Rules for Time Travelers

With the new Star Trek out, it’s long past time (as it were) that we laid out the rules for would-be fictional time-travelers. (Spoiler: Spock travels to the past and gets a sex change and becomes Kirk’s grandfather lover.*) Not that we expect these rules to be obeyed; the dramatic demands of a work of fiction will always trump the desire to get things scientifically accurate, and Star Trek all by itself has foisted half a dozen mutually-inconsistent theories of time travel on us. But time travel isn’t magic; it may or may not be allowed by the laws of physics — we don’t know them well enough to be sure — but we do know enough to say that if time travel were possible, certain rules would have to be obeyed. And sometimes it’s more interesting to play by the rules. So if you wanted to create a fictional world involving travel through time, here are 10+1 rules by which you should try to play.

0. There are no paradoxes.

This is the overarching rule, to which all other rules are subservient. It’s not a statement about physics; it’s simply a statement about logic. In the actual world, true paradoxes — events requiring decidable propositions to be simultaneously true and false — do not occur. Anything that looks like it would be a paradox if it happened indicates either that it won’t happen, or our understanding of the laws of nature is incomplete. Whatever laws of nature the builder of fictional worlds decides to abide by, they must not allow for true paradoxes.

1. Traveling into the future is easy.

We travel into the future all the time, at a fixed rate: one second per second. Stick around, you’ll be in the future soon enough. You can even get there faster than usual, by decreasing the amount of time you experience elapsing with respect to the rest of the world — either by low-tech ways like freezing yourself, or by taking advantage of the laws of special relativity and zipping around near the speed of light. (Remember we’re talking about what is possible according to the laws of physics here, not what is plausible or technologically feasible.) It’s coming back that’s hard.

2. Traveling into the past is hard — but maybe not impossible.

If Isaac Newton’s absolute space and time had been the correct picture of nature, we could simply say that traveling backwards in time was impossible, and that would be the end of it. But in Einstein’s curved-spacetime universe, things are more flexible. From your own personal, subjective point of view, you always more forward in time — more technically, you move on a timelike curve through spacetime. But the large-scale curvature of spacetime caused by gravity could, conceivably, cause timelike curves to loop back on themselves — that is to say, become closed timelike curves — such that anyone traveling on such a path would meet themselves in the past. That’s what respectable, Einstein-approved time travel would really be like. Of course, there’s still the little difficulty of warping spacetime so severely that you actually create closed timelike curves; nobody knows a foolproof way of doing that, or even whether it’s possible, although ideas involving wormholes and cosmic strings and spinning universes have been bandied about.

3. Traveling through time is like traveling through space.

I’m only going to say this once: there would be no flashing lights. At least, there would only be flashing lights if you brought along some strobes, and decided to start them flashing as you traveled along your closed timelike curve. Likewise, there is no disappearance in a puff of smoke and re-appearing at some other time. Traveling through time is just like traveling through space: you move along a certain path, which (we are presuming) the universe has helpfully arranged so that your travels bring you to an earlier moment in time. But a time machine wouldn’t look like a booth with spinning wheels that dematerializes now and rematerializes some other time; it would look like a rocket ship. Or possibly a DeLorean, in the unlikely event that your closed timelike curve started right here on Earth and never left the road.

Think of it this way: imagine there were a race of super-intelligent trees, who could communicate with each other using abstract concepts but didn’t have the ability to walk. They might fantasize about moving through space, and in their fantasies “space travel” would resemble teleportation, with the adventurous tree disappearing in a puff of smoke and reappearing across the forest. But we know better; real travel from one point to another through space is a continuous process. Time travel would be like that.

4. Things that travel together, age together.

If you travel through time, and you bring along with you some clocks or other objects, all those things experience time in exactly the same way that you do. In particular, both you and the clocks march resolutely forward in time, from your own perspective. You don’t see clocks spinning wildly backwards, nor do you yourself “age” backwards, and you certainly don’t end up wearing the clothes you favored back in high school. Your personal experience of time is governed by clocks in your brain and body — the predictable beating of rhythmic pulses of chemical and biological processes. Whatever flow of time is being experienced by those processes — and thus by your conscious perception — is also being experienced by whatever accompanies you on your journey.

