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.”]
I think 2 rules should be added to your list
11) Temporal reset valve.
If you go back in time and change your past, and then return to your time +1 nanosecond after you left, you will find the world to be in same temporal event condition as you left it. Why? If you negate your reason for going back in time, then you negate the event of going back in time to begin with, thus canceling out whatever changes you made to necessitate going back in time in the first. To make your changes “stick” you have to make them small enough to necessitate your going back in time to begin with. Or leave your past/soon to be time traveling self a “note” of some sort (like “build time machine, go back in time, do this, do that”).
You can take advantage of the temporal reset valve in the event you don’t like the changes you made (i.e. assassinating the wrong guy) by doing enough damage to either prevent you from building the time machine in the first place, or preventing civilization from existing in the first place (If you REALLY screw up in ancient times).
12) Time travel doesn’t occur in a vacuum.
Just like Watson and Crick weren’t the only ones working on DNA, you won’t be alone in working on Time travel. That said, you likely have competition. But you have to keep in mind that your actions won’t go unnoticed, even if you’re quiet about them. For instance, do you really think no one’s going to notice you fending off the Mongol hordes in a futuristic Abrams Tank? Someone in your altered future, alternative world or not, is gonna find out about this, put 2 and 2 together, and take action to STOP YOU. Or help you. Or both. And they may have a bigger gun than you…
Writing as a benighted lib’rul arts major with little to no aptitude for science but a love of science fiction in general and time-travel stories in particular, I also found this post interesting. And I had a question re: point six, but in writing it out I answered it for myself. Maybe not so science-challenged, after all.
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I always love these posts where afterwards readers sprint to Google and Wikipedia to earn their 5-Minute Doctoral degrees then come back here to make a lame effort at arguing against one point or another. Really, if you truly knew something about it you wouldn’t be wasting your time debating a blog post.
What exact thing are you only going to tell us “onece”?
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I’d like to join the endorsement of the movie Primer, it has a very realistic feel to it and any sci-fi/science geek is sure to enjoy it.
gogogo
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Sticking to these rules would probably make for boring time travel.
Imagine you start building your own time travel capsule. Seconds after completing it, the door of the capsule opens, and a future version of yourself steps out. It’s you that traveled one year back in time.
At this point, there are now three versions of you. The current you that is traveling normally through time and in one year from now will be stepping into the time machine to travel back to this point in time. The second you from the future that has just stepped out the time machine after traveling back one year in time. And a third you that is now currently still inside the time machine, but is still currently traveling back in time and will be doing so for the next year.
With the time machine occupied by your time traveling self, you’ll have to wait a year before you can actually get in it. Your future self that has traveled back in time faces an even bigger problem. With the current time machine occupied, he’ll either have to wait a year to get to the future, or think about building a second time machine and hope it is still empty once it is completed.
To the outside observer, the time traveling capsule would simply just be sitting there with inside it, what appears to be a frozen version of yourself (if we suppose that for the inhabitant inside, traveling back in time feels instantaneous). One could even freely move the time capsule around if they liked.
The question then is, what if in one year time from now, you decide not to step in to the time machine or are prevented from doing so? This can only be possible if we then enter a parallel universe if we haven’t already done so.
Unless of course time is three dimensional just like space is. This would change everything. Currently, we experience time as one dimension and traveling down it in one direction. If we could move freely through it in three dimensions, it would be possible to visit the same place and time you were an hour ago. And you would experience it exactly in the same way as you did an hour ago too inside the same body. The only difference is, you would then be free to move in a different direction in time from that point on. Our memory of time probably won’t be chronological anymore but be very similar to how we remember things from a spacial view point.
http://illinoisairship.tumblr.com/post/86741868/ill-never-own-a-time-machine-i-know-this-because
“I’ll never own a time machine. I know this because no famous photos exist with me in the background.
And if I owned a time machine they certainly would.”
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What utter drivel.
“7. There is no meta-time.”
That is an unsubstantiated, currently unverifiable claim.
It should read “7. There APPEARS to be no meta-time.” In which case it’s not a rule and shouldn’t be written here 😛
double post, see 170.
“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.”
Actually once you crossed the event horizon you would be instantly incinerated by the orbiting light.
Doesn’t rule number 6 effectively preclude traveling into the past altogether? If you can’t change the past then wouldn’t traveling into the past and trying to convince yourself to do something differently really be changing the past – even if you couldn’t convince yourself to act differently? Just by going back in time and having a conversation with your highschool-self which never originally happened would be a change so according to number 6 – that can’t happen. Right?
We do live in a multiverse, so any attempt to change the past would flip to an alternative timeline. However, supposing someone altered the past in a way no-one could know. For example, produce a dead clone of a person and replace it with a person about to die and then brought them to the future-effectively resurrecting them. Or what if an advanced future civilization came back in time and put all of us in a matrix style simulation, making all death an illusion?
Assuming there are no natural wormholes or pre-existing advanced alien civilizations with such technology, building a time machine to go to times before it was built is a tough one, as Ron L Mallete has found out with his ring laser wormhole (assuming it ever works) but using a device that manipulates probability I have been able to achieve bursts of intelligible superluminal communication and believe that infinite improbability drive type time machines are probably the only way to get back to before the machine was built. Using ultra-fast true random number generators and software with my formulae for extracting negative entropy, it might be possible to reverse the arrow of time in the real world, rather than just predict zeros and ones with fewer consecutive miss sequences than chance level.
james.edward@tesco.net
As a frequent time traveler I can tell you quite frankly that most of your points are quite off base. The good news is that in a few short weeks you realize this and post a wonderfully witty article about your new found knowledge. I’ll check back in once you get there.
Im not too happy with rule 8.
…i just stopped and continued reading and points 9 and 10 makes me feel better now LOL
Nice post
Nice article! I almost agree with it completely. Without reading all of the other reactions, let me respond to two of the rules, though.
— 3. Traveling through time is like traveling through space. —
Hm… Consider this: If time-travel is not instantaneous, then if you travel into the past, your body and time-machine will be present in that point in space from the arrival time to the departure time, aging backwards, as it were. (Well, they will at least occupy that point in space directly preceding departure.)
The you that is preparing to start the journey (you-A) would therefore be aware of the ‘you in transit’ (you-B). You-A would in fact be completely hindered by you-B in taking up his position. Unless those ‘two’ people could be completely unaware of each other and occupy the same point in space-time, this *is* a paradox. So if your rule 0 AND your rule 3 are true together, that would mean time travel is not possible (assuming a single universe).
In any case, for the purposes of fiction, it would be best if a time-machine could also teleport through time, but not necessarily through space (relative to the earth). Seems like a small enough concession. I’m currently designing an RPG with time-travel in it, which follows these rules pretty exactly (minus rule 3 and 8). At least I’m trying to design it, but I never have the time.
— 8. You can’t travel back to before the time machine was built. —
This one I just don’t get. Is there some significance to the “building of” the time-machine? Isn’t it really about the presence of the time-machine in some point in space-time? Is it not possible (for arbitrary values of ‘possible’) that the future me will build a time-machine, travel back to before I was born and do only stuff that is already part of history? For example, perhaps future me dropped the apple on Isaac Newtons head (then hid).