Rules for Time Travelers

By Sean Carroll | May 14, 2009 7:55 am

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."]

CATEGORIZED UNDER: Entertainment, Science, Time
  • Rob Knop

    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.

  • George Musser

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

  • greg

    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.

  • Aaron F.

    “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.

  • cmt

    “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.

  • Sean

    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.

  • wolfgang

    >> 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.

  • moshe

    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

  • rob

    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.

  • Holly

    There’s a lesson there somewhere.

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

  • Sean

    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.

  • nobody

    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 :(

  • rob

    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.

  • wolfgang

    >> 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) .

  • Aaron

    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)

  • Sean

    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.

  • Bob

    You take all the fun out of time travel.

  • wolfgang

    >> 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.

  • Michael D.

    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?

  • Successful Researcher: How to Become One

    Nice post :)

  • wds

    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?

  • curious

    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.

  • Phillip Helbig

    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:

  • magista

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

  • tacitus

    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).

  • Sean

    I should have said “grandmother.” On the internet, take nothing for granted.

  • Phillip Helbig

    A footnote on Nahin: Anyone who has taught a course on the history of radio and has a cat
    named Heaviside has to be a great writer. The book is very readable. Nahin has a good
    command both of physics and of the science-fiction literature, and the book is about both.
    More technical stuff is relegated to very large appendices.

    I’ll sign off with a quote from Kip Thorne:

    “CALIFORNIA magazine, in an article on “The Man Who Invented Time Travel”, even ran a photograph of me doing physics in the nude on Palomar Mountain. I was mortified—not by the photo, but by the totally outrageous claims that I had invented time machines and time travel.”

    I now see that Amara (who came to my wedding back in 2002) has already quoted it
    in this very blog:

  • Bee

    Sean, a question to you that I’ve recently been scratching my head over. It’s not really about time travel but about superluminal signaling, but we know both are related to each other. If you leave aside problems with defining a QFT, superluminal propagation isn’t a priori a problem as long as curves are, as you point out, consistent. Now that’s all well and fine, and one can show that tachyons aren’t necessarily problematic (and I believe there was a SciAm article recently with Albert that made a similar statement in relation to collapse of the wavefunction).

    But I am not sure this can work once you take into account the arrow of time. Thus, since you are the expert on the issue, what do you think? I was thinking roughly the following: take a lightbulb. It will emit photons outwards on the lightcone. Now let it emit some sort of superluminal particles. They will move outwards on a wider cone. Thermodynamics would tell you you are likely to observe these shells of light to increase in radius and decrease in amplitude but it’s extremely unlikely to find some set of photons moving inwards and collecting to a point. At least I’ve never seen that. The problem is then of course that there are Lorentz transformations that would topple part of the the superluminal lightcone upside down, depending on the direction of the boost, such that these particles would indeed collect to a point rather than expand. That would certainly look odd, but it isn’t really clear to me whether it’s problematic?



  • tacitus

    I would settle for just being able to observe the past.

    Of course, given that the past includes what happened seconds ago, there would be horrendous privacy issues, but the chance to observe historical events as they really happened would have profound ramifications for the present and future.

    Religious adherents of all faiths would rightly be very nervous about what we would find about their origins. Criminals would find it almost impossible to get away with any crime. All forensic disciplines would be revolutionized–archaeology, history, paleontology, etc.–and evolution could be studied first hand. Reality TV would become the standard as producers rush to edit all the great moments in history down to hour-long segments, not to mention what it would do for the porn industry…

  • Sean

    B, I think that it is very problematic, if I understand what you mean. One way of saying it is: if there are closed timelike curves, the entropy can’t increase monotonically along them. I.e. a consistent arrow of time can’t be defined. And I think there would be a similar problem for tachyons, basically as you point out, although it’s not as straightforward.

    Which is why I tend to think that superluminal propagation and CTC’s are very problematic in the real world. Even if you can consistently define propagation in their presence, it’s extremely restricted, in ways we would ordinarily consider to be wildly anti-thermodynamic. It’s exactly like having a low-entropy future boundary condition, because your future is now effectively tied to your past. And future boundary conditions are something we can contemplate, but they lead to funny acausal behavior. Not logically impossible, but incompatible with conventional causality. (See chapter six of the book!)

  • Low Math, Meekly Interacting

    “No Paradoxes” seems to imply weird things. It’s almost as if my intention to go back in time and commit some paradoxical action would bring some calamity upon me to prevent it, whereas if I set forth pure of heart and adhering scrupulously to the Temporal Prime Directive, all would be well.

  • gandeep

    Hi Sean

    I had a question similar to Bee’s question above. As you said traveling in time must be a continuous process. So in order to get to a past instant one will have to travel back through time. As you once explained earlier (when you wrote about ‘The curious case of Benjamin Button’) it is not possible to have incompatible arrows of time. So for the person ‘going back in time’ all biological processes like aging, remembering must still occur in the same ‘direction’ as the rest of the universe. So if we ‘go back in time’ it should not feel like going back in time all as still only the past would be remembered by us. Also we should get younger as we go more into the past. Is this reasoning correct?

    A broader question: Even if it is possible for a single particle to travel in a CTC in a certain spacetime, are new problems introduced by Thermodynamic considerations if a macroscopic object has to travel in a CTC?

  • greg

    @Low Math – when dealing with non-MWI time travel, you run into questions about free will and determinism more than problems with paradoxes. if at some point in the future, you travel into the past, it’s because at that point on the time line leading up to the time you left, you appeared, whether you knew about it or not. Your appearing in the past already happened. You can’t not appear at the point in time!

    For those of you who enjoy time travel science fiction, if you haven’t already you should go read Robert Heinlein’s “All You Zombies”. It’s one of the best consistent time line stories out there.

