop, not a paradox. Further it requires no outside intervention from the “universe”. I never said the loop was paradoxical. I only said that we’ll never know if our experiment was successful or not. ---
**Author:** ![Nobody](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@Nobody](https://boards.straightdope.com/u/Nobody)\ **Post date:** [December 29, 2010, 8:14pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/26 "2010-12-29T20:14:56Z")
> [@Agent\_Foxtrot](#): > > Yes, but what if you decide not to? That was part of my argument. I was only replying to what Eonwe said. As for your post, I guess you’re saying that at 10:00 you pull the switch. At 9:55 if you see an orange in the second cylinder and decide not to pull the switch at 10:00 what would happen? In that case I have to agree with everybody else, it would create a loop. And I think the same thing would happen with the “grandfather paradox”. You go back in time and kill your grandfather, thus, you’re never born. You’re never born so you couldn’t have gone back and killed him, so you’re born and you go back and kill him. Repeat to infinity. You’ve created a loop. ---
**Author:** ![Gedd](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gedd/32/4938_2.png) [@Gedd](https://boards.straightdope.com/u/Gedd)\ **Post date:** [December 29, 2010, 8:24pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/27 "2010-12-29T20:24:55Z")
Just yesterday I watched the episode of \*The Big Bang Theory \*where Sheldon decides to stop working on time travel for a while because if he ever did invent a time machine he would just travel to the past to give it to himself. I’m going to go with that too. I’ll just remind myself to show up right . . . about . . . now! . . . . Dang. Lazy no-good future me. ---
**Author:** ![Grumman](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@Grumman](https://boards.straightdope.com/u/Grumman)\ **Post date:** [December 29, 2010, 10:09pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/28 "2010-12-29T22:09:39Z")
> [@Damuri\_Ajashi](#): > > Who polices this consistency? God? That’s the massive flaw in this idea. In order for paradoxes to be theoretically possible but prevented, no matter how much you try to cause one, you need an omniscient, omnipotent agent who can identify and disarm _any_ attempt to cause a paradox. And _all_ time travel to anywhere within the origin’s light cone (less than one light-year away for every year into the past you travel) could potentially cause a paradox. The only rational conclusion is that paradoxes are impossible theoretically (because time travel is impossible, because the universe will flip-flop between two states or due to some other cause) as well as in practice. ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [December 29, 2010, 10:26pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/29 "2010-12-29T22:26:04Z")
I think the cosmic history censor stems from a belief of an underlying logic to the space time layout of universe. I don’t buy it but I can see a case for it. It’s formally know as the [Novikov self-consistency principle](http://en.wikipedia.org/wiki/Novikov_self-consistency_principle) > [@](#): > > In response, another physicist named Joseph Polchinski sent them a letter in which he argued that one could avoid questions of free will by considering a potentially paradoxical situation involving a billiard ball sent through a wormhole which sends it back in time. In this scenario, the ball is fired into a wormhole at an angle such that, if it continues along that path, it will exit the wormhole in the past at just the right angle to collide with its earlier self, thereby knocking it off course and preventing it from entering the wormhole in the first place. Thorne deemed this problem “Polchinski’s paradox”.[4] > > After considering the problem, two students at Caltech (where Thorne taught), Fernando Echeverria and Gunnar Klinkhammer, were able to find a solution beginning with the original billiard ball trajectory proposed by Polchinski which managed to avoid any inconsistencies. In this situation, the billiard ball emerges from the future at a different angle than the one used to generate the paradox, and delivers its younger self a glancing blow instead of knocking it completely away from the wormhole, a blow which changes its trajectory in just the right way so that it will travel back in time with the angle required to deliver its younger self this glancing blow. Echeverria and Klinkhammer actually found that there was more than one self-consistent solution, with slightly different angles for the glancing blow in each case. Later analysis by Thorne and Robert Forward showed that for certain initial trajectories of the billiard ball, there could actually be an infinite number of self-consistent solutions.[5] > > Echeverria, Klinkhammer and Thorne published a paper discussing these results in 1991;[6] in addition, they reported that they had tried to see if they could find any initial conditions for the billiard ball for which there were no self-consistent extensions, but were unable to do so. Thus it is plausible that there exist self-consistent extensions for every possible initial trajectory, although this has not been proven.[7] This only applies to initial conditions which are outside of the chronology-violating region of spacetime,[8] which is bounded by a Cauchy horizon.[9] This could mean that the Novikov self-consistency principle does not actually place any constraints on systems outside of the region of spacetime where time travel is possible, only inside it. ---
