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Thinking about one directional nature of time

  1. Jan 27, 2012 #1
    Hi all,

    I am from electronics background, yet I have studied quantum mechanics. Lately I have been reading Prigogine I., "From being to becoming". (If I am correct) the book deals with the one directional nature of time as it flows from past to present and onwards to future. But I did not get it how it was explained (or proven) in the book.

    The book suggests that in microscopic world the irreversibility can not be seen. But in macroscopic world when we deal with ensembles and averages the irreversibility is observed. Thus the relevance of time change can be understood. But how to really perceive this time change in microscopic world? Can irreversibility not be observed there?
  2. jcsd
  3. Jan 27, 2012 #2


    Staff: Mentor

    I don't know that reference but I think you would enjoy Chapter 5 of Feynman - The Character Of Physical Law - The Distinction Of Past And Future. Basically the arrow of time is a statistical thing - since the number of disordered states is much greater than ordered ones and roughly they have equal probability this means systems tend to disorder and is what gives the arrow of time. Physical systems are generally symmetric with respect to time reversal but the greater number of disordered states always wins.

  4. Jan 27, 2012 #3
    Are you asking how the 'arrow of time' emerges at the macroscopic scale from time reversible physics at the microscopic scale?
    If so then the most sensible answer I have read is that the arrow of time appears to flow in the direction towards higher entropy. And this change from order to chaos is a macroscopic effect rather than a microscopic one.

    Why would time appear to flow towards larger entropy rather than the other way around?
    Speculative, but possibly because any organism that has a memory would need to be storing low entropy data as that is less data than the present. i.e. storing the low entropy data, which is less information and guessing the high entropy data would be the better strategy, and that would equate to the past being perceived as the low entropy direction.
  5. Jan 27, 2012 #4
  6. Jan 27, 2012 #5
    I've already read that chapter quite a many times before reading Prigogine. :) I understand how irreversibility comes into picture in macroscopic picture. What I wanted to know was what causes "irreversibility" in microscopic picture. Like when two atoms are colliding, we can determine the trajectory. But why do they follow the direction where time increases?

    This is the thermodynamic point of view I've read about. The entropy of any closed system must always increase. But sometimes the entropy is seen to decrease also (described in my reference). However these are macroscopic point of view.

    The author described a Lyapunov function to define the attractor state of a system where the function is minimum. The state always tend to go towards the minimum position of that function to achieve equilibrium. Does this Lyapunov function applies for microscopic states? Will the system's behavior be different if we replace t -> -t ? Will this changed behavior prevent the microscopic state to go back in time?
  7. Jan 27, 2012 #6


    Staff: Mentor

    Just as pure conjecture - but maybe because we as human beings being a macroscopic body subject to the arrow of time of entropy perceive it that way.

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