The Flow of Time

By Sean Carroll | August 18, 2011 7:27 pm

I Tweeted the following inscrutable remark. Probably best left unexplained, but upon reflection I can’t resist.

My consciousness freely travels up and down my world line, but sadly it only carries the memories appropriate to the moment it inhabits.

The point is that (some) people don’t think about the flow of time in the right way, and this leads to a couple of unfortunate consequences: a difficulty in understanding the psychology of time, and a scattering of entertaining but illogical science-fiction scenarios.

Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing. That includes our own personal path through it, which defines our world line. This seemingly conflicts with our intuitive idea that we exist at a moment, and move through time. Of course there is no real conflict — just two different ways of looking at the same thing. There is a four-dimensional universe that includes all of our world line, from birth to death, once and for all; and each moment along that world line defines an instantaneous person with the perception that they are growing older, advancing through time.

But if you don’t play too much attention to the way these two views fit together, you are tempted to imagine that “you” might actually, in some set of laws of physics if not actually in our own, go visit different moments in your own life, carrying along the consciousness of your “present” self. Something like that happens in SF stories from Slaughterhouse-Five to Back to the Future. But it’s not consistent — it requires the implicit introduction of a kind of “meta-time” that keeps track of when we visit the ordinary time with which we are familiar. That’s not how nature works; my tweet was trying to point out the inconsistency of taking this idea seriously, subject to the strictures of 140 characters or less. (To be earnestly explicit: if you did manage to travel up and down your world line at will, you would indeed have whatever memories were appropriate to the moment you were inhabiting — which means it would be exactly like not traveling at all.)

Sometimes, unfortunately, people go further than science fiction. I’ve run into folks who believe that our conscious perception of time passing is actually evidence against modern physics — arguing that we need to change the known laws of physics to account for the flow of time. It’s always conceivable, in principle, that what we think we understand at a basic level is completely wrong. But the evidence had better be pretty overwhelming. The brain is a complicated thing, and I don’t think that our present inability to provide a complete and comprehensive theory of conscious perceptions should be held as compelling evidence that the laws of physics are in need of overthrowing.

  • John Kubie

    But a huge unknown, in physics and neuroscience, is consciousness. One or both of these worlds will be overturned when and if consciousness is unravelled. My guess is that both will have major change.

  • Matti Pitkanen

    No need to overthrow laws of physics. Just generalize them. Biology and consciousness is something with which the text book physics cannot cope with.

    It is of course extremely non-realistic to think that all of relevant physics had been understood five hundred years after Newton. To get some historical perspective it is good to recall that the general belief for about 110 years ago was that classical physics was all that is needed! It would be easy to ridicule these ancient colleagues but our fate might be the same unless we are very very cautious;-).

  • Neil

    By analogy, all of the information for a movie exists as a block on a DVD, but my DVD player, like my consciousness, makes it appear to me that the information (scenes) flow through time.

  • Chris

    Couldn’t my “future” worldline (that is, the part that my conscious self has not yet remembered), conceivably have some loop-de-loops in it? After all, I don’t yet remember whether my far future self has visited my slightly less far future self. I have no idea what my consciousness would remember at the moment where the loops start to inhabit the same slices of time, but is anything precluding that possibility? Obviously I can’t visit my past self, since I don’t remember having any visitors, but i think that I might be able to visit my future self once he’s become my past self.

  • Charon

    @Matti Pitkanen

    Biology and consciousness is something with which the text book physics cannot cope with.

    What’s the basis of this claim? And… you should probably read Sean’s “The Laws Underlying The Physics of Everyday Life Are Completely Understood” Part 1, Part 2, and Part 3 before you answer this question.

    Your argument appears to be “in the past people thought they knew some stuff that it turned out they were wrong about, so we’re probably wrong about stuff we know now – and, interestingly, not just any stuff, but this very topic of conversation.” Actual science is hard – you have to know lots of stuff, think about it, do tricky experiments, and all that. If we could reach valid conclusions with just a superficial knowledge of history instead, we would do that. But that doesn’t, you know, work.

  • Matt

    Sean, I wonder what the implications of this on discussions of free will/decision making are? If the four-dimensional universe includes our world line are we simply deterministic machines with our consciousnesses providing an “illusion” of the ability to make decision when from the full-dimensional view it is simply following its worldline?

  • Ray Gedaly

    Wow, I think I’m experiencing deja vu. :)

  • Mike


    Your statements hold only classically, assuming the world is deterministic. QM greatly complicates this picture, as you know. Even if you insist on viewing QM as being a deterministic theory of the wave function of the universe, the notion of unique classical worldines existing in a unique classical spacetime geometry doesn’t exist at a fundamental level. For those who follow the Everett interpretation, the insufficieny of unique classical worldlines and unique classical spacetime geometries is even more striking.

  • jpd

    “What’s the basis of this claim? And… you should probably read Sean’s “The Laws Underlying The Physics of Everyday Life Are Completely Understood” Part 1, Part 2, and Part 3 before you answer this question”

    i read them, they are just his opinion, not scripture put down on the internet.

