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Time Travel Simulation Resolves “Grandfather Paradox”

• idlewords 12y ago
  Two nice links for anyone who wants to nerd out about the physics of closed timelike curves: http://plato.stanford.edu/entries/time-travel-phys/ http://plato.stanford.edu/entries/time-travel-phys/ http://philsci-archive.pitt.edu/1673/1/TMArchive.pdf http://philsci-archive.pitt.edu/1673/1/TMArchive.pdf
• aaronbrethorst 12y ago
  My favorite "layman's" argument for why time travel, as it's been classically described in science fiction, can't exist is that we haven't been overrun with tourists from the future.
  • randallsquared 12y ago
    Sure, but CTCs only allow one to travel back to a time when the CTC existed, so I don't think that's a good argument unless there was a CTC-generating device in our neighborhood. Of course, as far as I know, all CTCs require infinities (Tipler cylinders, for example: http://en.wikipedia.org/wiki/Tipler_cylinder http://en.wikipedia.org/wiki/Tipler_cylinder ), which is an argument against constructing one...
    • pyre 12y ago
      > CTCs only allow one to travel back to a time when the CTC existed So... Quantum Leap rules of time travel then? Oh boy...
  • th3iedkid 12y ago
    but the full question being is we = all ?
  • beachstartup 12y ago
    and my favorite refutation is that they've been here for a while now.
    • ghayes 12y ago
      E.g. the fellow in this photo from 1940: http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef01347ff60cea970c-pi http://www.dailygalaxy.com/.a/6a00d8341bf7f753ef01347ff60cea...
      • ekianjo 12y ago
        Nice fake picture :)
        • ghayes 12y ago
          Consensus seems to be that the photo is actually real, just not so out of place. From the Wikipedia page "Time travel urban legends" [1]: > Further research suggests that the modern appearance of the man may not have been so modern. The style of sunglasses first appeared in the 1920s. On first glance the man is taken by many to be wearing a modern printed T-shirt, but on closer inspection it seems to be a sweater with a sewn-on emblem, the kind of clothing often worn by sports teams of the period. [1] http://en.wikipedia.org/wiki/Time_travel_urban_legends#Modern_man_at_1941_bridge_opening http://en.wikipedia.org/wiki/Time_travel_urban_legends#Moder...
          • ekianjo 12y ago
            Not sure how one can confirm the photo is actually real, it's so easy to fabricate picture nowadays and to add noise to make it seem like it belongs to the original. It's be interesting to actually have an in-depth article on the methods to detect fake pictures, I have seen a couple on HN but they only focused on a few examples.
    • eridius 12y ago
      It strains credulity to believe that we do in fact have time travelers living among us and 100% of them have managed to completely escape detection. For what it's worth, there actually have been serious attempts at determining whether there are time travelers living among us, including at least one study[1] looking at Twitter for evidence of time travelers tweeting [1]: http://www.cnn.com/2014/01/03/tech/innovation/time-travel-twitter/ http://www.cnn.com/2014/01/03/tech/innovation/time-travel-tw...
  • tzs 12y ago
    I'm sorry I cannot give a better cite than this, but sometime in the last 20 years there was a terrific short story, probably in Analog (but possibly in Asimov's) about the first time traveller. His first shock was when he found that the person in the past they selected to visit, Shakespeare, did not seem at all surprised when visited by a time traveller. In fact, he seemed to find it routine. The traveler at one point says he doesn't understand how Shakespeare can already know about time travelers, since he is the first. Shakespeare tells him that he may have been the first to leave, but he wasn't the first to arrive. Shortly after, many time travelers arrive...but not to see Shakespeare. They are reporters, from all throughout the timeline, coming to try to interview the first time traveller, and Shakespeare, an old hand at dealing with time travelers, steps in to protect the first time traveller and prevent him from being overwhelmed.
    • atmosx 12y ago
      "The Merchant of Stratford" by Frank Ramirez (amazing approach, really unique and ingenious).