  • Low Math, Meekly Interacting

    @greg: Well, sure, but it’s still weird. So OK, you’ve got your block universe, and free will is an illusion. That doesn’t stop its inhabitants from thinking they’re making a choice when they set forth to screw with causality, or whatever, only to be somehow thwarted, as they simply must be, to avoid paradox. The rule is, then, that anyone who thinks they can influence the past such that it creates a paradox is wrong, always, but what’s not obvious (to me, anyway) is how nature goes about insuring this is always true.

    Many worlds gives me hives.

  • Sean

    gandeep — The arrow of time can be defined locally; it’s okay (in principle) to have an arrow that points one way in one part of the universe, but differently from where it points in some very far-away part of the universe. The problem arises when two regions with purportedly different arrows are interacting with each other; that is very hard to sustain (probably impossible).

    So: if there were a closed timelike curve, we would age along it, and feel older when we finished our journey than when we started it, just as for any other journey. But we arrive at a part of the universe that was “earlier” in some approximate global time coordinate. The problem arises, again, if things moving along the closed timelike curve start interacting with the rest of the universe.

    There is a different but related problem for a macroscopic object that describes a closed loop all by itself. That is very difficult to imagine, since the state of the object at the “end” of its journey would have to be precisely the same as the state at the “beginning,” which for macroscopic objects is hard to pull off.

  • rob

    Of course the real world doesn’t do analytic extensions, because the real world doesn’t have coordinates. But the real world does, at some level, solve the Einstein equations. So all that matters to me is whether it’s a solution or not. Obviously Kerr is an unrealistic model of the real world, at the very least because the real world isn’t stationary. But at a mathematical level we have very little handle on the fully nonlinear dynamics of GR, especially when there are Cauchy horizons involved.

    Now, I will admit, and I’m sure you know much more about the details of this than I do, that there are strong observational astrophysical and cosmological reasons to doubt the physical existence of white holes, even on top of their inherent instability. So this could moot any talk about exiting a black hole into the same universe. And I’ll also admit, as you pointed out, that even if a physical black hole were tied to a white hole in the past in the same universe, the United Federation of Planets would not have the power to bring that white hole into the vicinity (in whatever sense the word is meaningful) of its partner. So it’s obviously not a practical method of time travel, especially when you can just use warp drive to follow a spacelike path into your immediate causal past. I guess my main point is that compared to other time travel scenarios Star Trek has used in the past (like flying around the sun at a really high speed, for example), this one is a hell of a lot better.

    But at any rate, we’re way off on a tangent space, so I’ll drop it.

  • Ken St. Andre

    Who made you God, and allowed you to decide the laws of time travel? Are you writing with the authority of logic or experience? My guess is logic, which is only as true as its premises. In the past there were things we didn’t know about space-time. My guess is there are still things we don’t know about space-time, and as our understanding of the universe changes/improves, our ability to move about in that universe will also change/improve. If we do live in a 4-(or 20)dimensional space time universe, and we can only move forward in time under normal circumstances, the solution to going to other parts of that space-time continuum is to get out of the space-time continuum completely and then reland in it wherever and whenever we want to. Your arguments about time travel suppose that there is an objective and unchanging reality. However, we each experience a subjective reality, and if, in my subjective reality I somehow manage to travel to far future or far past, then who is to say that did not happen to me? Again, I maintain, that your laws of time travel are only true if your premises are true, and that hasn’t been proven.

  • TimG

    Like wds, above, I’m wondering how things look to the non-time-traveling observer.

    I get that for the time traveler inside the rocket ship time always appears to move forwards (unless she looks outside the window). But for the “stationary” observer watching from outside the rocket ship, wouldn’t there be some point where the rocket “doubled back” in time? That is, if he were to plot the rocket’s motion on time and space axes corresponding to his “stationary” reference frame, then there has to be some point where the time coordinate of the rocket had its maximum value. (That’s what we mean by traveling backwards in time, right? At some point your motion in spacetime takes you towards the past of the observer outside the rocket, rather than towards his future.)

    How would things look to the observer at that point? I’m imagining something like a particle anti-particle annihilation, in which one backwards-flying rocket ship collides and annihilates with the forward-flying rocket ship. 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.

  • dmduncan

    Regarding rule 9: We can’t travel to the “past” of an alternate universe either. What you might be trying to say is we might be able to travel to the present of an alternate universe which resembles the past of this universe, assuming that such alternate universes exist at all and if they do that they parallel the development of our own but are not parallel in time, so that a lateral shift from this one to the other brings us into a similar universe at an earlier stage of development. But that’s not time travel. It’s weird, but not time travel.

  • Gabe

    So is #8 how one would get around a time-travel version of the Fermi Paradox?

  • rickb

    One of the best time travel movies has to be “Primer”. While the rules of time travel in the movie appear to make it possible to change the past–which means parallel universes, I much prefer the single universe, can’t change anything time travel story– it gets the actual mechanics of moving back in time dead on.

  • Hugo

    Not a problem really. The arrow of time, as the second law of thermodynamics are just statistical. Nothing Prohibits you of arranging light rays that would eventually converge to a metal filament and generate electrons. It’s highly improbable. though, unless you have a small number of electrons or a very small temperature. If something prevents the existence of tachyons I bet it would be causality, not thermodynamics.

  • rickb

    In Feynman-Wheeler absorber theory, both advanced and retarded waves are emitted symmetrically both backwards and forwards in time, as long you have the appropriate boundary conditions correctly, i.e. you have perfect future absorber, all the advance waves get canceled out so you get an arrow of time with only retarded waves. Also John Cramer’s transactional interpretation of quantum mechanics uses this trick to explain non-locality in QM, it’s not superluminal, the “transaction” happens at the speed of light, it’s just that its backwards in time.

