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Actual reason for 2nd law of thermodynamics

  1. Mar 19, 2004 #1
    Hi, I was just wondering what the actual proof is for the second law of thermodynamics that would disprove a situation where all particles are simply reversed in direction creating lower entropy? Is it a consequence of another scientific principle or where does it come from?

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  3. Mar 19, 2004 #2


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    Its so important of a concept that its used as a postulate: a starting assumption for theories, but it is easily tested, so its backed up by pretty much every thermodynamics experiment ever done.
  4. Mar 19, 2004 #3


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    The fact that you do not see a broken vase spontaneously reform back into the original vase without any input in energy is a proof for the 2nd Law. Other types proofs are more esoteric - heat engine, etc. - and requires some knowledge of classical thermodynamics.

    Whether it is a consequence of another principle is hard to say. There are many speculations that it is tied to our time arrow, but saying that will get us into the chicken or the egg question, i.e. is entropy the consequence of our 1D time arrow, or is the 1D time arrow the consequence of entropy?

    There is a very good website that describes and, more importantly, debunk several myths regarding entropy (i.e. entropy and "disorder"):


    It may not answer your question directly, but it's still a fun read! :)

  5. Mar 19, 2004 #4
    Ok, think for just a minute...where does the energy come from, to make the particles change direction?
  6. Mar 19, 2004 #5


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    Really there is no reason in physics preventing any of these things from happening, we can even compute the likelihood of such events occurring. When all is taken into consideration, the time time you would have to wait for these unlikely events exceeds the life time of the universe, thus are considered impossible.

    Entropy is statistical in nature, but in the statics of large numbers, such as the number of particles which constitutes the universe, the events that are most probable are what happens.
  7. Mar 19, 2004 #6
    Perfectly worded.
  8. Mar 20, 2004 #7
    An illustration of the 2nd law that I like to think about is ships sailing across the Ocean. Underneath them, there is a vast source of energy - the heat in the water amounting to billions of Joules of energy. So why don't ships use heat pumps to get hold of this free energy?
    The 2nd law tells us that heat flows from a warmer environment, to a colder one, never the other way round. The only way to tap into the sea's free energy therefore, involves having a colder heat sink for the heat to flow into. Other than towing an iceberg behind the ship, this is impossible.

    It is a bit like a wind powered sailing ship sailing over a Pacific ocean of free Diesel fuel, but not being able to use it!
  9. Mar 20, 2004 #8


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    I've noticed that entropy and the Second Law are favorite topics in some Creationist tracts and articles--which brings to mind the saying, "A little learning is a dangerous thing."
  10. Mar 21, 2004 #9
    You'll like this then from this weeks Onion:
    http://www.theonion.com/news/index.php?i=1&n=3 [Broken]
    Last edited by a moderator: May 1, 2017
  11. Mar 21, 2004 #10


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    Funny stuff, Adrian.

    I think I recognize Cosmic Stan as one of our members.
  12. Mar 22, 2004 #11

    Chi Meson

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    I have a "2nd Law" experiment in my classroom that has been ongoing for three years now: a sealed jar of water with food coloring in it. As soon as the coloring separates into a "clear" half and "green" half of the jar (as was the original condition), then the 2nd law will be disproved. I'll keep you posted.
  13. Mar 22, 2004 #12


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    It should be said that while the 2nd Law of Thermodynamics is a solid and verified principle, it is built on several major axiomatic conditions, one of which is that it isn't in a "transient" regime, or that it assumes a classically-defined space/vacuum configuration. Once we understand those boundaries, physicists, being the curious beings that they are, will always try to test the validity of anything beyond those boundaries and see where such thing will fail.

    .. and such things have been done, especially in the quantum critical regime, and when vacuum fluctuations effects are significant:


    I was hesitant to post this because there is always the potential "abuse" of such reports. The fact that this occurs in only very extreme and unusual situation where the 2nd Law wasn't meant to be applied should always be kept in mind. The 2nd Law reminds valid and fully intact.

  14. Mar 22, 2004 #13
    It might do it at night when you aren't looking!:smile:
  15. Mar 22, 2004 #14
    Are you saying that you are the kind of person that would turn off a guy's fuel petcock on the starting grid? lol. -Mike
  16. Mar 23, 2004 #15

    Me? As if!
    A long time racer told me a good trick of his for unnerving the opposition.... if you get past someone you are having a real battle with on the track, point at his engine and shake your head as you go past....

    Nothing to do with the 2nd law, but a good tip nonetheless!
  17. Mar 23, 2004 #16
    The practical application of physics is a blast! I'm too old for racing now, but I have many fond memories of Thompson, Ct Laconia, NH
    and Bridgehampton, NY. I still have a Suzuki GS 1000. The footpegs are about 1/2" long(That's the way I like them).

    We've gotten pretty far off subject. We should do this in the general discussion area.

    -Mike AAMRR #984
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