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Arrow of time

  1. Jun 19, 2006 #1
    This is a topic relating to physics but philosophical in nature. Physicists are talking about explaing why the "arrow of time" flows forward the way it does, instead of flowing in any other direction. My questions are these:
    1) How do we know that time flows at all? is it not possible that we simply experience time to be flowing as a feature of our consciousness and that all moments in time simply exist with no flow from one to the next. Is there any scientific way to distinguish between time flowing or not?
    2) Does it make any sense at all to ask why time flows in the direction it does? surely the direction of time being labelled as "forwards" is arbitrary. What would be strange is if time suddenly changed direction. But even then, would we even notice? If time were to change direction, we would have no idea as we ourselved would be going back in time, retracing our steps.
    Basically, i have no idea what physicists mean when they ask why time has an arrow
  2. jcsd
  3. Jun 19, 2006 #2
    I think it is because most scientists seperate between what we perceive and what the physical reality of the situation is.
    If the arrow of time was to change direction, then that would be a physical event, as such scientists want to figure out the math and logic behind that event.

    If however the arrow of time has had the same direction since the big bang or whatever started it all, then it is a pointless theory, unless it changes direction.

    As for your first question, it is my belief that we do need a physical time dimension for mass to be able to move, and that this time is different from our mental time, the time we create by retreiving memories and experiencing time with our senses.
  4. Jun 19, 2006 #3


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    1) When some scientist, or popular book on science, talks about time flowing, you can't take it seriously. They are not using language critically there, but just trying to communicate with ordinary people who use the image of time flowing.

    2) What scientists really mean about the arrow of time is the direction of cause and effect. You can run a movie of a breaking egg backwards to show the egg reassembling itself, but this does not happen in reality. This gives a preset direction to physical events, and that direction is "the arrow of time".

    There is another example, related to the above; the direction in which entropy increases. Entropy is a measure of disorder, or "lots-of-ways-to-wiggle-ness". If no other cause is acting, the number of ways to wiggle just keeps getting bigger. That gives an arrow of time too, wich always points the same direction as the cause and effect one.
  5. Jun 20, 2006 #4
    That doesnt really answer my original question. What I want to know is how physicists can objectively establish that time flows at all. Time could equally be a concept invoked by our consciousness. For example what does it mean to say that a particular moment in time is the present, other than to say that we are consciously experiencing that moment. What is the distinguishing feature about the present moment that sets it apart from all other moments in time.
    Secondly, surely the reason an egg never reassembles itself is because of the extremely complex interplay of forces required to do so which are immensely unlikely to occur naturally. If time were really to reverse for a while so that the egg did reassemble, we would never know since we would be simultaneously travelling back in time, retracing our thoughts and feelings exactly as they occured but in reverse.
    I see no objective way to show that time really does flow and addresses these problems.
  6. Jun 20, 2006 #5
    Time is relative. It's true, when we say we are looking at something in the present, that's not really accurate. It takes time for the light reflected off of what we are seeing to hit our eyes. Therefore we are really looking into the very recent past when we are looking at another person or our hand or something. If someone from way out in outerspace could see us on earth somehow, they would see the earth as it was a hundred years ago, if that's how long it took the light to get to their planet from earth. So, it is possible to go back in time, but it's not possible for time to go backwards. There is no specific direction that light is traveling in. Different light rays travel in all different directions. But whatever direction a light ray travels in, it is not traveling in the opposite direction from itself.
    I guess you are not so much concerned with the direction in which time flows, which is used as a visual aid, but more with the fact that there really is no time whatsoever.
    Some physicists beleive that without light there would be no time. So you may be right and time is only something specific to our universe or any universe who's living would depend on light for life. I don't see why that's not possible, since time is relative even within our universe.
    As far as how we know one moment is different from the last - I suppose that would also be the nature of the universe in which we exist. Once there was nothing, then there was a universe.
    Since time really has no affect on the existence of matter in its smallest states, then time is only an issue when talking about states of matter. That would be where one moment would be different from the previous, when matter is no longer in its previous state.
    Of course, I am far from knowing the answer, but that's my stab at it.
    Last edited: Jun 20, 2006
  7. Jun 21, 2006 #6
    We can objectively say that time flows because things move.
    We move, the universe moves.