  • Mark


    You might qualify your discussion by saying that you mean *intentional* time travel. After all, (spontaneous) time travel occurs all the time in relativistic quantum mechanics.

    The argument is trivial. Heuristically, the uncertainty principle implies that a particle can occasionally (but always uncontrollably) have greater speed than c. Equivalently, you can just compute the amplitude for a particle to be found outside its light cone, and you find that the answer is exponentially small but nonzero. A particle can jump backward in time, in which case it looks like its antiparticle.

    The whole reason why quantum mechanics and relativity are so conflicting (and why imposing both simultaneously is so constraining on the allowed physics) is precisely because of this light-cone leakage. Leakage of particles outside their forward light cones (which looks like backwards-time-traveling to certain Lorentz observers) explains why antiparticles are necessary (see, for example, Weinberg’s old GR book), as well as why particle creation/annihilation, particle indistinguishability, and the spin-statistics theorem are inevitable. (Though some of those are harder to prove.)

    So what you’re describing is the question of *controllable*, *deliberate* time travel. But it would be interesting to examine the spontaneous time travel of relativistic quantum mechanics in light of your list of 10+1 principles of time travel!

  • doublechateau

    What a cogent, succinct, well written piece. As a member of the “educated public” I thought I would respond to your post since, although most of the time your posts are written so we can understand, it seems like the responses all come from other scientists. I look forward to your book.

  • Fernando

    I’m ignorant on the physics of space and time and, well, of physics in general, so maybe what I’m going to say is completely wrong because of something I have no idea. But for me, time travel makes no sense.

    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?

    Second, does time really exist? When I think about it, the only thing that seem to exist is motion. Just particles moving, and we call time our perception of this sequence of movements. Why would there be something like time travel if time is a human concept?

    Third, wouldn’t time travelling mean creating matter and energy? And isn’t that impossible? When you travel through time you bring atoms to a time where those atoms simply weren’t part of that universe. Say, if I travel from the year 2000 to 1999, I will have created entirely new matter on the year 1999, duplicated matter actually. That matter would only cease to be duplicated at the date of my time travel, when the me from before the time travel went away to 1999. And in away, it is almost like a teletransportation, because the matter that composed me from 2000 would simply disappear and reappear at where the time-traveler me is at on the exact moment of the time travel.

    And finally, isn’t any time travel paradoxical on it’s own? If I travel to the past, the reasons that made me want to travel are due to what has already happened, considering we live in a deterministic universe. My actions in the past affect the universe and set in motion the sequence of events that have created the me from the future which decided to travel back in time. So what caused me to travel through time had already happened, which was my time travel in the first place. It is retro-causality, can such thing even exist?

    Long post with confusing ideas that not even I’m sure it makes sense, probably won’t even be read.

  • Guy

    I know one thing for sure, Every time we state absolutes, someone comes along and shatters that statement and proves otherwise. The only true statement would be that Mankinds dreams and imagination open doors that no one ever thought possible in the past. The Future holds Infinite inventions and discoveries. No one can say for sure today what is and what is not possible to do in the future. Endless inventors have proven that over and over.

    All of creation is none other than an endless amount of extremely complicated data and all we ever do while we are here is find ways to manipulate it and cause a ripple in the fabric of space time. remember ALL things are possible. The trick is discovering how to do it.


  • Fernando

    Also, the idea that it is impossible to create the grandfather paradox, at the same time it makes sense, is so incredibly unlikely. I mean, if I try a terrorist nuclear attack on the past manhattan, the attack will fail because there was never a nuclear explosion on manhattan. So it means that no matter how many times I try to send a nuke back to the past, when the device gets there it will inevitably fail. Every single time. Let’s suppose those are bombs that have been tested and proved to work, somehow the bombs I pick to send back will always fail, even if they are produced the exact same way as the functional ones.

    And the grandfather paradox also makes it impossible for me to send back in time some gigantic object that would collide against earth. The grandfather paradox restricts time travel of objects of a specific mass to a specific position at a specific time.

  • Low Math, Meekly Interacting

    If you’re boring, you could simply go with the “time travel* is impossible” option. Mother nature wouldn’t fault you.

    *of the deliberate, backwards, sci-fi variety, just to be clear

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  • Time Traveler

    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.

  • boxfiddler

    “…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.”

    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.

  • Pope Maledict XVI

    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:


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  • 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.

  • Bee

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

  • Mark S

    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.

  • Plato

    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?”


  • Denise

    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.

  • Marcus

    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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  • andreas

    “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

  • 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”?

  • greg

    @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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  • Lobster

    There are no paradoxes?
    Maybe there are only paradoxes . . .

    Here are some top tips for Time travelers

    Ozmonauts (dimensional brane skippers) need to enter the realm of paradox

    Time travelers may need to be genetically modified

  • Mr. Wonderful

    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).

  • bruce

    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…

  • TimG

    greg, thanks for the very interesting link.

  • Strawzenberry

    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?

  • Pieter Kok

    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.

  • Paul T

    Time flies like an arrow.
    Fruit flies like a banana.
    - Groucho Marx

  • Neal J. King

    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.

  • Cynara

    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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  • Mr. Wonderful


    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.

  • Martin McFly

    Wait a minute. Wait a minute. Doc… Are you telling me that it’s 8:25?

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  • doug

    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?”

  • Joe Knows

    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.

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  • Oliver

    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.

  • Just G

    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…

  • Mike

    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.

  • Fernando

    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.

  • “gunner”

    “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.

  • Pete N

    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.

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  • mrwizardly

    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.

  • Tony_wants_it


    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?)

  • Keanna

    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. 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.

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  • TimG

    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.

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  • Sam

    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?

  • Paul Selhi

    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

  • airplane11

    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?

