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The arrow of time

  1. Sep 25, 2006 #1
    I have only recently started reading about QP and there seems to be a big question mark on whether time has an arrow (direction). There seems to be a tendency leaning towards entropy having an influence on this topic. This is that, time moves in accordance to order vs disorder - the arrow facing in the direction of objects moving from order to disrder. My question: is it possible that the time arrow could point relative to objects loss of energy?
  2. jcsd
  3. Sep 25, 2006 #2
    I'm struggling to understand what you're asking here.

    As I understand it, the 'arrow of time' is simply a philosophical explanation for why processes happen one way but don't immediately undo themselves. We postulate that entropy can never decrease; so that's a constraint on the passage of time - that processes can't happen in 'reverse' as seen from our perspective, because you'd have to get rid of some entropy, which can't be done.

    However, there's no such constraint on entropy. You can - in theory - put energy back if you take it away, and vice versa - there's no explicit law that restricts the 'direction' of energy flow. Energy can be constrained by means of entropic variation - and almost always is in practice, but that's a property of entropy, not energy.

    In other words, because energy has a conservation law, things can 'run in either direction' because you end up with the same amount of stuff as you started with. Entropy isn't conserved and has a law that defines how it changes, so that produces the 'arrow of time' effect.
  4. Sep 25, 2006 #3
    Thanks. Formulating the questions is almost as difficult as coming to grips with some of the principles.
    I have read that it is a goal of all matter to achieve the lowest energy state possible. When objects move from order to disorder, are they always achieving this goal?
  5. Sep 25, 2006 #4


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    There is no such thing as order and disorder. Precisley, such concepts might exist in philosophy. Terms which would match 'order' and 'disorder' in physics could be 'equilibrium' (with zero velocity) and 'motion'.
  6. Sep 25, 2006 #5
    I have heard the terms "order and disorder" mentioned when regarding the universe. Is the universe (that most people think is expanding) moving from a more ordered situation to a more disordered one or the other way round. The terms order / disorder form part of the entropy priciples, where combined with probability, eggs for example are more likely to break than unbreak (order/disorder). Maybe I am barking up the wrong tree, and need further expanation. My original question was that time "seems" is percieved to move in a certain direction even if Einstein and Newton´s equations do not stipulate this and maintain that in physics, time, as per their theories, can move forward or backward.
  7. Sep 25, 2006 #6
    radou is right about the use of order and disorder. We may look at quantities like radial distribution functions when systems undero phase transistion but these have been shown to be pretty poor indicators of phase, just look at the whole class of amorphous solids.

    Also matter does not try to minimise its energy if it is not in equilibrium. Classically what is minimised is the action. For time independent systems this correspondes to minimisation of energy.

    There is no evidence for a time-reversal symmetry, just as there is no evidence for tachyons. All symmetries in physics do not have to be realised, but we should ask that if they are not, then Why?
  8. Sep 27, 2006 #7
    shaun: time doesn't actually "seem" to move in any direction. There's no forward or backward about it. That's just linguistic baggage, what you've been brought up with, what you're used to. What's really happening here is that things move, not time. Time is the measure of this motion against other things in motion, including our clocks, or the workings of our brain.
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