Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time: A-series or B-series?

  1. A-series (Presentism, Growing Block Theory, Moving Spotlight Theory, etc.)

  2. B-series (Eternalism, Block Universe, etc.)

  3. Undecided

  4. Other (Please Explain)

  1. Jan 17, 2012 #1
    I just want to know what the consensus is by people here in general on PF, although the only place where I felt this question appropiate is at the philosophy subforum.


    Anyway, in a nutshell, the A-series, or A-theory states that the passage of time is real and that reality is really changing while the B-series or B-theory states that there is no flow of time at all and that all past, present and future are fixed in a static state.
  2. jcsd
  3. Jan 17, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

  4. Jan 17, 2012 #3
    Via the conventional usage of the word reality, and the observationally supported fact that reality is changing, then the most reasonable conclusion is that reality is transitory.

    The present refers to our experience. The past refers to our indexes of our experience. And the future refers to our projections based on those indexes.

    So, it would be incorrect to say that "all past, present and future are fixed in a static state", because, clearly, there's ample evidence to the contrary.
  5. Jan 27, 2012 #4
    I see outgoing signals as the past with incoming signals the future, the present is the interaction of these signals with matter. I see nothing that is static in my view of time other than those bits of matter moving in time relative to myself.
  6. Jan 27, 2012 #5
    The entire question implies a false dichotomy as far as I'm concerned. The latest evidence supports contextual theories of quantum mechanics and I see no reason time can't also be framed contextually. If so such archaic metaphysical arguments of dubious value might finally be relegated back to the theologians and others for debate.
  7. Jan 27, 2012 #6
    Ryan - I dont agree with your reasoning. I think the arrow of time implies an asymmetry in time, but not necessarily a flow or privileged "present moment". Things can perfectly well be static and asymmetric. In fact, I think the laws of relativity are entirely at odds with the idea of a present moment or flow of time and rather support a "block universe". I am therefore inclined to think that the feeling of time flowing is a subjective trick, related to memory and other functions of our brains.

    However, I am also careful to see physical theories as only models of the world, and would therefore be hesitant to take this view given by relativity too literally here.
  8. Jan 29, 2012 #7
    I would think the subjective trick of time is all the motion we do not see, from this constant acceleration I feel on earth, to the duration of a photon which I see as space, but just like Thomas said it does take a functioning brain to interpret the signals.
  9. Feb 8, 2012 #8
    Going from you 'nutshell' definition, I definitely go with 'B'. Actually Im surprised to find it currently lagging behind A on a physics forum, though only a handful of votes are in I suppose.

    I don't see the need of a present to explain anything within physics. Also, physicists tend not to like privileged coordinates or frames of reference. Show me a reason to introduce the special case of a present into my beautiful symmetrical physical laws!

    (the rest that follows is really just my feelings)

    We all have this strong sense of time passing at a (outside moments of stress) constant rate, but does that even make sense? What units would the passage of time even have? It seems to me this rate is at most the ratio between your observations and your internal model. perhaps seconds per thought.

    Personally I favor the notion that the direction of time is simply the direction of increasing entropy. Given that you exist in a part of the universe that is not totally uniform you could expect to be on a gradient between more and less ordered states. Even if non-uniformity is incredibly rare, all other points are probably to boring to support intelligence. We call the past the past because we can remember it. For example if the universe started as a big cube of ice, and the ice was smashed into smaller ice cubes, you could point to one of these cubes and say 'this is a memory of the original state', but the original ice cube is too simple to hold a model of higher entropy states.

    As an aside: If our past is always in whatever direction order increases from our current coordinates then, regardless of the shape of the universe, all consciousness will perceive themselves to be in a present with a past that was slightly more ordered, and looking back further a past even more ordered than that, and so on perhaps until some sort of singularity is encountered.


    Just wanted to add that disbelieving the present is a necessary concept for physics does not necessarily mean believing the future is predetermined. For example perhaps not just one but every future is played out somewhere. Each future self would feel that just one future happened and could assume they were marching down a single linear predetermined path.
    Last edited: Feb 8, 2012
  10. Feb 10, 2012 #9
    Depends on your frame of reference.

