Time Travel Done Right: A Book Excerpt

By Sean Carroll | February 3, 2010 8:42 am

From Eternity to Here addresses the problem of the arrow of time — why is the past different from the future? But Chapter Six is all about time travel, and in particular the interesting version in which you travel backwards in time. Whether it’s possible, what rules it would have to obey, and so on. And now — even though I’m sure there aren’t more than two or three of you out there who haven’t purchased the book already — you can get a sneak peek of part of that chapter. It’s going to be the cover story in the March issue of Discover, and the story is already available online.

clockmedia And here’s a bit of multimedia bonus: to get the cool exploding-clock image, the intrepid editors worked with Biwa Studios to film high-speed video of exploding clocks, and you can see the whole videos online. They run the events forwards and backwards, just in case your personal arrow of time needs to be calibrated.

One may ask, why is there a chapter about time travel in a book about time’s arrow? Just couldn’t resist the temptation to talk about everything related to “time”? In fact there is a deeper reason. In the real world, the laws of physics may or may not allow for closed timelike curves — physicist-speak for time machines. (Probably not, but we’re not as sure as we could be.) But apart from the difficulty in constructing them, time machines boggle our minds by offering up logical paradoxes — what’s to prevent you from traveling into the past and killing your parents before they met? There is a consistent way to handle these paradoxes, simply by insisting that they never happen. (And we’re still hopeful that the folks at Lost adhere to this principle, regardless of the surface interpretation of last night’s Season Six premiere.)

The reason why that’s hard to swallow is because we can’t imagine anything that stops us from killing our parents, once we grant the existence of time machines. We conceptualize the past and future very differently — the past is settled once and for all, while we can still make choices about what happens in the future. That, of course, is the arrow of time. At the heart of what bothers us about time-travel paradoxes is the difficulty of establishing a uniform arrow of time in a universe where time loops back on itself.

Of course the easy, and probably correct, way out is to simply believe that time machines don’t and can’t exist. But disentangling the demands of logic from the demands of common sense is always a rewarding exercise in its own right.

  • José

    What would have if time had two dimensions? Could it be that traveling past in time as a result of moving in a second time dimension, appears as though the entropy would decrease but in fact the entropy in both dimensions increases? More generally if time had infinite dimensions one could travel back and forth decreasing the entropy in “inferior” dimensions but increasing the entropy in all the dimensions. Does this make sense?

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    If time had two dimensions, all hell would break loose. Time would be a 2-dimensional plane, not just a single direction. It would be easy to travel backwards (or sideways) in time. It’s hard to know exactly how physics would work, but things would certainly be different.

  • marc

    Sean—have you been watching “flashforward”? Everyone on Earth gets a “vision” of themselves at a particular time 6 months in the future. They are all consistent (if A envisions meeting B at that time, then B’s vision is of meeting A). But then people do things to ensure that their vision doesn’t come true….

  • Doug

    Hi Sean,

    1 – As a fan of science fiction, I have a fascination with the possibility of time travel.

    Yet on a practical level, I have ever increasing doubts that there is a means to travel by time.

    Would it not be necessary to reverse the flow of neutrinos?

    2 – Neutrino time would seem to be different from anthropic time.

    Quantum mechanics might not seem so strange if events could be measured on neutrino time (speed near c).

    Measurement by anthropic time (speed <<<<< c) necessitates the use of probabilities for events moving at relativistic speeds over quantum distances.

  • Chris W.

    This is one of those topics that tends to generate word salad.

  • Stanley H. Tweedle

    Do you think temporal paradoxes are possible?

  • doug

    Time travel is simply like a movie that can be played forward or reverse. Nothing can be changed no matter how often the movie is played. We are in a state of matter (left handed positive particles), to travel in reverse would require (a person) to move in an antimatter state (right handed negative particles). Since matter and antimatter can’t interact the antimatter person would simply be on observer without the ability to change anything. We could document the past for accuracy but not change it. Once the time line is written it’s done, no paradox. Thus time travel is possible, the difficulty lies in transforming the left handed positive particles into right handed negative particles. This can be accomplished by speeding a (person) faster than the speed of light.

  • José

    I just tend to think of time as having 3 dimensions same as space to better understand time dilation, and I see it as a curved motion instead of linear, like as if two particles traveling the same time line and one starts to describe a curve because of some great mass and the other is not caught in the gravitational field, they both individually experience the same time speed but in relation to each other time passes differently. Taking multiple time dimensions to the theme of time travel it sounded compatible with the second law of thermodynamics. But as I am not a physicist it might just be silly talk :)

  • Wondering

    Hi Sean,

    Why aren’t the grandfather paradox or the paradox of duplication ( if i travel 1 week into the past, i was already there so then there are 2 of me which, at minimum, seems like matter creation which violates Quantum number conservation) fully accepted as rigorous (dis)proofs by contradiction if we take physical reality to be a ‘formal’ system in action ?