      • andyjohnson0 12y ago
        Looks like it was in Isaac Asimov's Science Fiction, July 1979 [1]. [1] http://www.abebooks.co.uk/book-search/author/ramirez-frank-r/sortby/3/page-1/ http://www.abebooks.co.uk/book-search/author/ramirez-frank-r...
      • tzs 12y ago
        The internet tells me it was in Asimov's, in the July 1979 issue, so I was way off on the time frame.
    • RyanMcGreal 12y ago
      Reminds me of this classic: http://www.abyssapexzine.com/wikihistory/ http://www.abyssapexzine.com/wikihistory/ "Everybody kills Hitler on their first trip."
    • Sukotto 12y ago
      Looks like this thread is gathering the list of good Time Travel stories. So here's one I like: "Dinosaur Beach" by Keith Laumer. A delightfully twisty and multi-layered novella. Read: http://www.baenebooks.com/10.1125/baen/0743435273/0743435273__18.htm http://www.baenebooks.com/10.1125/baen/0743435273/0743435273... Reviews: https://www.goodreads.com/book/show/584217.Dinosaur_Beach https://www.goodreads.com/book/show/584217.Dinosaur_Beach
  • rtpg 12y ago
    I've always found this argument a bit bizarre, because in my mind any time travelling device would work via a pair of machines: a sender and a receiver. So nobody could come to the present simply because there's no receiver yet. The reason we haven't been overrun by tourists because there's no stop in this time period.
    • coldtea 12y ago
      >I've always found this argument a bit bizarre, because in my mind any time travelling device would work via a pair of machines: a sender and a receiver. Were did you get that impression from? Not most Science Fiction or Physics, because neither postulates such a thing.
      • afafsd 12y ago
        Actually the only vaguely plausible-ish (though by no means convincing) designs I've heard of for physically realisable time machines work like this. You do some horrendous things with spacetime curvature and you get something that can send you back in time, but no further back than the point at which you constructed your horrendously curved spacetime thingy. Paul Davis has a short book How To Build A Time Machine which does it this way, if I recall correctly (it's been a while since I read it).
      • snowwrestler 12y ago
        Isn't it the premise of the movie Primer?
      • dark_photon 12y ago
        Science Fiction is not relevant here. As for physics, you are mistaken. The only kind of "time travel" that has any basis in physics is the one using Closed Timelike Curves (which is what the article is about) and those are limited to the time the CTC was created in the past. So, in fact, this is a very good reason to reject the argument of "If it is possible, where are the time travelers?".
  • inclemnet 12y ago
    I hate this argument, not because it isn't correct as far as it goes, but because it's very often represented as much more broad than it really is. That is, yes, we can be reasonably sure that time travel mechanisms whereby tourists definitely could and would become known in our time period will not exist (even this isn't the same as can't exist). But this is a very small sampling of time travel scenarios you could imagine, and I think it's far from the only thing described 'classically' by science fiction.
  • gambiting 12y ago
    And my favourite "layman" counter argument to that is that plenty of time travel theories only allow travel as far back as to when the time travel machine was first created.
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• zrm 12y ago
  > In the presence of CTCs, quantum mechanics allows one to perform very powerful information-processing tasks, much more than we believe classical or even normal quantum computers could do That seems like an understatement if I'm understanding it correctly. Imagine you have a "time machine" and you want to solve some arbitrary computable problem. You try a prospective solution and see if it's correct. If not, you send the next prospective solution back to be tried on the next go around the time loop. Once you arrive at the correct solution, you send that same solution back so that the iteration stops there. Then the loop repeats indefinitely with the correct solution so that the probability of exiting the loop at the correct solution approaches infinity. It would literally be the end of the world as we know it. P=NP. Forget about quantum cryptography, that would break all public key cryptography, all cryptographic hash functions. It would obsolete algorithmic complexity theory by effectively turning every finite space algorithm into an O(1) algorithm. It would probably bring Strong AI. But it still couldn't break a one-time pad.