**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:** [December 29, 2010, 10:33pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/30 "2010-12-29T22:33:49Z")
> [@Nobody](#): > > We’ll say the time is 10:00 (am or pm, it doesn’t matter). You send an orange back to 9:55. > > At 9:55 there are now two oranges, the orange that was always there, orange A, and the orange that was sent back in time, orange B. From 9:55 until 10:00 there are two oranges. At 10:00 you place orange A into the machine and send it back and we go back to only having the orange again. Doesn’t sound paradoxical to me. When the second orange appears, have you been robbed of all free will? What makes you put the orange in the machine at the right 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:** [December 29, 2010, 10:35pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/31 "2010-12-29T22:35:44Z")
> [@Der\_Trihs](#): > > I recall that in a recent discussion one of the more physics minded posters mentioned that that doesn’t hold because the law of conservation of energy is a local law, not a global one. Really? That’s interesting, local to what I wonder? Time to do some searching. Still leaves things the same. If it works, there isn’t much to it, just some future oranges appearing in the present. The question I have: Does going back in time undo the future you (or the orange) came from. Can you decide not to send the orange back the next time you are in the future? I still think it’s all impossible, paraphrasing someone, if time travel were possible we would be inundated by future oranges. ---
**Author:** ![Skald\_the\_Rhymer](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Skald\_the\_Rhymer](https://boards.straightdope.com/u/Skald_the_Rhymer)\ **Post date:** [December 29, 2010, 10:37pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/32 "2010-12-29T22:37:16Z")
> [@Damuri\_Ajashi](#): > > Who polices this consistency? God? No one “polices” the consistency, any more than there is a barrier that prevents travel at speeds greater than _c_. It would be a function of the structure of the universe. [Here’s the wikipedia article.](http://en.wikipedia.org/wiki/Novikov_self-consistency_principle) ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [December 29, 2010, 11:05pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/33 "2010-12-29T23:05:36Z")
> [@The\_Tao\_s\_Revenge](#): > > It’s formally know as the [Novikov self-consistency principle](http://en.wikipedia.org/wiki/Novikov_self-consistency_principle) > [@Skald\_the\_Rhymer](#): > > [Here’s the wikipedia article.](http://en.wikipedia.org/wiki/Novikov_self-consistency_principle) I won! ---
**Author:** ![Skald\_the\_Rhymer](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Skald\_the\_Rhymer](https://boards.straightdope.com/u/Skald_the_Rhymer)\ **Post date:** [December 29, 2010, 11:13pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/34 "2010-12-29T23:13:06Z")
> [@The\_Tao\_s\_Revenge](#): > > I won! You only say that because I retired both the bees & the monkeys earlier. 🙂 Anyway, I mentioned Novikov back in post 6. ---
**Author:** ![The\_Tao\_s\_Revenge](https://avatars.discourse-cdn.com/v4/letter/t/73ab20/32.png) [@The\_Tao\_s\_Revenge](https://boards.straightdope.com/u/The_Tao_s_Revenge)\ **Post date:** [December 29, 2010, 11:18pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/35 "2010-12-29T23:18:59Z")
> [@Skald\_the\_Rhymer](#): > > You only say that because I retired both the bees & the monkeys earlier. 🙂 > > Anyway, I mentioned Novikov back in post 6. Oh. Well I’m still the winner at getting second place! ---
**Author:** ![Nobody](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@Nobody](https://boards.straightdope.com/u/Nobody)\ **Post date:** [December 29, 2010, 11:24pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/36 "2010-12-29T23:24:00Z")
> [@Czarcasm](#): > > When the second orange appears, have you been robbed of all free will? What makes you put the orange in the machine at the right time? You put it at the same time because that’s how you did it in the first place. In order for the orange to arrive at 9:55 you have to put it in at 10:00. And thinking about it, when you first go to place the orange in the machine, there’s probably two to begin with; The orange which is in its proper time, and its five minute older twin which traveled back. Yeah, causality is broken, but all becomes right when you send the first orange back. All of this is reminding me of [Timecrimes (American title for the Spanish film Los Cronocrímenes)](http://en.wikipedia.org/wiki/Los_Cronocr%C3%ADmenes). ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [December 29, 2010, 11:39pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/37 "2010-12-29T23:39:19Z")
