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.”]
Pingback: Kepler reaches its destination, time travel and the Overview Effect « Cog and Helix
Cynara-
We’re always looking into the past. It takes sun light eight minutes to get here. If the sun blew up right now, we wouldn’t see it for eight minutes. Even the light from the computer monitor takes some (tiny) amount of time to reach our eyes.
The only purely instantaneous thing I can think of, in that sense, is what happens in quantum entanglement, when measuring one particle instantly causes a change in another. And no one can figure that out yet.
Wait a minute. Wait a minute. Doc… Are you telling me that it’s 8:25?
Pingback: cobalt » time travails
Once again, Sean Carroll writes a gloriously informative and accessible post about very challenging material.
And, once again, the post reduces to:
“Listen up, people. Everything I’m telling you is the truth; we know it’s the truth because it has to be. The only way this stuff could be wrong is if we don’t actually know what we’re talking about. And, to be honest, we really don’t know what we’re talking about with most of this. So we could be, like, way wrong. We’re pretty sure we’re not, though…
Either way, isn’t it cool to think about?”
While I disagree with many of the statements made in this article, there is one especially that I would like to point out is (probably) false: “real travel from one point to another through space is a continuous process.” Since energy is quantized by the spin of the electron, that means that mass, and location, are also quantized. As such, travel through space is not continuous but is instead characterized by moving through a finite number of jump discontinuities. The fact that this occurs on scales too small to be observed does not affect its validity.
Pingback: What’s awesome this week: Let them eat dog food « A Fistful of Science
Pingback: Time Travel Is Real, Physicists Told Me « 24 Percent
Interesting article and also to read through comments. I really don’t know a whole lot on the scientific level, although I’m working on it, but there are two books that relate to some of the comments worth mentioning.
1. The End of Time by Julian B. Barbour – pretty much proposes that there is no such thing as time. How plausible it is, I have no idea, but a few posters wondered whether there is such a thing as time and whether others have advanced the idea.
2. The Light of Other Days by Arthur C Clarke and Stephen Baxter – plays with the idea of being able to view the past, not change it, but just see it. Almost exactly like one of the above comments, this would have quite the implications in terms of religion, crime, porn, etc.
I love the fact that apparently every little kid who fell in love with science fiction movies, grew up inspired by said movies, went to college to become a scientist capable of debating the validity of the very same “fantasy” concepts that were in the movies he/she loved as a child.
Don’t get me wrong. I love a good debate and educated geek speak. I just thought that it was an interesting tangent in my train of thought’s very own “timelike” path.
Well, got to go. My time machine is coming to a spot in the timeline that’s full of wormholes…
What the hell are you talking about? We’re all just human-batteries living in pods and plugged into the Matrix. None if this is really happening.
Greg, I thought that the argument against presentism was mistaken. What is used in there as a supposed proof that time exists is how time is articulated in language. But that doesn’t mean time exists at all. It simply means that we keep track of past events, we create records that are continuous in the present, records that tells us what happened before we arrived at where we are now. I’d say that we perceive time because we evolved to do so. We evolved to keep records of what happened before just so we can better plan our actions. And planning isn’t assuring that the future exists. Our verb tenses are not proof that time exists, but rather that we were made to think that it does.
A creature that is capable of keeping track is the conditions that lead to the current situation is, I believe, more sucessful than one that doesn’t, because the creature with memory is capable of planning and avoiding past mistakes. Memory can be anything though, even a bacteria not reproducing because it has received chemical signals that stops it from doing so.
Like I said before, I’m mostly ignorant when it comes to physics, so I might be mistaken, but my guess is that time doesn’t exist at all, only motion does.
“there was a young lady named bright,
who travelled much faster than light.
she set out one day, in a relative way,
and arrived the previous night.
What a lot of rot these rules are. Based on these rules, Dr Who and his Tardis would not exist and the world would now be overrun by Daleks. As it isn’t, it is obvious that Dr Who is real and really can travel through space and time without any of the restrictions mentioned here.
And Cynara, we really can see into the past. Just watch your television during the non-rating period.
Pingback: links for 2009-05-16 « pabloidz
forget time travel. it will be hashed and rehashed until a working time machine is built, or proven unbuildable.
instead, look up the experiment where somebody arranged a source for one photon at a time and then gave it two equally probably paths. dang photon went both ways.
