Matter-antimatter annihilation

  1. Why, or perhaps how, is 100% of the mass in matter-antimatter annihilation turned into energy? I thought no reaction could be 100% efficient, i.e. 2nd law of thermodynamics? Isn't the Carnot heat engine or the conversion of matter into energy in the accretion disk of a black hole the most efficient systems observed? Entropy, as I understand it, is a time biased quantity, which can increase or stay the same in a system as time moves forward. Does this mean that matter-antimatter annihilation is a reversible reaction and that subatomic particles aren't biased by the direction of time(like macroscopic objects are)? Can these reactions reduce the entropy of a system? I believe that I may be missing out on a fundamental aspect of the physics underlying this reaction.
     
  2. jcsd
  3. olgranpappy

    olgranpappy 1,273
    Homework Helper

    The 2nd law of thermodynamics doesn't just fall out of any microscopic theory of a few particles, whether it is your exotic matter/anti-matter example or whatever else. Entropy is a quantity that only makes sense in macroscopic systems.

    Same goes for all the other question in your post. Maybe ask again in the condensed matter section.
     
  4. This links describes a type of entropy occuring in quantum mechanics: http://en.wikipedia.org/wiki/Von_Neumann_entropy

    Does this have anything to do with the questions I presented?

    Would it be possible to move this entire thread to the condensed matter section? If so, how?
     
  5. olgranpappy

    olgranpappy 1,273
    Homework Helper

    The link shows a definition of the entropy of a system in terms of the density matrix which desribes the system. And there is other stuff shown on the linked page. Yes, the link is vaguely related to your first post.

    I don't know. If I were you I would just take some time and think about your own question and then rephrase and revamp the question and start a new thread. It is an interesting question, but I think there have been several similar questions that you can find by searching the threads here (sorry, I dont have a link). After reading other threads and other responses you will be able to ask a better and more thoughtful question that gets better and more thoughtful responses from the forum scientists.
     
  6. Mentz114

    Mentz114 4,231
    Gold Member

    The laws of thermodynamics, as is pointed out above, relate to heat exchange processes taking place in bulk. The theorem you quote is about extracting mechanical energy from hot gases, or heating or cooling matter.

    When an electron and a positron interact to make light, these laws are irrelevant. 100% of both particles becomes energy.

    You're applying bulk theorems to quantum-mechanical point processes.
     
  7. Also, matter/anti-matter anihilation on a large scale would not be 100% efficient. Lots (perhaps 50%) of the energy would be released as neutrinos, which are for all practical purposes worthless. All of the actual photons released would, as soon as they interacted with matter, likewise be affected by entropy. Much of the energy would become heat and (if the reaciton happened in the atmosphere) sound as well.

    So you could never get 100% efficiency from an antimatter power generator or antimatter weapon, or anything else involving antimatter on a macroscopic scale. Although, as others have said, on a microscopic scale, ie one anihilated pair at a time, entropy does not factor into isolated reactions, even a single photon will eventually give way to entropy, once that photon has been detected or has interacted with anything else in the macro world.
     
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