> [@TriPolar](#): > > Really? That’s interesting, local to what I wonder? Time to do some searching. Still leaves things the same. If it works, there isn’t much to it, just some future oranges appearing in the present. The question I have: Does going back in time undo the future you (or the orange) came from. Can you decide not to send the orange back the next time you are in the future? I still think it’s all impossible, paraphrasing someone, if time travel were possible we would be inundated by future oranges. In general realativity it’s a local law. ‘Local’ in physics is a word that can have precise meaning, i.e., from mathematics, a local property is a property that for every point on a manifold there exists some neighbourhood of that point for which it is true (manifolds by definition are locally Euclidian as for every point on a manifold there exists a neighbourhood which is homeomorphic to E^n). Though when physicists say conservation of energy is locally only, it’s more vague statement, which means something along the lines of it’s true for a vanishingly small volume of space (you could probably could be formalize the statememt in terms of tangent spaces). In general relativity the kind of spacetimes that contain for example wormholes (which theoretically can allow for a sort of time travel) are precisley those kind of spacetimes for which global conservation of energy is going to run in to the most trouble. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [December 29, 2010, 11:42pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/38 "2010-12-29T23:42:03Z")
> [@Grumman](#): > > That’s the massive flaw in this idea. In order for paradoxes to be theoretically possible but prevented, no matter how much you try to cause one, you need an omniscient, omnipotent agent who can identify and disarm _any_ attempt to cause a paradox. And _all_ time travel to anywhere within the origin’s light cone (less than one light-year away for every year into the past you travel) could potentially cause a paradox. > > The only rational conclusion is that paradoxes are impossible theoretically (because time travel is impossible, because the universe will flip-flop between two states or due to some other cause) as well as in practice. I don’t see what is particualrly special about time travel that you need God to make everything self-consistent. Physical theories without time travel manage to be self-consistent. ---
**Author:** ![wintertime](https://avatars.discourse-cdn.com/v4/letter/w/c68b51/32.png) [@wintertime](https://boards.straightdope.com/u/wintertime)\ **Post date:** [December 29, 2010, 11:45pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/39 "2010-12-29T23:45:30Z")
> [@The\_Tao\_s\_Revenge](#): > > I think the cosmic history censor stems from a belief of an underlying logic to the space time layout of universe. I don’t buy it but I can see a case for it. > > It’s formally know as the [Novikov self-consistency principle](http://en.wikipedia.org/wiki/Novikov_self-consistency_principle) I guess you are aware that the problem with an approach supported by General Relativity is the disregard of quantum effects that are, of course, not part of the theory in the first place. But when we talk about wormholes, we can’t ignore such effects. Which means that we can’t say anything with certainty without a working idea about quantum gravity. Still, you might like Matt Visser’s introduction to _[The quantum physics of chronology protection](http://arxiv.org/abs/gr-qc/0204022)_ as much as I did. ---
**Author:** ![Skald\_the\_Rhymer](https://avatars.discourse-cdn.com/v4/letter/s/ecccb3/32.png) [@Skald\_the\_Rhymer](https://boards.straightdope.com/u/Skald_the_Rhymer)\ **Post date:** [December 29, 2010, 11:51pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/40 "2010-12-29T23:51:32Z")
> [@Grumman](#): > > That’s the massive flaw in this idea.\*\* In order for paradoxes to be theoretically possible but prevented,\*\* no matter how much you try to cause one, you need an omniscient, omnipotent agent who can identify and disarm _any_ attempt to cause a paradox. And _all_ time travel to anywhere within the origin’s light cone (less than one light-year away for every year into the past you travel) could potentially cause a paradox. {bolding mine} What you’re missing, I think, is that the self-consistency principle says that paradoxes are _not_ theoretically possible. That’s the entire point. And God is no more required for Novikov than he is for _c_ being a time limit. Just as attempts to exceed _c_ are frustrated by the structure of spacetime. [Previous page](https://boards.straightdope.com/t/time-travel-quandry/565717.md?page=1) [Next page](https://boards.straightdope.com/t/time-travel-quandry/565717.md?page=3)