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A Question of Time?

  1. Jul 9, 2012 #1


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    I was wondering whether there is any general consensus about the nature of time within physics, as a whole. For example

    • Classical mechanics seems to suggest that time is an absolute parameter, which is essentially reversible in terms of its equations. Within this description, cause-and-effect appears to be linked in a deterministic manner, although it is not clear that such a description offers any real explanation of time or addresses the problem of free-will within such a rigid deterministic model.

    • Classical thermodynamics in its description of entropy appears to suggest that time cannot be reversed, although this premise may be said to be a statistical approximation of an aggregated system, i.e. an ensemble of particles. So while the system, as a whole, suggests the irreversibility of time, it does not necessarily exclude the reversibility of time when applied to individual particles within the system. This may leave the door open for quantum physics – see below.

    • Special relativity obviously redefines time as a relative parameter and possibly as an integrated parameter within spacetime. However, while special relativity can lead to different relative measures of elapsed time, causality appears to be maintained, such that there can be no obvious reversal of time, although some interpretations of the maths associated with negative energy may contest this argument.

    • Quantum mechanics (QM) and its extension into Relativistic-QM (RQM) appears vague on some matters. As understood, time was originally an operator in QM, but subsequently ‘demoted’ to a parameter in QFT. It is also not clear that RQM has anything more to say than QM or special relativity, i.e. if causality is maintained, then time must remain essentially irreversible. However, there may be some implicit reference to the granularity of time, i.e. non-continuous, in the definition of Planck time; although this would not necessarily change the overall ‘arrow of time’.

    • Quantum Field Theory is possibly open to more interpretations based on the conceptual nature of virtual particles and the Feynman-Wheeler idea of advanced and retarded waves, e.g. as defined by John Cramer’s transactional interpretation. With reference back to classical thermodynamics, entropy suggests that time is irreversible, but doesn’t quantify what might happen at the individual quantum particle level. Again, as far as it is understood, virtual particles can conceptually travel backwards or forwards in time within the limits of the Heisenberg uncertainty principle, when quantified in terms of energy and time. However, virtual particles as defined within Feynman diagrams are said to be unobservable and therefore we appear to be forwarding a conceptual ‘probability’ not a verifiable argument. Again, without understanding all the details of the Feynman-Wheeler idea, one counter-argument against this idea appears to come from cosmology in that for the advanced and retarded waves to cancel out in the present, the size of the universe in the past and future would have to be the same size, which is a bit of a problem within the Big Bang model.
    Clearly, human beings have an intuitive sense of time, although we invariably struggle to describe the concept in any rigorous manner. However, in many ways, physics also appears to have some difficulty in reconciling its idea of physical time with the conscious experience of the ‘arrow of time’ as supported by thermodynamics; while at the same ‘time’ our ideas about free-will appear to reject cause-and-effect determinism. Purely as a bit of speculation for debate and by way of summary:

    It would seem that quantum physics might argue that time cannot remain continuous as it approaches Planck time. It might also argued that time might be reversible within quantum uncertainty limits; although such processes are unobservable. In contrast, all observable processes confirm time to be irreversible. If so, the issue of determinism and free-will are not necessarily directly connected via time. Therefore, as a speculation, might all macroscopic systems cease to be totally deterministic simply because of quantum uncertainty, i.e. all outcomes are underpinned by probabilistic processes. So, as a consequence, even if two systems could be created in exactly the same initial state, e.g. |A>, and then subject to apparently identical conditions, quantum probability could always lead to a different outcome, e.g. <B| or <C|, at some level. As such, it is the probabilistic nature of quantum ‘reality’ that supports the non-determinism required by free-will, while for all practical purposes, systems continue to follow the ‘arrow of time’.

    Any thoughts or clarifications?
    Last edited: Jul 9, 2012
  2. jcsd
  3. Jul 9, 2012 #2
    Good, and interesting, post mysearch.

    Since the discussion/questions are at the edge of our knowledge, the discussion would have to be imaginative extrapolations based on limited human knowledge at this point in time.

    The time reversal within the narrow quantum limits might be possible because

    -perhaps interaction, with any of the elements of/within time-space, has not happened
    -or perhaps those interactions are reversible without any loss of information/entropy/energy etc

    Quanta/photon might be the smallest particle within time-space, however perhaps even that EM carrier has to manage its internal processes.

    on a separate note: the randomness of quantum mechanics is not necessary to justify free will. i think a framework of conceptual loops in the brain is enough and even some animals have (relatively) limited amount of free will.
    Last edited by a moderator: Jul 9, 2012
  4. Jul 9, 2012 #3


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    Could you clarify what is meant by 'a framework of conceptual loops in the brain'?
  5. Jul 9, 2012 #4
    I don't agree with part of the quote. In my opinion, the strongest argument put forth for the possibility of "free will" are positions that are able to challenge the following premise by Carl Hoefer:
    If it can shown that there exists the possibility for some type of 'downward causation' between the macroscopic/microscopic domains, then maybe "free will" can occur? Determinism or non-determinism is irrelevant, in my opinion. So, I can't see how a indeterministic universe would help the "free will" position anymore than a deterministic universe. It seems that would just lead to a "random will"?
  6. Jul 9, 2012 #5
    Many folks try to relate randomness with free will.

