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Sound in Bose Einstein Condensates.

  1. Jan 14, 2013 #1
    Hello there
    I love physics, so I normally read a lot of physics books. I am fascinated about BECs and their possible applications. I've read about BECs and BEC vacuum theory, also that is possible to create black holes in superfluids (as BEC) using sound (sonic black holes).
    As far as I know (sonic black holes in perfect fluids as BECs) are possible because phonons in perfect fluids exhibit the same properties of motion as fields, such as gravity, in space and time.
    And reading Wikipedia I get... In physics, a phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, such as solids and some liquids. Often referred to as a quasiparticle, it represents an excited state in the quantum mechanical quantization of the modes of vibrations of elastic structures of interacting particles. As far as I know... the name phonon is coined because long-wavelength phonons give rise to sound.
    So my doubts about this are... that if we need a vacuum chamber (MOT Magneto Optical Trap) and we need phonons to create sonic black holes in a BEC...
    are we having "sound inside the BEC" ?
    if we have sound, how is it possible that we have sound in a BEC if sound cannot travel in vacuum?

    Please, correct me in all you want, I only want to understand this well.
    Last edited: Jan 14, 2013
  2. jcsd
  3. Jan 15, 2013 #2


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    You have a BEC, not vacuum. That you have vacuum around the BEC does not mean that sound cannot travel within the BEC.
    A spaceship is also surrounded by vacuum. Nevertheless the astronauts inside can talk to each other.
  4. Jan 15, 2013 #3
    You explain it crystal clear, but... I thought the BEC itself was the vacuum also... because then what is SVT - Superfluid Vacuum Theory or BEC vacuum theory? quoting wikipedia says: "Superfluid vacuum theory (SVT), sometimes known as the BEC vacuum theory, is an approach in theoretical physics and quantum mechanics where the fundamental physical vacuum (non-removable background) is viewed as superfluid or as a Bose-Einstein condensate (BEC)."

    So if a BEC is not the vaccum then what is BEC vacuum theory? and how the sound fit with all this to create a sonic black hole in a BEC?

    Again, please, correct me in everything (I only want to understand this).
  5. Jan 15, 2013 #4


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    Just as a disclaimer: this is a physics model beyond the standard model. This means that it is one of many competing theories leaning towards unification and quantum gravity. Pretty much all of them are rather hypothetic at current as they do not make predictions which are testable by experiments today. It is also not a majority point of view.

    In the theory mentioned above, vacuum is considered a BEC. But just because a dog is an animal, that does not mean all animals are dogs. The same goes here. Just because vacuum is considered as a BEC in some theory that does not mean all BEC are vacuum. In fact, none of the typical BECs created in a lab are.
  6. Jan 16, 2013 #5


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    More concretely, in fundamental theories like QCD, the vacuum is modelled as a BCS condensate of e.g. gluons.
    In contrast the condensates experimentally observed in magnetic traps are condensates of real atoms at a density different from that of the surrounding vacuum (=0).
  7. Jan 18, 2013 #6
    I think my mistake was understanding the BEC as a vacuum when it is not a vacum, what led me to think in this way, was the BEC vacuum theory. My confussion was how all this fit with propagating sound inside a BEC. Quoting Wikipedia talking about the physics of sound it says: "The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum."
    However what is interesting in this case is we are having here a new kind of matter, it is a gas, but also a new kind of matter...
    So... I think BEC is a medium (perhaps elastic medium?) in which we can propagate sound.
    BEC is not vacuum, but (am I wrong?) it is an interface to get vacuum excitations. A BEC would be a very interesting "interface (method to)" to play with light and excite the vacuum.
    I am telling this, because I have another doubt about all this. As far as I know (am I wrong?) to create quantum entanglement, do we need vacuum? I think yes? so... if we can create quantum entanglement in a BEC... and BEC is not vacuum, how is that possible?

    I always thought a superfluid (BEC is a superfluid) was also a kind of vacuum...

    This paper is very interesting:
    On de Sitter radiation via quantum tunneling
    Direct link: http://arxiv.org/pdf/0803.3367v5.pdf

    I quote verbatim: "All these quantum effects may take place not only in the atsrophysical objects: for example sound waves propagating in a moving medium are similar to the light propagating in the curved space [5]. It is possible to create artificial horizons and other nontrivial spacetimes including those which cannot occur in astrophysics, for example the white hole horizon, and also to study radiation by objects moving in the quantum ‘vacuum’ of a superfluid liquid or Bose-Eistein condensate (BEC)."​

    So I was thinking we were having vacuum inside a BEC?
    Am I understanding wrong this paper? do we have perhaps an annalogy of sound and light ?

    Please, take a look also to: Natural Entanglement in Bose-Einstein Condensates, arXiv:quant-ph/0110114

    Direct link:

    I quote verbatim: It is worth noting that in relativistic quantum field theory even the vacuum is an entangled state. See e.g. S.J. Summers and R.Werner, Phys. Lett. A 110, 257 (1985); L.J. Landau, Phys. Lett. A 123, 115 (1987); H. Halvorson and R. Clifton, J. Math. Phys. 41, 1711 (2000); B. Reznik, quant-ph/0008006 (2000).​

    And regarding a BCS condensate, I really don't know how it works, I am still trying to understand well the BEC. I found another interesting paper about BCS-Condensates here: "Gray" BCS condensate of excitons and internal Josephson effect, arXiv:1202.5976 [cond-mat.quant-gas]

    Please, don't understand my message as if I was defending a particular theory or paper, I only want to understand all this well. Please, correct me in all you consider necessary.
    Last edited: Jan 18, 2013
  8. Jan 19, 2013 #7
    I forgot to mention this paper also:

    Propagation of Sound in a Bose-Einstein Condensate

    Sound propagation has been studied in a magnetically trapped dilute Bose-Einstein condensate.
    Localized excitations were induced by suddenly modifying the trapping potential using the optical dipole force of a focused laser beam. The resulting propagation of sound was observed using a novel technique, rapid sequencing of nondestructive phase-contrast images. The speed of sound was determined as a function of density and found to be consistent with Bogoliubov theory. This method may generally be used to observe high-lying modes and perhaps second sound. [S0031-9007(97)03665-X]​

    So my confussion is some papers state BECs have quantum vacuum and other talk about propagation of sound inside a BEC.
    I really don't know how to make these two statements have sense...
  9. Jan 20, 2013 #8
    Are you misunderstanding the difference between a spatial vacuum and the vacuum state in quantum theories?
  10. Jan 27, 2013 #9
    Perhaps... ??? I'm confused about this... if someone could give me a clear definition about: spatial vacuum VS vacuum state, I really would love this.
    Quoting Wikipedia: VACUUM STATE: In quantum field theory, the vacuum state (also called the vacuum) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. Zero-point field is sometimes used as a synonym for the vacuum state of an individual quantized field.

    So I understand in a BEC we have the vacuum state because we have a Ground state.

    Quoting Wikipedia: GROUND STATE: The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. The ground state of a quantum field theory is usually called the vacuum state or the vacuum.

    However I did not find any clear and simple definition about "SPATIAL VACUUM" ?

    I'm a bit confused about this ???
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