  • kirk

    Wm James explained this sense of time by a ‘specious present’ or 3 second interval that our time-bound self followed. This temporal window flows in the direction of increasing entropy. Outside of this specious present we use memory for both a look back at the past (against the arrow of time toward decreasing entropy) and a memory of the future (faster than increasing entropy is aging us). The look into the future or the guesses about the future hijack the use of memory to reconstruct ‘what must have happened’ and ‘what will happen next’. The extent to which I can make correct guesses (the inevitable shattering of a glass vase dropped to the floor) determines my ‘temporal fitness’.

    So it doesn’t matter *how* I guess what *really* and *accurately* happened in the past or what is about to happen next, it only matters that my guess about what happens next is more fit . – it only matters that I make better guesses than the guy next to me. This frees my 4th dimensional self from the chore of ‘carrying along my present self’ when I need to remember a useful (fit) version of the past or future.

  • Naked Bunny with a Whip

    they are just his opinion

    Everything is opinion until evidence is presented.

    Nobody said anything about scripture but you. Sean explained and supported his claims, rather than tossing off a vague, one-sentence assertion, yet you’re leveling your tedious sardonicism in his direction instead of Matti’s.

    The claim was made that consciousness and biology are incompatible with our current knowledge of physics. How so? Where is the “aether” for biology? Where is the “ultraviolet catastrophe” for consciousness?

  • Alan

    This is so interesting: Lloyd Rudy, cardiac surgery expert.

    Especially because, as he says near the end, his colleague surgeons have had similar experiences with patients. How can a confined biological brain account for this? What is consciousness that allows such veridical cases?

    As he says, “he was up there!” (above the operating table). “He described the scene (seeing?), things that there is no way he knew…what does that tell you…is that his soul up there?”

    The patient described the operating room accurately and then there is this “presence” staff felt in the second case he described. What was that?

  • Gene

    Mike @ 9: Of course you’re right that the classical worldline is a fuzzy path at the quantum level. But to my lights, the human notion of the ‘flow of time’ exists at a classical level and not the fundamnetal level, so I think Sean’s basic points still stand.

  • Jens

    Sean: “To be earnestly explicit: if you did manage to travel up and down your world line at will, you would indeed have whatever memories were appropriate to the moment you were inhabiting …”

    What is your evidence for this assertion?

  • Jens

    Sean: “…it requires the implicit introduction of a kind of “meta-time” that keeps track of when we visit the ordinary time with which we are familiar. That’s not how nature works…”

    How do you know?

    I’m not trying to coy. I just haven’t heard about experiments which have addressed this, one way or the other.

  • Jens

    It is amazing to me that we still (in my view) have absolutely no handle on what time actually is. Transforming it into a measure of entropy helps not at all. And saying that time didn’t exist before the Big Bang is even more nonsensical if one bases that view on the Big Bang occuring due to a quantum fluctuation. How does a fluctuation occur without time? At some point no Universe existed. At some point, one did.

    Kindly explain.

  • Count blis

    Any attempt to cut away anything but the present moment would fail, because you are free to expand the current state in any basis. If you apply the time evolution operator to the basis states, you obtain the future states, but by this argument, this then exists also in the present moment.

  • JimV

    RE: @12, out-of-body-experiences, operating tables:

    There is a semi-scientific study underway in several hospitals in the UK and USA:

    From what I read elsewhere on this study, it has been running for some time and no positive (confirmatory) cases have yet occurred. We should wait for a peer-reviewed report, of course, but my money says that it will be found once again, that the supernatural never occurs in reproducible, double-blind experiments, and that anecdotes aren’t data. After all, eye-witness accounts are often found to be inaccurate in criminal trials when people’s lives or liberties are at stake. Why trust them when they contradict all that is known in science?

    @16 – IANAP, but I think there is a distinction between time in our universe and time in some other universe from which a quantum fluctuation may have seeded ours. Time is relative within our universe, and General Relativity says that time stops (does not flow) within a black hole, so if our universe started at such a singularity, there is no physical way to get through it to experience time on the other side (prior). Nonsense? Well, but GPS devices don’t work accurately unless GR theory is used in their calculations, and there are many other experimental confirmations.

  • jpd

    “Biology and consciousness is something with which the text book physics cannot cope with.”

    What’s the basis of this claim?

    i looked for both terms in my physics text and they are not there

  • Alan


    Updates will be here: the main site for this scientific study.

    Of course the first case above by Lloyd Rudy wasn’t just anecdote but confirmed, it seems – he was on the spot during this and knew the sensory capabilities of his patient – around zero?
    And what does one do with the second case – an apparent healing? Dump the data?
    If the “anecdotes” keep coming like this, then eventually with this kind of subjective data you’re going to be raising your confidence levels. My money is the other way – I think Nature will surprise!
    Maybe the solution is somewhere in the physics – a la Feynman and the “qualitative content of equations”?

  • spyder

    Barring the presence of mental illness, i have noticed, now in my near dotage years, that my own memories of the long past have been reframed in a much more positive tone. Blessed be the elder status.

