# 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:** 11\ **Page:** 3
**Author:** ![Grumman](https://avatars.discourse-cdn.com/v4/letter/g/43a26b/32.png) [@Grumman](https://boards.straightdope.com/u/Grumman)\ **Post date:** [December 30, 2010, 12:01am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/41 "2010-12-30T00:01:16Z")
> [@These are my own pants](#): > > 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. It’s because the self-consistency principle does not create a self-consistent universe: it creates an _inconsistent_ universe that arbitrarily judges some changes as paradox-inducing and others as not. Either that, or it warps reality so that every photon and molecule that would have entered the time machine if not for the time traveller is “coincidentally” replaced with an exact duplicate. > [@Skald\_the\_Rhymer](#): > > {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. No, I was making it clear that there’s a difference between a universe where paradoxes don’t happen because they are averted 100% of the time (by the self-consistency principle) and one where paradoxes don’t happen simply because paradoxes don’t happen. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 30, 2010, 1:03am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/42 "2010-12-30T01:03:21Z")
> [@The\_Tao\_s\_Revenge](#): > > Well you’re not thinking it all the way through. > > If you don’t put the orange in the past because the past has an orange from the future, then the past won’t have an orange from future so you will put it in as before which results in you not. > > Basically you’ve initiated an infinite loop cycling between the orange being there, or not. Infinite loops are not logically impossible, as anyone who’s ever forgetting to increment the variable in their while loop knows. I’d say that’s more like a circular reference in Excel. Cell [A1] is defined as the value of [B1]+1 , and [B1] is defined as [A1]+1, or perhaps more aptly, a configuration of logic gates where X = Y, but Y=NOT(X) ---
**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 30, 2010, 2:00am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/43 "2010-12-30T02:00:53Z")
> [@Mangetout](#): > > I’d say that’s more like a circular reference in Excel. Cell [A1] is defined as the value of [B1]+1 , and [B1] is defined as [A1]+1, or perhaps more aptly, a configuration of logic gates where X = Y, but Y=NOT(X) This is a good example. Clearly the definitions A1 = B1 +1 and B1 = A1 +1 are paradoxical. The definitions though A1 = B1 +1 and B1 = A1 -1 are not, they are consistent. Though they don’t define A1 and B1 numerically. Let’s say Excel displayed A1 to have a value of 4 and B1 a value of 3, we’d be left wondering how and why Excel displayed those values, but they’d still be consistent with the definitions in the cell. ---
**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 30, 2010, 2:21am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/44 "2010-12-30T02:21:32Z")
> [@These are my own pants](#): > > This is a good example. Clearly the definitions A1 = B1 +1 and B1 = A1 +1 are paradoxical. The definitions though A1 = B1 +1 and B1 = A1 -1 are not, they are consistent. Though they don’t define A1 and B1 numerically. Let’s say Excel displayed A1 to have a value of 4 and B1 a value of 3, we’d be left wondering how and why Excel displayed those values, but they’d still be consistent with the definitions in the cell. Excel can actually be configure to allow circular references. IIRC what it does is loop through a hundred iterations and then give up. However time isn’t a quitter. Edited: Kind of irreverent but checked the options, 100 is the default but it can configured for more or less ---
**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 30, 2010, 2:26am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/45 "2010-12-30T02:26:41Z")
> [@wintertime](#): > > 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. Btw thanks for the link. Got something to read later now. ---
**Author:** ![Siam\_Sam](https://avatars.discourse-cdn.com/v4/letter/s/d78d45/32.png) [@Siam\_Sam](https://boards.straightdope.com/u/Siam_Sam)\ **Post date:** [December 30, 2010, 2:41am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/46 "2010-12-30T02:41:25Z")
> [@The\_Tao\_s\_Revenge](#): > > Well you’re not thinking it all the way through. > > If you don’t put the orange in the past because the past has an orange from the future, then the past won’t have an orange from future so you will put it in as before which results in you not. > > Basically you’ve initiated an infinite loop cycling between the orange being there, or not. Infinite loops are not logically impossible, as anyone who’s ever forgetting to increment the variable in their while loop knows. This is similar to my own pet theory of dividing universes. If you go back in time and kill your grandfather, you won’t be born. So then you can’t go back in time and kill him, so you are born. So then you do go back etc. My theory is when you go back and kill your father, you cause the universe to split off into a separate reality. So both events occur, but you have two universes proceeding from that track. I think it’s the same in _Groundhog Day_. Each of those days goes hurtling off into its own direction, with each of however many Phils living with the consequences of each of the days. ---
**Author:** ![wintertime](https://avatars.discourse-cdn.com/v4/letter/w/c68b51/32.png) [@wintertime](https://boards.straightdope.com/u/wintertime)\ **Post date:** [December 30, 2010, 10:21am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/47 "2010-12-30T10:21:25Z")
> [@Siam\_Sam](#): > > This is similar to my own pet theory of dividing universes. Frankly, I’m not sure whether you talk about one of the MWI interpretations or bubble inflation multi – looks to me like a mix. Could you specify your idea a bit more? Btw, the one multiverse-hypothesis I know that produced falsifiable oberservational predictions ([Smolin’s conjecture](http://arxiv.org/PS_cache/hep-th/pdf/0612/0612185v1.pdf)), might have been .. falsified: [A two-solar-mass neutron star measured using Shapiro delay](http://www.nature.com/nature/journal/v467/n7319/full/nature09466.html). This is a neutron star too massive to fit into Smolin’s hypothesis, isn’t it? ---