    Time isn't so much a defined static dimension, as it is an elusive set of rules that emerges from mass moving.
    Finding out those rules is of utmost importance, but nobody seems to have done it yet.
  8. Jun 21, 2006 #7
    Time has a number of arrows which is what makes it meaningful
    to ask about time reversal. Off the top of my head, they are:-
    The expansion of the universe since the BB; the arrow
    entropy; the arrow of causality; and the subjective arrow.

    Of course they are not necessarily all different under some
    final analysis.

    Consciousness is generated by the brain, which is a physical
    system ? Does it fall under some special set of physical
    laws giving it a unique gift of time-flow-generation ? Or, if it
    falls under the same laws as everything else, why shouldn't
    those laws be capable of generating an objective flow of time ?
  9. Jun 21, 2006 #8
    Whilst that may not be the ideal metaphor, it does not
    help to ignore the flow of time entirely and insist on
    a static universe. Dismissing the flow of time a subjective doesn't
    help either, since it suggests cosnciousness is exempt from physics.
  10. Jun 21, 2006 #9
    Why must time always "flow" as an arrow. If time is a "movement", why only flow either one direction or another (forward, backward), why not also move (oscillate) in place, perhaps at speed of light. Thus no "flow" to time at speed of light, more like a spinning wheel ?
  11. Jun 22, 2006 #10
    of course consciousness is exempt from the laws of physics. It would be meaningless to try to explain thoughts and feeling and perceptions in terms of matter and energy. The physical processes in the brain are subject to the laws of physics, but science has yet to explain how these processes give rise to our subjective experiences. I find problems with viewing time as a physically occuring process because there seems to be no physical explanation for what gives the present moment its uniqueness. According to physics the present moment is no different to any other moment in time. The reason we see time flowing could be a facet of our consciousness. Is it not possible that all moments in time simply exist, and our consciousness gives us the experience of the "present", which has no physical difference to any other moment in time
  12. Jun 22, 2006 #11


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    I hate to use such a harsh expression, but this is just bosh. I have explained above in this thread how physics has perfectly good accounts of how different moments may be ordered. As for representing feelings and such, neurobiology has already managed a lot with feelings, through studies of serotonin and such. Did you know that the instance of suicide has fallen in recent years? Since the introduction of Prozac, in fact.
  13. Jun 25, 2006 #12

    The "arrow of time" is a metaphor used by Arther Eddington for the first time in 1928 to represent the asymmetric properties of time that have no analogue in space.

    It's important to distinguish between the arrow of time (why events seem to be asymmetric in time), and the apparent illusion of the "flow of time". These are two very different things.

    There are in fact many "arrows" of time, including the psychological arrow (we can remember the past, but not the future), the thermodynamic or entropy arrow (the one described by selfAdjoint above), the cosmological arrow (the fact that the universe is expanding), and many more.

    We may ask : Why does our psychological "arrow" always seem aligned with the thermodynamic "arrow"? The answer is (imho) as follows :

    The Origin of the Psychological Arrow of Time
    At the microscopic level of individual particles it turns out that time seems perfectly symmetric – we say that the dynamical laws of physics are invariant (do not change) under time-reversal. Thus there seems to be no fundamental difference between past and future directions of time at the level of individual particle interactions. But at the macroscopic level (the level at which our brains, and almost every other object we interact with, work), it seems to be a fundamental characteristic of our world that there is an inherent “entropy gradient” within spacetime. What this means is that if we take different “timeslices” through 4-dimensional spacetime we find that we can identify an overall asymmetry wherein the total entropy of the universe increases in one time direction (and conversely of course decreases in the reverse time direction). Unlike space, therefore, the time dimension contains an inbuilt arrow, often referred to as the thermodynamic arrow of time.