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  • Jim Voris

    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?

  • Rick

    “Interested in time travel? Meet here last Tuesday”.

  • DigitalAcolyte

    “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!!!

  • Greensleeves456

    “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.

  • llahsram

    Regarding the distinction of Least Realistic Time-Travel Movie:

    For your consideration, may I submit: “Superman: The Movie”

  • TimG

    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.

  • TimG

    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 . . . .

  • TJ

    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.

  • meglodave

    “Your crime is time, and it is 18 and life to go.”

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  • Luis Frost

    speaking of fiction, sounds like Lost got it right somehow

  • confused in the early morning monday

    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.

  • dave

    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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  • Matthew

    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.

  • Bob

    “I’m only going to say this onece: there would be no flashing lights. ”

    ONECE, spelling mistake, BOOOOOOOOOOOOOOOO

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  • Dave M

    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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  • chris

    “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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  • ben

    Even if it were theoretically possible to travel back in the time dimension, how would one be able to determine where in the other three dimensions you would end up? If you instantly went back in time two seconds, the spot you are now standing was somewhere else then. The planet is spinning, orbiting the sun, the solar system is traveling through the galaxy, the galaxy is moving relative to other galaxies, and the universe is expanding. Where was “here”, two seconds ago?

  • Samson

    In 6 you said: “every event in spacetime is characterized by certain things occurring, and those things are fixed once and for all once they happen.” If that is true, how could you even get the attention of your younger self – make them see you etc. They already “saw” something else during their conscious moments and slept during their unconsious ones. Indeed, you mere physical presence would be changing what happened in that time on that day on billions of levels from sub-atomic on up. You would have displaced and moved things just by being there. Until you could resolve the ability to make those changes, talking about more gross changes like human actions and decisions is kind of beside the point, right? What am I missing?

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  • Jonatan Funtowicz

    If the goal of time travel is to ‘change the past’ that is relatively easy to achieve in at least a couple of ways and we can do that in the present:

    a) We can create ‘fictional’ accounts of the past that become more powerful and accepted than what actually ‘happened’. This happens a lot, see history books and Fox News

    b) We can alter our memory of the past (with complex gadgets or beer).

    We cannot change physical reality but we can change our consciousness of physical reality.
    i.e. we cannot prevent global warming for instance but we can figure out a way to blame someone else for it and thus make ourselves feel better about it.

    If you can get through Neil Stephenson’s Anathem you can find some interesting ideas about multiple universes and consciousness.

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  • Matt

    Ben: you raise an interesting point, with our current expanse of exploration, any time travel machine would have to be “geosynchronous” or that it would have to move through the time dimension relative to the physical/spatial dimensions as well. Samson, if time travel is indeed possible, the quantum interpretation of the “universe(s)” would have to be the case, for reasons I am about to lay out. Also, no traveling through time would allow the reversal of the machine’s creation. You could always travel “behind” the creation of the machine if you take the schematics with you and design it in a parallel universe(it would be extremely improbable and most likely fiendishly difficult to return to the original closed time loop) , however, this will bring said traveller to a parallel universe. What perplexes and frightens(somewhat) me the most is “how do you acquire a time machine and how do you return to verify that it works?” None of us in this version of reality possesses the ability to travel through time, and the person who has the ability and is able to travel in other directions will not be in our version of reality for long. If we are able to return to the original loop, we would have to be sure that the actions we take in other times/universes would undo the creation of the time machine(i.e. we wouldn’t change much). However, if we do not/cannot return to the to the original loop(which, without a significant advance in knowledge of physics is most likely), then that person is lost to the reality they left behind from that point from which they travelled. Also, our choices affect the future of the singular closed loop outcome, with this ability, we could affect much more than our own existence, we are not able to predict the consequences of our actions across several dimensions and will likely never be able to (see, the refutation of Lovelace’s demon). While I fear the time when the ability for time travel becomes widespread, however, the people who travel through time won’t be able to come back and tell us how.

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  • Matt

    The person who designs a working time machine won’t be around very long, however, if they leave the schematics behind, a time machine could be acquired. But all theoretical discussions aside, we as a species and a form(s) of consciousness are nowhere near ready to be time travelling. I think we should work on our knowledge of the fabric of space time/FTL engines(in a closed time loop) first.

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  • Big Time Patriot

    ” real travel from one point to another through space is a continuous process. Time travel would be like that.”

    Except for the fact that sometimes travel through space in NOT alway continuous, you may have heard of this new fangled “Quantum Theory”?

  • Dave

    It’s people like you who take all the fun out of Science Fiction.

  • Emanla Eraton

    A black hole itself may not be a time machine, but you can theoretically use its properties to acheive time travel. If you somehow find a way to hover right above the event horizon of the black hole, you will travel forward through time at a much faster rate than normal.

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  • bob

    Why are you not able to travel in the past beyond the point when the time machine was made? I figure you are in a different time frame while traveling back. I do not understand how that is a limiting factor? Can someone smarter than me explain this?

  • mofo

    Would you be willing to rate noteworthy time-travel movies, with conformance to your rules as the touchstone (whatever that is)? I’m a fan of 12 Monkeys, and think its concepts hold up to logical inquiry, but I’m curious as to your more inetnsive analysis.

  • Sean

    I don’t know enough time-travel movies to make up any sort of useful list. Heck, I don’t even know about this new fangled “quantum mechanics” which is apparently all the rage with the kids these days.

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  • AgentX

    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…

  • hwickline

    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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  • adb

    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.

  • Joe Clark

    What exact thing are you only going to tell us “onece”?

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  • rasmus

    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.


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  • Patrik

    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.

  • Sven

    “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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  • Jelahd

    What utter drivel.

  • Colin

    “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 :P

  • Mike

    double post, see 170.

