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Quantum vaccum question.

  1. Sep 22, 2004 #1
    When QM physicists refer to virtual particles being created spontaneously out of the vaccum, which particles are the referring to? I know of examples with photons, electrons and positrons, but does this occur with other particles as well - gravitons, quarks, WZ Bosons? Have these been observed?
     
  2. jcsd
  3. Sep 23, 2004 #2
    You gotta be careful here on what them virtual particles really are. Basically they are NOT physical thus you will never observe them. Then why are they used ???

    Well, they are nothing more then a mathematical trick used to describe the interactions between all possible elementary particles. So virtual particles occur in ALL interactions between ALL elementary particles.

    Now, there is a possibility to turn the virtual particles into real physical particles for a very short time. What you need is enough "external" energy that is used to make them real. The energy gives them a reason to exist, so to speak...This is possible because in QM and in QFT, energy-conservation can be violated for a very short while due to the Heisenberg-uncertainty-principle...

    Now in QFT the vacuum is not empty, it is constituted out of an infinite amount of virtual particles that can become real when there is enough energy available. These virtual particles can be all the possible force carriers like photons, gluons, vector-bosons because they mediate the interactions between the fermionic matter-fields like the electrons, positrons, and quarks.

    Ofcourse fermions can also be virtual and then become real when enough energy is available...You only gotta make sure that the conservation laws for spin, letpon-number, and so on are always respected. Only energy-conservation can be violated for a short while...
    regards
    marlon
     
  4. Sep 23, 2004 #3

    ZapperZ

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    I would like to point out that there is a risk in indicating that something isn't "real" or "practical", and that it is simply a "mathematical trick" or convenience. If that is true, then the concept of virtual particles would not predict anything new or deviate from conventional picture of "fields". Now we know that this is certainly isn't the case. Using such virtual particle interactions, QED can make some amazing predictions BEYOND classical field theory, so much so that we are gasping at the accuracy of the value of the electronic gyromagnetic ratio between theory and experiment. The concept of virtual particles also play a role in predicting the Casimir effect, which simply would not come about if we just stick to classical fields.

    So are virtual particles "real" or not? Are they merely a mathematical invention? Can a mathematical artifact predict new physical phenomena? They may not be "observed" directly (but then again, what does?), but if their presence can affect and correct existing measurements, and also predict new phenomena, I certainly would not want to say they are not "physical".

    Zz.
     
  5. Sep 23, 2004 #4
    Hmm,
    I think this post will make this thread evolve into a discussion on personal opinions...let's focus on the facts...

    Stating that a mathematical trick never leads to something new is just wrong. The virtual particles were introduced in QED in order to calculate the interactions between fermionic matterfields using perturbation theory and the corresponding Feynmann-diagrams. They are certainly not to be looked at as real physical particles. This is a misconception that occurs very often so it is necessary to stress this difference, especially to student who are just getting to know these concepts.

    Besides nobody is questioning the accuracy of QED here, i think everybody studying this subject will be familiar with Feynmann's analogy of the distance between the top/bottom of our head to the moon.

    Virtual particles are NOT REAL yet they do mediate interactions and physical phenomena that ARE REAL and observable. The Casimir-effect is explained using the virtual particles but it does not show that virtual particles are real...
    A particle being not real does not mean it does not exist...
    In the description of the interaction between elementary particles in quantum field theory, a virtual particle is a temporary elementary particle, used to describe an intermediate stage in the interaction. A virtual particle can never be the end-result of a process, that is what i mean that they are not real.

    I also pointed out that virtual particles can only become real for a very short while.

    If they were real then they would be they result of some interaction and they would not be an intermediate stage during the interaction...

    regards
    marlon
     
  6. Sep 23, 2004 #5

    selfAdjoint

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    Marlon, as you say, this view of virtual particles is a personal one, if increasingly popular with the younger generation of physicists. But it is not universally held. I would like to repeat my question, what is your opinion of the dressed electron, and its different mass from the bare electron? Are these real differences? Since there is experimental measuremant how can they not be? And then what is your account of the Casimir effect?
     
