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.”]

229 Comments

229 thoughts on “Rules for Time Travelers”

  1. I don’t know what the big deal is, I travel backwards in time as a matter of habit, it just has a tendency to decay very quickly. Of course I use this to my advantage because it allows me to change a lot of history fairly arbitrarily. Doesn’t always work, but predicting the future doesn’t always work either.

  2. “…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. ”
    “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.”

    So…
    We don’t know enough to determine whether or not the thing is possible, but we know enough to set rules in case it is?
    Gotta love that.

  3. I know that this is boring, but I have to correct this:

    ” But once you got close to the center of the hole,”

    Everyone [especially Samir Mathur] repeat after me:

    BLACK HOLES DO NOT HAVE CENTERS.
    BLACK HOLES DO NOT HAVE CENTERS.
    BLACK HOLES DO *NOT* HAVE CENTERS!!!

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  5. Just A Thought

    Although energetically intense, you can create local time machines. Just take a reasonable region of space and accelerate to near the speed of light in some stable orbit. Although it will began to evolve slightly differently then the space its embedded in, an outside observer could visit the accelerated region, and have the sense of time travel, without violating any laws.

  6. Sean: Thanks. Roughly what I thought too, I just can’t really pin it down. Will have a look at chapter 6 🙂

  7. Oh man, this post captured so many of my views on the subject. THANK YOU for a much-needed dose of reality on the subject. Which will of course never be noticed by the writers in question.

  8. A Gedanken Experiment

    Around a black hole, virtual particles and anti-particles can be separated by the event horizon. Unable to annihilate, they become real. The properties of each pair are linked, or entangled. What happens to one affects the other, even if one is inside the black hole.The elephant and the event horizon 26 October 2006 by Amanda Gefter at New Scientist.

    In essence “Time travel?”

    Best,

  9. I have to take issue with Item 3 to some degree. To the non-traveling external observer, the time-machine would pop-out and pop-in into their perception. This is because the observer is not wired to perceive time.

    The external observer can only perceive the presence of an object that co-exists at the point in space-time. Once the object begins to move at a different velocity in time (Vt), the observer will see it wink out of their perception.

    We ride the breaking wave of now, always about to collapse into the future. We can neither see where we have been or where we are going.

    Before someone takes issue with my comment and say they can remember the past, I point out that memory of a space-time event is not the same as being able to perceive it. We can look out a window and perceive objects in near us separated by space, but the same can not be said of objects near us in time.

    Various SF gimmicks have had the object diminish to a point as it slips ahead or falls behind on the time-line, but this would mean the observer could perceive objects nearby in the time-line in some Cassandra-like manner.

    What the time-traveler would experience is entirely open to debate due to the speculative nature of the method of time-travel.

  10. All this verbage is based on the belief that time has actual existence. The past and the future are only thought forms in your own mind. You are always only present. Read Julian Barbour. Easier to accept the mass delusion called time, I suppose, if everyone else enjoys the conversation. No time, nothing to travel in.

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  12. “0. There are no paradoxes.” Well, yes, there are. Look into quantum mechanics, which has found a number of logically impossible situations, yet they really happen. Particles leap from one position to another without passing through space; particles pass through barriers (quantum tunneling), particles disappear, and so on. So… does time travel obey the laws of logic? Perhaps not. — yrs, andreas

  13. Low Math, Meekly Interacting

    But we’re talking about things that are bigger than elementary particles. Why shouldn’t classical logic hold for a classical system than only uses quantum theory to provide unobtainium like “exotic matter”?

  14. @TimG – But with macroscopic objects, this seems a bit bizarre, since from the non-time-traveling observer’s perspective both rockets would be partially overlapping with each other at the moment before the collision.check this out

    @Fernando – For starters, does the past really exist? Is every moment in time a frozen moment, a “place” that can be visited? How would that be? I mean, how is that the specific position of every particle on the universe at a set time is somehow recorded and stored for eternity? – you might want to read this

    in fact, reading this site is a good place to start for anyone who is interested in the metaphysical questions about time travel. Various philosophers have been dealing pretty seriously with the non-physics related aspects of time travel for about 35 years now.

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  16. I think Fernando asks some good questions.

