The Moving Light of Time

“In a word, the Future is, of all things, the thing least like eternity. It is the most completely temporal part of time — for the Past is frozen and no longer flows, and the Present is all lit up with eternal rays.”

—C.S. Lewis, The Screwtape Letters

In the above excerpt, C.S. Lewis expresses a very common view of time: that it “flows” from future to present to past like a filmstrip passing through a movie projector, with each frame of the film momentarily becoming the present as it is illuminated by the projector’s light. Christian apologists such as William Lane Craig use this view of time to argue that the universe must have had a beginning, and hence a creator, because a universe with an infinite past could never have traversed the infinite number of past moments to reach the present moment (the kalam cosmological argument).

This is the most widely held view of time, for obvious reasons. It seems intuitively true, in accord with our experience; we do observe time apparently flowing, with one moment following the next in succession. In the present, we can act and make choices that seemingly affect the future, while the past is frozen and beyond our ability to alter.

This view is also, indisputably, false.

This may seem a strange thing to say. How can time not flow? We feel it flowing, don’t we? If time does not flow, then what does it do, and why don’t we notice? If time does not flow, how can there be such a thing as cause and effect? How can there be such things as choice or free will?

These are all valid and important questions. Nevertheless, the widespread perception that time flows is – must be – an illusion. Consider the following points.

First: If the “moving light” of the present, in fact, moves, how can this be? Ex hypothesi, that “moving light” is outside time, because its motion is what defines what time is. And yet, how can there be motion without time? The very idea of motion implies a time at which an object is in one place and a different time in which it is in another. If there is no time, there can be no motion. For this light to move at all, there must be some “meta-time” within which it moves. But if that is the case, then does this meta-time dimension have its own moving light determining the present? And so on, in an infinite regression. The kalam argument, invented to avoid the supposed impossibility of an infinite, ends up running right back into that same impossibility.

But in addition to committing its user to an infinite number of temporal dimensions, this view of time has another problem: the moving light is utterly superfluous. We do not need it. To see this, imagine a thought experiment: What would happen if the spotlight of the present suddenly started moving in reverse? Would we notice anything?

As a moment’s thought should make clear, the answer is no. If there is a moving light, then time must be divided into some number of discrete slices – like the frames in a filmstrip – each one representing a snapshot of the universe at a single moment. But the people – more accurately, the people-slices – inhabiting each moment only have the memories leading up to that moment, regardless of whether it is “replayed”.

Consider three frames in the filmstrip of my life. In moment A, I’m graduating high school; in moment B, I’m starting college; and in moment C, I’m putting up the first post on this blog. If the spotlight of time started moving backward, so that these moments happened in “reverse” order, would I notice? Of course not. The spotlight of the present cannot change the content of these moments; even if it moves backward to reilluminate a particular moment, that moment will reoccur exactly as it did the first time. And in each of those three moments, I only have the memories of the events preceding it. Regardless of which way the spotlight of the present is “actually” moving, the sequence of my memories imposes a consistent order on my experiences, making it seem as if time is flowing in the “right” direction. And this works exactly the same way if there is no such light at all. The spotlight of the present is an unnecessary explanation, conferring no additional ability to explain our seeming perception of time’s flow. Thus, by Occam’s razor, it should be eliminated.

But all this is not just a matter of armchair philosophy. It is also a matter of empirical evidence. And in this regard, the evidence strongly confirms the arguments I have put forward. Physicists have known for over a century that the commonly held view of time is incorrect.

For example, if there was a spotlight of time, highlighting moments in succession so that each one briefly becomes the “now”, this implies that there is one true present in which everyone shares. All observers should agree on what is happening “now”. But this is not the way the universe actually works.

Consider a classic example. Imagine a moving train car, with a light source in the exact center. At a predetermined moment, the light source switches on and fires two photons, one toward a detector on the front wall of the car, one toward a detector on the back wall. Which detector will be triggered first?

An observer on the train, moving along with its motion, will observe both detectors trigger simultaneously. After all, the emitter was in the exact center of the car, so the two photons have to travel the same distance to their respective detectors. This is undoubtedly a correct answer.

