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Time arrow and newton

  1. Apr 16, 2006 #1
    Once in while I read that Newton laws do not forbid the unbreaking of eggs. The only reason that we don’t see it and similar events is the second law of thermodynamics.

    Is that correct? If an egg falls off the table, isn’t then the reverse process also forbidden by Newton, then how should the egg shell and yolk go up against gravity and get to the table? Where should the energy come from?
     
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  3. Apr 16, 2006 #2

    dav2008

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    Let's look at what happens when the egg falls. Gravitational potential energy is converted into kinetic energy. As the egg hits the ground, that kinetic energy goes into changing the internal energy of the floor and the egg (deforming aka breaking the egg) and into things like heat and sound.

    The point is that energy is conserved according to the first law of thermodynamics. It would not be a violation of conservation of energy if all of that heat and internal energy went back into the egg to put it back together and give it enough kinetic energy to jump back up to the table.

    However, like you said, according to the second law of thermodynamics that will never happen.
     
  4. Apr 16, 2006 #3
    I also look at it this way: There is no "superball" that will ever bounce back to the same height as dropped(not throwing).
     
  5. Apr 17, 2006 #4

    Andrew Mason

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    The gravitational energy was converted into kinetic energy of the egg. When the egg crashed, that energy dispersed into the surroundings as heat, ultimately. It is still there. However, the second law prevents it from being used to do work to lift the egg back up to where it was.

    If all the molecules in the surroundings under the egg suddenly moved in the same direction (up), the egg could move back to where it was. But it is statistically so improbable that it will never happen in 100 gazillion lifetimes of this universe.

    So, the arrow of time is determined not by any law of physics but by the law of probability.

    AM
     
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