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Is nuclear fusion reverse entropy?

  1. Sep 14, 2014 #1
    Certainly fission is a prime example of entropy. But what about fusion? Doesn't fusion look like a process that goes from a disordered state to an ordered one? Out of chaos comes order!
     
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  3. Sep 14, 2014 #2

    Nugatory

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    No. Part of the problem here is that the notion that entropy is just disorder is one of those pervasive pop-sci oversimlifications - there's a lot more to entropy than that.

    A bigger problem is that you're failing to consider the energy that is released by the fusion reaction, which travels away from the site of the reaction increasing the disorder everywhere else.

    [This is not a quantum mechanics question, so I'm moving it to general physics]
     
  4. Sep 14, 2014 #3
    Well I understand that entropy has its own laws and rules to predict how it changes over time. But fusion doesn't seem that way to me. Fusion looks more like setting up the right conditions and then crossing your fingers. And fusion, to me, seems far more quantum mechanical this fission (highly probabilistic, quantum tunneling, etc.).
     
    Last edited: Sep 14, 2014
  5. Sep 14, 2014 #4
    If you look up a diagram or illustration for the process of fusion, as seen in the sun for instance, you'll see that it's not just two particles coming together and that's that. Fusion also releases energy in the form of neutrinos, positrons, photons, etc. Furthermore, the process doesn't stop when two hydrogen nuclei (i.e. protons) fuse, but requires another step for those helium 3 nuclei to fuse and make a helium 4 nucleus, again releasing particles back into nature including protons which go on to continue the fusion process elsewhere. It's not entropy in the sense of things breaking apart and never being put back together (like fission), but it spreads out the total disorder in the environment. The reason we get sunlight on Earth is because of entropy when stellar fusion spews energy into the surrounding space.
     
  6. Sep 14, 2014 #5

    PeterDonis

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    Not really. It's like struggling to set up the right conditions. Under the right conditions, fusion happens quite predictably; for example, in the Sun's core. The problem is that the right conditions are very difficult to set up if you don't have a Sun's mass worth of gravity to confine the plasma.

    I'm not sure where you're getting this impression, but I don't think it's correct. Both fusion and fission can be modeled without getting very deep at all into quantum mechanics.
     
  7. Sep 14, 2014 #6
    Fusion releases energy, so you must take into account the radiation given off in the entropy calculation.
     
  8. Sep 14, 2014 #7
    I think you guys can't see the forest for the trees. The idea I'm presenting is that the whole fusion and fission reactions are a cycle. An uranium atom is a an ordered state, unstable yes but still ordered. It's instability leades to fission. Instead of an atom of uranium, we now have lighter atoms. But those same lighter elements can be be fused back into an uranium atom (with sufficient time and energy)- thus getting you back to an ordered state.

    Wouldn't it be a self-sustaining loop forever if dark energy wasn't possibly leading to a cold death for the universe.
     
  9. Sep 14, 2014 #8
    But if it's not releasing energy then it is mass. And it's mass has an effect everywhere else.
     
  10. Sep 14, 2014 #9

    Drakkith

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    No, fission and fusion are both irreversible processes and will increase the entropy of the system as a whole. You cannot return the products of either reaction to their initial state without losing some amount of energy in the process, usually as EM radiation or heat.
     
  11. Sep 14, 2014 #10
    But the system had to consume energy in the first place. Why is energy only considered when it is lost and not considered when it is gained? That kind of sounds like my bank. No problem when I put money into my account. But issues (like fees) when I take it out.
     
  12. Sep 14, 2014 #11

    Nugatory

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    No - google for "binding energy curve". Fusion reactions generally release energy as we build up larger nuclei until we get to iron-56, and fission reactions generally release energy as we turn larger nuclei into smaller until we get down to iron-56. We can't get away from iron-56 in either direction without adding energy from outside, and whatever process produces that energy has to be included in your total entropy calculation.

    This would be a really good time for you to look up the definition of entropy according to statistical mechanics. You're still talking like someone who has only seen the pop-sci definition of entropy as "disorder".
     
  13. Sep 14, 2014 #12

    Drakkith

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    The system is the universe, so no energy is gained or lost from the system. However, energy is lost from the nuclei in both natural fusion and fission. In cases where we force fusion on heavy nuclei or fission on lighter nuclei, most of the energy required to get the particles to fuse or fission is lost in the process. Yes, the products of these reactions have gained energy, but much more was used and lost as heat or radiation into the surrounding environment.
     
  14. Sep 14, 2014 #13

    PeterDonis

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    So just to make sure I understand: you want to fission some uranium, then fuse some hydrogen into helium, and use the energy from the fusion to re-combine the fission products back into uranium, correct?

    What I described above would not, no. The overall process still consumes fuel (for the fusion) and increases entropy.
     
  15. Sep 15, 2014 #14

    D H

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    You need to read our rules on personal theories.

    What you wrote is not true. Fusion is exothermic up to the point of producing iron or nickel. Beyond that, fusion is endothermic. This is why very massive stars undergo core collapse at the end of their lives.

    At the opposite end of the periodic table, large unstable nuclei undergo a series of decays, typically resulting in some form of lead after a series of decays. There is no decay chain from uranium all the way down to only hydrogen or helium nuclei. Most unstable nuclei spit out a helium ion, an electron, or a positron (alpha and beta decay). The resultant nucleus is still rather massive, and might or might not be stable. The process repeats if the resulting large nucleus is not stable. A few large nuclei split into larger chunks (i.e., fission). There is no decay chain involving fusion all the way down to only hydrogen or helium nuclei.


    In short, there is no cycle. There's just a dead end.
     
    Last edited: Sep 15, 2014
  16. Sep 17, 2014 #15
  17. Nov 7, 2014 #16
    This YouTube clip by Sixty Symbols is a nice explanation of the point I was trying to make in my orignal post (i.e., entropy creating ordered states).

     
  18. Nov 7, 2014 #17
    I can't watch the vid right now but from what I understand entropy and chaos are not really related, entropy is the tendency to dissipate to consistent distribution, while chaos is random distribution... total disorder so to speak.

    Nuclear fusion on the other end of the spectrum is matter's insatiable desire to attract and combine to higher atomic states which do release energy up to iron, but after that threshold more energy is required to create higher elements which become increasingly less stable, more or less, depending on molecular structure.

    I would guess with nothing else involved nuclear fusion into perhaps diamond, having the strongest most stable molecular structure, might be considered the opposite of entropy... but the universe is far more complicated than that.
     
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