    Next we need to consider how memories are formed in the brain. A memory is in general terms simply a record or a form of “representation” of one part of a timeslice (let’s say t0) within another timeslice (let’s say t1). This applies to things like computer memories, photographs, drawings, descriptions, and human memories. To form a “memory” requires an extraordinary and inherently improbable correlation between the “worldlines” of many particles. A footprint on a beach, for example, might have appeared spontaneously (there is a vanishingly small chance that this might happen), but the patterns (which represent information) within a footprint are so highly correlated that there is an incredibly overwhelming probability that the footprint is associated with some identifiable “cause” at some other timeslice which we say “formed” the footprint. The “directionality” of this correlation, in the way that the correlations between worldlines change on going from one time-slice to another, is almost always aligned with the directionality of the overall entropy gradient between those timeslices.

    We see cups break, but we never see them spontaneously re-assemble, because our internal psychological arrow of time is aligned with the thermodynamic arrow of time (the entropy gradient). And the reason our psychological arrow is aligned with the thermodynamic arrow is precisely because the processes which produce “records” in our brain, representations of one timeslice within another timeslice, are themselves thermodynamic processes. Our psychological arrow is “locked” into the same directionality as the thermodynamic arrow because both arrows share a common source – the entropy asymmetry of the time dimension.

    The upshot of all this is that for any macroscopic record of time t0 contained within time t1, the entropy at t0 is always lower than at t1. All of our “memories” therefore are of times of lower entropy. We simply do not have memories of times where the entropy is higher. It is simply by convention that we refer to the timeslices where entropy is lower as “the past”, and timeslices where entropy is higher as “the future”. Hence we have records and memories of the past, but not of the future.

    Best Regards
  14. Jun 25, 2006 #13
    Not at all. The conscious illusion that we have of time "flowing" from past to future may be explained very simply on the basis that we have memories only of the past, and never of the future (and the reason why this is the case is explained in the previous post).

    Assuming that the universe is deterministic (and this is another topic entirely) then the future is just as "fixed" as the past. But what the conscious agent perceives is that the past is fixed, but the future seems to be open, or not fixed. Given these perceptions, it would not make rational sense (it would in fact be irrational) for the agent to believe that time flowed from future to past (because the agent perceives that the past is fixed).

    It also would not make rational sense (from the agent's perspective) for the agent to believe that time was not flowing at all, and the agent is simply "frozen in time".

    The only rational interpretation that the conscious agent can place upon its perception that the past is fixed whereas the future seems "open" is that it (the agent) is actually "flowing through time", from past to future.

    Best Regards
  15. Jun 25, 2006 #14
    Only in the same way that anything else can, e.g., distance, tables, sound...
  16. Jun 27, 2006 #15

    if the universe is not merely deterministic, but a static, eternal
    4D structure, then the future "already" exists, then I "already" exist in it.
    But that doesn't explain the feeling of getting from one state to
    another (which must be an illusion, but the source of the
    illusion is not explained.)

    OTOH, if future states are continually coming into freshly-mineted
    being, the feeling of flow reflects the phsycial situation and is
    therefore not an illusion.

    I know of no theory that explains the feeling of flow as an
    illusion. It is either illlusory and inexplicable or explicable
    and non-illusory.

    The question of why there is a "flow" at all is separate from the
    question of the direction of flow.

    Not in face of the feeling of flow, but the feeling is not
    explained under the block-universe theory.

    It is perfectly possible to conceive an agent which has
    a fixed set of memories of the past no knowledge of the future..
    and no "flow". The flow is something extra.
  17. Jun 27, 2006 #16
    It's not botch. You didn't explain what makes the present moment a unique moment in time, or how time flows in a universe in which can just as equally be considered as 4-dimensional and static. Neurobiologists may have discovered what is happening in the brain when we feel certain emotions, but how something material actually gives rise to mental formations is not known.
  18. Jun 28, 2006 #17
    Understanding Time

    madness asked excellent probing questions at the top of this thread. Let’s see how they can be answered.

