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Is the universe a fermion or a boson?

  1. Jul 11, 2012 #1
    From introductory QM class, my understanding is that one can consider any number of "fundamental" particles as a composite system and pretend it is a particle. When 2 fermions are considered as a single composite system/particle, it becomes a boson. When 1 fermion and 1 boson are considered together, it is a fermion. When 2 bosons are considered together, it is a boson.

    So my question (that have been on my mind for a couple of months now) is that, when you considered all particles that exist in the universe, do you in the end get a fermion or a boson? Do you have an odd number of fermions in the world.

    Phrased in another way, when you add up the entire angular momentum in the universe, do you get an integer spin (boson) or a fractional spin (fermion)?

    Is this a valid question? Is there a fatal flaw/assumption that I made somewhere?

  2. jcsd
  3. Jul 11, 2012 #2
    The universe is not in a spin eigenstate, so neither.
  4. Jul 11, 2012 #3


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    After a brief time from the big bang there were only Photons, so Boson ?
  5. Jul 11, 2012 #4
    Spin of a composite particle is the sum of angular momenta of it's constituents, not just sum of spins. I.e. Saying there were only photons is not sufficient (and false) to calculate the spin.
  6. Jul 11, 2012 #5


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    An odd number of fermions makes the system a fermion. An even number makes it a boson. So tell me the number of fermions in the universe and I'll answer your question.
  7. Jul 11, 2012 #6
    it would be a meaningless answer, because we wouldn't understand anything about it's nature.
  8. Jul 12, 2012 #7

    I do not know the answer to your question, but I think there is a misunderstanding, the fact that two particles in a system does not mean that the ensemble will behave as a new particle (either like boson or a fermion). This only happens when the mentioned particles are bounded to each other (eg, cooper pairs in superconductors), when it is not the case (which is most of the time, at least at 300 K in Earth) an ensemble of N fermions will behave as fermion independently if N is odd or even, because the particles in the system are not bounded.
    Other problem is even if we pretend that there would be a mechanism to pair particles, distance between then (is the case in the interstellar space) is too large to keep some coherence or entanglement to provide the bounding, at least is what I know.
    But your question, I think it was formulated a little bit different by X-G Wen (Quantum Field Theory of Many-Body Systems, Oxford Editions), he formulated the question of what is more fundamental bosons or fermions as the fundamental particles are bosons or fermions. In the book he explain his theory of string condensation, which could be helpful to answer your question, but I am not an expert in the subject.
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