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What causes the arrow of time ?

  1. The projection postulate / Born rule: Waves collapse only into one direction.

  2. Non-unitarity: Unitarity is not exactly 100%

  3. Imperfect entanglement: The conservation laws are not exactly 100%

    0 vote(s)
  4. Spontaneous symmetry breaking: We might see Universes where time goes the other way

  5. The time dimension itself is simply one way: The future does not yet exist.

  6. Other (explain or suggest)

Multiple votes are allowed.
  1. Nov 5, 2005 #1
    Precisely this is the reason that ALSO the problem of arrow of time is still unsolved

    It is simply false that a non-unitary evolution can be derived from an unitary evolution as a kind of "good approximation". It is mathematically imposible and physically wrong. This is the reason people seriouly working in arrow of time (specialists in the topic) is proponing nonunitary evolutions. For example Prigogine theory, CSM, etc.

    That a non-unitary evolution cannot be obtained from an unitary evolution was already adressed many time ago. In words of specialist van Kampen: irreversibility cannot be obtained from reversibility except by an appeal to manthematical funambulism. He clearly emphasized the word funambulism. In fact all 'derivations' in literature beggining from unitary physics have wrong mathematical steps of kind "since 2 + 2 = 5 then A > B". People is doing is adding wrong mathematicals teps for deriving the corerct answer from a incorrect beggining. That is, NOBODY is deriving irreversibility from unitarity.

    All supposed 'derivations' i know from literature are mathematically wrong and physically unsustainable.

    Your example of decay of atomic states is simply wrong as is well-known in literature on the problem of time. There is a couple of mistakes in standard elementary textbook 'derivations' (i remark supposed derivations). Literature on why standard elementary approaches are wrong when one study details is excesively huge i can cite all relevant papers on the topic. But i can say some of typical errors.

    First the use of a continuum of radiation does not introduce irreversiblity since QFT is time-simmetric. The quantum states are not defined in standard QM and QFT and one uses approximation that a state is described via Dirac kets, which is not true, because the Dirac state is valid only when interaction is EXACTLY zero. Some authors are exploring more general states like Gamov ones.

    The use of a pure continuum is an approximation known like 'thermodynamic limit'. In standard approaches resonances between discrete spectra and that ill-defined continuum spectra are simply ignored. In rigor, standard QM does not work in that continuum. In fact, as proven by Prigogine and colleagues the Hilbert space structure of QM collapses and wavefunctions loose probabilistic interpretation, for example the norm of density matrices is NOT the unity -they solve this introducing a more general RHS-. The relationship between the non-hermitian 'Hamiltonian' and the original Hermitian one is NEWER addressed. One can prove that the solution choosed in textbooks is incomplete (in a similar manner like ignoring negative energy states in relativistic Schrödinger equation does not work). The total system atom + field continues to be reversible and production of entropy computed is zero, which is wrong, etc.

    As said the derivation of the nonunitary law from the unitary one is mathematically wrong. People DOES is really substitute the unitary law by the nonunitary one at some specific point of the computation, but this is 'hidden' is usual presentations -however one can prove that is that people is really doing-. Etc, etc.

    1) Precisely the problem with QM -as already noted by Einstein- is that QM is incompatible with classical mechanics. Precisely Born explicitely splitted universe into two parts, classical and quantum, with QM applying only to the latter. The problem of quantum measurement is that people is attmepting to derive measuremente from QM only when one needs introduce some classicality concept from outside of QM. 2) Precisely Prigogine approach is the construction of a generalization of QM ALSO applicable to classical systems. It is also Penrose approach who argues that GR cannot completely quantized and that classical residue is hidden element for explaining measurement.

    I think that both approaches (Prigogine and Penrose) are good but are not the final answer.
    Last edited: Nov 5, 2005
  2. jcsd
  3. Nov 5, 2005 #2
    This popularity of this 'explanation' is only suppered by its incorrectness.

