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Boson questions

  1. Feb 6, 2006 #1
    Do we know for certain that bosons exist?

    Do we have experimental evidence that bosons exist?

    How in the world can bosons be real and yet not obey the pauli exclusion principle?

    Does the fact that bosons do not obey the pauli exclusion principle mean that it is impossible to tell one boson from many bosons?

    Please explain to an outsider - this idea that multiple things can exist in the same place at once is confusing...
     
  2. jcsd
  3. Feb 7, 2006 #2
    :smile: Bosons are particles which have integer spin and which therefore are not constrained by the Pauli exclusion principle like the half-integer spin fermions. The energy distribution of bosons is described by Bose-Einstein statistics. The wavefunction which describes a collection of bosons must be symmetric with respect to the exchange of identical particles, while the wavefunction for a collection of fermions is antisymmetric.

    At low temperatures, bosons can behave very differently than fermions because an unlimited number of them can collect into the same energy state. The collection into a single state is called condensation, or Bose-Einstein condensation. It is responsible for the phenomenon of superfluidity in liquid helium. Coupled particles can also act effectively as bosons. In the BCS Theory of superconductivity, coupled pairs of electrons act like bosons and condense into a state which demonstrates zero electrical resistance.
    :smile:
     
  4. Feb 7, 2006 #3

    dextercioby

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    If you had asked about Higgs bosons, it would have been a more interesting question...

    Daniel.
     
  5. Feb 7, 2006 #4

    Gokul43201

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    Yes we do.

    Photons and phonons, for instance are bosons. They exhibit the bose statistics, as predicted. The heat capacity of many insulating solids has been experimentally measured to very high accuracy - and is in very good agreement with the calculation based on the bose distribution of phonons.

    Very simply ! The Pauli Exclusion Principle applies only to fermions; and is a result of the more general Spin-Statistics theorem, also due to Pauli.

    No, it is not, in principle. For instance, if you had a very good photodetector, you could distinguish two photons from one photon.

    This is only true of fundamental bosons. The atomic bosons that you more often hear about (He4 or alkali metal gases) are only approximately bosons - they are called "composite bosons". Thie limit in which the approximation holds is when the atoms are far away from each other. If you try and bring the atoms closer than a critical separation, they "realize" that they are in fact made up of an even number of fermions. So, in reality, a bose gas involving composite bosons will never have two particles in the same place.
     
  6. Feb 7, 2006 #5
    Thanks!

    So is it accurate to say that, while one (fundamental) boson or one million (fundamental) bosons can exist in the same place, it would still be possible to tell if there were indeed a million (fundamental) bosons or just one (fundamental) boson in any given space?
     
  7. Feb 7, 2006 #6
    With fundamental boson, you mean a boson arising from an elementary field excitation, right ? Like a gluon, photon, ... The "non-fundamental boson" would be something like the Cooper pairs ?

    Bosons follow the Bose Einstein statistics which for example predicts that all bosons want to sit together. This means, they want to occupy the same energy level (eg Bose Einstein condensation). However, this has NOTHING to do with "they wanna sit together" in the sense that they "wanna occupy the same spatial region". The "sitting together" part refers to an energy base, not a spatial coordinate base.

    Also, for example gluons are totally different in nature than Cooper pairs. Just look at how a Cooper pair moves through a lattice in the Cooper pair animation of this page. This mechanism does not happen in the case of gluons. They just pop up "out of nothing" and dissapear shortly after.
     
  8. Feb 11, 2006 #7
    spam?

    Why are you spamming the movie 'Hostel'? It looks pretty sick and has nothing to do with physics.
     
  9. Feb 11, 2006 #8
    Einstein-Bose condensates

    As you cool Bosons near absolute zero, they can all occupy the same energy state, but they do not occupy the same physical space. Picture and onion with many spherical layers.... In the case of an electron, each layer can contain only a certain amount of electron states. The innermost layer can hold 2 electrons, the 2nd layer 8, etc. With bosons, a huge arbitrary number of bosons can occupy the lowest state (ie. the near absolute zero energy level), meaning that they are not bound by the rules of the Pauli Exclusion Principle like electrons are. However, the bosons aren't all superimposed on one spot in this innermost layer, they are free to occupy the whole sphere of this onion layer, but they do so at exactly the same energy level.

    That's the idea.
     
  10. Feb 11, 2006 #9

    ZapperZ

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    Note that photons and phonons do not need to be cooled to near absolute zero (and neither do Cooper Pairs in high-Tc superconductors). They behave as bosons no matter what. The low temperature requirement for most "particle" bosons is to reduce the thermal fluctuation that destroys long-range coherence and/or to allow the "glue" to "adhere".

    Zz.
     
  11. Feb 11, 2006 #10
    Okay, some of the responses are well above my pay-grade, but there are some fundamental points here that confuse me.

    "As you cool Bosons near absolute zero" & "Photons and phonons, for instance are bosons."

    I was under the impression that photons were in a different category than things that can be cooled or heated to change energy levels. I can understand the principle of reducing the excitation of atoms or subatomic particles (cooling) but I am confused about how a photon can be cooled and what that process would look like?

    Oh and to clarify a point of confusion, with "fundamental boson", I was only trying to be more specific by adopting a new (to me) distinction, presented in this response:

    "This is only true of fundamental bosons. The atomic bosons that you more often hear about (He4 or alkali metal gases) are only approximately bosons - they are called "composite bosons". Thie limit in which the approximation holds is when the atoms are far away from each other. If you try and bring the atoms closer than a critical separation, they "realize" that they are in fact made up of an even number of fermions. So, in reality, a bose gas involving composite bosons will never have two particles in the same place."
     
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