    If you are travelling at less than the speed of light, then Series A is true - events are dynamic and unfold as a series of interactions in time.

    If you are travelling at the speed of light, e.g. a photon, then everything happens at once and Series B is true.

    They're both true. The cat is alive AND dead.
  11. Feb 10, 2012 #10


    User Avatar
    Staff Emeritus
    Science Advisor

  12. Feb 11, 2012 #11
    As far as I know, there is no need for this concept within physics. I understand there are rules ordering cause and effect etc, but these are true of all points in history, not just true at one point that we call the present for example. I don't think the B theory excludes such notions as one point being near another in time, or defined as before or after with respect to some coordinate system.

    Also my understanding that the laws of physics are all reversible with the qualified exception of entropy, mentioned above. Anything that physics models going forwards could also describe another possible system with time reversed. It seems to me that all we need to explain entropy is the notion that the universe is not entirely homogeneous and then the common experiences of past and future, such as the inability to remember the future, can be deduced.
  13. Feb 12, 2012 #12
    Time does not exist.
    There is entropy and cause and effect. They result in a sequence of events that we perceive as time. But time is really just a concept so A and B theory are both equal as they are both nothing but sometimes useful concepts. For physicists B theory is obviously more useful but that does not change that it is nothing but a conceptual tools to create models of reality.
  14. Feb 13, 2012 #13
    I voted A because of the reference to "presentism", namely the idea that the present moment is all that exists. I wouldn't however agree with the notion that the passage of time is real, insofar as I would see time as being a manmade concept, not something that is real. I think the idea of time arises form our ability to remember the past and project the future; it is essentially a mental construct.

    Obviously these ideas aren't original, but I think they are reasonable.

    Just read the following, which I would agree with (to a certain extent)
  15. Feb 16, 2012 #14
    I think the main arguments against B-series, which only slightly favour A-series - insofar as A-series postulates that the present moment is all that exists - are:

    - A clock doesn't measure a physcial property called time; a clock provides a regularly occuring, repetitive process which is used as a standard unit, which facilitates the comparison of other physical processes, and the expression of those processes in standardised units. At no point in the process is there a physical property called "time" actually measured.

    - There is no experiment that can, or ever will, be conducted that isn't in the present moment. All that can ever be experienced by anyone is the present moment.

    - Following on from that; there is no direct evidence that "the past" or "the future" exist; what we have are the mental constructs of "past" and "future". Without such direct evidenece, both have to be assumed to exist, and "time" along with them.
  16. Mar 5, 2012 #15
    A bit late to add my own take on the issue, but in my opinion, the philosophical case against the A-series is quite weak to say the least. However, physics apparently points in the other direction on this issue (though I am not certain just how relevant the science is to philosophical issues such as this one), which sort of leaves me in the middle.
  17. Mar 11, 2012 #16
    I voted "B Series" though elsewhere I have seen "A versus B" explained in terms of relative versus absolute time which is slightly different.

    IMHO, the Twins Paradox of special relativity argues strongly against most forms of presenntism. If as enduratism says, an object is "wholely present at each instant", then the travelling twin will be in the future of the stay-at-home twin after the trip even if they agree to meet at the same location. This means that a "future" stay-at-home can only meet a "past" traveller. On that basis, I am a perdurantist. Until a quantum substratum can be developed from which spacetime becomes emergent, I would have to adopt the postulate that time and distance are continuous hence I would further suggest that worm theory is the scientifically favoured model.


    The question about whether time is "real" is that of substantivalism. In that area, I think the observations by Hulse and Taylor for which they won the Nobel Prize are crucial. They showed by careful timing that a pair of stars including a pulsar was losing orbital energy at the rate expected to result from gravitational radiation. That means that ripples in the metric of spacetime carry off energy. Recent advanced simulations of the merger of two black holes can, if their spins are aligned correctly, impart a thrust to the merged product. The effect is known as gravitational wave recoil. If ripples of time can transport energy and momentum, it is hard to see them as anything but real. For that reason, I support Metric Substantivalism. However, I would argue against Manifold Substantivalism on the basis of Leibniz Equivalence and the Hole Argument (on the latter, note that the SEP does not differentiate between the two forms and really only addresses Manifold Substantivalism).