**Author:** ![iamnotbatman](https://avatars.discourse-cdn.com/v4/letter/i/858c86/32.png) [@iamnotbatman](https://boards.straightdope.com/u/iamnotbatman)\ **Post date:** [December 30, 2010, 10:53am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/48 "2010-12-30T10:53:05Z")
> [@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. [This](http://boards.straightdope.com/sdmb/showthread.php?t=588688) thread perhaps. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 30, 2010, 11:31am UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/49 "2010-12-30T11:31:26Z")
> [@Siam\_Sam](#): > > This is similar to my own pet theory of dividing universes. If you go back in time and kill your grandfather, you won’t be born. So then you can’t go back in time and kill him, so you are born. So then you do go back etc. My theory is when you go back and kill your father, you cause the universe to split off into a separate reality. So both events occur, but you have two universes proceeding from that track. > > I think it’s the same in _Groundhog Day_. Each of those days goes hurtling off into its own direction, with each of however many Phils living with the consequences of each of the days. The only problem with this idea (and it’s not really a problem, well, maybe not) is the question of: what scale of event causes new universes to be spawned? For example, how many universes are spawned during the action of me rolling a die? Six? - Because there are six possible results of the die roll? What about the possibility of the die rolling off the table? Do new universes get spawned when the die bounces three, four, five times before settling? What about all the subtly different positions and angles/orientations the die may pass through, or end up in? And so on, in ever-finer detail - and that’s just for my one activity - everything else happening everywhere else is also similarly variable. As far as I can see, the only way the splitting universes thing can work is if _infinite_ universes are spawned at every moment, but then each of those infinite universes also gives rise to its own infinite set of universes the next moment, and so on. That’s a lot of universes. Where do they all go? ---
**Author:** ![Siam\_Sam](https://avatars.discourse-cdn.com/v4/letter/s/d78d45/32.png) [@Siam\_Sam](https://boards.straightdope.com/u/Siam_Sam)\ **Post date:** [December 30, 2010, 4:09pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/50 "2010-12-30T16:09:48Z")
Well, I’m not sure I can specify my idea very much more, as I haven’t really fleshed it out all that much. But it seems to me that if you go back and kill your grandfather, that has happened. And if you kill your grandfather (before he has children, of course), then your going back cannot have happened. An endless loop was mentioned before, but I prefer to think of it as a new universe splitting off with each incident. We’d never know it. Only an outside observer would ever see it. But in _Groundhog Day_, I think the Phil Connors who found himself in jail after asking if it was too late for flapjacks will continue on that course, not wake up again at 6am (actually 5:59am). And the Phil Connors who made love to the fake high-school “buddy” will follow on that. Etc. I am not a physicist, nor do I play one on TV. ---
**Author:** ![BlinkingDuck](https://avatars.discourse-cdn.com/v4/letter/b/eada6e/32.png) [@BlinkingDuck](https://boards.straightdope.com/u/BlinkingDuck)\ **Post date:** [December 30, 2010, 4:31pm UTC](https://boards.straightdope.com/t/time-travel-quandry/565717/51 "2010-12-30T16:31:46Z")
> [@Agent\_Foxtrot](#): > > Seems there’s quite a bit of Schrödinger in here. I postulate that even if we can manipulate time, we will never know if we were successful or not. Case in point: > > In my mad scientist lab, I have two clear cylinders. The first cylinder should send objects back in time five minutes to the second cylinder. Let’s say, for argument’s sake, that I’ll know my experiment was successful five minutes before I flip the switch if an orange appears in the second cylinder. I then have a choice: I can choose whether or not to proceed with the experiment. If I don’t, I create a paradox. And the universe will prevent me from creating that paradox, so my experiment is a failure. > > However, suppose I can’t see what’s inside either cylinder until I flip the switch. The orange will either be in the first cylinder or the second, but I won’t know until I open them. Therefore, I’ve deprived myself of the ability to create a paradox five minutes before I flip the switch. It now doesn’t matter whether my orange traveled time or not: If it’s in the first cylinder, then the experiment is a failure. If it’s in the second cylinder, all I can be sure of is that I teleported an orange. Whether or not it was there five minutes ago is impossible to know. > > Ergo, even if we could invent a time-travel device, we’ll never know if it works or not. Q.E.D. 🆒 Agreed that you would never know, but not your reasonings… IF time travel is possible and IF sending the time machine with the orange back 5 minutes in the past creates its own ‘future’ (i.e. the timeline ‘splits’) then it will never come back to you in your timeline. You would just send off the machine never to see it again. If YOU stepped into a time machine while I watched saying to me “I’ll be right back” the same would happen. Sure, you would come back and say hello to ‘me’…but the me that watched you go would never see you again. [Previous page](https://boards.straightdope.com/t/time-travel-quandry/565717.md?page=2)