But an observer on the platform, watching the train pass by, will observe something different. To that observer, the back wall of the train was moving toward the photon, while the front wall was moving away from it. The difference in distance is miniscule, but it exists; so the back detector should trigger first. This, too, seems to be a correct answer.

Which observer is right? As Albert Einstein first demonstrated, the answer is that they both are. Strange as it seems, there is no one absolute answer to this question. Rather, simultaneity is relative: it depends on the perspective of the observer. Observers who are in motion relative to each other will disagree on which events are simultaneous – in other words, they will disagree on what is happening “now” – and there is no way to say that one is right and the other is wrong.

And this is not a unique, contrived case, applying only to this one example. On the contrary, this is the basis of Einstein’s theory of special relativity. Observers in relative motion will always disagree on which events are taking place in the present. At ordinary time and distance scales, this effect is so small as to be almost undetectable, but at very high velocities or over astronomical distances, it can become very significant. Physicist Brian Greene, in his book The Fabric of the Cosmos, gives a startling example: an observer 10 billion light-years away from you, who moved in a direction away from you at a mere 10 miles per hour, would have a “now” consisting of events 150 years in your past. And if this observer moved toward you at the same speed, his “now” would consist of events 150 years in your future.

This shows that moments cannot be partitioned into “past”, which have already happened and cannot be changed, and “future”, which have yet to be decided. These terms do not correspond to any fundamental attribute of reality. In fact, one observer’s past may be another observer’s future, and vice versa. Even more importantly, there is no single, universal present, no one moment somehow highlighted in a way that differentiates it from all other moments. Quite the contrary, there are only the moments themselves – each one existing eternally, each one unchangeable, and each one just as real as all the rest. We do not experience this because, as already explained, the causal order of our memories produces an illusion – a convincing illusion, but an illusion nevertheless – of time’s mutability and flow.

This realization demolishes the kalam argument. The supposed impossibility of traversing an infinite number of past moments to reach the present simply evaporates, because nothing is traversing anything – there is no spotlight moving in sequence from one moment to the next. The term “now”, rather than some special metaphysical significance, becomes, like “here”, a term of purely indexical significance. “Here” is wherever I am when I invoke that word; similarly, “now” is whenever I am when I invoke that word. You do not need to traverse anything to reach “here”, and nor do you need to traverse anything to reach “now”.

The last important question is what place there is for free will in such a scheme. The apparent fixity of the future would seem to deny the possibility of choice, but this ceases to be a problem when we realize that free will does not require the ability to have done otherwise. We choose in accordance with our natures, and our natures are shaped in turn by those choices, regardless of whether that process takes place in a single, special “now” or in one of an eternal succession of moments.

Though the commonly held view of time is wrong, this does not deprive us of anything important. We should always bear in mind that unaided human perceptions are not necessarily, and in fact not usually, a reliable guide to the true nature of ultimate reality. But using science and reason, we can surpass our limitations and arrive at a more fundamental understanding of the orderly laws by which the cosmos operates.

About Adam Lee

Adam Lee is an atheist writer and speaker living in New York City. His new novel, Broken Ring, is available in paperback and e-book. Read his full bio, or follow him on Twitter.

  • ex machina

    I always disagreed with the Kalam Cosmological argument. Why would does infinity negate the existence of now, or the ability to move progressively along it. A line on a graph (x=2y, for example) goes on forever into infinity, but that fact in no way prevents me from moving from one point to the next. Am I missing something Math/Philosophy experts?

  • ex machina

    I always disagreed with the Kalam Cosmological argument. Why would does infinity negate the existence of now, or the ability to move progressively along it. A line on a graph (x=2y, for example) goes on forever into infinity, but that fact in no way prevents me from moving from one point to the next. Am I missing something Math/Philosophy experts?

  • http://stupac2.blogspot.com Stuart Coleman

    There’s one problem with your physics, you forget about entropy. Since entropy must always increase, we know that there is an arrow of time that points to the future. If time started to run backward, we’d either have to say that entropy always decreases or we’d know that time is running backwards.

    At least, that’s how I understand it. Entropy is complicated and so is time, so I could be wrong.