Time
Was TIME born with all the sky?
Can TIME suffer and also die?
Is TIME flat? Or is TIME round?
Where is TIME? Can TIME be found?
Is TIME there with each beat of the heart?
Was TIME there from the very start?
Must TIME go to that being made?
Does TIME show by that being fade?
Can TIME give and also take?
Must TIME be for that to make?
Does TIME provide a way dimensions are spaced?
Are all creatures of TIME somehow inwardly paced?
Is all manner of TIME with all matter that be?
Does TIME flow endlessly?
How does the future with TIME flow past?
Does TIME move slow? Can TIME move fast?
Is TIME rough? Or is TIME smooth?
Can TIME have inflame? How does TIME soothe?
Is TIME bound? Or is TIME free?
Is TIME that what is meant to be?
Tony Avila Sampson
(How many of these questions can be answered?)
Um….Einstein said e=mc2. BUT, he also said it was his best theory at the time and was likely not completely correct. I am of the personal opinion, and no one has been able to successfully prove my opinion incorrect that there is NO reason we can’t go faster than the speed of light. It’s merely a speed, a velocity, a distance traveled over a certain period of time.
Before we ever broke the sound barrier people were afraid of what would happen if we actually did go faster than the speed of sound. Some people said it couldn’t be done. Some people said the world would end. Nothing happened. Well, something did…we actually went faster than the speed of sound. Woooo. Big deal. We do it all the time now.
There should be no reason we can’t go faster than the speed of light. It seems simple, really. I invite people to actually ponder this and to comment on it. Keanna1970@yahoo.com. And please, I don’t want to hear “you’re wrong” or “that’s a stupid idea”. Explain to me why I might be wrong but also look and try to explain why I could be right.
Pingback: Yeah dude, explain me this… « Gillian’s Blog
Pingback: walterhisownself's status on Saturday, 16-May-09 17:57:25 UTC - Identi.ca
The fact that you can’t prove something exists isn’t a good reason to believe it doesn’t. Maybe the past never happened, and we all sprung into existence memories and all at this very moment. But if so, it seems awfully peculiar that we would have memories, especially ones so thoroughly consistent with our having existed in the past. The assumption that we actually had a past does a lot more to explain that data. Plus, if the universe sprung into existence in its present state, why did it bother with the fossils and cosmic microwave background and what not? If we want to do anything resembling science, step 1 needs to be assuming that the universe isn’t designed to trick us.
As for the argument that we can’t go to the past because the past doesn’t exist anymore, that’s begging the question. The whole question of “Is (backwards) timetravel possible” amounts to asking if we can visit things that “don’t exist anymore” in the present.
Pingback: popurls.com // popular today
When you go back in time, the earth is not in same location as it was when you started. So, if you are on earth and then go back in time, the first thing you need to do is figure out where the earth is…Even the sun may not be in the same location. It seems to me that if you go backward too much, it would be difficult to find the earth and sun.
Is that the case, or am I missing something?
It find it wryly amuzing when people start claiming indisputable rules about something we know so little about. Througout “time” mankind has constantly thought that the knowledge they had was the definitive truth. From the ancient greeks view on matter, Newtons ideas about physics to the pre darwin ideas about life on earth.
“Time” and again we are remined that we really know very little about “Life the universe and everything” to claim that we are even close is abusred unless you believe that in the next 100,ooo years ( or even millions of years) of scientific progress we come to find that in the year 2010 we had achieved almost total enlightenment and the rest of our knowledge gained was merely refining that which we attained in 2010
History tells us we have a lot.. a lot.. to learn and to the people of the year 9010 our knowledge/beliefs will be titilating at best.
My personally hunch ? time travel into the past is not possible but hey who know ?
One thing to bear in mind, if time travel into the past IS possible then the future has already happened. If a time machine was built in 2000 and we in 2010 travel back to 2000 then to the people of 2000 the future is already mapped out. Similarly it then would be feasable that someone from 9010 could travel back to 2010 in which case our future is already mapped out
Yesterday, I took off in an airplane right after sunset, and it was so surreal – the sun actually rose again! Did I travel back in time?