    If the choice is (truly/quantum) random then the human does not have a choice, it's not under human control......but then that's not the definition of free will...

    Bohm posted same thought, similar time and I agree with him.

    As to "framework of conceptual loops (tied to survival) in the brain.....will post a bit later
    Last edited: Jul 9, 2012
  7. Jul 9, 2012 #6
    If you want an entertaining discussion of free will, but keeping it within the domain of physics rather than philosophy, the Conway Kochen free will theorem is an entertaining read.
  8. Jul 9, 2012 #7
    It is my understanding that the opposite is true. I.e. in quantum mechanics time is not considered a variable, not an operator. Its only in QFT that time is promoted to the statis of an operator.

    Sure. In my graduate quantum mechanics class we used Quantum Mechanics by Cohen-Tannoudji, Diu and Laloe. On page 252 the authors write
    The italics are the authors.
  9. Jul 10, 2012 #8


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    To be honest, I really started this thread just to see whether there was any consensus about the definition and description of time in physics. In this context, I had not really considered the issue of free-will in any depth as it mainly arose due to the possible source of non-determinism in quantum physics. In this respect, quantum physics seemed to offer an alternative to the determinism of classical physics, which I simply assumed contradicted the idea of free-will. However, I am beginning to see that this subject extends way beyond my original naïve assumptions. So let me comment of some of the posts by way of my own education, which I have initially anchored in a quick reading of the following link: Causal Determinism: Stanford Encyclopedia of Philosophy.

    However, up-front, there are aspects of this article that seem quite contentious to me, e.g.

    “Physics, particularly 20th century physics, does have one lesson to impart to the free will debate; a lesson about the relationship between time and determinism. Recall that we noticed that the fundamental theories we are familiar with, if they are deterministic at all, are time-symmetrically deterministic. That is, earlier states of the world can be seen as fixing all later states; but equally, later states can be seen as fixing all earlier states.”
    Sheaf, thanks for the link, but based on a quick read I am not sure that I am any the wiser of what this theorem is really proposing. While aspects of free-will may well come under the heading of philosophical debate, it would seem that if physics cannot explain free-will, it doesn’t understand a key mechanism at work within the universe. In this context, can it be argued that this is just as much a topic of physics as anything else?
    San K, I think I agree with this premise, although would appreciate any further insights.
    Bohm2, you seem to be arguing that free-will cannot be directly connected with non-determinism. However, it is not clear to me what you (and Carl Hoefer) actually mean by ‘upward and downward causation’. Based on only a quick read of the Stanford link above, I am not pretending to argue with any authority. However, physics appears to basically confirm the observation of the classical laws of physics at the macroscopic level, such that an aspect of nature appears to be essentially deterministic and causal, i.e. time always follows the arrow of time. This said, we might need to qualify this apparent determinism with the potential chaotic nature of deterministic systems as defined by statistical mechanics and thermodynamics. In addition, we also have a perception of some underlying non-deterministic quantum processes plus the apparent existence of some undefined process labelled 'free-will'. Is it unfair to say that collectively, we have no coherent explanation of how all these aspects underpin the world in which we 'appear' to live?
    Despite what I might appear to be arguing above, this seems to be a very good point, which I am assuming that San K supports, i.e. non-deterministic or chaotic processes are not enough to explain free-will? Again, would appreciate any further clarifications on offer.
    Hi, let me see if you agree with the following corrected statement. In QM, position [x] was considered as an operator, while time [t] was only treated as a variable. However, this asymmetry was problematic in terms of special relativity. Therefore, in QFT, position [x] and time [t] are now both considered as variables, while fields are elevated to operators? Thanks
    Last edited: Jul 10, 2012
  10. Jul 10, 2012 #9
    A nice read on the topic can be found in Davie's "the physics of downward causation" but the link isn't working as before but the interesting/relevant quotes:
    Davies suggests a "different mechanism of downward causation that avoids the problem of coming into conflict with existing local theories":
    The physics of downward causation

    I'm not sure of the merits of his arguments although I've seen similar arguments before. For example:
    For whom the Bell arguments toll
    Last edited: Jul 10, 2012
  11. Jul 10, 2012 #10
    The free will theorem is discussed in a bit more detail here. The essential content is:

    I think the paper had a bit of a tongue-in-cheek element about it, but such things are nevertheless worth thinking about. I only mentioned it because you mentioned free will in the OP.

    For a short summary of the various meanings of time in physics, I like Carlo Rovelli's article "Analysis of the distinct meanings of the notion of "time" in different physical theories"
  12. Jul 10, 2012 #11
    That's what my quantum mechanics saysm yes. And I fully agree with it. It's just nice to have a text agree with a person.