  • andrea

    I agree with Stuart. Time is, to me, simply the action/reaction of events. it always seems that relativity forgets that things are indeed relative. Time still occurs, it’s just the observer’s observation of events that is relative.

  • andrea

    I agree with Stuart. Time is, to me, simply the action/reaction of events. it always seems that relativity forgets that things are indeed relative. Time still occurs, it’s just the observer’s observation of events that is relative.

  • ex machina

    Hrrrmmm. . . . I’m not sure if that impacts my argument or not. Let me reboot:

    Imagine a train on tracks that are infinite. Forever one way, forever the other. . .
    The train moves along the tracks, and, of course, never reaches “the end” of the tracks, or makes any meaningful progress relative to the tracks. But it will make progress relative to whatever finite objects it happens to pass by. Lets say it goes through your hometown, you could hop on while it was nearing one point in the city and get of as it neared the next. You and the train would move regularly through your town, but the tracks would still be infinite.

    Kalam seems to think that infinity would not allow you to reach “now”, or (sticking with the analogy) allow the train to ever reach your town. But while the track might be infinitely long, it does not follow that the distance between the train and another finite point of reference (your town) is also infinite.

    Also, I don’t really see how the behavior of entropy as time hypothetically moves backwards is important. We’re willing to say that entropy always increases, so why aren’t we willing to say that time always moves forward?

    So I guess I’m trying to say that you can have your cake and eat it too. The universe could be infinitely old and time always moves “forward.”

  • Vince

    Ex Machina is definitely onto something.

    The argument that an infinite past being impossible is false. The example of the film reel is actually pretty good except for one main difference. A film strip runs at 24 frames per second. Life, or time, runs at an infinite “frames” per second. Well, infinite frames per… anything might be better.

    There’s an infinite amount of time slices between between every key that I press to write my sentence, yet my sentence can still be completed.

  • Vince

    Ex Machina is definitely onto something.

    The argument that an infinite past being impossible is false. The example of the film reel is actually pretty good except for one main difference. A film strip runs at 24 frames per second. Life, or time, runs at an infinite “frames” per second. Well, infinite frames per… anything might be better.

    There’s an infinite amount of time slices between between every key that I press to write my sentence, yet my sentence can still be completed.

  • http://www.patmuchmore.com p4limpsest

    Exactly. It’s analgous to the old Zeno motion paradox. Zeno argued that Achilles could never catch the tortoise in a race, because before he could reach the tortoise, he would have to travel halfway to the tortoise. The distance is now half as long, but before he can cover the shorter distance, he has to travel half of it. And so on. In fact, before he can even reach the first halfway mark, he has to travel half of that distance, etc. Therefore, motion is impossible. QED

    Of course it just a fun mental game. I think that Hofstadter deals with this a lot in Godel, Escher, Bach–with arguments that could probably apply equally well to the Kalam version. Making it more cosmic does not make it more viable.

  • http://www.patmuchmore.com p4limpsest

    Exactly. It’s analgous to the old Zeno motion paradox. Zeno argued that Achilles could never catch the tortoise in a race, because before he could reach the tortoise, he would have to travel halfway to the tortoise. The distance is now half as long, but before he can cover the shorter distance, he has to travel half of it. And so on. In fact, before he can even reach the first halfway mark, he has to travel half of that distance, etc. Therefore, motion is impossible. QED

    Of course it just a fun mental game. I think that Hofstadter deals with this a lot in Godel, Escher, Bach–with arguments that could probably apply equally well to the Kalam version. Making it more cosmic does not make it more viable.

  • Mrnaglfar

    I would argue it all comes down to how we perceive time and how we use the way our mind interprets events to project the reality (be it false) that this is how things really work. In other words, we’re using the model under which our mind operates to interpret something to which that model does not apply. Infinity is an idea throughly un-understandable by our minds; we have an idea of what infinity is but we have no conceptualization of it (in other words you cannot picture an infinity of something, it’s along the lines of a two dimensional figure imagining a three dimensional one, or a three dimensional one imagining a four dimensional one etc).