    Before considering the arrow of time let’s be a little more basic and clarify our understanding of time. When we see the sun at sunrise, noon, and sunset, we remember not only those observations but also the order in which they were made. Each long term or short-term memory is ‘tagged’ with its sequential position in a string of memories. Our notions of the past, present, and future are based on our recognition of memory sequences. That is the source of our concept of time.

    We are not the only animals that recall memory sequences. Animals that hunt often base their present actions on the action they anticipate of their prey. To distinguish between present and future requires some understanding of time.

    In a few sentences I will use the term ‘abstraction.’ so let’s be clear about what I mean by that term. An abstraction expresses a quality or characteristic apart from any specific object or instance. For example, there are red apples, red sunsets, and red light. Red is an abstraction. Red does not have existence independent of the things it characterizes. There is no such thing as red itself.

    Time is an abstraction we make from the motion of things. We cannot conceive of time without conceiving of things that move, such as the rhythmic beating of our hearts, the motion of the sun across the sky, the swinging of a pendulum, the oscillations of a quartz crystal, and the alternating electromagnetic field of a photon. We do not sense time; we sense things and observe and remember their changes. Therefore, time without things can have no meaning for us. Just as there is no such thing as red itself, there is no such thing as time itself.

    Since there is no such thing as time itself, it is meaningless to consider time to have physical existence. Time, therefore, cannot have physical properties; it cannot spin, dilate, shrink, or flow. When we say that time flows, we are speaking metaphorically as if time were similar to the water flowing in a river. But water is a real thing, time is not.

    The idea of going forward or backward in time presumes that the time traveler would be spared the effects of the travel, that he could even go to times before and after his own existence. I guess he would don a cloak that shields him from time.

    The notion of time running backwards implies that all motions would be reversed and that all history would retrace its steps backward. It’s not possible. There are too many things that prohibit the reversal of time. Water can’t change its direction through a check valve. Electrons cannot change their direction through a diode or transistor.

    The earth would have to change its direction of rotation in order to make sundials tell time backwards. If it changed its direction instantaneously there wouldn’t be a human-built structure left standing and there would be horrific flooding. Instead of people getting younger they would be killed; that wouldn’t be a backward replay of history. And if it changed direction slowly, there would be terrible destruction as equatorial oceans moved towards the poles during the reversal. That, too, would not be a backward replay of history. The notion of time reversal, of history running backward, is self-contradictory.

    As I said, it’s not possible. It’s as funny as a man jumping up upside down from a swimming pool and landing feet first on a diving board. Just because we’ve seen films run backward or wished we could take back some words we’ve said doesn’t make time running backwards possible, let alone a good idea The idea of the reversal of time, of going back in time, is fantasy.

    If the arrow of time is stationary like a traffic sign, it can point only toward the future. If the arrow of time flies, it flew from the past to the present and now flies toward the future. The past no longer exists. The future does not yet exist. We are forever stuck in the present, observing the past, and anticipating the future.

    Now let’s answer the questions posed in the opening of this thread.

    Time can flow metaphorically but not physically. In the same way that time is an abstraction from the motions of things, the flow of time is an abstraction from the continuity of the motions of things. The continuity of things and their motions is based on our concept of object persistence, a concept formed during infancy.

    If science deals with reality and not metaphors, there is no way for science to probe that question.

    That question is equivalent to asking why all things move. I don’t think it makes sense to ask that kind of question.
    The question is pointless since irreversible processes preclude time reversal. Think of light and heat going back to a lamp to create ac electricity. Think of the check valve and diode.

    Physicists who ask that question are incorrectly taking metaphoric descriptions of time literally. The direction of time is implicit in Newton’s first law: “Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon”. The notion of continuity of motion is implicit in his (now archaic) use of the word ‘perseveres’ and his use of the term ‘uniform motion.’
  19. Jun 29, 2006 #18
    Tournesol writes: "I know of no theory that explains the feeling of flow as an illusion. It is either illlusory and inexplicable or explicable
    and non-illusory."