    Also–although it’s been a while since I saw Back to the Future–it’s not that changing the past “instantaneously” changes the future. To the people “back in the future,” the changed past that they recall IS the past. Wouldn’t the normal chain of cause and effect still unfold?

    The violations and paradoxes occur when time-bound things appear at a, uh, time when they have no business being there. If you sent an AK 47 back to the Battle of Bull Run, stuff would get weird. (Although, according to the “rules,” it might not work.) But changing human decisions per se might not have that disruptive/violative effect. For all we know, Amelia Earhart didn’t “originally” go on that fatal flight. But someone from our future went back, persuaded her to do it, and we’re now stuck with *that* sequence of events.

    I’m still waiting for someone to suggest that there is no such absolute thing as time anyway. If every quantum event spawns a new universe, then what we perceive as being a smooth, “linear” unspooling of time may only actually be a series of universes that exist for one-over-infinity seconds, and then disappear, to be replaced by the next one. Just as a line is actually a series of points (which have no dimension).

  17. I’m also puzzled about how a time traveling rocket would look to an earthbound observer. Since it’s headed on a timelike path it would look like any other rocket, though it might be headed for some twisty region of spacetime. Would we lose sight of it as it makes its timelike loop? Would there necessarily be an event horizon?

    I’m even more troubled by how it would look to the people whose time was being visited. Would the rocket emerge from a twisty region of space? Would the twisty region of space always have had to be there? If not, how would the past folks have seen it to form? As mentioned above, it’s pretty clear that the time-traveling spaceship cannot just blink into existence, since this would be an apparent violation of a whole lot of conservation laws and probably various civil statutes as well.

    And these are closed timelike loops. Does this mean that the ship has to return to the moment it had departed? Or can you decelerate out of the loop whenever you want? How much time will have passed for you? If any time passes, this would mean that the arriving ship would no longer be the same as the departing ship, which would be a paradox. I suppose if a short enough time had elapsed the two versions of yourself could be compatible in a quantum mechanical sort of way…

  18. I’ve always been of the belief that we are constantly time traveling not only forward but also backward while we are in our present. We are constantly changing our past and thereby our future within our present perspective.

    If we were to experience an embarrassment at the age of 10 we would thereafter avoid circumstances that would cause us this embarrassment as long as we perceive the event as an embarrassment and when the perspective of the past event is changed from an embarrassment to, say, a thing of humor instead, then from that point of changing the past we now change our future by the actions we will take based on our perspectives.

    Therefore if you want to travel back in time you need only the conduit of your mind and the ability to understand perspective and it’s power over everthing.
    Then I think with this theory all your rules above could apply quite easily.

    Have any books on this subject been written?

  19. Nitpick (that may have already been raised): a paradox is an apparent contradiction, i.e., not a real contradiction. Real contradictions are bad, but a paradox just means that you have not considered the situation carefully enough. For example, the twin paradox in special relativity is a genuine paradox (and a great pedagogical tool), but it is not a contradiction. So special relativity has paradoxes, but it is perfectly consistent. Similarly, the grandfather paradox is a real paradox in time travel, but it just means that somewhere there has been a shortcut in the reasoning.

  20. On “6. If something happened, it happened.”:

    One of my favorite time-paradox stories is by Isaac Asimov, entitled “The Endochronic Properties of Resublimated Thiotimoline.” Thiotimoline is a solid with the property that it dissolves 1.2 seconds prior to the introduction of a solvent. By arranging a chain of thiotimoline dissolution systems, you can arrange for for something to happen an arbitrarily long time before the initial cause (the logically first, temporally last, spill of the solvent).

    The problem comes when you get the result, and then try to cheat nature by refusing to pour the solvent at the end. All hell breaks loose.

  21. While we may not be able travel into the past, I maintain that, at some point, we may be able to see into the past. Of course traveling to, and merely seeing, are two very different things, but since we’ve established cannot change the past – they would accomplish the same result.

    Even today, we already look into the past by looking at the outer most reaches of the universe. I imagine at some point we will be able to look into the past for much closer objects – i.e. things that happened on the earth. And that would be amazingly exciting.

    We could see our own evolution, watch the migration of the human race, solve historical mysteries, and as a commentator above mentioned, it could be a huge invasion of privacy, since if we could look back to any point in time, we could look back to last week, yesterday or two hours ago.

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