    It is completely false that arrow of time can be explained via initial conditions alone. It is also false that appeal to "improbable". Probability is computed from wavefunctions or classical distributions functions. Since basic evolution law is time-simmetric, transitions from less probable to more probable are theoretically permited.

    When one solve Schrödinguer equation one uses an initial state

    Phy(t) = exp(-iHt) Phy (0)

    That do NOT introduces irreversibility because the equation is time symmetric. On ANY application of above equation evolution is reversible and production of entropy is zero.

    However, in Prigogine theory the basic equation is irreversible and applied to the same initial state Phy(0) evolution IS compatible with experimental data: irreversible and producting entropy that verifies second law.

    Take the irreversible process A ---> B

    Irreversibility does not mean that initial condition A explains transition to B. Irreversibility means that when the system was in B, newer returns to A.

    The process B ---> A is newer observed.

    Therefore the evolution is

    A ---> B ---> C

    if B is an equilbrium state

    A ---> B ---> B

    Moreover, one would remark the paradox that those 'highly improbable' initial states are ALWAYS observed, just in the initial state of the irreversible evolution. If A was really highly improbable (so improbable that newer will observed, why do we observe always? At t =0 the state is precisely that 'highly improbable' state A).
    Last edited: Nov 5, 2005
  4. Nov 5, 2005 #3

    Hans de Vries

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    What causes the "arrow of time" ?

    What causes the "arrow of time" ?
    Multiple Choice and Public.
    Alternative suggestions welcome.

    Regards, Hans
    Last edited: Nov 5, 2005
  5. Nov 5, 2005 #4


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    I think you've forgotten the standard textbook explanation in statistical physics: the very special initial condition of the universe...
  6. Nov 5, 2005 #5


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    Do a simple simulation on a computer, with a totally reversible dynamical law: you can very simply simulate "entropy increase".
    For instance, put classical elastic marbles packed in one corner of a cube, all with the same momentum, and let it evolve. You get soon a totally messy distribution which looks a lot like a classical perfect gas. The dynamics is perfectly reversible. The initial condition was special. Liouville's equation applies. No singularities in the dynamics. No magic.
  7. Nov 5, 2005 #6
    I think the "arrow of time" is cause by the 2nd Law of theromodynamics. The amount of entropy in a system will always increase; the way it increases is if it travel forward in time. I believe if without the 2nd law of thermodynamics we would not be able to tell the difference between forward in time and backwards.
  8. Nov 6, 2005 #7
    Thermodynamics, self-contained determinismus of the evolution
  9. Nov 6, 2005 #8
    Causality requires time to be one-dimensional and unidirectional (although one could try multi-dimensional time, but the other dimensions must be compact variables), else one can construct scenario that defy causality.
  10. Nov 6, 2005 #9
    That the time dimension is one way seems to follow from the big bang model. The concepts of causality, the past, the future, evolution, etc., have the meanings that they have, and are physically meaningful, because the universal wave front created by the big bang is moving isotropically away from its source, and the flotsam and jetsam (which constitute us and the rest of the physical universe) moving in the wake of this expansion must follow this general, universal trend. (ie., any direction of motion follows the general omnidirectional expansion)

    So, disturbances move, on any scale, away from their points of origin. If the disturbance is in a more or less homogenous, isotropic medium, like water or air or light, then the disturbance moves more or less isotropically away from the origin. It simply can't be any other way in an expanding universe. In order for phenomena to spontaneously return to previous states (eg., the evolutionary process that led to a broken cup suddenly reversing and the cup assembling again, or the observation of advanced waves) it would seem necessary to reverse the universal, isotropic expansion -- and at least one way of interpreting the available evidence suggests that this is impossible (at least in our universe).

    These considerations don't depend on positing a certain set of initial conditions, but only on observations of how the universe at large (and medium and small) is behaving.

    The observed expansion is the fundamental physical reason why there is any motion at all in the first place, and observations suggest that that motion is constrained in certain general ways {including the i) necessary evolutionary direction of any process, ii) inertia, iii) and a universal speed limit on any evolutionary process, any propagation}.
    Maybe the speed limit hasn't been nailed yet, maybe it isn't defined fundamentally by electromagnetic phenomena, but if the universe had a finite beginning, is finite in extent and energy content (even if it's constituents are continually evolving according to local interactions), and is expanding, then it seems to me that a universal limit on the rate of any evolutionary process is required.
    Last edited: Nov 6, 2005
  11. Nov 6, 2005 #10
    Isn't Time just the product of change?
  12. Nov 6, 2005 #11
    Time is change. The question concerns an apparently general characteristic of, and constraint on, change. We observe an 'arrow of time'. Nature never runs in reverse of this 'arrow of time'. Why??

    Statistical physics says that Nature can and will run in reverse, but that the probability of this happening is so small that FAPP it will never happen.

    I would rather assume that Nature *can't* run in reverse, and consider why that must be.
    Last edited: Nov 6, 2005
  13. Nov 6, 2005 #12
    The cause for the arrow of time may be just entropy, because if a closed system is at maximun entropy and you were to say record it on tape and watch the tape backwards, you would not know that your watching the tape backwards because the entropy would not increase anymore. thus, the arrow of time is just a system going from low entropy to high entropy and it's just the because of the second law of thermodynamics

    for example take a box filled of half footballs and soccerballs each on one side, then shake it up to increase the disorder(entropy) and if you were to tape it on video and watch the tape backwards you would not be able to tell if it was forwards or back, but at the beginning when the balls was all organized, you would because it was going from a state of low entropy to high entropy.
  14. Nov 6, 2005 #13
    Systems tend to evolve toward equilibrium. Drop a pebble in a flat pool of water and the disturbance will propagate outward until the pool is flat again. It never happens that a, say, 50 meter diameter, wave front spontaneously appears in a flat pool, propagates inward toward a central point, gradually increasing in amplitude and decreasing in diameter, until suddenly, the pool is flat again.

    Just as Newton's gravitation law doesn't tell us the physical reason why gravitating bodies behave accordingly, and just as the first law of motion doesn't tell us why there's any motion in the first place or the fundamental physical reason for inertia, the second law of thermodynamics doesn't tell us why there is an arrow of time. It's just one way to describe it.

    The alternatives in the poll aren't physical reasons, per se, for the arrow of time. That "the time dimension itself is simply one way: The future does not yet exist" is simply a restatement of the arrow of time that our collective experience tells us is a fact of Nature.

    The way of talking about it that I've learned is that the fundamental physical reason for the arrow of time is the isotropic expansion of the universe.
    Last edited: Nov 6, 2005
  15. Nov 7, 2005 #14

    Hans de Vries

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    That's the GR version indeed, but as you say it's an initial condition.
    There might be some equivalent border condition at the end of time as well.

    I'm interested in how this keeps working each and every moment, what kind
    of processes, if any, are responsible... quantum mechanically or other.
    Off course this poll is more of a gut-feeling kind poll rather than a "what is
    the answer" poll.

    The big bang response reminds me of an amusing answer I once read, on
    physycs.research, on the question where the 'missing' anti-particles are
    (the particle/anti-particle asymmetry in the universe) The response was:

    "They all flew off into the other direction, BACKward in time" :rolleyes:

    Regards, Hans

    P.S. I suppose it's only Greg who can edit/patch thread titles. It's what
    you get when you're paying attention to a five-year old at the same time :smile:
  16. Nov 7, 2005 #15


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    Apparently, super mentors can do it too :smile:
    (didn't know until I tried).
  17. Nov 7, 2005 #16
    All other options (except the latter which is not defined) on the poll follow directly from the non-unitarity approach.

    In fact, Prigogine theory is a non-unitary approach. Penrose theory is non-unitary, etc.

    Many time think that projection postulate alone explain arrow of time. Well that is not true, and this is the reason of in more than 100 years the quantum measurement problem has been not solved and decoherence approach is in a dead way. Prigogine has shown as the projection postulates follow from his nonunitary theory. One begin with a quantum system in a superposition state, then the system contact with a measurement systems (an LPS in Prigogine theory). The theory clearly shows how the wavefunction collapse.