5. Black holes are not time machines.

Sadly, if you fell into a black hole, it would not spit you out at some other time. It wouldn’t spit you out at all — it would gobble you up and grow slightly more corpulent in the process. If the black hole were big enough, you might not even notice when you crossed the point of no return defined by the event horizon. But once you got close to the center of the hole, tidal forces would tug at you — gently at first, but eventually tearing you apart. The technical term is spaghettification. Not a recommended strategy for would-be time adventurers.

Wormholes — tunnels through spacetime, which in principle can connect widely-separated events — are a more promising alternative. Wormholes are to black holes as elevators are to deep wells filled with snakes and poisoned spikes. The problem is, unlike black holes, we don’t know whether wormholes exist, or even whether they can exist, or how to make them, or how to preserve them once they are made. Wormholes want to collapse and disappear, and keeping them open requires a form of negative energies. Nobody knows how to make negative energies, although they occasionally slap the name “exotic matter” on the concept and pretend it might exist.

6. If something happened, it happened.

What people want to do with time machines is to go into the past and change it. You can’t. The past already happened, and it can’t un-happen. You might wonder what’s to stop you from jumping in your time machine, finding your high-school self, and convincing them that they really shouldn’t go to the senior prom after all, thereby saving yourself all sorts of humiliation. But if you really did go to the prom, then that can’t happen. The simple way out, of course, is to suppose that travel into the past is simply impossible. But even if it’s not, you can’t change what already happened; every event in spacetime is characterized by certain things occurring, and those things are fixed once and for all once they happen. If you did manage to go back in time to your years in high school, something would prevent you from dissuading your younger self from doing anything other than what they actually did. Even if you tried really hard.

7. There is no meta-time.

The least realistic time-travel movie of all time might be Back to the Future. When Marty McFly changes the past (violating Rule 6), the future “instantaneously” changes. What the hell is that supposed to mean? Time measures the temporal interval between different events in spacetime, and can be quantified by clocks. There is no set of clocks outside the universe, with respect to which you can go muck around in the past and have effects propagate into the future “at the same time.” Likewise, your brain is not going to change to remember things differently, nor will any other record-keeping device such as diaries or photographs or embarrassing sex tapes. Sorry about that.

8. You can’t travel back to before the time machine was built.

Right now, at the particular place you are sitting, at the time when you are sitting there, one of two things is true: either there is a closed timelike curve passing through that point in spacetime, or there is not. And that situation will never change — no matter what clever engineers may do in the future, if they create closed timelike curves they cannot pass through events in spacetime through which closed timelike curves did not pass (corollary of Rule 6). Or in plain English: if you build a time machine where there wasn’t one before, it may be possible for future travelers to come back to that time, but nothing can help you go back to times before the machine was built.

9. Unless you go to a parallel universe.

Parallel universes — the kind we contemplate in the many-worlds interpretation of quantum mechanics (MWI) — provide potential loopholes for some of the above rules. According to the MWI, there exist different “branches” of the wave function of the universe, distinguished by different observed outcomes for the measurement of quantum events. In the celebrated Schrödinger’s cat thought experiment, there is a “universe” where the cat is alive, and one where it is dead. Some imaginative (but respectable) physicists, especially David Deutsch, have speculated that we could combine this idea with the possibility of closed timelike curves to contemplate travel into the past of a different universe. If time travel is unlikely, this idea is (unlikely)2, but it’s not inherently paradoxical.

If you could travel to the past in a different branch of the wave function, then we are allowed to contemplate changing that past in a self-consistent way, because it’s no longer really “your” past. So almost all cinematic invocations of time travel — where they are constantly mucking about, changing the past in crucial ways — would have to appeal to something along these lines to make any sense. But even if you can change what you thought was the past, all of the rules of continuity and sensibility still apply — no flashing lights, no disappearing, no sudden changes in the future, no re-writing of your memories, etc.

10. And even then, your old universe is still there.

Remember Rule 0: no paradoxes. If you have reliable records of having made some unwise decisions regarding your social life in high school, then those decisions were made, and can’t be un-made. Even if you go into a different branch of the wave function, where you bestow some wisdom-of-experience on your younger self, you would only be changing the history of that universe. There is still the universe you left behind, with all of your bad decisions still intact. That’s life in the multiverse for you. It remains for future scholars to write Ph.D. theses along the lines of Utility Functions and Moral Dilemmas in an Ensemble of Multiple Interacting Universes. But it’s just a matter of time.

