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Is the vacuum the same as a quantum vacuum?

  1. May 5, 2009 #1
    is the vacuum the same as a quantum vacuum? Minor question but just wondering.
     
  2. jcsd
  3. May 5, 2009 #2

    malawi_glenn

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    Re: Vacuum

    define "the vacuum"
     
  4. May 5, 2009 #3
    Re: Vacuum

    I must say i'm Sorry for my weak English.
    What is Vaccum?, me I do not know, and doubt if anybody knows.
    But, when we say quantum vaccum we mean that sheild of nothing surroundin every particle,; it is the discontinuoty of the body density.
    Serious difficulties of comprehension appear when we try to describe that by quantum treatement, because in fact when we describe a vaccuum in QM we make it very crowded, and very energytic which appear like a contradiction.But it is not, ther is just some adjusment of our comprhension of things wich has to be adapted.If we consider a zone of 0 enrgy, for a short time Heisenbergs implyes that it shoul have infinity value of energy ,
    this folows immediatly by the application of incertitude relation. I think it just( gives us more raison to adjyst our definition of vaccum,
    I think the othor vaccum wich is not QM, is the one describe in GR by the density fonction, where we estimate the energy of the univers by adding the energy of radiation, the energy of mass, and the energy of vaccum. This en,ergy of vaccum implyes the addition of a constanc in the einsteins equation noted ( lambda), that is why we name the present model (lambda)cdm = means cold dark matter ( + the constant lambda, wich is due to the vaccum energy)
    measurment by time are progressing in the favor of a plate universe ( not spherical or heberbolic) wich means that there is truely this vaccum enerhy and it is even estimated in pourcentage of the total universal mass, but whar is it,how to explain it?, I think that is still the subject of reseaches - as far as I know-,
    Experimntally concerning the Qm vaccum, we find the Casimir effect, observed in laboratory; two objects distant of 1 micrometre, infuelece each othor by a force proportionnel to the surface of the subjects. This force is due to the vacuum betueen the objects.
    Finally befor to answer QM vaccum= vaccum or not, we have to firt knowwhat is vaccum, and we have to know sufficient about the QM vaccuum.
     
  5. May 5, 2009 #4
    Re: Vacuum

    i guess another question I would ask is what is the difference between the quantum field and the vacuum?
     
  6. May 6, 2009 #5
    Re: Vacuum

    I think you seem to be using terms in ways that aren't quite in line with standard usage. Until you get some of the standard relationships between concepts a little more straight in your head, you will get answers to the questions that people think you might have asked, or that they think you ought to have asked. Keep at it! You probably need to do some more reading in parallel with asking questions on Physics Forums. When I get confused, I often try to read a different book on QFT, because it's often quite illuminating when you notice differences between presentations.

    When considering quantum field theory at an elementary level, people often just say that the vacuum state of a quantum field is the lowest energy state. However, just as important is the invariance of the vacuum state of a quantum field under Lorentz transformations and under translations --- so the quantum field vacuum state looks the same to you no matter what constant velocity you're going and no matter where you are. Don't accelerate, however, because then the vacuum state of a quantum field does look different.

    The vacuum state looks very similar to a thermal state insofar as there are fluctuations, but a thermal state is different because there is a special observer who sees the thermal state as moving at the same constant velocity, where all other observers see it as moving relative to them. Thermal states are invariant under translations, however, just like the vacuum state; that is, they look the same wherever you are.

    It's famously the case that the vacuum state of quantum field looks like a thermal state to an accelerating observer, which is known as the Unruh effect. There are some tricky details, but this more-or-less means that you can reasonably think of the vacuum state of a quantum field as very similar to a thermal state of the same quantum field, with the distinction that the vacuum state and a thermal state have different properties under Lorentz transformations. Speaking loosely in terms of concepts from general relativity, by changing to an accelerating frame of reference you can turn the vacuum state into a thermal state.

    All the above is fairly conventional. For quantum field theory it's also possible to think of Planck's constant as a measure of fluctuations in the vacuum state, just as temperature is a measure of fluctuations in thermal states. Sadly, we don't know how to reduce Planck's constant the way we can reduce the temperature when we want to make our measurements more precise. Don't put what this last paragraph says in a term paper, however, even though you can find mathematics supporting it in a published paper of mine.

    Enjoy!
     
  7. May 6, 2009 #6
    Re: Vacuum

    I think you're talking about concepts which arise in SSB (spontaneous symmetry breaking). In general, the classical vacuum is not the same as the quantum vacuum, because loop corrections affect the construction of the quantum action, so that the ground state is modified a little.
     
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