  • terhechte 12y ago
    There's a chapter in the brilliant fan fiction "Harry Potter and the Methods of Rationality" where Harry gets access to a Time-Tuner, i.e. a time machine from the Harry Potter universe, and this is the first experiment he envisions and performs. HPMOR is a really great book. https://www.fanfiction.net/s/5782108/17/Harry-Potter-and-the-Methods-of-Rationality https://www.fanfiction.net/s/5782108/17/Harry-Potter-and-the...
    • deleted 12y ago
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    • Semaphor 12y ago
      fair warning: This, absolutely awesome and actually well written, fanfiction is by now longer than all the books combined.
      • eru 12y ago
        It's a decent read, but you have overlook that the author is very fond of himself. Harry Potter and the Natural 20 is also highly enjoyable.
• vbit 12y ago
  Say I go back in time. Does it mean I disappear from the time I was in? And I'm in two places at a point in the past (the me from that time and the me from the future)? Now if I want to go back to the time I came from, do I join back at the exact time I left (which means no one would notice my travel), or would I have to travel back to the time I left + the elapsed time I spent in the past? And if I really did return to the exact time I left, does that really mean no one noticed my leave? Because I'm sure when I left time continued and somebody noticed.
  • afafsd 12y ago
    In the form of time travel being talked about, the answers are: a) Yes, you disappear from the time you were in. b) Yes, you're in two places at once for the duration between your arrival and your departure c) Forwards travel in time doesn't cause too many philosophical problems (since we're already doing it) which is why nobody talks about it. But there's no requirement to wind up exactly where you started.
• jaza 12y ago
  Time to dust off my old flux capacitor again...
  • wuliwong 12y ago
    lol, can't believe this gets down-voted. God forbid there is humor on HN!?!
• stinos 12y ago
  Can someone enlighten me: what is this simulator they speak of? It doesn't seem answered anywhere in the article what it is, whether it's software, let alone how it works. Yet the whole article relies on it. Does it act on real protons? Or are they simulated as well? And, most important, why do they trust a simulation device built by humans who hardly understand the matter completely, when the device is then used to make assumptions about that same matter? Is it somehow proven this simulator is correct? And how did that happen?
  • jessriedel 12y ago
    This is a poor article. It is reporting on a simple table top device created by researchers eager to misrepresent their work. It tells us essentially nothing about quantum mechanics or CTCs. We don't know how quantum mechanics behaves in the presence of CTCs. There are multiple, mutually incompatible proposals for extending QM in this regime that all make identical predictions in normal, causal spacetimes (i.e. the only sorts of spacetimes we have access to). Deutch has one extension of QM, and it suffers from serious internal consistency problems, but there are others. Since this experiment takes place without CTCs, it can't tell us anything we didn't already know. It's literally just a toy device that is sorta described by the same math as a certain system with CTCs in QM_Deutch would be, if you squint your eyes hard enough. But you could say the same thing about a computer simulating QM_Deutch in software. It tells you nothing about whether the universe actually obeys QM_Deutch in the presence of CTCs ... if they exist at all. Similarly deceptive bits of "research" in the news often appear that claim to find quantum gravity effects on table-top experiments. http://www.space.com/5052-black-hole-effect-created-lab.html http://www.space.com/5052-black-hole-effect-created-lab.html In all these cases, the scientists will eagerly over-represent their work to the journalist, and the journalist will eagerly gobble it up and write eye-catching headlines about new breakthroughs in physics, but the heart of the claim will be buried in the article and subtly couched in weasel words. And if you actually try to drive down at it by asking probing questions, the journal and scientist will retreat to much, much less exciting claims and you will never be able to prove they intended otherwise.