  • Mike

    “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.

  • Steve

    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?

  • Trevor Loughlin

    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.

  • Meh_Gerbil

    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.

  • The Stig

    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

  • Michiel Helvensteijn

    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).

  • Pug

    If traveling through time is like traveling through space, then if I was able to go back in time, say 6 months, then would I still be standing on the planet Earth? The Earth would be on the other side of the solar system and I would be stranded in space.

  • Blackbird Crow Raven

    You can download a free pseudo-science-fiction time travel (parodic/satiric) novel (.PDF, etc.) from here:

    or here:

    (“the Zany Time Travels of Warble McGorkle”)

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  • Stewart

    Okay, I admit I started skimming the responses about halfway up, but it occurs to me that given rules 1-3, 6 and 10 (which presupposes 9 and 0), any universe (fictional or otherwise) that allows for time travel to a fixed point in the past must also have a defined future. On the face of it, this violates rule 7, but rule 7 *only* applies *within* the context of a given universe.

    Still, the future is only unknowable from the relative perspective of a person contained inside a specific “bubble of spacetime”. A significantly multidimensional being or, as stated, someone from a parallel universe would not be limited to the linear interpretation of time that the inhabitants of that universe experience.

    To look at it another way, if spacetime is a painting, it’s one that is constantly being painted. If you think of it as a mural, the events of the past are fixed an immutable, but the events of the present are still being painted. The future is an indeterminate length of blank canvas. But only the brush sees it that way. The brush only lays down paint in a predetermined pattern.

    People don’t like this aspect of time travel because they think it negates their free will. But simply knowing the past doesn’t mean that the people in the past didn’t have free will. The thought that some future (or other-universe) observer knows what decisions we choose to make doesn’t mean that we didn’t make those decisions.

  • doug

    Two points:
    1. You can scrap all the talk about going into the past of a parallel universe. It is a parallel universe, how could you tell if you were in the past, or just the present of that universe. I suppose if you went there, came back, waited a l-o-n-g period of time then went back to same parallel universe and found it exactly the same, then maybe, but then maybe you are just at another instance of a different universe at its’ present day.

    2. You can’t change the past? How might you know? Maybe someone has already gone into the past and prevented Hitler from being killed? Maybe this happens a lot, sometimes killed, sometimes saved. But each time, that line of history moves forward, and there is no record of the other line at all. Maybe it has only happened once, Hitler was saved, and the resulting timeline caused the person who created the time machine to not be born. And what if this was done because in that other future, if Hitler WAS killed, something else ocurred preventing the JIHAD from succeeding, so, some unnamed group or person went back in time, and commited a ‘man-made catastrophic act’ (not terrorist, being PC here), and let Hitler have his day. And now the coming JIHAD has at least a chance of succeeding.

  • george james ducas
  • I own a Time Machine

    We already know how to produce time machines, in fact the typical american has one in their kitchen.

    A properly sealed refrigeration system is a perfect example of a time machine. Since the system is finite and the internal entropy fluctuates in closed cycle you effectively have a time machine. Since the molecules are indistinguishable, finite in number, and have a finite number of configurations, the system will eventually explore all possible configurations (poincare recurrence) and end up cycling back through any arbitrary starting configuration.

    Hello time machine

  • Science_Boy

    I love fiction, especially science fiction. I have always enjoyed Star Trek in all its incarnations from the beginning to the latest movie. I like imagining warp drive, and time travel. But…

    Time is just a way of keeping track of motion and change. The entire universe is constantly in motion and changing.

    If you imagine traveling back in time to a specific day, let’s say the day of a solar eclipse perhaps, what has to happen? Where is the energy going to come from to stop the universe in its tracks and rewind it so that the earth, the moon, the sun, the whole galaxy, the other galaxies, and every subatomic particle of everything is right back where it was on that day? The galaxy has rotated away from that position, as has the earth, moon, sun, and everything else. Whatever physics equations may tell us about what we can do mathematically with the time variable, nothing is going to rewind and reposition every particle in the universe.

    Now let’s imagine what reversing the arrow of time would mean. Gravity would now be a repulsive force. Chemical reactions would have to happen in reverse. An explosion would mean that specific gas molecules and precise little bits of things would have to assemble into a cloud and then violently assemble themselves into a stick of dynamite while absorbing a lot of heat.

    No. If nothing else keeps the arrow of time firmly locked into the forward direction, the momentum of the entire universe should be enough.

  • andreas

    Look, this “There are no paradoxes” issue is important. Many of the arguments against time travel are based on this No-Paradox Rule: A) Time travel creates paradoxes, B) The real world doesn’t allow paradoxes, C) Therefore, time travel is not possible.

    Sean (the author) writes: “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.”

    Well, yes, in the actual world, paradoxes do occur. In quantum mechanics, things happen that don’t make sense, yet they happen. The best-known example, which anyone can duplicate with a piece of cardboard and a razor, is the double-slit experiment, which proves that light is definitely a wave (and not a particle) and definitely a particle (and not a wave.) You’re right: that’s a contradiction, that’s illogical, that’s a paradox, and yet… that’s what happens. You can see this with your own eyes. The human mind can’t accept logical paradoxes, but the universe isn’t the human mind.

    There are paradoxes in the world. Therefore, the No-Paradox Rule does not apply to time travel. Time travel may indeed be impossible, but it won’t be because the No-Paradox Rule doesn’t allow it.

    For more, see “Double Slit Experiment” in Wikipedia.

  • Michiel Helvensteijn

    Andreas: The double slit experiment would be part of the the “our understanding of the laws of nature is incomplete” bit.

    Our current understanding of waves and particles might not allow for one phenomenon to be both. But obviously the double slit experiment tells us that our understanding is incomplete. Not that paradoxes are possible.