  • jbtul

    As an avid reader and a most interested layman I offer my first comment. If God ( or whoever bestows mathematical ability) had been most kind I would have been a Physicist/Cosmologist. Sadly that was not the case so I merely observe and often ponder the deep mysteries of the universe and our existence. Time/Gravity to the layman are still mostly unanswered questions from an explanation in a common language perspective. Einstein changed the way that we can consider space/time. Massive bodies warp space/time.
    Ok easy enough to follow .. but then how do you get from there to the proposed graviton ( search still continues for this force (sic) carrying particle ). Or is Geometry enough to carry the day however complex (point particle being a geometric object).

    Back to the subject at hand… just a thought who is the I that i speak with in my thoughts and dreams. Tough to answer.


  • Craig

    But the particles that make you up are constantly being replaced over time. How do you square that with the world line idea?

  • Jens

    #18: I have no problem with GR (aside from the fact that it is incompatible with QM) and as you say, time does stop at the event horizon of a black hole, but I don’t see what this has to do with the current discussion. As to your idea that our Universe was spawned from another Universe which already had Time, that in my view is like an argument for God, i.e. ad infinitum.

  • Peter Lynds

    Hi Jens,

    “It is amazing to me that we still (in my view) have absolutely no handle on what time actually is.”

    I think some people do. Indeed, I think Einstein (at least late in his life) was one of them.

    “How does a fluctuation occur without time?”

    I think that’s a perfectly valid question. If the so-called quantum vacuum before the big bang is said to not be in time, it can’t be said to have caused, or be causally connected to, the big bang. If it is, one is faced with the contradiction of an infinite past.

    Best wishes


  • Ray Gedaly

    To the folks talking out-of-body experiences … floating above the surgical table and looking down, etc …

    The problem I have is that I think I understand a little about light, optics, and how the eyes work; and it all obeys physical laws. But if your consciousness floats up out of your body and you look down and can “see” yourself, then how are you “seeing” this reflected light without physical eyes. And if your consciousness can see without physical eyes, then why do we need eyes in the first place?

  • Ray Gedaly

    Oh yea, floating above the surgical table and “hearing” the surgeon talking to the nurse presents the same problem. If your consciousness can sense these acoustic waves without ears, then what’s the point of physical sense organs … or physical laws? I wonder if when your consciousness continues to float upward beyond the atmosphere it can still hear sounds?

  • Ray Gedaly

    If you need glasses to see clearly, and your consciousness floats up out of your body and looks down at your physical self on the operating table, do you see yourself in or out of focus?

  • Ray Gedaly

    Sean, when I was first exposed to quantum theory decades ago, I considered the possibility that information projected into the past better explained such phenomena as non-locality (spooky action at a distance) and the paradox of Schrodinger’s Cat than the more outlandish (to me) Many Worlds (or multiverse) interpretation then being bantered about. After all, Many Worlds may seem reasoable for coin tosses, but ask me to write down a real number between zero and one, and we’ve just split off an infinity of new worlds. Furthermore, I conjectured that the Big Bang could have produced not just our universe moving forward in time, but an equivalent (antimatter?) universe moving backwards in time … analogous to pairs of virtual particles materializing from the vacuum. Of course this was mere speculation; no chalk was wasted attempting to take it beyond bong babble. And in fact I don’t think it could fit with your proposal that entropy produces an Arrow of Time. But what am I missing that seems to be obvious to more educated physicists which rules out information projected into the past as a plausible explanation of quantum phenomena?

  • wes

    I have heard that closed timelike worldlines are mathematically possible
    in GR. If so, and I could follow one, it seems I could – in principle –
    “visit” my own past and meet my past self. (No doubt the actual
    world has no such worldlines, but this is perhaps not known for

  • Nullius in Verba

    Why stick to just your own world line?

  • Anchor

    “…(some) people don’t think about the flow of time in the right way, and this leads to a couple of unfortunate consequences…”

    This reminds me of another common misapprehension. Many if not most people have the impression that when we gaze outward into intergalactic space, we are looking at what we call ‘the universe’, potentially, at the whole of everything. It’s easy to forget that we can never obtain more than a cross-sectional and fleeting ‘snapshot’ of all that’s ever transpired in the universe – that is, just the info that’s just arrived at our position that has traveled on the anterior light-cone. Even our view of the potentially ‘observable universe’ is grossly restricted.

    It’s also worth noting that the conventional western way of thinking about the direction we are heading in time, FACING the future ahead of us and turned away from the past behind us, doesn’t reflect our situation very accurately: at any given point in the present, we don’t see the future before us but only have a view (a ‘memory’) of the past. Some isolated cultures have cultivated this ‘past-facing’ tradition in their impression of time, with the unknown future out of sight behind them. Their ‘backing into the future’ way of looking at it is, to my mind, superior to the one western societies assume. But the western way is well-entrenched: they are ‘forward-looking’ cultures, I suspect, because of the constant preoccupation and feedback stimulus of looking ahead down the road, navigating toward a destination, negotiating to reach a goal, etc.

  • Anchor

    Craig #23 says, “But the particles that make you up are constantly being replaced over time. How do you square that with the world line idea?”