    Also, is there not a claim by serious researchers that if a time machine were to be invented, it could not travel back to any time before its invention. So, a time machine invented on a Wednesday could not travel back 24 hours until at least Thursday. Why isn’t such an assertion in outright contradiction of SpaceTime Invariance wherein the rules of physics ( or the function of devices ) should not depend on the day of the week or their location etc ? Or was that limitation of time travel not a serious proposal ?

    Thanks for any input on this and forgive me if you have addressed this in your book which i do not yet have.

  • Matt

    I don’t understand why the grandfather paradox is a paradox at all. If you’re in your own past, it’s already by definition a different past. Even if it weren’t, the odds of all the various wavefunctions collapsing in the same way as they did originally is so small as to make the thought of an identical repetition of history absurd in the first place. Right?

  • Wondering

    HI again Sean,

    My reading of your excerpt and the entropy discussion reminds me of another thought regarding the inevitable changing of the timeline not matter how we exercise our free will in exploiting a Closed Timelike Curve or other time travel device in a single universe context. I hope the reasoning will be correct.

    In departing the NOW we reduce the entropy of the NOW and increase the entropy of the PAST by our arrival. No matter how we choose to interact with the environment we increase its (and our) entropy. We spend X time in the past interacting and return to the NOW where the NOW regains the entropy lost when we left but also now gains the extra entropy we personally gained while in the PAST. Meanwhile, the extra entropy gained by the environment (the universe) due to our visit constitutes a higher baseline entropy which has been compounding the whole time that it takes for the PAST to catch up to the NOW we returned to. The farther back the PAST is, the greater the entropy of the universe we return to. We are aging the universe with this device no ? If we invoke the argument that we travelled to another universe, aren’t we then ageing the other universe ?!

    While this may be taken as the entropy cost of exploiting the machine or CTC, Isn’t it also therefore inevitable that doing so constitutes a de facto alteration of the timeline in this fashion no matter what the free will considerations may be ?

    Thanks so much.

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  • Rohan

    Speaking of the book, what’s the deal with that identically titled, similarly type-faced yet clearly very ‘different’ book? That can’t be a coincidence.

    (also, when can we order the paperback? :)

  • Aaron Sheldon

    To time travel in the past, wouldn’t you need to annihilate with your anti-particle releasing a flash of gamma rays in the present, and then re-coalesce in the past from colliding gamma rays into yourself and your anti-particle? Or is this taking path diagrams to literally? Seems to be entropicaly unfavorable to just happen across your own anti-particle in the present, and then at a precise point in the past to just happen to coalesce from colliding gamma rays.

    At the quantum scale time travel can be viewed as a form of the Heisenberg Uncertainty principle, where the ‘borrowed’ energy of the virtual particle is actually in an infinite time travel loop, which points to all the difficulty with renormalization of Quantum Gravity.

  • Nancy

    I can understand why Memento is one of your favorite films–as I was reading your book today, I was thinking about it, and how Memento shows this so well: how unless we know what has come first, (if we only are operating from this point backward without prior knowledge), our interpretations of data and ensuing decisions may be horribly wrong. And I was thinking of the book, The Time Traveler’s Wife, and how Henry interacts with himself in the past (Audrey Niffenegger, the author, has Henry time travel back to his own wedding and the real-time Henry steps aside and yields the vows to older Henry). But, were time travel a possibility, wouldn’t we be able to travel to certain times as multiple selves? (I do not recall reading that happening in Niffenegger’s novel.) However, were time able to flow in both directions, there is no reason we could not appear simultaneously in one specific point, the 30, 40, 50 year old self appearing all together with the real time 10 year old self. But this makes me wonder, as you have written on page 14 of your book, “There are an infinite number of such slices” (regarding the concept of time) that this means that, were we able to time travel, there are an infinite number of “selves” simultaneously appearing at infinite moments. Not just the 20, 30, 40–whatever–year old me, but the 30 years 13 days 2 hours 5 min 1 second year old me…and the 30years 13days 2 hours 5 min 2 seconds year old me (etc) time traveling back to the exact other moment and vice versa. No wonder time can only travel in one direction–can you imagine the traffic jams at rush hour?!

  • Wondering

    Yes Nancy -

    I remember reading a wonderful science fiction novel in the ’80s which i wish i could remember the title of. It had the word ‘circus’ in the title i think. If anyone knows, please post it.