    This is very much along the lines of p4limpset’s post above mine ( I was actually going to use the same argument). What logically makes sense to our mind does not always translate into the reality of things; I would go so far as to argue it never truly does. I remember a talk by Richard Dawkins in which he said ” In this sense, really is whatever the organism needs it to be in order to survive in the orders of magnitude useful to it’s survival” (roughly paraphrasing). So while we might experience a rock as being solid, had we have evolved to life at the relative size of atoms we would experience a rock as being mostly made of empty space, and while we’re very attuned to force of gravity we are almost oblivious to the force of surface tension (as a fly would reverse these priorities).

    If anyone wants to hear that full talk, here’s the link:
    http://www.youtube.com/watch?v=1APOxsp1VFw

    Point of all this being we are applying our version of our mind (of limits and sizes) to something to which it does not apply, and I believe this is a conversion that will fail. In order to truly conceptualize what time and infinity are we need a model that operates outside the boundaries of our minds.

  • Herb

    Very nice post.

    If I may chime in the modern physics perspective… Our current understanding is that time and space are unified into a single entity called spacetime. That is, the Universe has 4 “directions” labeled (x,y,z,t). Different observers are on different paths through spacetime (physicists describe each path using a 5th quantity that is essentially meaningless – a placeholder). The statement that time and space are relative simply means that different observers disagree on the (x,y,z,t) labeling of each point. Brian Greene’s Fabric of the Cosmos has the best layman’s explanation I’ve ever seen – he compares the disagreement of labeling points to two navigators, one using the north star and one using magnetic north.

    @Stuart, you’re forgetting that if time “ran backwards” we could not be aware of it, as Ebonmuse points out. The increasing of entropy picks out a special direction in spacetime, and we can only ever have memories corresponding to points with lower entropies.

    @Andrea, saying that time “occurs” is tantamount to saying that it “flows”. Ebonmuse has done a good job showing that this idea is flawed.

  • Herb

    Very nice post.

    If I may chime in the modern physics perspective… Our current understanding is that time and space are unified into a single entity called spacetime. That is, the Universe has 4 “directions” labeled (x,y,z,t). Different observers are on different paths through spacetime (physicists describe each path using a 5th quantity that is essentially meaningless – a placeholder). The statement that time and space are relative simply means that different observers disagree on the (x,y,z,t) labeling of each point. Brian Greene’s Fabric of the Cosmos has the best layman’s explanation I’ve ever seen – he compares the disagreement of labeling points to two navigators, one using the north star and one using magnetic north.

    @Stuart, you’re forgetting that if time “ran backwards” we could not be aware of it, as Ebonmuse points out. The increasing of entropy picks out a special direction in spacetime, and we can only ever have memories corresponding to points with lower entropies.

    @Andrea, saying that time “occurs” is tantamount to saying that it “flows”. Ebonmuse has done a good job showing that this idea is flawed.

  • Herb

    And for the Zeno thread, once you realize that time is unified with space, the kalam argument becomes moot. There is no requirement that anything traverse all of space or all of spacetime.

    This probably makes no sense to people, but I’m having fun. :-) Read Brian Greene.

  • Mac

    Doesn’t current quantum mechanics hold though that time is not an infinite scale, ie the time between two events at it’s smallest can be 1 unit of Planck time?
    (a similar restriction holds true for distance).

    Planck time would remain constant for all observers though as it is defined by the time it takes light to cross a distance of 1 planck length.

    I just say this, as it would seem that quantum mechanics suggests a universe that goes “from one frame to another” like with a movie projector – but running such that the time between frames was 1 planck time.

    However – I am not a qualified physicist, so I may be interpreting it wrong, and it may only imply that while time is indeed “fluid” and constantly in motion we have a universal limit on how accurate our measurement of time can get…

    I also like some of the analogies about the train on an infinite track still traveling, as I find them a good explanation.

  • Herb

    @Mac, Current quantum mechanics does not say there is a smallest unit of time. You are perhaps thinking of “quantum gravity”. It is believed that a successful combination of quantum physics with gravity will, as you say, lead to finite chunks of spacetime with Planck-sized dimensions. But no theories that do this have been confirmed. String theory is the leading candidate, though it’s not testable yet.

