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Fermions vs bosons

  1. Aug 16, 2012 #1
    why is that at relatively low temperature bosons can occupy the same state while the fermions cannot?
    and how does we macroscopically see the effects of bosons (with explanations)?

    a theoretical answer is preferable
     
  2. jcsd
  3. Aug 16, 2012 #2

    Simon Bridge

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    Um - because Fermion obey the Pauli exclusion principle?
    That's pretty much part of the definition of "fermion" ... which leaves: how come spin 0.5 particles also hot their quantum states?

    "because that is how the Universe works" is the bottom line.
    Physics does not answer "why" questions very well.

    I remember there is something about the symmetry involved in spin half particles that means that they cannot share states. But it is just modelling the physics we find in the real world. You realize that Physics is an empirical science right?
     
  4. Aug 16, 2012 #3
    let me phrase it in the way physics would like

    In the early stages of universe, just after the big bang at the moment when particles start to get their properties and dimensions how it is that a particle will become a Fermion or a boson?
    or is there any possibility that a fermion can become a boson?
     
  5. Aug 16, 2012 #4

    Dale

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    Certainly. You can have a positron and an electron which decay into a pair of photons, and you can also have the reverse.
     
  6. Aug 16, 2012 #5
    So your question is about the begining of the universe then?

    Since we do not know the physics or the initial conditions of the universe at the earliest scales then we cannot certain how particles "gained" their statistical properties.

    If we try and approximate using field theory however we can get an intuition about it. Here is my intuition about it:

    1) In field theory particles live in representations of a symmetry group.

    2) Depending on which representation it lives in then it will have either integer or non-integer spin.

    3) For various technical reasons a field with integral (half-integral) spin will obey commutation (anti-commutation) relations.

    4) the commutation relations make explicit which statistics a particle will obey (note: statistics implies the exclusion principle).

    As for how many and what type of fermions and bosons were created at the early universe that has to do with the mechanisms of inflation and reheating processes.
     
  7. Aug 16, 2012 #6
    getting higgs theory into the picture, i think that higgs feild has something more to do than adding the mass, there must some asymmetry before the particle could take their statistical properties but i am unable to find the event that will bring sufficient asymmetry.
     
  8. Aug 16, 2012 #7
    can you give me an example where matter-antimatter anhilation is not involved in producing a boson fron fermions or vice versa
    and the boson produced in the process must have some physical properties instead of being an energy packet.
     
  9. Aug 17, 2012 #8
    As for what causes symmetry breaking in the standard model at least to my knowledge I dont think there is a specific mechanism that is widely accepted. In supersymmetry the runing of the higgs mass tends toward negative values in a natural way and it is believed that some higher susy theory will be involved in spontaneous symmetry.
     
  10. Aug 17, 2012 #9
    Hi,

    I think your question does not have an easy answer, the raison why bosons and fermions are different is their statistic. From this we get the fermi and bose statistic, which is related to their spin. The Pauli exclusion principle tells us that the wave function of a set of fermions must be antisymmetric and that bosons have symmetric wave functions.
    The problem is that to demonstrate this amazing insight Pauli had, it is necessary to use QFT, actually there is a draft of explanation in wikipedia that I like:

    Then there is a second question, about how do we see this difference macroscopically and then I would say that the most spectacular effect is the Bose-Einstein condensate. Other difference are more subtle like what happens when a fermion interacts with a boson (eg the Higgs mechanism).

    Finally, you ask if we can transform bosons into fermions and vice-versa, and eventually, if I understood correctly your question is which of both is more fundamental. I am not sure that there is a positive answer for that, what I think we can say is that all the experimental results show a Universe made of boson and fermions (Standard Model) and if this model is the correct one then both fermion and bosons are fundamental.

    Cheers
     
  11. Aug 17, 2012 #10
    i read a article when the LHC announced the foundings for higgs boson that just after the big bang the particles were shapeless, without any physical properties and then they interact with higgs field to gain mass and shape. i don't know how this shapeless particle came in the picture.
     
  12. Aug 17, 2012 #11

    Simon Bridge

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    That is a good question which nobody knows the answer to. There are some guesses ... the area is work in progress.

    We can speculate of course - but that would not be allowed in the forums.
    Basically that is just how the Universe is.

    I think we missed one:
    Whole atoms can behave like bosons ... see liquid He II for example.

    http://en.wikipedia.org/wiki/Superfluidity

    At higher temperatures you can observe the different effects of Bosons and Fermions statistically. Very macroscopically, you need only look to neutron (boson) stars vs White dwarf (fermion) stars.

    So did you have a particular effect in mind?

    Is there a specific aim to these questions or are you just curious?
     
    Last edited by a moderator: Sep 25, 2014
  13. Aug 17, 2012 #12
    i am not sure but i think the expansion of the universe and the higgs feild have something in common to create asymmetries that could explain the action
     
    Last edited by a moderator: Sep 25, 2014
  14. Aug 17, 2012 #13
    Actually I am not a supporter of the High Energy Physics (HEP) but when comment results we have to be a little bit more objective and to comment the technicalities of the problem we should be more formal about the physics and mathematics behind it.
    I am not going to explain in full detail how it works the Higgs mechanism, for that a short comment will not be enough, instead of that you can find a large literature of excellent quality on the web (arxiv is a good site and for free).

    My point about mentioning the higgs mechanism, is the fact that as in superconductivity or superfluidity, the interaction with boson is essential. I am pointing this out because you were asking which are the difference between both. It does not matter if the higgs boson exists or not to make this point, the fact that many fermions can interact with boson all of them in the same state is the essential. Which if you look in detail gives some linear dependence that allow the this phenomena to happen.

    Cheers
     
  15. Aug 17, 2012 #14

    Dale

    Staff: Mentor

    Certainly, emission of photons from an atom returning to the ground state from an excited state is an example of producing a boson from fermions without any anhilation.

    The bosons produced must always conserve all conserved quantities, not just energy. For example, spin must also be conserved.
     
  16. Aug 17, 2012 #15
    can a boson have charge?
     
  17. Aug 17, 2012 #16
    the example you are giving me doesn't transform a fermion into a boson it instead produces a boson from its energy.
     
  18. Aug 17, 2012 #17

    jtbell

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    W+ and W-, for example.
     
    Last edited: Aug 17, 2012
  19. Aug 17, 2012 #18
    A photon produced in annihilation is not just a packet of energy. Photon is a full-fledged particle with physical properties (like the spin, momentum). There is no reason to treat photons as inferior to other particles.
     
  20. Aug 17, 2012 #19

    Dale

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    Ah, I misunderstood, to me "transform" doesn't mean the same as "produce".

    As far as I am aware there is no particle reaction which has as inputs only fermions and as outputs only bosons other than matter-antimatter annihilation. However, I am not a particle physicist, so there may be some of which I am not aware.
     
  21. Aug 17, 2012 #20
    so i would come at the same question again...
    an example where a fermion can transform into boson giving him charge, momentum and energy?
     
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