Impossible Is Real

Fermi famously argued that either aliens aren’t out there, or they can’t or don’t want to get here, since we see none around us. An undated Stephen Hawking lecture makes a similar argument about time travel:

If sometime in the future, we learn to travel in time, why hasn’t someone come back from the future, to tell us how to do it. Even if there were sound reasons for keeping us in ignorance, human nature being what it is, it is difficult to believe that someone wouldn’t show off, and tell us poor benighted peasants, the secret of time travel. … If governments were hiding something, they are doing a pretty poor job of extracting useful information from the aliens. … Once you admit that some are mistakes, or hallucinations, isn’t it more probable that they all are, than that we are being visited by people from the future, or the other side of the galaxy? If they really want to colonize the Earth, or warn us of some danger, they are being pretty ineffective.

Many people seem quite resistant to the idea that fundamental limits might apply to our descendants, limits that continue even after trillions of years of advancement. But if we have vast hordes of descendants over trillions of years, almost none of them have the ability and inclination to travel back in time to visit us now. Because almost none are visiting. Some things really are impossible.

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  • Nathan

    What if the future has only gotten to the year 3,000 and they are only a year away from developing time travel?

    • Anonymous

      I lol’d.

  • DB

    I don’t doubt your overall point – that there are some impossible things – but there are several explanations as to why we have not met any time travelers.

    One possibility is that a mechanism is required on both ends, so that you cannot travel to a time before the invention of the machine itself (think Primer).

    Alternatively, the cost in terms of energy or some other resource could vary depending on how far back you go. If time travel is at all similar to our travel though spacial dimensions, then surely it would be cheaper to travel 1 year backwards than a thousand or a million years.

    • Robin Hanson

      If a mechanism is required on both ends, then time travel is not possible from the future to now, when we have no mechanism.

      • DB

        Yes. My point is that evidence that time travel from the future to now is impossible is not necessarily good evidence that all backwards time travel is impossible.

      • Shawn

        Maybe the “future” does not exist beyond the present period.

  • Robert

    If time travel is possible but has a high chance of destroying the universe, you are more likely to find yourself living before any time tourists’ vacations than after.

  • Matt

    My understanding (very amateur) is that, according to the current belief in physics, time travel is physically impossible. Therefore, an informed person today shouldn’t be able to “imagine” time travel. But if you can’t imagine an occurence or phenomenon, how can you project the rules that govern it? In other words, if there were some undeniable fact that we are all completely ignorant of today that allows time travel, wouldn’t we also be ignorant to the rules subsets?

    Also, we can travel through space right now, but we can’t make it to the next galaxy. Maybe future time travelers can only go back so far…

    • TGGP

      In Sean Carroll’s “From Eternity to Here” he said that self-confirming/self-causing loops in time are considered possible by the standards of modern physics. But any sort of “grandmother paradox” is not allowed.

    • Cas

      Time travel is real. You’re doing it right now. You are moving forward in time.

  • Lord

    This requires broad assumptions as to their motivations that tell us more about ourselves than them. Perhaps aliens would be more interested in evolutionary adaptation than shaping it. Perhaps time travelers would like to return to their own time than be forever lost to it.

  • Steve

    A fascinating topic.

    As we “move forward” to the future, it seems we know ever more about the past, we time travel already, and are getting better at it. With a near infinite number of people, ems, whatever, to divide labor into ever smaller bits, many multiples of our current world civilization could be devoted to discovering minute clues to the past to the point where we’d think we are actually there.

  • richard silliker

    You can not map inward as in time travel. Perhaps “they” are already here and they are indifferent to us because of our behaviors. .

    • Stephen

      I agree. It is like when we study single cell livings (our evolutionary ancestors), the single cell livings do not know we are studying them! And we do not boost

  • Nolan

    Maybe the fact that we have not been visited by time travelers is proof that we destroy ourselves before we figure out the secrets of the universe.

  • Mark M

    Our location changes over time. Perhaps travelling in time doesn’t also change our location. Due to the rotation of the earth, the orbit of earth around the sun, and the orbit of our sun around the galactic core, our future time travelers could easily land in a hostile environment – most likely outer space. The further back in time they travel, the further they are from Earth, Sol, or even the Milky Way, making it increasingly difficult to reach us. They may only be able to go back a year to remain in radio distance.

    I’m just throwing out the idea that even if time travel is possible there may be very real reasons that our descendants don’t drop by to say hello.

    And anyway – the time cops have literally all the time in the world to stop miscreant time travelers before we find out about them. And if they don’t stop the time travelers the first time, they’ll try again the next day, or after they get back from vacation, or in a few years.

  • Anonymous

    Time travellers from the future may know that they can’t change the past and they may know that the past leads to the future they know, so there is no incentive for them to communicate with us in an obvious or highly impacting way, or try to change the past deliberately – because they know such projects would fail.

    So maybe they conform to the well-known timeline constraints and send cloaked nanoprobes etc. for historical study only.

  • Eric

    I echo DB and invoke T. McKenna’s assertion that: when a time travel device is actually invented, all of history will happen at once and we will be flooded with visitors.