  7. Sep 23, 2004 #6
    Rhizo,

    This site gives a good example of the socalled vacuum fluctuations involving virtual particle - antiparticle pairs that are created out of "nothing" and die a short period later. During their existence they can influence real physical phenomena...Check out the famous Casimir-effect.

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

    regards
    marlon
     
  8. Sep 23, 2004 #7

    ZapperZ

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    I'm not saying they are real. I just don't think they should be looked at simply as a mathematical artifact or "unphysical". By saying that you are implying that we made it up, and that there shouldn't be any problem if we simply eliminate them, since they weren't "real" in the first place. The fact that we do not "observe" them directly should not be used as a criteria for something to be "real" or not.

    Let's discuss this another way to make sure we are not simply talking about preferences. Let's say I have a charge q at a distance d from an infinite conducting plane. The typical issue here would be to find the E field in the space between the charge and the conducting plane, and the charge distribution on the conducting plane itself. Now if we try to solve the Poisson's equation for this, it will be a pain. However, we know via the Uniqueness theorem that if I can find a similar situation that satisfies the same Dirichlet or Neumann boundary conditions, then I will have found a unique solution up to an additive constant. That is why we can replace the conducting plane with an image charge. This is a much simpler situation to solve. The image charge isn't real, it is unphysical, and purely a mathematical convenience/artifact. If the conducting plane is at z=0 and z>0 represents the space that the charge q is in, then z<0 is an unphysical region. We can solve for z<0 region, but this solution is purely a mathematical artifact. There's nothing meaningful about it.

    Now, if for some odd reason, by replacing the conducting plane with an image charge, I find that not only did it satisfy all the Dirichlet and/or Neumann boundary conditions as the original problem, I also find that by doing so, it gave EXTRA values that was not in the orginal situation for z>0 region, then something is different here. There is now a distinct deviation between the two situations. If I make a measurement and verify that this deviation is actually present, then what I orginally thought to simply be a mathematical entity (the image charge), is no longer that. There is a component of some reality to this situation that is closer to what Nature is then the original situation of solving a charge q in front of a conducting plane. Replacing the plane with an image charge is CLOSER to reality than the original description. So in this case, while the image charge is STILL not something you "observe" directly, its description is more accurate!

    Of course, this doesn't occur in E&M with image charge, but this is what is occuring when we replace the classical fields with virtual interactions. By including them in, we get a zoo of ADDITIONAL interactions that simply were not predicted by classical fields. Real particles can scatter off these virtual particles, producing corrections that are simply not there in the classical picture. These particles may be virtual, but they have a real set of properties that we can detect and measure via their interactions with others.

    While calling them real, unphysical, mathematical, etc. may be a question of semantics, I don't think their existence is a matter of tastes or preferences.

    Zz.
     
    Last edited: Sep 23, 2004
  9. Sep 23, 2004 #8

    jcsd

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    Usually whether or not these 'mathematical conviences' are 'real' or not is just a matter of ontology, but aren't virtual particles artifacts specifically of applying peturbation theory to QED and therefore not necessarily a prediction of non-peturbative QED?
     
  10. Sep 23, 2004 #9

    ZapperZ

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    The application of virtual particles is more general than that - it is more of a QFT, rather than for a specfic QED technique. So anything that uses QFT - even in condensed matter - would make use of virtual interactions.

    Secondly, I don't think it is restricted to simply perturbative scenario. Even without including any higher order corrections, the "coupling" constant at each vertex of interaction involves the emission or absorption of a virtual particle.

    Zz.
     
  11. Sep 23, 2004 #10
    Well, what is renormalization ???
    If someone would ask me this question the answer would be : Renormalization is a "mathematical" system that turns the infinite unphysical bare parameters of the Lagrangian of the field theory into finite dressed parameters that are physical. Why are the dressed parameters the fysical ones ??? Well because they are the "quantities" that we measure in experiments. The naked parameters are unfysical because they do not describe properties of the particles that are DIRECTLY measurable...