  • satanist

    “At ordinary time and distance scales, this effect is so small as to be almost undetectable, but at very high velocities or over astronomical distances, it can become very significant. Physicist Brian Greene, in his book The Fabric of the Cosmos, gives a startling example: an observer 10 billion light-years away from you, who moved in a direction away from you at a mere 10 miles per hour, would have a “now” consisting of events 150 years in your past. And if this observer moved toward you at the same speed, his “now” would consist of events 150 years in your future.”

    So two guys 10 billion light years away decide to start moving; one moves towards us at 10MPH, the other moves away at 10MPH. These guys are merely moments away from each other yet 300 years apart according to my observations? I don’t get it.

  • Polly

    I thought at 10-43 seconds, time ceases to be. Moreover, below a certain threshold of quantum distance all there is, is not even “regular” space but some kind of foaming space with no time component.

    I’ll put up the same disclaimer: I’m not a physicist, heh, obviously.

  • Polly

    I thought at 10-43 seconds, time ceases to be. Moreover, below a certain threshold of quantum distance all there is, is not even “regular” space but some kind of foaming space with no time component.

    I’ll put up the same disclaimer: I’m not a physicist, heh, obviously.

  • http://www.patheos.com/blog/daylightatheism/ Ebonmuse

    These guys are merely moments away from each other yet 300 years apart according to my observations? I don’t get it.

    The analogy Brian Greene uses is that your perception of “now” can be likened to a “slice” through space-time. When we’re right next to each other and moving at the same speed, our slices overlap; we agree on what is happening now. But if I’m in motion relative to you, my slice is at an angle relative to yours. It will encompass events that are either in your past or in your future, depending on which way I’m traveling.

    Normally, the way to see a substantial difference is for me to travel very quickly relative to you, at some appreciable fraction of light speed, so that my now-slice of spacetime will be steeply angled compared to yours. But even a very slight angle (a very low relative speed) will do if I’m far away from you, so that the difference can accumulate over distance.

    This is, of course, a very poor analogy to describe something that can only truly be described mathematically, which is what the special theory of relativity does.

  • http://www.patheos.com/blog/daylightatheism/ Ebonmuse

    These guys are merely moments away from each other yet 300 years apart according to my observations? I don’t get it.

    The analogy Brian Greene uses is that your perception of “now” can be likened to a “slice” through space-time. When we’re right next to each other and moving at the same speed, our slices overlap; we agree on what is happening now. But if I’m in motion relative to you, my slice is at an angle relative to yours. It will encompass events that are either in your past or in your future, depending on which way I’m traveling.

    Normally, the way to see a substantial difference is for me to travel very quickly relative to you, at some appreciable fraction of light speed, so that my now-slice of spacetime will be steeply angled compared to yours. But even a very slight angle (a very low relative speed) will do if I’m far away from you, so that the difference can accumulate over distance.

    This is, of course, a very poor analogy to describe something that can only truly be described mathematically, which is what the special theory of relativity does.

  • http://elliptica.blogspot.com Lynet

    These guys are merely moments away from each other yet 300 years apart according to my observations? I don’t get it.

    No, the two people are not 300 years apart according to your observations. But their observations of your position in time differ by a total of 300 years.

    I believe the example assumes that according to your observations, the other guys are at that position simultaneously with you. Their observations disagree with you on that — they think you’re forward or behind.

    Getting used to the idea that time differences in one frame of reference can never be simply transferred into another frame of reference takes a while, because we’re so used to our ordinary idea of time that we have to learn to notice we’re using it.

    If you want to get the hang of things, I suggest Minkowski diagrams, a lot of practice, and an automatic understanding that the first three ‘paradoxes’ you come up with will merely be flaws in your understanding. Oh, and Brian Greene, of course.

  • prase

    So two guys 10 billion light years away decide to start moving; one moves towards us at 10MPH, the other moves away at 10MPH. These guys are merely moments away from each other yet 300 years apart according to my observations? I don’t get it.