• elseless 12y ago
  Scott Aaronson has done some work regarding CTCs and quantum computers: http://www.scottaaronson.com/papers/ctc.pdf http://www.scottaaronson.com/papers/ctc.pdf Apparently, in the presence of CTCs, quantum computers are no more powerful than classical ones.
• smokel 12y ago
  Apparently, Stephen Hawking organized a time travel experiment in 2009. There was also the Time Traveler Convention in 2005 [1]. If people keep organizing such events, future time travellers may be at a loss as to which one to attend. [1] http://web.mit.edu/adorai/timetraveler/ http://web.mit.edu/adorai/timetraveler/
  • narag 12y ago
    future time travellers may be at a loss as to which one to attend Ahem, all of them at once?
    • coldtea 12y ago
      That they can do that, technically, doesn't mean the would. Just like we don't attend all parties available to us, they would soon get bored.
    • Dylan16807 12y ago
      Do you really want to go to 10000 conventions for the same thing?
  • stevewillows 12y ago
    The best part of this event was that they turned people away. I feel sorry to those time travellers who didn't go back far enough.
  • swalsh 12y ago
    Imagine you're a time traveler from the future, and you've read all about the NSA from 2009. Would you want to risk making your presence known? What do you think the government would do if they got a hold of a real life time traveler?
  • 3pt14159 12y ago
    The first one?
  • u124556 12y ago
    We all know no one attended the party, so maybe time travelers just didn't want to disturb that part of history?
• DCKing 12y ago
  Well, the argument is that quantum indeterminism implies quantum time travel is possible, because the grandfather paradox is solved/migitated by indeterminism in which the grandfather paradox is still a valid outcome. However, quantum indeterminism itself is not yet completely undisputed; some theoretical physicists such as Gerard 't Hooft maintain that quantum mechanics can be both deterministic and consistent with current experiments. This also raises the following question - if quantum mechanics is undeterministic, then the macroscopic world is at least probabilistic. If probabilistic systems allow the possibility time travel on the quantum scale, would the grandfather paradox also not be solved on classical scales? Since it is possible that me killing my grandfather in the past would fail with some very small probability (due to various quantum effects adding up), would the grandfather paradox then not also be solved in this case? I'm not sure if I'm interpreting the article correctly.
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• axilmar 12y ago
  If time travel exists, then it cannot be a simple rewind of the universe's tape. Because if that was the case, then rewinding the tape to a time that your father was not born would mean that you will also not be born, and thus there wouldn't be any time travel, since neither you nor anyone else would be able to witness it. Thus, in order for time travel to work, it means to rewind the tape and add yourself to the tape at a moment you did not exist before. Thus, if you killed your father, your replica wouldn't be born, but you wouldn't stop to exist. Therefore, the whole 'grandfather paradox' is not a paradox at all: if time travel exists, then you would simply exist in a universe where a replica of you will exist or not, depending on how events play out.
  • afro88 12y ago
    Not so fast - who says you become a "replica" in the first place? Assuming time travel replicates things, then yes, what you said holds true. But as I understand it you don't replicate, your time just loops back on itself. So looping back to a time before you were born and killing your father would create a paradox.
    • axilmar 12y ago
      You seem to have not understood what I said. I did not say time travel would replicate anything. You would simply be transferred to the past, without any connection to the outcome of that past. I.e. Let's say that you are transferred back in the 50s and you go kill your father before you were born. Then nothing will happen, there would be no paradox, you will continue to exist in that universe.
• deleted 12y ago
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• TeMPOraL 12y ago
  I'm starting to adopt the idea of Scott Aaronson that since we have a lot of evidence suggesting that P != NP, we should treat physic ideas as less probable of being true if they make P=NP as a side effect.
• sandycheeks 12y ago
  When I was younger I thought a lot about the physics of traveling back in time but I always seemed to hit a brick wall with spatial coordinates. Where we are in the universe today is very far from where we were in the universe yesterday based on the movement of the earth alone. Add to that the movement of our solar system, galaxy, cluster, supercluster and movements I am not even aware exist and it becomes really far away. Grandpa would probably be light years away from me and my time machine. Am I missing something here, because I've never heard this mentioned by anyone else?
  • jdjb 12y ago
    If you can travel backwards in time (or even just stop time) you can be at any spatial location you desire in the entire universe (since you have essentially infinite speed).
  • growse 12y ago
    Moreover, imagine I write down some information, then wait for exactly 6 months for the earth to take me far away from that point in space. If I now travel 6 months minus 1 nanosecond back in time with that piece of paper, but preserve my spacial position, haven't I just 'transmitted' information through space faster than light can?
    • TheLoneWolfling 12y ago
      FTL travel is synonymous with time travel, and vice versa. Relativity is weird.
  • kevincennis 12y ago
    It's been almost 10 years, so I may be mistaken... but I think the director of Primer mentioned this in the DVD commentary. EDIT: Right, not really part of the plot. I just think I remember him (in the commentary) talking about some of the "trickier" aspects of writing a story about time travel, and I think he mentioned the fact that nobody ever really addresses the positional aspect.
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• TheLoneWolfling 12y ago
  My pet theory: You create a paradox, it just keeps looping (you go back in time and kill your grandfather -> you don't exist -> you don't build the time machine and go back -> your grandfather lives -> you exist -> you build the time machine etc). But every time around the loop there is a certain amount of actual randomness. Perhaps that atom decays this time around, or that transistor erroneously conducts due to shot noise. So all we see is the final "fixed point". The iteration where everything ends up going just right to avoid a paradox - perhaps the gun fired prematurely, or the time machine didn't work, or a passing airplane dropped an engine on you, or... From the perspective of any time traveler, it ends up being as though the universe is conspiring against you if you try to do anything that would cause a paradox.
  • DennisP 12y ago
    This sounds like the Novikov self-consistency principle. Say you have a wormhole on a billiard table. It curves around and goes three seconds backward in time. You roll a ball into the wormhole, aimed such that after it exits the wormhole, it will knock its earlier self off the path so that it never enters the wormhole. Paradox. Except when you try it, instead of emerging along the pathway you aimed, it emerges along a slightly different path, and strikes its earlier self only a glancing blow. And why did it emerge along a different path? Because it was struck a glancing blow. http://en.wikipedia.org/wiki/Novikov_self-consistency_principle http://en.wikipedia.org/wiki/Novikov_self-consistency_princi...
    • TheLoneWolfling 12y ago
      I suppose having a superposition of all (self-consistent) loops that collapses into a single result is equivalent to the result of what I said... Hmm. Thanks for the link!
    • cscheid 12y ago
      David Deutsch also proposed a similar resolution for closed timelike curves, which say that if you have a probability distribution of the present, all that's necessary is that when you travel back in time, you get the same probability distribution. In short, if you flip a (true!) random coin to before shooting your grandfather, no paradox exists: http://journals.aps.org/prd/abstract/10.1103/PhysRevD.44.3197 http://journals.aps.org/prd/abstract/10.1103/PhysRevD.44.319... This is described with more intuition and background in one of (always excellent) Scott Aaronson's lectures: http://www.scottaaronson.com/democritus/lec19.html http://www.scottaaronson.com/democritus/lec19.html
• im2w1l 12y ago
  http://en.wikipedia.org/wiki/Closed_timelike_curve http://en.wikipedia.org/wiki/Closed_timelike_curve "In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime that is "closed", returning to its starting point." Now, go to http://en.wikipedia.org/wiki/Virtual_particle#Vacuums http://en.wikipedia.org/wiki/Virtual_particle#Vacuums and look at the picture "One-loop diagram with fermion propagator" It seems to me that virtual particle pairs satisfy the CTC criterium.
• onion2k 12y ago
  It's quite possible that the problem with paradoxes stems not from them being 'impossible', but from our limited cognitive tools failing to equip us to think about them. Sort of like people in ancient times believing you can't predict the motion of the planets because they didn't have calculus yet.