    I never took quantum field theory before. However I did see the opening remarks about t and that appears to be the case. Perhaps someone who is proficient in QFT will chime in on that point.
  13. Jul 10, 2012 #12
    Since space and time are the same entity I don't think there will be a consensus to the nature of time until we have a working model of quantum gravity.
  14. Jul 10, 2012 #13
    Space and time are parts of spacetime. In many ways space and time are treated on the same level. However physically they have different meanings physically.
  15. Jul 10, 2012 #14
    I think your confusing time with entropy. But the again there is a paper out there deriving gravity from entropy.
  16. Jul 11, 2012 #15
    I'm refering to the different physical natures of space and time in physical reality with the similar mathmatical roles that they play in relativity. On this point I refer you to two articles on the subject

    From A Brief Outline of the Development on the Theory of Relativity by Albert Einstein, Nature, Feb. 17th, 1921 No. 2677, Vol. 106
    From Relativity, Thermodynamics and Cosmology by Richard C. Tolman, Dover Pub, (1934), page 29
  17. Jul 11, 2012 #16


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    In part, this thread seems to have opened up 2 aspects of time, i.e. its 'fundamental nature' and the 'effects of causation'. My opening post was originally focused on the nature of time as described by physics, i.e. classical, thermodynamics, relativity and quantum. In this context, it might be argued that time is absolute or relative, discrete or continuous, reversible or irreversible etc. However, it would seem that the description of time in physics might influence how we come to interpret the issue of causation, which several of the previous posts have now touched on. As such, I will split my comments of some earlier posts along these lines in 2 following posts.
    Last edited: Jul 11, 2012
  18. Jul 11, 2012 #17


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    Nature of Time:
    It is possible that I am misinterpreting the full scope of of your statement, but I tend to agree more with the argument put forward by the Richard Tolman quote given by Boston_Guy in post #15. For while space and time might be integrated within the mathematical description of a Minkowski diagram, the perception and description of space and time, as separate entities, seems to be persistent. While I do not know the current status of quantum gravity, might it also be argued that the idea of a ‘working model of quantum gravity’ is still an assumption that may never be realised?

    Regarding the various descriptions of time in physics, i.e. absolute or relative, discrete or continuous, reversible or irreversible; is it possible to converge to any practical consensus? Here are some possible issues:

    • If I assume that most people accept the basic principle of relativity, then time is relative, not absolute, but causality is always preserved. However, does the universe age at the same rate if different galaxies are moving relative to each other? If not, is there some concept of a time ‘synchronised’ to the preferred frame of the Cosmic Microwave Background (CMB)?

    • While the mathematical premise of classical physics might lead to the deductive conclusion that its equations are time reversible, empirical observation and thermodynamic appear to support the inductive conclusion that time must be irreversible. From the perspective of a scientific methodology it would seem that the latter should be preferred?

    • The issue as to whether time can remain continuous as it approaches the Planck scale is to some extent hypothetical in as much that this scale is way beyond any practical verification at this stage and does not appear to affect the overall assumption about the arrow of time, although it may have some bearing on the non-deterministic nature of quantum systems?

    • So while quantum physics might argue that time is irreversible within its conceptual description of virtual particles and the limits of the uncertainty principle, should it concede to verification, i.e. all observed quantum states undergo time evolution in the forward direction, as described by thermodynamics?
    Last edited: Jul 11, 2012
  19. Jul 11, 2012 #18


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    Effects of Causation:
    Sheaf. Thanks for the extended references. Note, there was a typo in your link to the Rovelli article that I have corrected above. I have not read either as yet, but will do so asap. Thanks.
    Bohm2: Thanks also for the extended references. Again, it will take me some time to read through this information. However, I found the following link very useful as an initial introduction of ‘Up/Down Causation’ and the example of a snow crystal might possibly more of a realistic starting point for the physics of causation than trying to address the complexity of causation in the human brain. However, those who prefer a more philosophical treatment might like the ‘Backward Causation’ article at Stanford. Again, many thanks.
    Last edited: Jul 11, 2012
  20. Jul 11, 2012 #19


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    By way of a footnote to discussion of time:
    I also very much liked this article, although it appears to have been written in 1994 and probably requires several reviews. However, it appears to provides a very useful summary of 10 notions of time and makes some reference to quantum gravity, which JPBenowitz might wish to clarify and update. However, I do have one fundamental issue with the article that was touched upon earlier, which I would like to raise in this thread, related to the methodology of scientific reasoning. The article does not seem to provide a clear distinction as to which notions of time are solely based on deductive reasoning, i.e. conclusion from premise, as opposed to being supported by inductive reasoning, i.e. conclusion from observation. In part, it seems that many of the notions of time are simply being forwarded on the basis of a mathematical premise, which have no direct verification in observation. Is this assertion incorrect or unfair?
  21. Jul 11, 2012 #20
    I read an article about this subject a while back. It was very helpful to me. The article is called Time as an Observable by J. Oppenheim, B. Reznik, W.G. Unruh and is found online at http://arxiv.org/abs/quant-ph/9807058

    I believe that if you're interested in the subject matter then you'd enjoy reading this article.
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