    Anyway, suppose time travel technology became the basis of a tourism ‘in time’ industry. So 6 months after the tech becomes available i visit the crucifiction and there are 5,000 people watching. But 6 years later, i visit again and this time there are 100,000 people watching the very same unique event in spacetime history because it is a popular time tourist destination. That’s the paradox of multiplication. We can’t invent a Fate Protection Principle so all visitors see the ultimate total who will eventually ever visit because then the universe has to somehow know how many that will be, a fact which only comes into being when the last visitor ever, travels to the crucifiction. How weird is that ?

    I really think this whole time travel notion needs to be glued to a plaque and hung on some wall somewhere, right next to the one for aether, lol.

  • Wondering

    Silly of me, i should have been more clear. The novel was all about widespread time travel tourism and explored all these time travel paradox issues in great detail. Doh ! I do hope Sean will be gracious and forgive my mention of an out of print scifi novel and also forgive anyone kind enough to post the title i’m trying remember from the meager clues provided.

  • http://www.timesplash.co.uk Graham Storrs

    You seem to be sticking with an Einsteinian model of time in your discussion but one or two of your commenters have already started talking about multiple timelines (via multiple worlds) to avoid paradoxes. And that’s one of the beauties of time travel. there is so much uncertainlty about how time works within modern cosmology that a lot of different models are still up for grabs. As long as a writer uses one model and sticks to it (keeps it all internally consistent) I’m always happy to read time travel stories.

  • Matt

    You think it might be possible to look at Vegas betting patterns for evidence of time travelers here? Maybe like a statistical sampling of long shots that win, and the people who bet on them? See if there’s a slight bias toward the better?

    If nothing shows up, would it be safe to assume one of the following is true:

    A) Time travel is impossible
    B) Humanity doesn’t progress to the point of travel travel capability
    C) Money is meaningless in a time traveling future
    D) Knowing the outcome of a betting event in your timeline does not guarantee the event playing out the same way when you travel back in time to bet on it.

  • Jesse M.

    I think people often feel that there is something suspicious about the idea that if you tried to kill your grandfather in the past, events would “conspire” to prevent it (like your gun jamming at just the moment you have him in your sights)–they think that this would seem to require a sort of intelligence on the part of the universe, knowing that the death of the grandfather must be avoided and “figuring out” how to stop it. I think the conceptual problem here is that people are thinking in terms of the “moving present” view where time is flowing forward, and the universe is deciding “what happens next” on the fly, based on past conditions. But another way of conceptualizing the issue which avoids this need for “intelligence” is to imagine just generating a whole bunch of random histories, and then throwing out all the ones that aren’t consistent with the laws of physics.

    Here’s an analogy. Imagine you want to write a computer program to generate a possible chess game. One way is to start with the pieces in their starting configuration, then have the program generate each successive configuration on the next turn from the configuration on the previous turn, using only legal chess moves. But here’s another, more elaborate way to do it. have the computer generate an entire series of configurations at once, completely randomly, so it just picks randomly which pieces to put in which positions on which turn. It is very unlikely that the resulting series will look like a legal chess game–a piece might randomly be on a particular square on one turn, but then the next turn randomly be on some totally different square that it shouldn’t be able to get to in one move by the rules of chess. But suppose you have access to an idealized computer with nearly infinite speed and memory, and you have it generate a gigantic number of random series this way–if your number is large enough, chances are at least some of the series would just happen to satisfy the rules of a legal chess game. So you could specify that the computer should throw out all series which violate the rules of chess, and be left only with series that represent legal chess games. But since you are dealing with an entire series at once, you could also place other constraints on them, like “throw out all series where white wins”, or “show me only series where the black rook checkmates the king in 25 moves”, whatever you want. For sufficiently detailed conditions, it might be very hard to generate a chess game that matched them in the traditional way of starting from the beginning and basing each new configuration of pieces on the configuration of the previous turn, but using this brute-force method of generating a near-infinite number of entire histories, and throwing out all but the ones that satisfy your constraints, it’s easy to get a game that satisfies any conditions you like without even having to think about it or plan the details of the game.

    Similarly, suppose you were using this incredibly powerful computer to generate a simulation of an entire universe–instead of picking some initial conditions and then letting it evolve forward according to some set of laws of physics, you could again specify your “laws” in terms of constraints on entire histories, with the computer generating a huge number of random histories and then throwing out all the ones that don’t satisfy the conditions. If the “laws of physics” you pick happen to allow time travel, then obviously any universe that respects the laws of physics locally at every point in spacetime must be globally self-consistent, and the computer will find some histories satisfying this condition. But the computer does not need to have any intelligence to do this, it’s just randomly generating a huge number of possibilities until it finds one that satisfies the constraints. From the point of view of a time traveler in this universe, it might seem like the universe was cleverly finding ways to “outsmart” them and thwart their plans, but it would actually be the result of a fairly simple rule, just not a dynamic rule.