    The view of QFT is that the naked property let's take the mass is surrounded by a cloud of virtual particles that undergo all kinds of creation, annihilation and scattering-processes and as a consequence of this they dress the naked particle into the real physical particle that we observe. Here it is again : the virtual particles on themselves are NOT real, yet their contributions due to their interactions is REAL. So virtual particles DO deliver a contribution that we can observe when it is summed up with the naked parameters. We do not observe the virtual particles themselves, we only see (indirectly) a manifestation of their INTERACTIONS (just like in the Casimir-effect). All these interactions are lumbed up into the socalled self-interaction, which itself can be infinite in perturbationtheory. Well, as a matter of fact it is infinite. We solve this by stating that the naked mass for example is infinite itself (with opposite sign as the infinity of the self-energy) so these two concepts (naked mass + self-energy) sum up to an finite one : the dressed real mass that we observe.

    These are examples of coupling-constant-renormalization (ie introduction of the surrounding virtual cloud) and mass-renormalization.

    Basically that is my view (and all other younger :wink: physicist born after 1975 :biggrin: )

    regards
    marlon
     
  12. Sep 23, 2004 #11
    i agree with this statement
     
  13. Sep 23, 2004 #12
    This is correct, this is how virtual particles were "born" yet they are certainly used in non-perturbative QED and QFT generally

    regards
    marlon
     
  14. Sep 23, 2004 #13
    With all the respect due to a scientist such as you sA, I was wondering how the question can be addressed at all ? We have to renormalize because we must admit our ignorance of what could happen at very high energy or very small distance. Then, how could we say the naked particle has a physical meaning at all ? We cannot measure it. The naked parameters are unphysical to me
     
  15. Sep 23, 2004 #14
    I share the same opinion as Humanino sA, i wrote this down in the above post on renormalization, although we can manage the properties of phenomena at high energies (certainly in theory)

    marlon
     
  16. Sep 23, 2004 #15

    ZapperZ

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    At the risk of putting words into SelfAdjoint's mouth, I think you are misinterpreting what he is asking.

    If a "naked particle" is unphysical, and yet, a "dressed particle" is real since we can measure it, but yet again, it is dressed due to all the virtual particles surrounding it, then how can we say virtual particles aren't real? I think that was selfAdjoint's original question to Marlon. If the dressed particle is real, how can we then turn around and say the dressing isn't?

    SelfAdjoint can correct me if I misread his intentions....

    Zz.
     
  17. Sep 23, 2004 #16
    You had 666 posts zZ, it scared me :wink:

    Thank you for the precision. To me (this is a personal opinion of a young person) the virtual particles are not real, neither the naked ones. Only the dressed ones makes sens. The dressing process by itself would not correspond to anything else than a mathematical clever trick, to make our model consistent with reality.

    This is all a matter of taste and color however :smile:
     
  18. Sep 23, 2004 #17
    I could not be so short it is confusing.

    I do attribute reality to the effectiveness of saying "one virtual particule is exchanged in a deep inelastic process". I do not attribute reality to imagining that this correspond to a single exchange of a massive photon for instance. As Zee put it "the vaccuum is boiling sea of emptiness, full of sound and fury, and significating a great deal" or something close. When we try to measure the charge of an electron by "getting a probe too close to it" we trigger many flucuations around which cannot be accurately computed, only effectively described by a dressing process.
     
    Last edited: Sep 23, 2004
  19. Sep 23, 2004 #18
    Well, this is my whole point. A real particle is NOT dressed by the virtual particles surrounding it, it is dressed due to the INTERACTIONS of these virtual particles...

    The interactions are real and observable, the virtual particles themselves are not...

    regards
    marlon
     
  20. Sep 23, 2004 #19

    jcsd

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    So then it is just a matter of ontology then! :smile:
     
  21. Sep 23, 2004 #20

    ZapperZ

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    Not if you accept the Higgs mechanism. Then a naked particle has NO MASS until it is "dressed" around a cloud of higgs bosons. In a condensed matter system, you NEVER measure the "naked" particles. All that you measure are the dressed particles. Based on this, an undressed particle is NOT real, since you never measure them, at least not directly.

    Zz.
     
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