  • ereilad

    It would seem the only safe way for a human to travel in time would be in a really good space craft with lots of power available, for lots of reasons already discussed.
    I would suggest any breakthrough otherwise would be in the area of communication. Some interesting experiments at the quantum level now.

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  • do be do be do

    according to an as-yet unpublished Time article, this thread is “…both perky AND obsequious…” so you must have heard that Heisenberg’s wife lived a rather unsatisfying life; after all, when he had the time he didn’t have the energy, and when he had the position he didn’t have the momentum. in any case, carpe diem friends. keep yer pink matter warm

  • paul pomeroy

    Do any of you ever think of Copernicus? Do you truly believe that we have learned even an iota of what is to be learned. Einstein suggested we use only 10% of our brain. I suggest we use less than .00001% of our brain power.(and that is a SWAG number) Within the next millennium, space ships for travel will be a laughing matter; medical doctors will be obsolete; Science, as it is now known will be spoken of like we speak of “falling off the edge of the world”, and “the earth is the center of everything.” We haven’t even learned how to learn yet. I think we are getting there, but it is a slow process and we shouldn’t take ourselves too seriously. There will be discoveries that will stand science and religion on there respective ears. We will travel anywhere in the cosmos in the time it takes to think of where we want to go. Our bodies will heal by themselves, because ‘they can.’ I think our great-great-greatgrand kids will be looking back at our science with great mirth. But, they had better watch it because learning/knowlege moves in a geometric fashion and prpbably will for a long long time.

  • CelticMinstrel

    I LOVE how this article brings up most of the things that irritate me. For example, time-travel in Stargate Continuum simply didn’t make any sense! The new Star Trek, on the other hand, definitely got things right, I think.

    Note: I could be wrong about some things in what follows, since I’m not a physicist or anything like that.

    Matthew said:
    > (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)
    People keep saying this, and it doesn’t ring true to me. It seems to be assuming that there is an absolute coordinate system – the very thing that relativity has shown to be false. If you are on Earth and you travel back in time, you’re most likely travelling with respect to the Earth’s worldline. As an analogy, consider a river. You are at point A on the river and wish to travel upriver to point B. Do you simply travel away from the river mouth in a straight line, or do you follow the course of the river? Similarly, in time travel you would follow the Earth back through time to your new location.

    It helps to think of the Earth’s “worldline” as something like the sum of its positions over the course of its history – a trail extending backwards in time. I’m not sure how accurate that is though. It also becomes confusing when you consider that, by this logic, one of two things is true: either the worldline extends into the future, or it is constantly extending as the Earth moves… neither of which quite seem right.

    dmduncan Said:
    >Regarding rule 9: We can’t travel to the “past” of an alternate
    > universe either. What you might be trying to say is we might
    > be able to travel to the present of an alternate universe which
    > resembles the past of this universe, assuming that such alternate
    > universes exist at all and if they do that they parallel the development
    > of our own but are not parallel in time, so that a lateral shift from
    > this one to the other brings us into a similar universe at an earlier
    > stage of development. But that’s not time travel. It’s weird, but not time travel.
    …what? In all honesty, I don’t see how this is any different from travelling into the past of a parallel universe. After all, there isn’t really an ordering of times in different parallel universes – if it looks like a past of your universe of origin, for all intents and purposes it IS a past of your universe of origin.

    Keanna Said:
    > 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. 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.
    No reason? I could name a few, in theory. Really, it probably depends on the attainability of infinity. In order for anything with mass to reach the speed of light, you need to apply an infinite force to it. It’s generally accepted that this is impossible.

    Of course, an alternative would be to somehow suppress one’s mass, which would allow you to travel at the speed of light. Or rotate it 90° in the complex plane (whatever that means) – then you could travel through space without regard to time, moving backwards and forwards through time as easily as you can normally move backwards and forwards through space… but at the cost of never being able to stop – with imaginary mass you can never travel slower than the speed of light. If the Higgs field turns out to be true, I suppose suppressing mass MIGHT be possible; the other, I can’t even begin to say (probably not though).

    chris said:
    > Doesn’t time travel imply greater then 3 dimensional space?
    Yes, indeed it does. In fact, it implies a minimum of 4 dimensions – the fourth being the time dimension (which is mostly just like the three spatial dimensions, except that when calculating distances between two points you subtract rather than add the square of the time difference).

    Emanla Eraton said:
    > A black hole itself may not be a time machine, but you can theoretically
    > use its properties to acheive time travel. If you somehow find a way to
    > hover right above the event horizon of the black hole, you will travel
    > forward through time at a much faster rate than normal.
    It’s called “orbiting”. ;)

    Science_Boy Said:
    > If you imagine traveling back in time to a specific day,
    > let’s say the day of a solar eclipse perhaps, what has to
    > happen? Where is the energy going to come from to stop
    > the universe in its tracks and rewind it so that the earth, the
    > moon, the sun, the whole galaxy, the other galaxies, and every
    > subatomic particle of everything is right back where it was on that
    > day? The galaxy has rotated away from that position, as has the earth,
    > moon, sun, and everything else. Whatever physics equations may
    > tell us about what we can do mathematically with the time variable,
    > nothing is going to rewind and reposition every particle in the universe.
    But why is rewinding necessary? Think of the Earth as an object that is very large in the temporal direction. If you consider spacetime to be static from an outside perspective, what would such an observer see from outside the universe? They would see Earth as an enormous spirally object, something like a helix, extending back through time and “fraying” as it pokes back beyond the point of its formation. If you travel back in time, you’re just moving “down” that helix to an earlier point.

    Trouble still arises when you extend that analogy to the future; the many-worlds interpretation helps there though.