    Last edited: Mar 11, 2012
  18. Mar 12, 2012 #17
    Some years ago I thought that this was a proof of the nonexistence of time, but then I realized that it is not necessarily the case, or at least there are some caveats.

    It is quite important, as you mention, that the past and the future are inferred from the present, and we do that through patterns and regularities (e.g. the laws of nature and other hacks). We never 'directly' observe them, true.
    However, the special status of direct observation applies to truly a small fraction of events/things (someone would say to none), especially in modern physics. We observe lots of things indirectly - we infer the existence of quarks or other subatomic particles starting from very indirect observations, through a certain set of patterns and regularities (e.g. QCD). We have reason to believe their existence because there is overwhelming, consistent evidence.
    So if we do accept 'existence by inference', I am not sure why we should treat the ontological status given by inference differently when dealing with the time direction rather than any other 'direction' (e.g. scale).

    Given this, my current feeling is towards an "extended presentism". That is, I think that the present carries some structure about some of the future and some - possibly a lot (but it might depend on the scale) - about the past, for this point see the 'time capsules' theory of Julian Barbour (even if he is arguing for the non-existence of time).
  19. Mar 12, 2012 #18
    The point may have been misunderstood; we don't infer "past" and "future" from the present; we can't even infer them from the present; it is always the present, and all we ever have are mental constructs of "past" and "future"; even a photograph of a "past" event exists only in the present, and isn't the event that was photographed.

    All we can infer, from the present, is that "past" and "future" are mental constructs.
  20. Mar 12, 2012 #19
    I suppose my point has been misunderstood.
    It is always the Present, yep, we agreed on that from the start.

    However we do infer - i.e. compute/construct through an inference process given some evidence and a set of rules - the 'Past' from the present. 'Past' is a useful label/index/category that we use to refer to some specific structure/ordering in the information in the present (e.g. the time capsules Julian Barbour talks about). A photography is a good example. A slightly different argument but similar in flavour applies to the Future.
    Anyhow, we never see the Past nor the Future.

    So yes, they are mental constructs, useful descriptions - as are 'electrons' and 'quarks'. Subatomic particles are inferred too (i.e. computed/constructed through an inference process given some evidence and a set of rules). They are too a useful label/index that we use to refer to some specific structure in the information in the present. You do not see an electron, you see a trace on a plate. Even less we can see of a quark.

    Subatomic particles are just an example. The point I was making is that we are typically happy to grant 'existence by (sound) inference', so I am not sure why we should deny it to the Past and Future on the basis that we never see them directly.
    OK, it can be argued that 'Past/Future' and 'particles' belong to completely different ontological categories, and so different rules apply, still I do not see why.
  21. Mar 12, 2012 #20
    The point is that there isn't even a trace of "the past" or "the future"; there is nothing from which to infer it. We construct models of the future and have records of the past, but all that can be inferred is that the present moment is ever changing.

    "Past" and "future" are useful concepts, but that is all that can be said of them, they are merely concepts. If we take the photograph example, how do we infer that "the past" continues to physically exist?

    The photo was taken in the present; the scene that the photograph pictured has since changed and can no longer be observed, so there is no evidence of its continued existence. The photograph only ever exists in the present. How can we infer that the past is anything more than a mental contstruct; or the same for the future?
  22. Mar 13, 2012 #21
    I'll try to unpack a bit my thoughts, since you're stating things I agree on as if I was saying the opposite, not sure I am expressing properly.