  • paul valletta

    Sean, great reading the cover page linked, if I was to go back and stand behind Isaac Newton, whistled to get his attention, and cause him to miss the apple being pushed_to-ground from the tree, then this would not cause a great deal of Entropic Distruption locally, at that moment..then. But as time passes, Entropy will increase to such a degree that, today, here and now..online and talking about the said subjuct would/could never take place?..the farther away from “changed” events, the more Entropy changes the present time?..thus things today would not be stable in any way, and like you say Magicians would rule all the Physical, and Relative Laws.

    I think your explination of the precision of atomic structure at moments in time is spot on!

  • matteo210

    Hello, just a quick question on your book, do you plan to have it translate into italian any time soon? are there any chances?
    thank you for the answer

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    An Italian edition is in the works, yes!

  • jeff parnau

    1. Suppose time travel becomes extremely inexpensive, and it’s built into your cell phone. You and 10 million other people decide to travel back 25 years for a reunion with yourselves. How do you account for the change in mass which must occur when 10 million people disappear from today, and their body masses arrive in the past? For an observer in the past, he or she could quickly prove that matter can be created, and when you leave, that it can be destroyed.
    2. Go bigger. Send North America into the past. Now we’d have two continents. The earth would rotate kind of funny.
    3. To be mundane/repetitive, I am strongly attracted to the argument that we will never travel into the past because nobody in the future has ever done it.

  • Jesse M.

    @ jeff parnau: on points #1 and #2, on a theoretical level we definitely know that general relativity allows time travel into the past in certain circumstances such as trips through traversable wormholes, so time travel automatically respects whatever version of mass/energy conservation exists in general relativity. Apparently the question of energy conservation in GR is actually somewhat tricky, it’s conserved locally but there may be problems with defining energy on a global scale, see this article by Michael Weiss and John Baez. But I’m not sure if these problems apply to “asymptotically flat” spacetimes like a spacetime containing a wormhole that gets arbitrarily close to flat spacetime at large distances from the wormhole. In the case of wormholes, I’ve read (see post #6 by ‘pervect’ on this thread) that when an object disappears through a wormhole mouth (like an object traveling into the past) the mouth itself actually gains some mass to compensate, and when an object appears from a wormhole mouth (like an object arriving from the future) the mouth loses mass to compensate (which can eventually result in the mouth having negative mass), so this might be how energy conservation works in that situation.

    As for point #3, wormhole time travel would never allow you to travel farther back in time than the date the wormhole was created, so even if this turns out to be possible and our descendants have the ability to make it work on a practical level, we shouldn’t expect to see any time travelers around today. It’s probably more likely the time travel solutions that appear in general relativity will just be ruled out by quantum gravity, as Hawking proposed with his “chronology protection conjecture”.

  • jeroboambramblejam

    Time’s “Arrow” may be a misleading metaphor – or at least insufficiently stringent. True, in flight an arrow is moving in a satisfyingly reliable direction; and it’s stopping also mimics the behavior of time at relativistic velocities, but neither the point nor the feathers prevent it moving backward. A more useful metaphor would invoke the irreversibility of the (seeming) passage of time, while admitting the infinitude of paths forward… something like a funnel… but with valves… and an adjustable length… Time’s Trombone’?…

  • http://www.sciencechat.net/wordpress SeanM

    Doesn’t it follow that if time travel was ever to become possible, we would already see the evidence of it? Or should we believe that there was a worse threat than Hitler that was displaced by a time traveller? That’s just one example but you must agree that there are things that have happened that any time traveller would try to change, thus disproving that time travel will ever be possible.
    As an aside, the only realistic treatment of time travel in media, books, films, etc. was (will be) Twelve Monkeys…

  • Jesse M.

    SeanM, on why there wouldn’t be any time travelers today even if it was eventually possible, read the second paragraph of my comment #25 above. And I can think of one other realistic treatment of time travel in movies (realistic in the sense of respecting the Novikov self-consistency principle)–Bill & Ted’s Excellent Adventure! (see in particular their complicated but self-consistent scheme for getting into the police station to rescue the historical figures that were arrested at the mall) In science fiction books there are plenty of other examples, check out the “Pre Novikov examples” section of the wikipedia article I linked to…

  • bill tony

    I found this related article by Marshall Barnes http://www.scientificblogging.com/temporal_mechanic/blog/“new_rules_time_travel_”_and_sean_carroll’s_gate_2 for those really interested in time travel physics.

  • Pingback: From Eternity to Book Club: Chapter Six | Cosmic Variance | Discover Magazine


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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] cosmicvariance.com .


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