    Simple: ‘you’ aren’t the particles but the dynamic interactions between them. Your mind is a ‘process’ generated by a brain consisting of a complex system of billions of interconnected neurons and your neurons and the rest of the cellular components in your body down to the molecular and atomic level are all processes in dynamic flux, but which have evolved the ability to maintain a relatively stable equilibrium configuration over your lifespan. The pattern of interactions persisting in time is ‘you’ and that’s the ‘worldline’ referred to, but if you examined that ‘worldline’ at increasing magnification, you would see that it isn’t a single line at all, but rather a gnarly hideously-frayed spaghetti-like cable with particles (other ‘lines’) leading to and from it, coming and going all the time, a cable always threatening to unravel while the biochemical pattern works to maintain its integrity with replacements and repair mechanisms. The ‘self’ is not as adamantine and pure as some of us would like to suppose.

  • Vincent

    My consciousness freely travels up and down my world line, but sadly it only carries the memories appropriate to the moment it inhabits.

    Do you have any empirical evidence for this, or is this merely a rationalist speculation. Come to think of it, can you even produce any empirical evidence that you are conscious. Suppose you are facing a panel of extremely talented but very skeptical scientists equipped with the best experimental equipment out there. Can you convince them you have consciousness.

  • CTC

    “There is a four-dimensional universe that includes all of our world line, from birth to death, once and for all; and each moment along that world line defines an instantaneous person with the perception that they are growing older, advancing through time.”


    There is a 4D solution at any instant that is consistent with all the information we possess. As that information is updated, the solution changes. It is the path through this solution space that is of interest.

  • Dan

    “Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing. That includes our own personal path through it, which defines our world line.”

    Let’s see what can know about the world by looking at it. Well we can know the past existed, since the information we directly observe comes to us, dependent on distance and via a constant speed of light. Since this information is being consistently updated, we can define the time that we receive it to be the present. We can only infer that the future will exist, since our experience and our knowledge of the laws of physics suggests that it will. Here’s the kicker. The future is not guaranteed to exist. Something unknown could happen to the entire universe – say a collision in the multiverse or “whatever” – that destroys our entire universe such that time ceases to exist. Bearing any unlikely such event, does the future exist and is it already determined? If it were determined, why doesn’t classical physics apply to all physical objects regardless of scale?

    Perhaps the future only exists in a superposition of possible futures and the past only exists in the information that it is encoded in the EM radiation that travels through space because the *map is not the territory*. With all due respect to you, Minkowski, and say Max Tegmark, the constructions we devise to model reality through the rationalization of physical phenomena with mathematics is only an *approximation* of reality, it is not reality, itself. Just as there are no known true spherical objects in the universe (only ones that are approximately so), there is no evidence that the past, present, and future exist as a whole in some four dimensional spacetime. This “block universe” is just one of our constructions to help us model physical reality. The “succession of present moments” is another concept of the universe and the nature of time that works as a map of reality and IMO fits better with the indeterminate aspects of QM observations.

    We all have direct evidence of the present moment, it’s what we know by looking at the world. The present moment flows at different rates dependent on our motions and positions wrt the proximity to massive objects in complete agreement with the theories of relativity, without the need for a block universe. We have no evidence that the past and future actually exist in parallel to the present moment, only evidence that such rationalizations have been useful, especially in modeling the physics of relativity.

    We do have evidence that the universe is expanding. What is the universe expanding into? It’s expanding into the future. Just as new space is created by the expansion, new time is created as well. Doesn’t this serve as a quite satisfactory explanation of what creates the succession of present moments?

  • Gene

    Dan: I don’t see how we can define a ‘moment’ let alone a present one, in any sense that is independent of the observer like Einstein said. It is true that the map is not the territory- but Minkowski’s map was better than Newton’s. To the extent the universe is truly evolving like some species, it’s certainly doing so not in our ‘present’ but some other frame of reference that has nothing to do with the human experience.

  • Alan


    Of course your questions are great. How about an extended mind? There was a conference in 2009 to do with this. Can this explain somehow? If people are seeing during NDEs can optics get bypassed? There are very good cases. Is there a subtle body doing the seeing?

    Lloyd Rudy (from above) – “he was up there!”

    Answers on one sheet of A4 only please.

  • Laurie Paul

    It’s worth distinguishing the world line of a person from a person’s passing through time and experiencing the evolving, unfolding universe (or at least having the phenomenology as of passing through time and the phenomenology as of the universe unfolding)

  • MAC

    What’s interesting to me is not that we have these fascinating discussions about the nature of time and how gravity works, but that we think our brains are evolved enough to understand the nature of the universe at all. We’re like goldfish in a bowl who think we know how the world works because we’ve seen our owner use a can opener. We see the universe from our own extremely limited perspective, and all we can do is form theories about it. It seems to me the height of hubris to imagine that, from our dusty little perch partway out one of our galaxy’s spiral arms, we can proclaim we have knowledge of how the whole, intricate mechanism is built. In fact, I’m sure we have much more to discover than we have ever learned. Invoking terms like “dark energy” and “dark matter” to try to explain things we don’t understand is proof enough. Not that we should stop striving to understand the nature of a universe in which we play a miniscule part, but doing it with a modicum of humility might more accurately reflect our place in it.