  • cLeEOGPw 12y ago
    It's also quite possible that the whole idea of time travel seeming probable stems not from being actually possible, but from human brain cognitive bias of memory. In our head, past events are almost as real as current events, therefore creating obvious illusion of possibility of traveling in reality just like you travel in memories.
• camelNotation 12y ago
  My pet theory: Quantum mechanics is a statistically based science because we have yet to discover the constants that would unify it with general relativity, making this entire thought experiment irrelevant. Imagine if you are watching 10 parallel strings ripple up and down at various frequencies. Viewed from the side, it would at first glance appear chaotic, but by observing the behavior long enough and recording where the activity occurred over time, you could statistically predict where the strings would be at any given time. However, if you were aware of the number of strings you were viewing and the frequencies at which they were moving, you could formulate a precise theory of exactly what you were viewing and what you would view at every point in the future. That is the current state of physics. Quantum mechanics is seeing without understanding the rules governing the system, so predictions and observations are made using statistics instead of constants. Once we discover the constants, the current theories will be entirely obsolete and articles like this will become relics.
  • ep103 12y ago
    yeah. I remember some ~7 years ago reading about a research team that stated that they had rewritten many of the fundamental equations of (some branch of) quantum physics to show that they could be written in a completely deterministic manner. They said that while such equations weren't likely to be useful (due to the limitations of our testing technology), it was an important result, particularly when it comes to understanding the underlying science. And they were very upset that basically no one paid their result any real attention.
• phpnode 12y ago
  All hypothetical time machines must also be space machines, i.e. they must take into account the movement of the planets, the galaxy, the universe to avoid materializing their passengers in the middle of space or the centre of the sun. My question is what frame of reference would they use?
  • claar 12y ago
    This is exactly what always bothers me in time travel movies. You never hear about the poor time traveler who got the calculation slightly wrong and ended up deep in space or inside of a mountain.
    • chippy 12y ago
      Only when you find their fossils.
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  • TheLoneWolfling 12y ago
    You can use a time machine as a very good space drive. "All" you do is jump back / forwards in time repeatedly so that gravity pulls you in the direction you want to go. (In actuality: good luck plotting a flight path) Want to get to 20% of the speed of light? Sit in freefall near Jupiter for 5 weeks (assuming 20m/s/s acceleration), jumping forward / backward in time as appropriate to stay in roughly the same spot relative to Jupiter.
• stevewilhelm 12y ago
  I would like to see a unified explanation that describes how our current modes of time travel work. How does the matter that comprises my car and myself successfully get from Palo Alto to The Mission every morning?
• chippy 12y ago
  I've enjoyed thinking about the following: It must be almost impossible to time travel back in time. But it could be possible, given unlimited time. Thus, given that at some point in the unlimited future, time travel would occur, how can we increase the chances now of being visited by these future time travellers?
  • testing3212 12y ago
    > But it could be possible, given unlimited time. Just because time is infinite, does not mean that all things are possible. My understanding is that the universe is in a bounded random walk. As space-time moves, the unexplored potential state space increases more quickly than can be explored. If there are multiple universes, then each universe could explore one of these states. So if there are more states than universes, not all things are possible. But if there are more universes than states, all things would be possible. Though there are infinite potential states and infinite universes, I feel like the potential states expand faster than the number of universes. This means that our likelihood of living in a universe where something like like time travel is possible, is incredibly low.
• u124556 12y ago
  If you had a program that generated a random number, and somehow send a hash of said number to the past to be used as a salt to generate itself. Would the new random number be truly random?
• axilmar 12y ago
  I do not think that there can be a Grandfather Paradox. If one goes back in time, he/she will simply be removed from the state of the universe as it is now and be added to the state of the universe at the target past date. He/she could then go on killing their father before they were born, but nothing will happen to them except not meeting with themselves as babies.