[* Update: Spock does not actually travel backwards in time and become Kirk’s grandfather, nor lover, nor does he write Shakespeare’s plays. That was a “joke.” I am reliably informed that the Spoiler Patrol and Internet Rectitude Society does not appreciate “jokes.”]

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229 Responses to Rules for Time Travelers

  1. Rob Knop says:

    Dude, I’ve seen flashing lights while traveling through space.

    I then also had to pay a fine and go to traffic school to get it off my record.

    I suppose the same could be possible when traveling through time.

  2. George Musser says:

    Can it be? Did you write a whole post about time without mentioning the word “entropy” or “arrow” even once? 😉

  3. greg says:

    Papers like that have been written. I took a metaphysics class in college and it dealt with the metaphysics of time travel. Covered pretty much all of the things you mentioned here, and we specifically dealt with non-MWI timelines. But the amount of paper spent on such issues of ethics and metaphysics of time travel (both singular time line and splitting) in philosophical circles was somewhat surprising.

  4. Aaron F. says:

    “8. You can’t travel back to before the time machine was built.”

    … Unless there are enough naturally-existing wormholes! In that case, you could probably find, in your near future, the mouth of a wormhole that will spit you out in the past light cone of the event you want to visit. Then you could kick your space machine into high gear, and arrive at your destination in an arbitrarily short time. Once you’ve had your fun, another highly relativistic journey will take you back to whern you started.

    ‘Course, it would be hella expensive. Unless you take advantage of monetary inflation to stock up on cheap gas from the past while you’re then.

  5. cmt says:

    “If you did manage to go back in time to your years in high school, something would prevent you from dissuading your younger self from doing anything other than what they actually did.”

    The problem with time travel is the inherent sloppiness of language. I think it would be more correct to say:

    If you did manage to go back in time to your years in high school, something DID prevent you from dissuading your younger self from doing anything other than what they actually did.

  6. Sean says:

    Every time I suggest “someone should write a Ph.D. thesis called…”, someone else points out that it’s already been written. There’s a lesson there somewhere.

  7. wolfgang says:

    >> if you fell into a black hole, it would not spit you out at some other time

    I think we are not so sure about that if the black hole rotates and/or is charged.

  8. moshe says:

    Somewhat contrary comment regarding item 3, and closed timelike curves. I’d characterize the existing evidence as uniformly pointing to them being impossible to both construct and pass through. This is an undecided question that ultimately requires knowing how to quantize gravity. Nevertheless, even with the tools we have we see that to construct them you need negative energy, or strictly infinitely long strings, or other exotics, and the resulting structures would be very unstable, etc., etc., not a single piece of good news, one test of the idea that did not fail, unless I missed something

  9. rob says:

    Very cool, but I’m tempted to quibble a little with point 5. The Kerr spacetime, as you know, contains closed timelike curves. They pass through the interior, but the singularity is timelike, and therefore avoidable, and the spacetime has a white hole horizon as well, so there is, in principle, an exit route.

    Now you can say that the Kerr interior is unstable to linear perturbations, and in particular so is the chronology horizon. However, to my understanding we don’t know precisely what the nonlinear completion of these instabilities is. The usual assumption — that chronology horizons generically collapse to null singularities — is certainly plausible, even likely, but not yet proven.

    But also, we have to remember that in this case we’re talking about Star Trek. In essentially all space operas, there is a ship that can travel “faster than the speed of light.” In other words, the ship can travel along spacelike worldlines. Once you can do that, the black hole horizon is no longer a point of no return, even in Schwarzschild.

    Of course, the real irony is, it’s easy to find closed *spacelike* curves, even in Minkowski spacetime. So strictly speaking, as soon as Gene Roddenberry (or George Lucas, or Greg Battlestar) decided to let his characters travel “faster than light” he was implicitly giving them real time machines that they just never thought to use.

    It’d be interesting if someone made some kind of mass-media sci-fi epic that actually dealt with relativistic causality properly. Maybe “Interstellar” will be the first.

  10. Holly says:

    There’s a lesson there somewhere.