    • DB

      If “all of history will happen at once” what does “when a time travel device is actually invented” mean? If an act in the future affects all of time, we don’t have to wait for it.

    • mjgeddes

      Experimental Journal:

      3:00: I open the time portal

      3:05: Hit in the face when my cat (black with white stripes) came flying out of the portal, meowing with all its fur standing on end. WTF?

      3:10: I need a test subject. Ah! My cat of course, I grab it, hoist it back and forth through the air, and toss it into the portal, setting the time for 5 minutes into the past.

      3:15: A future version of me came through the portal, assaulted me and tried to chloroform me, before running off! WTF?

      3:20: Unfriendly AI came through the portal. I’m going to apply Hanson’s economic theories, I’m going to trade my way out of this mess by offering to sell it a lovely piece of pottery for just 5.99…

      (All entries end)

  • Chris

    How can we assume that human nature will remain the same? I mean I guess we have to use the data at our disposal to extrapolate about future ‘humans’, but I just don’t see how we can base anything reliable off of those assumptions. I sincerely hope advancements are made in human nature despite how ugly the rammifications may be.

  • Nancy Lebovitz

    We could be a lot less interesting than we think we are.

    More generally, I expect that people (or the equivalent) in the future may be governed by fundamental constants we believe in, but they’ll also have weird stuff we can barely imagine.

  • arch1

    Yes it’s hard to believe that future entities with a significant link to us, and with the ability to do so, don’t at some point travel back and make themselves known. So there probably are no such entities.

    But that doesn’t cinch the case that the requisite flavor of time travel is physically impossible. It could simply be the case that all entities with a significant link to us die out before acquiring the capability.

    (Which suggests among other things that one of the headlines you don’t want to see in tomorrow’s paper is “time travel believed feasible in near future.” :-)

  • superflat

    yeah, they have the technology for time (or interstellar) travel, but not the technology to hide their presence. is there some reason you think that assumption makes any sense? or some reason we should feel comfortable guessing at the motivations and tech available to vastly advanced people and/or aliens?

  • superflat

    the other thing that’s so funny about this set of posts on (im)possibilities of tech/physics is that, with respect, they seem clearly to be signaling — that you are thoughtful, have interesting ideas about a wide variety of topices, etc. — rather than any sort of serious contribution to consideration of the possible. they might also be more along the lines of entertainment — interesting to speculate about what might be, even if ultimately silly (given that no one will ever offer $1 million for you to go to school naked, you’ll never see the inside of a black hole, etc) — but they’re presented as something akin to proofs.

  • Andy

    There’s lots of science fiction written that has time travel into the past without the people in the past realizing it. For example, Connie Willis has a few notable books of that type. So it’s possible that “historians” are studying our present right now, but of course they blend in so as not to attract attention.

  • ChrisA

    It could be that our descendants already have the required information on today (thanks to digital record keeping) to simulate it in sufficient detail at much less cost than travelling back in time. Maybe is the case that they are visiting other older times, but we wouldn’t know that since we have only have records of the original, unchanged times.

    Or it could be that they are visiting us, but every time a timetraveller arrives, they change history, so we only have one visit in current history.

  • Robert Koslover

    Why no time travel? Well… you can’t travel into the past because it doesn’t actually exist anymore. It has been replaced by the present. (But you can see/detect plenty of signals from the past, because those signals still exist in the present.) Similarly, you can’t travel into the future (although you can certainly wait for it to happen) because it doesn’t actually exist yet. :) This is perceived as a terribly unsophisticated view of course, in light of the curved space-time of general relativity, the rather remarkable symmetries and rules of quantum electrodynamics, multi-universe theories, arguments about entropy destruction or preservation in black holes, etc. And yet, despite all of that, it seems that no real-world contradictions (aka, something you can measure) to the aforementioned unsophisticated view have been found.

  • Robert Koslover

    …Of course, some people do presently live in the past. E.g., my mother has often remarked, “Thank God I’m not living nowadays!” :)

  • Gulliver

    Five possibilities I can see:

    1) Time travel into one’s own past light cone is impossible – at least unbounded by a past receiver such as the other mouth of a wormhole – whether we inhabit a multiverse or one universe.

    2) Time travel into one’s own past light cone results in all retrocausal particles (including the traveler) tunneling to a different Everett sheaf of the multiverse than this one.

    3) There is no multiverse, at least in the quantum decoherence sense, and any universe that allows mechanisms for time travel into one’s own past light cone eventually evolves Feynman paths that intersect its moment of origins, altering its constants, so that only universes that prohibit unbounded time travel are temporally stable and can manage to decohere into macroscopic reality.

    4) We are in one of Nick Bostrom’s simulations and the simulation’s creators and/or controllers are arranging it so we either have or detect no retrocausal time travelers.

    5) Some other sufficiently advanced agency – such as god(s), aliens with a big head start or something of which we haven’t yet conceived – is making sure we either have or detect no retrocausal time travelers.