  • Dan

    Gene #37,
    OK, let’s assume that the block universe exists. If A is a random event in the set of all events that creates the block, we can trace back in time to the singularity to say that A occurred when the universe is XX.X billion years old. If we hold the time constant, and vary the three spatial dimensions, we trace out the surface of a hypersphere, with a radius of XX.X billion years. This surface intersects the world line of every particle in the universe, even though the local time of each particle is determined by local conditions and the laws of GR (all physics is local), this hypersurface represents the “present” state of the universe. As this hypersurface expands, it creates new space and new time. We can not prove that addition hypersurfaces, interior and exterior to the “present” hypersurface, exists in actuality (there is no evidence for their existence in reality) rather than in the Platonic sense. We do not need to posit their existence to adequately describe the universe. Therefore by Occam’s razor, it is simpler to assume that they do not exist.
    If you would like to learn more about this model and its surprising consequences, I have written a essay for FXQI’s essay contest that can be found here:

  • Terry Bollinger

    Sean Carroll noted that “… Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing … This seemingly conflicts with our intuitive idea that we exist at a moment … Of course there is no real conflict — just two different ways of looking at the same thing.”

    A direct question to Sean Carroll if you are still reading this thread:

    Both special relativity and quantum theory include strongly deterministic threads, e.g., the reciprocity of “now” in SR frames and the advanced EM wave solutions of Feynman’s thesis.

    If you have a moment, would you share your views on how to reconcile the idea of a real, non-trivial “now” with those parts of physics that seem to imply predetermined worldlines?

  • Terry Bollinger

    Ouch, I just looked up a Perimeter Institute paper on alternatives to the block universe.

    As someone with a long-term fascination about methods and heuristics for determining when complex information is worth analyzing closely, and conversely when it should be discarded quickly, that paper reminded me of simple point: If an argument is based on a huge number of ideas that individually sound impressive, but which on average have never been well verified, the real chances of the argument being valid are… well… vanishingly close to zero. (This is also a simple way of explaining why I am am incapable of being impressed with the enormous work and detailed reasoning that has gone into string theory over the past several decades.)

    So, if I may add a qualifier to my last question: Sean Carroll, would you happen to have any simple, well-focused, and perhaps even experimentally meaningful approaches to reconciling your idea of a real “now” with the idea of a block universe? My emphasis here is on “simple.” If an issue this fundamental to how existence operates cannot be explained by using only a very small number of conceptual building blocks, it’s time to go back to the whiteboard and start over again.

  • Tom Clark

    Matt in 6:

    “Sean, I wonder what the implications of this on discussions of free will/decision making are? If the four-dimensional universe includes our world line are we simply deterministic machines with our consciousnesses providing an “illusion” of the ability to make decision when from the full-dimensional view it is simply following its worldline?”

    Our ability to make decisions and the decisions themselves are just as real as anything else in the block universe, not illusory. But of course the decisions are all there in spacetime, it’s just we experience them serially.

    We wouldn’t gain anything in terms of control were we somehow able to extricate ourselves from our worldlines, since exerting control is a particular sort of pattern in the worldline itself, namely when getting what we want follows from what we do,

  • Pontus Gagge

    As Wes comments, GR does not rule out the possibility that points on your worldline may have a spacelike separation, sharing a single ‘now’ according to some observers. At one of the points you would have memories of being at the other. Are those memories appropriate to the inhabited moment? I’d say so: most would call the person with the extra memories a time traveller…

    Whether nature or intelligence can fiddle with the spacetime curvature enough to allow this is of course still unknown. Does anyone have a massive near-infinite cylinder rotating at relativistic velocities handy?

  • Gene

    Dan: A most interesting theory, and good luck in the contest. Still, I don’t understand its advantages. It is true that we can construct a hypersurface and watch it expand ‘as if’ it were like a balloon in a room that fills up with air as we look at with our watches. The problem is there is no unique way to do it. In a sense, its not that I maintain all hyperspheres exist, its that I don’t think any of them do in any fundamental sense. Even the ADM formalism has its limitations, and I doubt that a future theory of quantum gravity will fix the situation.

  • Dan

    Gene #46,
    Thanks, for the feedback. Unfortunately, the contest is already over and I didn’t receive a prize. I felt like a winner nonetheless, since I managed to make it thru to the judging, which was no easy task. My essay was a little off the topic of the contest and it was a lot to cover given the limitations of the essay length.

    I’m presently working on a more rigorous version to submit to a respectable journal. I believe that the Concordance Model has some definite fatal flaws that can be amended by a new model without a wholesale revision of our fundamental laws. I will need a little luck, a lot more hard work, and some ironclad proofs, if I want to have any effect on the status quo.

  • R

    Re: Sean’s Tweet.

    Sounds like bad poetry written by a bored 15 year old after flipping through A Brief History of Time.

  • Terry Bollinger

    Time Flow in Stephen King’s “The Langoliers”

    In the 1995 a sort-of science fiction novel by Stephen King was made into a movie called The Langoliers. As you would expect for a Stephen King book, the set up was primarily a horror-drama with the usual selection of ornery and outright demented people, as one of King’s scary-smart young girls with telepathic powers that he seems to favor for many of his stories.