    Science_Boy Said:
    > Now let’s imagine what reversing the arrow of time would mean.
    > Gravity would now be a repulsive force. Chemical reactions would
    > have to happen in reverse. An explosion would mean that specific
    > gas molecules and precise little bits of things would have to assemble
    > into a cloud and then violently assemble themselves into a stick of
    > dynamite while absorbing a lot of heat.
    All possible, except for the bit about gravity, which is false. Run a chemical reaction backwards in time, and it still follows all the laws of physics; same with the explosion. The reason we don’t ever see a reverse explosion is simply because the starting state is more complex than the ending state; we don’t have the resources or patience to set it up. That doesn’t mean it’s breaking physical laws. As for gravity, no, it wouldn’t be repulsive. However, consider a meteor heading for the Earth. It has a certain velocity as it approaches, and that velocity increase as it comes closer. Now reverse time. Suddenly, its velocity is reversed – it’s moving AWAY from the Earth. So gravity is repelling it, right? No, it’s not. As it moves away from Earth, it SLOWS DOWN due to the ATTRACTIVE force of gravity – just like we would expect. It doesn’t fall to Earth because its velocity exceeds escape velocity.

    Whoa, long post. I hope I didn’t say anything stupid…

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  • bill 1932

    Intuite says: How does time exist outside of the cogent mind? Is it only a creative tool that works well for the measured mind? Imagine a universe with no cogent mind, no time pieces; what part of that universe, micro or macro would be effected as to where it is, is it moving or stationary, what speed, would speed exist? Eternity has no measurement, it has no fixed place. Space is no different. If it is infinite every point is the center, there is no alternative. Without our handy tools, of which one is vision, every point may be the same point; like the eternal now.

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  • Brewmaster

    Obviously, IF time exists in the manner in which we perceive it (or, I should say, in the way we understand our own and each others’ perception of it), and IF we could travel from one point to another within it, then Bill and Ted had the right idea. If we need a set of keys, we just need remember to get them in the future, travel back to before we need them, and place them in the bushes. If we had seen them in the bushes first, we wouldn’t have brought them back, so they wouldn’t have been there and we would have brought them back. Paradox? Naah. We didn’t look in the bushes until we thought to put them there.

    IF the ability to do this in a controllable manners exists/existed/will exist, spacial location is not at all a problem. All we would need to do is be able to perceive one extra dimension and move through it, heading for a desired 4D point just as we would head for a 3D point within the confines of our current limitations.

    Of course, then we would start trying to figure out how to move from one 5D point to another…

  • Science_Boy

    To CelticMinstrel. You’re right about the gravity. I should have left that out because reversed time would not make it behave like a repulsive force. Your description of how the meteor would move is more accurate.

    My point was simply that time travel, as pictured in popular fiction, is completely, utterly, totally impossible. The past no longer exists. You can’t go back to a previous point in time because there is nowhere to go to. Any past time you can name represents a previous arrangement of all the particles in the entire universe. You can’t put the entire universe back into the previous arrangement.

    We can *imagine* the earth as a spirally object in 4 dimensions, but that is all it is: a way of imagining it.

    Sean’s article has it right. Time travel is something that happens in a “fictional world”. That’s the fun of science fiction. We say what if something impossible were possible, then what? When we do that, we need rules to keep the internal logic of the story consistent, given the original premise for the fiction, that time travel can happen.

  • CelticMinstrel

    ScienceBoy: I don’t think you and I are going to agree; you have more of a “presentist”" view whereas I have more of an “eternalist” view. I don’t know of significant evidence to support either viewpoint; however, one of the reasons I take the eternalist stance is because general relativity treats time as just another dimension (albeit a dimension with slightly different properties), and if it’s just another dimension, then theoretically it would be possible to travel backwards through it and hence return to an earlier moment.

    Of course, general relativity is not a perfect model. Perhaps the perfect model will agree about time being a dimension; perhaps it won’t. Only time ( ;) ) will tell.

  • Cheldric

    If time travel to a time before a time machine was invented were possible we would already have evidence of such travel. Humans are way too careless not to leave evidence behind. The same goes for travel to another dimensional timeline. (Not us going there – but them coming here.) Since we see no evidence of interdimensional time travelers from “there” it’s probably not possible. Imagine the evidence of a Tourist Time Spot (significant events in our history both natural and human oriented). Temporal Archeologists would have blown their cover “long ago.” ;-)

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  • Mike

    I think a great example of how time travel would work is season 5 of the tv show Lost. Traveling back in time without a time machine but exotic matter causing workmhole like effects all in a fixed remote location with know one knowing what originally happened at the poin of time they traveled to or only a little bit and actually cause and witness all events that originally took place.

  • Brewmaster

    To Cheldric: Sadly for us, time travelers have gone/come back and cleaned up all convincing evidence of previous (to them) time travellers. They have also inserted flawed and confusing scientific theories and discussions into our culture to further delay our progress in the field. For a while, it was (is/will be) quite the multi-dimensional Chess game, but finally (or so it seems) that group won (is winning/will win), eliminating all time travel competition and convincing the rest of us that it doesn’t exist. I feel confident in telling you this, because somebody will come back and convince me not to post it and no harm will be done. Hopefully, “convince” does not mean prevent my existence, but I digress…
    :D Just kidding (maybe) :D

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  • Melissa in viewing the past

    If you travelled faster than the speed of light by say…an hour. Had set up a system to keep light from splitting off along your viewing area, and landed on alpha centari (with the correct equipment) and was able to view through a machine the planet Earth, would you see the past (for an hour)? Or some similar set up? Truely when we look up at the stars, we see the past light…and light is matter, so if we could keep it from dispersing…it would work. I can’t really think of a benefit…other than big brother, but we could video from right here on the planet instead.