    For me the Past/Future have no independent existence from the Present. They are 'derived quantities', not fundamental. As I already said, I am a Presentist at the moment. (Of course I couldn't be at any other moment.) I'll make a couple of examples to explain myself:

    1) If a demon changed all the present evidence regarding a 'past event', I mean all the information regarding the event which has decohered in the universe, people would typically think that our available information regarding the Past has been changed, but not the Past itself. My opinion would be that after the demon's intervention the Past has been changed, since the Past sits 'on top' of the Present (or in other words, it from bit, if you change the information regarding the thing, you change the thing).
    There is no thing as an 'independent Past', lying somewhere behind us (or in front of us, according to the Aymara).

    2) If the structure of the Present does not support the existence of records of any type, or inference systems, Past and Future lose meaning. Past and Future are labels we give in the presence of specific patterns of structure. I want to mention though that these are not arbitrary labels, they do depend on objective features of the patterns (independent observers would agree on the ordering of causally related events).

    So if by existence you mean 'independent existence', I think we more or less agree from the start on that, Past and Future are not 'out there', they are (re)constructed from the Present. What I wanted to point out with my first post were only a couple of remarks:

    a) Existence does not mean only independent existence. We typically do grant existence to things which depend on other things to exist.
    But I agree that it is good to keep the distinction in mind, and since most people do believe in an independent existence of Past/Future from the Present, I see why it is easy to overreact and take the opposite side, that they do not exist at all (I did that).

    b) I do not think that the 'mental construct' position is a particularly useful one; if you dig deep enough (almost) everything is a mental construct. So it's not false, I appreciate that, but I don't see the specifics that make it non-vacuously true.
  23. Mar 13, 2012 #22
    On the contrary, all we ever observe is the past. If I stand 3m from someone, it takes light 10ns to travel from them to me so I see them as they were 10ns into the past. From that and our experience that objects persist in some manner (conservation laws), we infer that they still exist. In fact QM permits the possibility that they ceased to exist 5ns ago (at a very low but non-zero probability).

    What you need to consider though is what the labels "past" and "future" really mean. Suppose one person, "Alice", walks east past someone else, "Bob", who is standing still. They momentarily brush arms as they pass which defines a reference event 'P'. There is another person "Charlie" standing 300m ahead of us who claps his hands defining event 'Q'. If the image of event Q arrives 1us after event P then Bob will conclude that P and Q were simultaneous, that is he considers Q occurred what he called the present at event P. Alice on the other hand finds the image arrives less than 1us after P hence she classifies it as having already happened before P, it was already in her past. Similarly, someone walking west who also brushed arms with Bob at P would consider Q happened later than P, it was in their future. The concepts of past, present and future are labels which can be arbitrarily assigned to events which are space-like separated. This is one reason why many people think that it is very hard to reconcile presentism with relativity.
  24. Mar 13, 2012 #23

    I understand that there can be delays in our perceptions to what is happening "out there", but I think it is a little misleading when people say that "we perceive the past" as though the past is still eixsting as we perceive it in the present. It is no more different than sending a letter across the world in my opinion.

    But I imagine that the situation in SR is more deeper than that, but that is just my impression from what I am learning about it currently.
  25. Mar 13, 2012 #24
    Absolutely, the acts of posting and receiving are "timelike" separated, in all inertial frames they will occur in that order, you can only read what was written in your past. That doesn't imply that it still exists though, as you say.

    Yes, in the example I gave, the events P and Q are defined as happening at the same time in Bob's frame. Those events each exist at some instant. What you have to think about is when they exist as viewed by the two people passing Bob. At the moment when they agree P is "now", one thinks Q is yet to happen while the other thinks it has already happened but they all agree the event actually happened. It is just one event but one considers it part of the past while for the other it is part of their future.
  26. Mar 13, 2012 #25


    User Avatar
    Gold Member

    If we take time as some general attempt to measure change, then the problem seems to be that change has two contrasting sources. So the "present" becomes a mixed state.

    One source of change is global constraint. That is the sum of everything that has gone before to create a history. And because it takes time - in a sub-relativistic world at least - for distant events to impinge on some particular locale, this history is always "just arriving". For a locale, some constraint - like perhaps the ripple of a gravity wave as mentioned - was not there a moment ago, but is changing things for that locale right now.

    This is the kind of deterministic change modelled by Newtonian mechanics. The past is already coming at you, like a piano dropped off a high building above your head, but you just don't know it yet.