    Now what was interesting about this purely-for-entertainment work was that King put some non-trivial effort into developing a theory of the flow of time, one in which he then places his characters to stress them out and make them do the sorts of things you expect in Stephen King novels. In a nutshell, King’s theory of time goes roughly like this: The “now” in which the universe exists moves not through some abstract definition of time, but through a quite literal empty space in which “now” is more like a spaceship in motion than a system in evolution. Change takes place within the spaceship, but in a way that is independent to some degree of motion through the literally space-like time dimension.

    That’s sort of interesting, but what really captured my attention about King’s model of time flow was that he placed incomplete versions of the universe both in front of and behind the moving location that defined what “now” is. King’s leading-edge incomplete universe was bright, fresh, and full of the promise of change, while his lagging universe was stale, fading, and clearly head towards a bad end. This being a Stephen King movie, he of course placed the crew of an airplane first in the lagging universe, where they would await… what?

    The Langoliers, of course. The Langoliers might best be described as the ideal nightmares of someone who is frightened horribly by large drill bits and empty spaces. Their purpose? Recycling of course! The Langoliers are the überscary (well, by 1990s CGI standards they were) chompers and eaters of the past, leaving it in place only long enough to (I guess) reinforce the real “now” further up the chain. Once the Langoliers recycle the ragged trailing edge of reality, however, it is gone forever — all that is left is a gaping, empty vacuum from which “now” is hurrying away at breakneck speed.

    And therein lies what I like most about Stephen King’s version of the flow of time: He simply creates an intuitive model that deviates in the most fascinating and heretical ways from “standard” spacetime physics. Why shouldn’t he? His only purpose was entertainment.

    Yet it’s a fascinating model about which to ask some of the same questions that perplex far more learned models of the flow of time. For example, can you travel in the past in King’s version of spacetime? Yes and no, since you can go a short ways back, but no further, and even that distance is ragged and unpredictable. Can you visit the dinosaurs? Definitely not, since they only exist as memories stored within the Present. The past in King’s universe is nothing more than the lonely and empty road left after the first, last, and only bus there will ever be passes over it. Only the memory of dinosaurs lives on in the images and information within the bus of now, just as the bones we find in our actual universe are nothing more than memories of a past that is no longer accessible as a existing place.

    In King’s universe, can you change the present by altering the brief past that lags behind it? No, because that past is only a subset whose impact on the present fades quickly as you move farther away from now. Can space be curved? Well, yes, but probably only in the sense that the bus that represents all of now can be curved or bent, since only that part contains space as we would perceive and measure it with instruments. Are there infinite worlds? No. Are there infinitely long world lines, what is often called the “block” (as in “solid like a block from start to finish”) universe? No, only short dangling world lines that extend a ways into both the future and the past. Can the waves of quantum mechanics grow without limit? No, because there is no room for them to grow; they are captured and must resonate within the bound of the Bus of Now.

    If Now can be bent, might the wormhole solutions of general relativity allow travel back in time within King’s flow of time? Probably… but you would only find empty space when you got there, since time in King’s universe is an infinite sequence of empty spaces through which the occupying mass of Now moves. Can Sean Carroll’s vision of complementary universes moving in opposite direction in time be accommodated? Sure, but in an almost trivial way: Both universes change in the same way, they just literally travel in opposite directions down the long road of empty spaces. In King’s universe time is not so much space like as it is two separate variables, one for distance along the path of empty spaces (t), and another, call it tau (that’s not standard incidentally) for “change like a clock hand moving.” That kind of choice is a programming or data representation issue, and so is not necessarily fundamental to physics if precise equivalence is maintained. Cramming multiple meanings into a single variable is what computer types call overloading. It can be a handy technique for expressing issues compactly, but some willingness to recognize provably equivalent alternatives can also be helpful for finding cleaner or more understandable ways of representing ideas.

    Now here’s a more hypothetical question that I don’t think King had in mind, but I don’t want to be too quick to judge him on that either: Is there a plausible physical interpretation for why a universe would have leading and lagging edges to it, that is, partial versions of itself that cannot exist or change in isolation, but which somehow assist the main Now of the universe to function and work?

    This more than anything else struck me about King’s flow of time, because I think you can make a surprisingly deep argument that whatever the universe is, it does indeed require roots that extend into both the past and the future. The reason is that at a profound level, much of physics seems to operate on a principle sometimes called minimization of action, action being an odd product that has the same units as Planck’s constant. One way to get a tiny bit of a graphical feel for how this minimization of action works is to imagine particles as rubber bands that are stretched between points in the distant past and other points in the distant future. Near the middle of these bands is the Present, where forces and potential energies cause the rubber bands to stretch and move sideways in interesting ways. The tension of the rubber bands can then cause them to interact and interplay with each other can, producing in many cases results that seem strangely prescient, almost as if the particles could see in limited ways into the future — which of course with those rubber bands extending into both the future and the past is sort of what happens.

    I should mention that this whole peculiar issue of particles being smarter than they have any real right to be, at least when it comes to responding to energy potentials, is one of the reasons why some physicists like to extend those rubber bands all the way from the start of the universe to the end. At that point they become what are called world lines, and the resulting universe of infinitely long rubber bands becomes the “predetermined” block universe I mentioned earlier.

    King inadvertently (?) proposes a scheme that is far more appealing to computer folks, which is to make the worldlines virtual. That is, to help the Present respond meaningfully to its array of diverse forces and potentials, King’s worldlines appear to extend as far as they need to into both the future and the past… but no farther. Once their action minimization duties are completed, the rubber bands in effect snap back towards the fabric of Now and become available for building new virtual world lines to explore new possibilities and potentials.

    And that gives another answer to a question someone might want to ask about King’s flow of time:
    Why is quantum computing so hard? Well, in King’s universe that’s easy to answer: The worlds being used to calculate more powerfully are virtual only, so they can only exist for as long as the lagging edges of the recent past can be coaxed into hanging around. Eventually, inevitably, and without exception, they will snap back into the fabric of Now upon which their existence depends. And when that happens, any quantum computing will be over. The overall result in King’s universe is that quantum computing becomes a matter of balancing cantankerous noodles on their ends. It can be done, for a while, but in the end it will always collapse back into the ordinary physics of Now.

    And last but not least, there’s an ugly beast — the Langolier. What would it be in such a universe? Would a real-physics analog to the destruction of packets of virtual worldlines a thing of beauty, or would it be as ugly, scary, nasty, and poorly understood as King’s Langoliers?

    I vote for ugly, because the beast already has a name in real physics, and it’s not much liked in many circles: wave collapse. That is, to make virtual worldlines real and a part of actual physics, you unavoidably must also make their destructors just as real. Many physicists do not like even the idea of operators of this type, which have never been well quantified and which have annoying properties such as changing or collapsing entire regions of spacetime. One answer to that is to avoid such operators. Another is to get creative and look for some entirely new ways to do math.

    And the end: Why have I bothered with all of this?

    Because I just wanted to point out that in physics, interesting and even entertaining ideas can lead to conclusions that are no more absurd than wondering if the ability to think and remember might require the redefinition of all of physics.[1] Three cheers then for Stephen King, who took the time to explore some wild ideas for how time works for the purpose of entertaining, yet surprisingly wound up with a collection of ideas that both resonate and conflict with modern physics thinking in a number of truly fascinating ways.

    [1] I know nothing of all that argument, but I would make this observation again: If Now is just a set of memories, as in the King universe I just described above, then any measurement of a time difference unavoidably depends in part on memory and intelligence. Why? Because the past cannot be observed directly, even if you believe it still exists. Thus you can at best only measure time as it exists between the real instruments looking at Now, and the intelligent memory constructs that we call Then. Forget about observers being needed in quantum mechanics: Every invocation of time or t in classical physics calls for an intelligent observer, and does so just as forcefully as do the precepts of quantum mechanics.

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  • Phillip Helbig

    A few comments. First, Pirsig notes in the foreword (or afterword, depending on the edition, though the earliest editions had neither) to Zen and the Art of Motorcycle Maintenance that most people think of their journey through time like they think of walking down the street: they are moving and they are looking toward the future. Pirsig says that the ancient Greek concept of time is much more accurate: we are standing still and time moves past us (we can’t influence its speed) and we are looking toward the past. This is a much, much better metaphor.

    I recommend of course The End of Eternity by Asimov, which is actually a spoof of time-travel stories in that it uses all the cliches and really goes over the top—but ends up being one of the best time-travel stories of all time. (In this respect, it is similar to Jethro Tull’s truly excellent Thick as a Brick which is a spoof on 1970s concept albums but actually turns out to be better, by their own criteria, than the things it spoofs.)

    There is a very interesting book which reviews time travel in science fiction and in science by someone who knows about both: Recommended almost as highly as Zen… and …Brick.

  • David

    Dear Mac #40,

    That people of science are not humble is among the most laughable of misconceptions. They are indeed the most humble simply by virtue of the fact that they – like no other group of human beings – agree that pictures about the way the universe is must be grounded in the most stringent experimental constraints and logical consistency. It is these constraints then, and not the feeble mechanisms of the mind, that legitimize the method and reflect the humility of its practicioners. Our planet is still a haven for explanations about our origins involving six days, and postmortem scenarios invoking flush gardens soaked in optical light. Scientists are hard to come by. Please go bother someone else.

  • Terry Bollinger

    Hi #52 Phillip Helbig,

    Great references, thanks! I think I may have read The End of Eternity once, but if so it was so long ago that I recall nothing at all about it. In a different series I do like the scene of Marvin pacing on that moon literally until the end of eternity in the HHGTTG series, when his time-traveling friends finally bother to pick him up. Somehow it feels like I’ve had friends like that… or maybe it’s just that I’ve been a friend like that? Like the time my brother called me after arriving at the airport, while I was a hundred miles away picking up my son? A bit of time travel would have come in sooo handy that time… :)

  • Jens

    My simple questions meant for Sean have not been addressed. Is it time to threaten? Perhaps threaten viral? I don’t think so.

    I just would like a reply to my very simple questions.

  • CuriousTech

    @CTC (35.) Awesome. Short and sweet.

    @Alan (38.) Having first-hand NDE experience I’ll explain something in short that probably nobody has. Your body floats up… no. We know that the subconscious does tons of work 24/7 in the background. Processing information, accessing memories, imagining what’s out of view, and so on. When the experience occurs synapses misfire due to lack of oxygen, blood flow or whatever, and you’re practically wired into it on a conscious level. It’s overwhelming. You can’t tell what’s a memory, real or a dream. “Vivid dreaming” comes to mind but a jumble of many at the same time even after you wake up. People halucinate (see/hear) ghosts from their past talking to them, spirits, the operating room, heaven, all kinds of stories. Keep in mind that all accounts are after the fact and the effects can take days, weeks, months to fade if ever and the stories are after the fact. So basically all the stories can be explained as situations constructed from what already existed before including memories and beliefs, and a powerful subconcious. Most just aren’t prepared for this mental state which can be quite scary if they don’t understand it.

  • Mike Cope

    I find it helps to think of time as a concept we use to understand change, rather than a thing, dimension, or some other entity.

  • Guest

    1. Introduction

    The problem of consciousness today occupies one of the most important points to be clarified by philosophy and science. By its nature, there is no consensus among both philosophical, as in science, with respect to its explanation.

    The main philosophical today are materialistic, where consciousness is explained as a result of neural processes (is the basis for scientific research on consciousness) or as an illusion generated by brain processes (deniers), the dualistic, which considers consciousness independently of matter and which postulates that consciousness is a fundamental entity of the universe (as fundamental as space-time and matter) and idealist, who believes consciousness as the basis of all physical existence.

    2. Theories of Relativity

    The Special Theory of Relativity “break”s the objective flow of time. According Belizário (2001) “The idea is that the” flow “of time is different for different observers.” With that, according to Einstein himself, the distinction between past, present and future is a persistent illusion, a illusion, that comes from our perception, which curiously depends on our consciousness.

    According Damour (2010) “General relativity has opened the door to an even deeper twist of the ordinary concept of time. However, the most popular treatments of science have a tendency, when speaking of General Relativity (GR), and especially when describing relativistic cosmological models (inflation, Big Bang), to use language that suggests that the GR reintroduces the notion of temporal flow, which had been abolished in Special Relativity.

    Far from it. The GR space-time is as timeless as the Especial Relativity. The Big Bang should not be referred as the birth of the universe and its creation “ex nihilo” but as one of the possible limits of a strongly deformed (and timeless ) block of space-time. ”

    3. General Theory of Relativity and the Philosophical Currents of Awareness

    The “materialists” consider consciousness as a result of neural processes. The hypothesis of consciousness as a result of one or more brain processes has as implicit premise (which is necessary), the “physically” existence of the flow of time.

    In a “Newtonian world”, where space and time are absolute, and the flow of time is a physical reality, the explanation of consciousness as a result of a process, adheres to this reality, but if the flow of time doesen’t exist (physically), so we have no processes and therefore we have no consciousness being “produced”; emerging from these processes.

    From the above we can conclude that, if we accept the premise that the GR is correct about the lack of flow of time, then all philosophical hypothesize that consciousness as a result of one or more neural processes should be compulsorily incorrect because it is premised on a reality incompatible with the reality described by TRG.

    In this case we have only two hypotheses to explain the phenomenon of consciousness. The idelasita and dualistic.

    The dualistic hypothese of Chalemrs consider consciousness as a basic entity, wich does not interfere in the matter.

    In this case, we can think of consciousness and its relation to space-time, similar to a game, and the set HD + player. Just as the game exists as a whole, inside the HD, and its sequencing is the result of the interaction of the reader with HD, we can consider that the spacetime contains the whole of our history (and in a sense, our immortality), and the illusion of the flow of time refers to the stream of consciousness “through” space-time, this flow allows us to experience our story sequentially.

    This modified Chalmers Hypotheses maintains compatibility with the GR: how we have to give up totally, our free will, compatibility with GR is assured since all our past, present and future are already determined.

    Sean Carroll, in a article “The Flow of Time” says that “Modern physics suggests that we can look at the entire history of the universe as a single four-dimensional thing. That includes our own personal path through it, which defines our world line. This seemingly conflicts with our intuitive idea that we exist at a moment, and move through time. Of course there is no real conflict — just two different ways of looking at the same thing. There is a four-dimensional universe that includes all of our world line, from birth to death, once and for all; and each moment along that world line defines an instantaneous person with the perception that they are growing older, advancing through time.”

    Notice that i don’t deny Seans claim (in what I am proposing), that every time we are aware of the “present moment” plus”moments earlier” The problem is: what makes us walk through these moments, so we constantly forward in time.

    His argument doesn’t addresses this question, and brings another problem: as he considers the existence of instantaneous awareness, consciousness – through a “materialist” explanation – is no longer the result of a brain process, and become a state.Because even with nothing happening (impulses coming and going), consciousness is still there. Even with frozen time!

    But if the illusion of time flow is the result of the flow of consciousness through a “frozen” space-time, them we can satisfactory explain the instantaneous awareness, and our movement forward in time.


    Whereas, as a premise, that the GR is correct about the physics lack of flow of time, then the whole “materialist” explanation of consciousness is necessarily wrong, due to incompatibility between the physical reality in which it’s need to be in order to be correct, and the reality described by TRG.

    The price to pay to have an dualistic explanation for consciousness is the total absence of free will.


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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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