    Maybe it would be good to formulate the things more precisely. First, it is better to speak about the time distance of events instead of objects (I mean it is far from obvious what does “two guys 300 years apart” mean). Imagine for example that these moving guys send a signal (this defining two events: E1 = guy no.1 pushing a button on his transmitter, E2 = guy no.2 doing the same thing on his apparatus) and imagine that from your point of view E1 and E2 are simultaneous (since the guys are placed symmetrically with respect to you you will receive both signals at the same time). The point is that from the first guy’s point of view E1 happens 300 years after E2 and from the second guy’s point of view E1 happens 300 years before E2.

    Note also that it is not so easy for the first guy to measure the actual time of E2. Either he knows what the distance to the second guy is (and as the signals propagate with the speed of light he can compute the time needed by the signal to travel from the point where it was released) or he has to have a companion who:

      1) has his clock synchronised with guy no.1
      2) has zero velocity with respect to guy no.1
      3) at the moment when guy no.2 pushes the button (event E2) is (almost exactly) on the same place as guy no.2 is

    In such situation if the companion observes E2 at time t, first guy would observe E1 at time t+300.

  • javaman

    excuse the dumb question, if there is no observer does time still exist? Is time something created by our congitive processing to put order into the signals our senses take in. In other words do we create time in our head? If time exists is there a particle that transmits it?

  • javaman

    excuse the dumb question, if there is no observer does time still exist? Is time something created by our congitive processing to put order into the signals our senses take in. In other words do we create time in our head? If time exists is there a particle that transmits it?

  • prase

    excuse the dumb question, if there is no observer does time still exist? Is time something created by our congitive processing to put order into the signals our senses take in. In other words do we create time in our head? If time exists is there a particle that transmits it?

    No, time is not any more constructed than the other physical quantities we observe. We usually construct the language we speak about the nature (this includes the definition of how we understand the word “time”), but if there were some aliens describing the nature in some completely distinct way, our and their description should be completely equivalent. The nature also should behave in the same way without any observers within it (at least if you believe in existence of external reality, what seems to be not believed by some postmodernist philosophers who however usually have very sparse knowledge of physics).

  • prase

    If time exists is there a particle that transmits it?

    There is no “time particle” as there is no “position particle” or “velocity particle”. On a level of classical theory each force that exist in nature is mediated by some field (like electromagnetic field), in the quantum theory the fields can be described in terms of particles. So there are particles which correspond to four fundamental forces (also called interactions) observed in nature: photon for electromagnetism, bosons Z and W for weak interaction, eight sorts of gluons for strong interaction and graviton for gravity (the last one being still hypothetical as there is no consistent quantum theory of gravitation so far).

  • troutski

    Maybe I’m missing something but I think you are making this too complicated. The 150 years is because that is how many light years you traverse in 10 billion light years at 10 mph: 10*10^10*365*24/5.878 = 149.0 That is how much longer it takes the light “now” signal to get there.

    I don’t know where the “moving light” came from, certainly not from the initial quote. It is true that time durations can be measured, unless you are saying days, weeks and years are also an illusion. And it is true that an infinite duration would be required for an infinite past.

    In the meantime, will one of you “other observers” fetch me a dinosaur egg and tell me a good stock to own 10 years from now?

  • troutski

    Maybe I’m missing something but I think you are making this too complicated. The 150 years is because that is how many light years you traverse in 10 billion light years at 10 mph: 10*10^10*365*24/5.878 = 149.0 That is how much longer it takes the light “now” signal to get there.

    I don’t know where the “moving light” came from, certainly not from the initial quote. It is true that time durations can be measured, unless you are saying days, weeks and years are also an illusion. And it is true that an infinite duration would be required for an infinite past.

    In the meantime, will one of you “other observers” fetch me a dinosaur egg and tell me a good stock to own 10 years from now?

  • troutski

    That is: The 150 years is because that is how many light years you traverse in 10 billion years at 10 mph: 10*10^10*365*24/5.878 = 149.0

  • semuzhang

    I come from china,majoring in philosophy .I think it is very good after reading your this paper.

  • zenbullet

    So this is super late in the game, but I thought i would just point out that it is always “now” to light.

    Search “Dr. Quantum Double Slit”

    The local speed of time is set by the conflux of large gravity sources in the area.

    Lightcones from stars are what set the Universal Now.

    {imho}

    I’m going to read Brian Greene, thanx for the suggestion.