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Process possible where the entropy of the universe decreases

  1. Oct 20, 2004 #1
    Is there no process possible where the entropy of the universe decreases. If so, then what is the need of using the word spontaneous in the second law of thermodynamics? Why don't we just write any process?

    Last edited: Oct 20, 2004
  2. jcsd
  3. Oct 20, 2004 #2
    The overall entropy of the universe always increases in any process. However, it is possible to locally decrease entropy, but to do so you must actively interfere in natural processes, it won't happen on its own. That's the reason for including the word spontaneous in the second law of thermodynamics.

    An example often used in this situation involves a teacup. A teacup which falls off a table and breaks (increasing it's disorder or entropy), will not spontaneously put itself back together. However, you could get out your glue and glue it back together thereby accomplishing a local decrease in entropy. But, the effort and resources you expended in gluing the cup back together increased the overall entropy of the universe.
    Last edited: Oct 20, 2004
  4. Oct 20, 2004 #3
    would the cup reform itself if time was reversed, ie. spacetime is contracting?
  5. Oct 20, 2004 #4
    There are many enunciations of the second principle. The condition of entropy growth is a consequence of the second principle. Clausius one says that its not possible to transfer heat ciclically from a cold body to a warmer one as unique effect. With this, we can get the definition of entropy and proof that the entropy of an irreversible proccess is positive, and the entropy of a reversible one is zero.

    We can have a system which entropy is decreasing. But the entropy of the universe (the region of space which surrounds our system) always grows. It has no sense apply the local because the universe is "all but our system".
  6. Oct 20, 2004 #5


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    :rofl: :rofl:

    Hey, I thought I was the only Clausius in the world. My name is Rudolph, Rudolph Clausius. I enunciated that principle: Clausius Principle of Thermodynamics. You know, I go for a walk over here and nobody get surprised of crashing into such a famous physicist :cry: !!. I'm only 200 years old.

    :rofl: :rofl:

    Good clarification, Migui. :biggrin:
  7. Oct 20, 2004 #6
    Just because spacetime is contracting, that doesn't mean the arrow of time is reversed. Time, and entropy will continue to "increase" (I'm not sure I can use the term increase with time), even if spacetime contracts.
  8. Oct 20, 2004 #7
    emm.... can someone explain the arrow of time, I thought time only flows forwards because our universe is expanding.
  9. Oct 20, 2004 #8
    This is a borrowed phrase. It refers to the fact that time seems to have a direction; we talk about a past a present and a future. The first place I encountered it was in Stephen Hawking's "A Brief History of Time." He refers in there to three "arrows" of time: the thermodynamic arrow of time, the psychological arrow of time, and finally, the cosmological arrow of time.

    The thermodynamic arrow of time points in the direction of increasing entropy. Our psychological arrow of time points in the same direction; we are not accustomed to seeing broken teacups put themselves back together spontaneously. The cosmological arrow of time points in the direction of the universe's change in size; currently we are expanding.

    Later in his chapter on the arrow of time in "A Brief History of Time," Stephen Hawking explains his conclusion that the thermodynamic and psychological arrows of time will not reverse if the universe begins to contract.
  10. Oct 21, 2004 #9

    One has to be very careful with entropy and Universe's, expansion-contraction paramiters. For instance the Dynamical Space-Time zones of our Visible Universe show in all mathematical inquires that Entropy is Increased, yet the Accelerated Expansion rate of Space itself, seems to lead to a Future that has no Entropy, (when the total mass-energy is zero), thus as the Dynamical Energy dissapates to the last Photon?..the Universe is at its optimum Equilibrium Phase, with respect to Thermodynamic Arrow!

    There are a number of recent advances in 'Observer dependant' Entropic Functions, just a visit to the pre-print archives shows the flurry of activity, some at least can be retraced to the "information-paradox" and Black Holes.
  11. Oct 22, 2004 #10
    If the arrow of time is reversed the cup reform itself, because all processes with a total entropy change less or equal than zero will be spontaneous.
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