    In fact, any other derivations of the arrow of time without the explicit use of nonunitarity are mathematically wrong and unphysical. This is the reason that Penrose also has choosed nonunitarity.

    Most of physicists do not like unitary because there is a theorem that links unitarity with conservation of quantum probability. The theorem of course is valid only on standard quantum mechanics in a Hilbert space. One can construct a nonunitary theory with conservation of probability.

    Any attempt to explain the arrow of time on function of 'initial conditions' or ratios of probabilities is completely wrong. One would read advanced literature before claim that solutions is in a basic textbook. One would read detailed analisys if those solutions before believe that are correct. The best valuation of those irrelevant explanations that textbooks explain was done by specialist in arrow of time and stochastic theory van Kampen:

    Last edited: Nov 7, 2005
  18. Nov 7, 2005 #17

    Hans de Vries

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    Thanks! :smile:

    Regards, Hans
  19. Nov 7, 2005 #18
    It appears you have an increased tendence to trivialize things. It appears you think that reading some basic textbook you are in the cutting edge of a specific research topic and 'all is known' or well you 'are solved the question'. Again you are wrong.

    Vanesch, there is different levels of literature from 7-years old coloured books to advanced very, very specific journals (as Chaos and Fractals). You would read published research level material on a specific topic before doing irrelevant claims. If your only basis is one elementary textbook, vanesch, you would be more 'prudent' on your claims.

    If you use an unitary propagator on the simulation, the simulation is reversible on all moment and does not increase entropy.

    If you use a nonunitary propagator (for example forcing 8bit digit arithmetic), then due to rounded errors of the simulation process, trajectories are breaked and the system cannot memorize the trajectory and simulation generates entropy. In a nonunitary simulation (which is the usual due to limitations on memory and digits of computers), when you reverse the simulation the computer does not obtain the initial state due to acumulation of rounded errors. Then one can prove that generates entropy.

    If you put classical elastic marbles packed in one corner of a cube, all with the same momentum, and let it evolve. If they evolve unitarity, entropy is, of course, conserved by Liouville theorem. If you use a nonunitary propagator (for example forzing 8bit digit numerical arithmetic or programing colisions probabilistically via a model of independent particles colliding at azar (as in a perfect gas) then you can simulate entropy increase. In both of last models the simulation is not dynamical and Liouville theorem does not hold.

    Any attempt to derive irreversiblity from a reversible law is subject to (in specialist on arrow of time van Kampen words)

    It appears that you like mathematical funambulism. But people doing research in the arrow of time has proved -in basis to rigorous published work- that simplistic approaches as yours are completely incorrect.

    As already explained above, initial condition is not the key to irreversibility, because i) if dynamics is unitarity by Liouville theorem entropy is conserved and violates second law of thermodynamics. ii) if one takes the final quasiequilbrium state B on (A ---> B), the use of initial conditions doe not forbid the unphysical return to 'A', which is newer experimentally measured.

    There are many publications in the topic proving that initial conditions do not solve the arrow of time. You would read research-level literature on a specific topic before doing irrelevant claims.
    Last edited: Nov 7, 2005
  20. Nov 7, 2005 #19
    If one observes a box of Gas molocules tucked away in one corner, over time the Gas tends to order, via a distributed thermal Equilibrium?

    If one now replaces the Gas molocules with Gravitational bodies, then things tend to evolve the other way, they tend towards collecting into clumps (like the Gas initial location Molocules in 1st example), as entropy increases, bodies collect tegether, finally there is a vast increase at the location of clumping as Blackholes form.

    From the Penrose book Road To Reality page 707.

    The 'initial' Arrow of Time can be manipulated if one has systems that are isolated, a Blackhole provides a technical isolation location, the Big-Bang has to have had an intial state, Gas, Liquid, Solid or other?
  21. Nov 7, 2005 #20


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