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  • Jon

    I agree pretty much across the board with all these rules, especially with the fact that certain people need to lighten up on what is or isn’t a joke. You only have one subjective existence, regardless of how many times you travel to the past or parralell universes, (either with a machine or through any other means) life’s too short to be anal about something as silly as a story.

    Zen Master Jon

  • CelticMinstrel

    Melissa in viewing the past said:

    If you travelled faster than the speed of light by say…an hour. Had set up a system to keep light from splitting off along your viewing area, and landed on alpha centari (with the correct equipment) and was able to view through a machine the planet Earth, would you see the past (for an hour)? Or some similar set up? Truely when we look up at the stars, we see the past light…and light is matter, so if we could keep it from dispersing…it would work. I can’t really think of a benefit…other than big brother, but we could video from right here on the planet instead.

    Whoa, this sounds complicated. Okay, basically you are proposing to travel to Alpha Centauri in some way that ensures you arrive one hour before the light showing your departure from Earth reaches Alpha Centauri (okay, you wouldn’t actually be able to see your departure from that distance, but that’s not the point).

    One theoretical way to do this (which may or may not be possible) is via a wormhole. I don’t think it needs to be as complicated as you make out – what’s all this about light splitting off a viewing area?

    Anyway, yes, if you did that and looked back at Earth, you would see it as it was an hour before your departure – though, at that distance I don’t think you would be able to make out much.

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  • Kilroy_of_Aus

    What has not been mentioned is …As we travel back in time, our memories would disappear, would they not?
    So would we ‘forget’ how to work the time machine. Forget is a vague term, what I want it to mean is “When we arrive back in the 1960′s our brain would not know how to work the time machine to get us back to 2050.”
    ( and press the Big Green Button)

  • CelticMinstrel

    No, your memories would not disappear. If you could “rewind” time, then that would happen, yes. But travelling back in time does not mean you become younger – in fact, you continue to age even as you travel through time. Time is relative, and even though you’re travelling back in time, you still experience relative time in the same way.

    Or something like that.

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  • Mayor McCheese, The Galactic

    I love how the author, Sean, makes hard assertions about a branch of science that no one really knows anything about with any real certainty.

    Sean, you seem to be pretty sure about all of this time travel stuff. I take it you have, in your infinite humility and wisdom, graciously traveled back in time to share your incredible knowledge with the rest of us. Unfortunately, you forgot one of your own rules that whatever happened, happened. Thus, this post’s benevolent intent is lost because the rules dictate it cannot affect the original flow of time. Your trip is redenered useless. This post, worthless. Your life, meaningless.

    …unless you traveled to another “branch of the wave function”, meaning that this not your original reality. In that case it IS possible to affect the happenings of this universe.

    Wait a minute. That also means that another version of you likely exists here, in which case you would have to kill him so that you could take over his life. Since I doubt you have the balls to kill this multiverse’s version of you, I’m going to go with one of my other two theories. The one listed above in which your life ends up completely meaningless or the much more likely possibility that you actually didn’t travel back in time (even though you never claimed to in the first place) and that you are just an arrogant @ss who thinks he knows something.

    Besides, we all know that whatever Mayor McCheese says is the final word – and I’m with Science_Boy (#185).

    I’ve tried so many times to figure out a way to go back in time and not get AIDS from that Bangkok hooker, but it’s just not possible.

    -Mayor McCheese

  • Trivi

    Great read! Thanks for the wonderful article.
    But i have one comment.
    This is a comment is in response to #9: “Unless you go to a parallel universe”
    You said…
    “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”.

    But just because we have landed in the past of a different universe doesn’t mean we can alter events of that universe because “whatever done is done” applies to that universe as well. So no matter which universe’s past we go to, altering events should be impossible because every universe has its own course of history and changing anyone’s history would be impossible. Right?

    “parallel universe” concept doesn’t necessarily give us access to alter events. No matter which universe’s past we go to, altering would be impossible. Something will prevent us from altering. This is just a thought. I might be wrong so I am open to suggestions. Thanks for reading.

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  • Joram Arentved

    My worst scenario is right now everything, I DIDN’T know, DIDN’T think, if I could count on & about my past, thus an issue, whereon I’m willing & maybe under obligation, which is just
    my sense of, what moral is, to let you receive some more info, e.g. that right now my labor situation is, what it doesn’t look like: It’s terrible for me, the best thing of mine as an honest man to mention it to you, so that I can of course tell & e.g. help us both etc. find out &, whoe-ever’s who, our best answers to, – the rest goes without saying, greetings, P.S.: The Corrupt still approve of no honesty, so whatever God exists or not, it’s not MY opinion that I can fail to become & believe in & so on.

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  • WilfredT

    Fernando, I read it. and I agree.

  • http://princessweb cherise

    how do you make a time a machine?


  • tom cotter

    I hate the rhetorical defense on this subject, ‘we don’t know all that is involved in how time travel will work.’ The general features of the current understanding of the laws of nature are sufficient that this will require more data and debate. Though a testable experiment; time or ‘moving clocks’ aren’t globally synchronous. Is there a ‘permissable kinesmatic translation’ that would be symmetrical in all global events?
    I enjoy the fantasy of ‘time travel’; but just because it is part of pop culture doesn’t put it outside of scientific investigation.
    My best guess would be ‘time travel’ is up there with engineering a star, teleporting a macrosopic object or explaining the Universe with a 2″ equation.
    We just live in a part of history where things aren’t fully developed. Keep at it till humans find something better?

  • Regina Karl

    can i go to past so that all these worse present tragedies would be changed into normal? I am so desperately, wishing that the time machine would be at my sight at least at the moment that i’ll use it to make my present worthy.
    first of all my mother died because of cancer, my whole family needs my mom so my father doesn’t not have to work unstoppable until he’s deadly tired.


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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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