    As my undergrad quantum mechanics prof said of philosophizing indeterminately about the interpretations if quantum maths, shut up and calculate! You should appreciate that, Hanson, being as you are a mathematician 

  • Dave

    There is the problem of past activities altering the future. No one knows how much a given activity could leverage into the future. For example,if you sneezed and Hitler’s mother caught cold,she might have stayed home the day she met Hitler’s father. So if someone slightly altered the past everyone on earth might instantaneously cease to exist and be replaced by an alternate population. I am here to witness that that hasn’t happened,though it might happen any second now. Given this ,the idea seems to reduce itself to absurdity.

    • Gulliver

      Dave, what you refer to is the classic “grandparent paradox” which was the first argument advanced against retrocausal time travel. While a useful argument for explaining the, as you said, apparent absurdities of such travel, it suffers from overspecification because it gives the impression that the paradox requires something salient to human (or even conscious) observers to edit the timeline…offing Hitler or your grandmother or some such. But from a quantum perspective, particle interactions decohere wave-functions with or without conscious observers. So even a single quantum particle, say a photon, leaking into its own pat light cone – all the spacetime coordinates from which, traveling at c (the vacuum speed of light) anything can possibly reach the place and time of departure and so influence it by, for example, stopping the time travel – will minutely disrupt its own history. This is thanks to Heisenberg’s famous Uncertainty Principle, the same one that says merely observing a particle system disrupts its quantum state because particles must impinge on the system to relay any information to the observer.

      This is why Stephen Hawking was so keen to find a mechanism for “cosmic censorship” to make the universe “safe for historians” despite General Relativity’s allowance for closed timelike curves (CTCs) via uneven time dilation of the ends of an Einstein-Rosen bridge (AKA wormhole). Mind you, Hawking was not necessarily convinced wormholes were practical, but he wanted to fill in the gap in the otherwise very consistent theory of Relativity. But wormhole time machines can’t trace back to earlier than the reference frame they are created in, so that is a boundary condition and, barring alien wormhole time machine relics or ultrahush government superscience (and these are the bright lights that let guerrilla militants hack into their aerial drones), one that stops time travel to now or earlier.

      Mel Brooks covered this a quarter century ago:

      For the unbounded type of time machine Hanson was talking about, the situation is much worse. The third possibility I mentioned above is the unbounded version of Hawking’s cosmic censorship, the universe’s very own grandmother paradox, so to speak.

      The book isn’t completely closed, and any honest skeptic must admit the limits of current theory. But without a multiverse, the prospects for bounded retrocausal time travel look dim; for unbounded DeLoreans, they look positively atrocious.

      Damn this site rocks! Great content. Great comment policy. Great clientele. Great Scott! Must. Not. Get. Sucked. Into. Another. Awesome. Blooooooooog…

      • Dave

        Quantum stuff makes things weird. I don’t have enough understanding of it to make any judgements of its effect.

        Multi- universality ,as far as I understand is even weirder. Correct me if I am wrong,but if there infinite universes being spawned at each juncture,for example me crossing a street and getting run over or not by a Ford or Chevrolet or each car, truck or bicycle driven by each person on earth who could ever have been conceived , there become insanely too many universes.

  • David C

    I would say that Gulliver’s 5th argument is pretty much guaranteed to happen if time travel is real. Access to time travel would be as dangerous a weapon as a nuclear bomb. If our future selves discovered, it would likely be incredibly expensive especially since it would likely require a spaceship to move through both space and time, per Mark M’s argument. If time travel was first discovered in the 24th century, it might take until the 29th century to develop a cheap and easily available means for anybody to do it. 70 years after their invention, nuclear weapons are still incredibly expensive to develop. By then, future governments would have likely developed a system for detecting and preventing dangerous time travel incursions. Some would likely slip through, but they would be small events or rare enough that our present selves could dismiss them. And for those paying attention, yes, that was a Star Trek reference.

  • Gulliver

    Dave, there was no reply button, so I’ll reply down here.

    What you refer to is Hugh Everett’s Many-Worlds Interpretation of quantum mechanics, which he proposed as an alternative to the Copenhagen Interpretation that holds that it is the act of observation that causes probability waves to collapse (or decohere) into particles. Many physicists prefer the more modern decoherence interpretation that posits each particle as a kind of “observer” collapsing others as it interacts with them. There are a few other interpretations as well. The problem is that while the system of equations known as quantum mechanics are excellent predictors of physical systems, the math doesn’t tell us anything about what that physically means. It’s even possible that all the interpretations are simply macro-centric attempts to stuff quantum physics into a philosophy we can grok, but which have no bearing on the actual behavior which the equations are calculating.

    If you think this is frustrating to non-physicists, imagine how exasperating it is to scientists whose whole…ah, universe is built on experimental verification of falsifiable hypotheses. Many prefer to apply Occam’s razor and not multiply universes beyond necessity. I prefer to keep an open mind so unfounded assumptions don’t blind theory to possible avenues toward a unified theory of QM and Relativity, but to bear in mind that without experimental data one way or another, all interpretations are speculative – fun, but speculative.