    But there is also a second source of change, and that is local degrees of freedom. Whatever is not past-constrained is a source of future-oriented spontaneity. It is a potential waiting to be realised in some locally independent fashion.

    Now mostly our large~cold universe seems to have very little spontaneity about it. It has so much past-history that the state of every locale is highly constrained, almost completely determined. The trajectory of particles is inertial, apparently tied to the track of its history and with no freedom to swerve or in any other way show accelerative change.

    But QM shows that there still is some kind of actual local freedom, some indeterminacy that complicates any simple measure of change.

    And for actually complex systems like humans, there is a lot of local freedom. We can look up and see the falling piano. Or for some other reason, just happen to step out of its path. The psychological notion of time thus emphasises the local freedom, the creative spontaneity, that comes with being able to anticipate - "look into the future" - and so respond according to imagined constraints rather than being just determined by past ones.

    So time is our attempt to model/measure change. And the model we have constructed actually reflects a rather particular "time" in the universe's development - it is the view when things have become generally so expanded and cold that almost all the local spontaneity has been squeezed out of the picture, nearly everything is narrowly determined by a history of past constraint. A deterministic Newtonian model works because it covers 99.99...% of what is "happening". Back nearer the big bang of course, the spontaneity of fluctuations ruled.

    Now a further feature of our modelling of time is that almost without comment, we take the sub-relativistic view as our natural baseline, as if a lack of change - being at rest - is the default state of things. But of course, it is the relativistic realm of the photon which is the real baseline state for the universe. When mass condensed out as the universe expanded~cooled, suddenly a new local freedom was created - the possibility to go slower, to lag behind. If we are talking about dimensions, this phase change added another general temporal dimension that had not existed previously.

    So we have a general story that the notion of time is our attempt to create some general or universal measure of change. It would be conveniently simple if it were just a Newtonian dimension - a metric without any complex structure. If every locale marched in lockstep with no local degrees of freedom, just a single universal history in which not just past-constraint completely rules, but also all action is being measured from the most extreme case of "locales at rest", the farthest possible end of the non-relativistic case.

    However, time does have this more complex structure. The balance between local freedom and global constraint changes simply because the universe cools and expands. The hot quantum possibilities get decohered, the universe becomes more classically past-determined. On top of this, there is the condensing out of mass that sets up a new dimension of change, the sub-light realm where it takes time for locales to learn about events (and so creates new local inertial degrees of freedom). On top of this again develops the kind of psychological degrees of freedom we have that come from being able to imagine our futures, and act on the resulting beliefs.

    Presentism recognises that there is this intersection between global constraints and local freedoms. It divides them into a map of the past vs a map of the future. But then it also wants to universalise this story - to apply it everwhere to create a universal moment.

    I think this can be done in reasonable fashion if anchored to a thermal notion of time (as being suggested by Carlo Rovelli, for instance). The temperature of the universe at some common "moment" does create an average benchmark of change (the relative remaining potential for change, as well as its direction).

    But still, a lot of the nagging ontological questions being asked here appear to arise from wanting to have some overly simple model of time - collapsing its complex and processual structure to a single universal dimension.

    Now it is quite possible that a more complex model of time is of no real practical use - physics could do without it. Yet I think as we come to understand the universe and the big bang in terms of entropy dissipation, and the fact that sub-relativistic mass is an emergent extra to the story, not a fundamental fact, then the modelling/measuring of time may indeed be in for a shake-up.

    As well as Rovell's thermal time, there are other interesting stabs at the problem, like Stu Kauffman's notion of the adjacent possible - an edge of chaos type balancing act story where, as I argue, the present moment for any locale is a dynamic balance of past constraints and future freedoms.

    The retrocausality implied by QM nonlocality also has to be fitted into the picture. The future seems to create its own past in some admittedly very slight degree.

    Debates around the nature of time usually stick to Newtonianism vs relativity. But QM and thermodynamics are also ingredients in the story. Boil it all up and what is our overall model of change then?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook