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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1. Wouldn’t entering (popping into) a point back (past) in time cause a rather catastrophic event? Here is mass and energy popping into a space that currently contains matter and energy. I always thought there was some Quantum rule about two things cannot occupy the same space at the same time.
2. If the past exists as a physical place then visa-vie the future must also have to exist. Both in infinite directions, from beginning of time until the end. If you could travel back in time the arrival point would then become your present. But you know there was a future to the time you are now in. If time travel is possible and someone pops in on you in the “present – now” then it again proves there is a physical future.
3. If as the article suggests you can’t change the past, then how about your future. Case in point, today I know what the Lottery winning number is so I travel back a day before the drawing and bet on the winning number. I haven’t changed anything in the past, but I took knowledge back and used it for my future (based on being in the past) advantage. Ahh you say… but when you got the winning lottery number and there was no winner you can’t change the past. (If so wouldn’t that again prove #2 above?) And if not and it showed that you won the lottery (because you went back and bet on it), then just go collect your winnings. You don’t need to go back and buy a ticket – you already have it! Paradox?
Anyone have some fun arguments to these points?
“Interested in time travel? Meet here last Tuesday”.
“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.”
Or perhaps the act of going back in time, to attempt to prevent it, was ultimately the reason why it happened in the first place. Dont mess with it!!!
“If you have a spinning black hole — and all the black holes we’ve seen in space are spinning very rapidly — then the math says they collapse to a ring, not a dot. If you fall through the ring, you wind up in a parallel universe. This solution was first found by Roy Kerr in 1963, and it is the most realistic description of a spinning black hole. There are many, many questions raised by this. If you could go through the ring, then who knows where the other universe is located? It could be backwards in time, it could be a parallel universe. But there are problems like radiation. The radiation would be very intense. And if you add radiation, there is a debate among physicists right now about, will it close up the wormhole, is it stable? Math says there is a wormhole at the middle of a spinning black hole. To keep the hole open so you can go through it, you need negative matter to stabilize the black hole. These are called transversable wormholes; you can go back and forth freely without too much effect.” — Michio Kaku
There are many beliefs/theories on black holes. I guess this article decided to take the route of once you enter a black hole, you get ripped apart, which is just a theory, and may not hold true.
Regarding the distinction of Least Realistic Time-Travel Movie:
For your consideration, may I submit: “Superman: The Movie”
Greensleeves456, I don’t think Kaku is suggesting that the sort of tidal forces Sean talks about don’t exist. Rather he’s speculating on what would happen if you could survive such forces and make it through a wormhole in a black hole of the Kerr type (supposing they even exist).
You’ll notice a lot of speculative language in proposed mechanisms of time travel. If the Kerr spacetime occurs in the real world, if you could get inside the black hole without being “spaghettified”, if you had access to some sort of exotic matter with negative energy that you could use to stabilize the wormhole, if such a form of matter even exists at all . . .
Since we (A) don’t know if time travel is even possible in principle and (B) don’t know what hypothetical time-travel procedures will be practical for future technology, this is all incredibly speculative.
Regarding the Star Trek non-spoiler, might I suggest it would have been both funnier and more clearly a joke if Sean had said “Spock travels back in time and kills Kirk’s grandfather.”
You know, because of the “grandfather paradox”. And also the fact that one character turning out to be the other one’s ancestor is stupid in just the sort of way that actually appeals to Hollywood scriptwriters. I mean, it’s no “Darth Vader builds C-3PO”, but still . . . .
And, if time doesn’t exist, there’s nothing to ‘travel through’. The Now is where things happen that condition where matter will be and how much energy it has. All the rest is, like the Sun revolving around the Earth, a vapor of imagination.
“Your crime is time, and it is 18 and life to go.”
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speaking of fiction, sounds like Lost got it right somehow
Bee: is what you’re asking along these lines?
1. Start near an omnidirectional blackbody radiator at rest with respect to you and with a temperature of 6500 K.
2. Keep your instruments directed at the blackbody radiator
3. Accelerate to relativistic, then ultrarelativistic speeds
4. Accelerate to superluminal speeds
5. Observe photons converging on blackbody radiator
My (2) is a problem for me because as one reaches the ultrarelativistic limit photons from the blackbody radiator will still ultimately reach you even if highly redshifted. If you travel on an only mostly radial geodesic when you pass the relativistic speed limit you’ll start seeing progressively blue-shifted photons emitted earlier.
Here I’d substitute a movie projector that emits a highly collimated beam that you can pass through a beam splitter and an adjustable lens that lets you watch on a movie screen. As you reach relativistic recession velocity with respect to the projector, your movie winds up in slow motion and and much redder, then a slide show, then an almost entirely still image in the ultrarelativistic limit. When you reach c, you have a dark screen. Adjust your lens/screen orientation and as you accelerate beyond c you start seeing the photons that got through the splitter initially, and now you can watch a reverse and very red slide show, then a slow motion red movie, then the film in reverse at normal speed and colour, then a backwards-fast-bluer movie and so forth until you pass the start of the projection.
I have more trouble with there not being a “0. Start in a large volume of perfectly flat ideal vacuum initially populated only by you and a blackbody radiator…” in the recipe than in the view as you accelerate, because it’s so tempting to think about being ionized by the particle inrush and shaken apart by gravitational waves.
The projector watching your screen would see the same Lorentz contractions as you but only until you are moving at c at which point you and the projector each lose sight of the other. You get the benefit of the “rewind” from the photons that passed through your beam splitter and outran you when you were travelling less than c, and relative to your local rest frame you’d describe their trajectory as being pretty highly curved, right?
Returning to the omnidirectional blackbody radiator, with you travelling superluminally you’d see the photons of the expanded shell that you subtend as you outrace them, doppler shifted as appropriate. You could only do this if you aren’t travelling perfectly parallel / exactly radially unless you were transparent. They were emitted at the blackbody radiator, sure, and they would appear to you to be travelling towards the blackbody radiator as you outrace them, but would they really appear to be falling in on the projector from your perspective?
Sorry this is so sloppily put; I lost a couple strands of thought while mis-editing in this text box.
Your rules are nice for what little we think we know,but if we discover something new in physics all your early 21st century dogma goes up in smoke especially the part about not gong back before the machine was built. Why would you think you would need a machine in both times. If you use a faster than light vehicle you could catch up to the past; like watching a star from a hundred years ago, you could watch the earth from 10 light years away and see what happen 10 years ago. You might not actually travel in time but you could see the past without interfering. There is also out of body mental projection Read silver mind control. Ill bet on parallel universes though. You just have to vibrate at the right frequency
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I’ve always thought that if you could travel through spacetime (as mentioned before, if you travelled only through time, you’d be in a void of space since the universe is moving – though, gravity might still affect you as you travel through time so space movement would be unnecessary) you could change the past.
The thinking is that if you changed something in the past, the changes wouldn’t affect your future because those changes would travel at one second per second. When you returned to the future (assumption) you would be travelling faster than the changes. So the people in your relative time instant would never know anything different.
But maybe, only that that timeline you changed experiences the changes, since in that timeline you didn’t travel back. So the time flow behind it erases the events of that time instant. So if people from that time instant went to the future or past, they would be in a future or past different from the one they experience.
“I’m only going to say this onece: there would be no flashing lights. ”
ONECE, spelling mistake, BOOOOOOOOOOOOOOOO
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I second rickb’s endorsement of Primer, in which occurs the line (one time traveler to the other): “Are you hungry? I haven’t eaten since later this afternoon.”
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“bruce Says:
May 15th, 2009 at 11:20 am
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 agree that this would cause an issue, I mean if the backwards travel through time was linear it would seem like the object would collide with itself at nearly the instant it left.
Doesn’t time travel imply greater then 3 dimensional space?
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