    Try having a look at the theory of emptiness as expounded by Nagarjuna, or dhamma theory as expounded in the 'Abhidhamma' (one of the 'three baskets of teachings' in Buddhism). In this view spacetime is a psychological construct, as I think it was for Kant.

    Nagarjuna's Fundamental Wisdom of the Middle Way is a proof that the notion of time as existent is inherently paradoxical. He makes a reductio argument similar to Zeno's. In this view, at the deepest level of analysis nothing really exists and nothing ever really happens (really!). Roughly speaking, he argues that the past does not exist, the future does not exist, and thus the notion of the present moment is incoherent (an argument related to the 'Dedekind Cut' in mathematics).

    Many people conclude his proof is successful. The philosopher Francis Bradley attempts a similar (but far more complex) proof in his metaphysical essay Appearance and Reality.

    The Wheeler-Feynman 'absorber theory' of time is, I think, still well thought of by physicists. In this entities can move both backwards and forwards in time quite happily, quite as if the flow of time is an illusion.

  20. Jun 29, 2006 #19
    Thanks to Canute and Drachir, you offered interesting answers to my questions. However, I am still a bit confused about our concept of the present. In, a deterministic universe, all moments in time are preset, and would seem to have some kind of existence. For example some theologians have argued that when God created the universe he created all moments in time at once, rather than creation being related to the beginning of the universe. So what does it mean to say that a certain moment is happening now? Or does it make more sense to view the present as the only moment in existence, and the past and future as illusory? In this view, problems such as the above would be solved.
  21. Jun 30, 2006 #20
    Is the Flow of Time an Illusion?

    The argument seems to be : Conscious agents perceive time as if the agent is “flowing in time” from the past into the future, and from this perception it is allegedly safe to infer that such agents are not simply under the illusion of “flowing in time”, but indeed (and objectively) they are “flowing in time” from the past to the future.

    I shall show that this inference is invalid.

    Experiments by Libet and Grey Walter on the differences in the objective and subjective (experienced) correlations between temporally separated events show that the mind can reconstruct or rearrange the actual temporal sequence of perceptual information coming from phenomenal events, such that the consciously perceived (the experienced) sequence is not the same as the objective sequence of events (see the paper by Dennett and Kinsbourne referenced below). Libet explains this in terms of “backwards referral” or “backwards projection” of certain consciously experienced events with respect to other consciously experienced events. Dennett describes this kind of mental manipulatioon of the objective sequence of events in terms of Orwellian and Stalinesque models of mental representation.

    (See attached figure)

    View attachment figure.doc

    As Libet remarks, there is
    Thus there is an “objective timeline”, and for each conscious observer we have an individual, subjective “experienced timeline”. The individual experienced timelines do not necessarily precisely match up either with each other, or with the objective timeline, in terms of their experienced sequence of events.

    Thus if time really does “flow” (and we are to believe that this flow is not an illusion), then all but one of the above timelines (since they reflect different sequences of events) must be an illusion. Which of the above timelines would one think represents the real “flow of time” – the objective timeline or one of the the experienced timelines? It obviously cannot be one of the experienced timelines (because we each experience different timelines, and none of us is in a privileged position of being able to claim to have direct access to the “absolute flow of time”), therefore (if any one timeline flows) it must be the objective timeline. But if this is the case, then it follows that we each sometimes perceive time as flowing in the opposite direction to the way it is objectively flowing! Thus, our subjective experience of the flow of time is indeed an illusion (whether the objective timeline really “flows” or not), and we thus cannot infer from our perceived or experienced flow of time that objective time is actually flowing at all.


    For Libet, see for example : Libet, B., 1981, "The experimental evidence for subjective referral of a sensory experience backwards in time: reply to P. S. Churchland," Philosophy of Science, 48, pp.182-97

    Dennett, Daniel C. & Kinsbourne, Marcel (1992) “Time and the Observer”. Behavioral and Brain Sciences 15 (2) 183-247
    Last edited: Jun 30, 2006
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