    Is it that no one reads Ph.D. theses except the people who write them and their supervisors? 🙂

  11. Sean says:

    I think the instability of the Kerr solution should be taken very seriously. It’s hard to prove things, but I don’t know of anyone who thinks that you really do get CTC’s inside the horizon of a real-world spinning black hole. And even if you did, they would still be inside the horizon. At best you could be spit out into another asymptotically flat region of space, not into our universe.

  12. nobody says:

    I agree with Holly… in my case I doubt if even my supervisor actually read more than ~20 pagess of it.

    I also know that neither any of my friends nor a very special (to me) person did read my thesis 🙁

  13. rob says:

    It depends on how you do the extension. There’s no reason the “future universe” after the white hole horizon couldn’t be identified with the “past universe” before the black hole horizon. Basically you’re putting the analytic extension on a cylinder.

    I agree, it’s implausible that the instabilities will lead to anything other than singularities. But weighed against faster-than-light travel, artificial gravity, landing on arbitrary planets without pressurizing or breathing equipment, quantum transportation of entire bodies complete with consciousness, or conversation in English with creatures who’ve never come in contact with humanity, the treatment of black hole interiors is the least of our problems.

  14. wolfgang says:

    >> it’s implausible that the instabilities will lead to anything other than singularities.

    As far as I know, it is not completely clear if the Cauchy instability is there if the BH sits inside a cosmological solution (e.g. dS or Friedmann) .

  15. Aaron says:

    By CPT isn’t time travel the exchange symmetry to reflective scattering? But that would mean that horizons black holes are the exits to time machines, not the entrances ;o)

  16. Sean says:

    rob– the real world does not do analytic extensions. You create a black hole through the collapse of a rotating star or some other localized physical mechanism. There is zero reason to believe that the interior spacetime will magically become identified with something in your past.

    Of course this whole discussion is predicated on piling speculations on top of each other. But it’s interesting to ask what the laws of physics would allow a civilization with arbitrarily advanced technology to accomplish — and identifying a black hole interior with the past is not on the list.

  17. Bob says:

    You take all the fun out of time travel.

  18. wolfgang says:

    >> You create a black hole through the collapse
    SciFi stories usually dont care about how the BH/wormhole was created
    and I think this paper
    should be sufficient scientific basis for a good story.

  19. Michael D. says:

    what about traveling back in time AND to a certain point in space? I’ve often thought (and I believe that there was a “Futures” short story in Nature a couple of years back along these lines) that if I sat here in my chair and went a few years back in time that I would probably be in the void of space, since this exact xyz point in space where I am NOW is not where I was THEN. Am I making sense?

  20. wds says:

    I’m having a little trouble with rule 3. I understand that traveling through time would be like moving through spacetime for you. But consider an outside observer at Point A where you leave and another observer at Point B where you arrive in the past. Wouldn’t you appear to blink out of existence for an observer at point A, and then blink into existence to an observer at Point B? If not, why not? How would it look then?

  21. curious says:

    dude: could the space between the “spoiler” warning and the actual spoiling content be made such that it is in fact possible to escape the spoiler next time? peripheral vision and reading momentum blew this one away.

  22. Anyone interested in time travel should check out Paul Nahin’s book TIME MACHINES.

    He’s also written several more books I haven’t read, but which look interesting:

    About 20 years ago, I read James P. Hogan’s THRICE UPON A TIME. The plot involves
    sending not objects but information into the past. One character remarks that one must
    be careful, since the consequences of one’s actions are far from clear. In the sense of the
    butterfly effect, seemingly small things could have huge consequences. Another
    character remarks that one has the same situation without any time travel or time
    communication—who knows what consequences our actions in the present will have?

    I think it’s time to read THE END OF ETERNITY again. Much as James Joyce packed all
    of Western thought into FINNEGAN’S WAKE, Asimov wrote this book as a time-travel story
    to end all time-travel stories:

  23. magista says:

    Not to worry, curious… he’s actually Kirk’s adopted older brother. 🙂

  24. tacitus says:

    Michael D. is correct. Unless you’re using a wormhole with “tethered” points of entry and exit, traveling through time and *not* traveling through space is all but useless. You would have to take account of Earth’s rotation, Earth’s orbit, the Sun’s motion, and the galaxy’s motion through space — all of which add up to hundreds of km per second.

    Even a short jaunt through time would leave you hopelessly adrift in space unless you can precisely account for the difference in location (not to mention differences in direction and speed, unless you want to arrive with a horrendous splat).