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Vacuum Energy: same inside a rock?

  1. Apr 18, 2010 #1
    Hi,

    Even though it is called 'vacuum energy'... do we know or do we have some theory on... whether the activities of virtual particle pairs is the same whether in the relative vacuum of space or... with in ther Earth's atmosphere (where tests confirmed it) or... say inside a rock?

    Rusty
     
  2. jcsd
  3. Apr 18, 2010 #2
    Could be wrong, but I don't think one can test for the actual presence or absence of virtual particles.
     
  4. Apr 18, 2010 #3
    Their effect alters the behavior of electrons in atoms in a measurable way.

    The vacuum energy is on a short distance scale so the inside of a rock
    looks a lot like outer space to the vacuum energy.
     
  5. Apr 18, 2010 #4
    I could be wrong but I believe the test that proved their existence was placing two metal plates next to each other in a vacuumed container. The greater vacuum energy on ether side of the metal plates (compared to the vacuum energy between them) caused the plates to move toward each other. I don't recall who conducted the experiment or when.

    I thought so. As to altering the behavior of electrons, since the vacuum energy is always there, how could you tell?

    Thanks for the quick replies!
    Rusty
     
  6. Apr 18, 2010 #5
    Right, I do understand that experiment.
    I was merely pointing-out(if correct) that virtual particles have never been DIRECTLY observed, due to their nature.
    Certainly not contradicting anyone here.
     
  7. Apr 18, 2010 #6
    The Casimir Effect is one thing... I don't see how a similar effect can arise in a solid. I also don't believe virtual particles can be directly observed because they are a mathematical approximation, not real particles. As Tiny Tim so often says, "The giveaway is in the name" 'virtual'.
     
  8. Apr 18, 2010 #7

    SpectraCat

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    I don't see why we should ascribe any particular significance to the name "virtual" ... that name was assigned by a human (or humans) after all, and we hardly have a stellar record with naming conventions. I have seen several posters on here go on multi-post rants about the stupid and confusing names and conventions that are accepted in physics. I am not nearly so passionate about it, but I do think that just because a particle is called "virtual" does not inherently prevent it from being a detectable physical object.

    "Virtual particles" is the catch-all name we give to the entities that bind quarks together inside nucleons, and those that bind electrons in atoms, so I personally think they are very real. If the standard model is correct, and gravitons exist, then I think that would also likely mean that the planets are held in their orbits by "virtual particles".

    As I said before, it is not clear to me why virtual particles would be characterized as a "mathematical approximation" ... approximation to what? (You didn't really answer this the last time I posted it :wink:). I thought they were a consequence of the necessary symmetry of the relativistically correct expression for energy: E2=(pc)2 + (mc2)[SUP2[/SUP], which led Dirac to postulate the "Dirac sea" of "vacuum fluctuations". Anyway, I don't think that QED could explain much without virtual photons to carry the electromagnetic force, so they seem pretty physical to me ... perhaps they are just poorly named?

    Regarding the Casimir effect, it seems to me that the jury is still out on that one. It is a beautiful idea, but as far as I know, there is no definitive proof that can distinguish the Casimir effect from van der Waals forces.
     
  9. Apr 18, 2010 #8
    Hello,

    I can see that at the level that virtual particle pairs are thought to occur, vacuum, Earth's atmosphere, metal, stone, plasma... whatever should be transparent. We're down below atoms and molecules.

    However, that acknowledged (and as I whack myself in the head), if the 'vacuum energy' effect completely ignored vacuum, air, solids, etc... the Casimir effect (assuming that this effect is from the vacuum fluctuations from virtual particles) would not work. The vacuum energy would pass through the Casimir's plates ignoring them. Dho!

    So... there is some difference. Perhaps virtual particle pairs do occur within all matter, it seems that it must at least drop off inside of solids. The two plates somehow separated the outside fluctuations from the inner to push the plates together, yes?

    Again, thanks for your help!
    Rusty
     
  10. Apr 18, 2010 #9
    They are certainly poorly named, but this is physics, not math. When I said mathematical approximation, I mean that it emerged from the math and has no apparant physical reality. I agree that the Casimir effect could be van der Waals forces, because we're observing an EFFECT, not the cause, and can't rule out other potential causes. Of course, van der Waals is a bit of a catch-all too in terms of naming conventions.

    To make QED work requires virtual pairs, but who thinks QED is a complete theory? By definition VPs are not observable, which is frankly a cute way of saying, "Whatever is happenign at/below the Planck Scale is a big mystery". If you discover a way to observe that tomorrow and find just such a particle, it won't be a "virtual particle" by definition. Hopefully some new physics would come along with it too, not just math to make the current model work. The only reason that's done is because it DOES work, and in a way, it is emblamatic of the "backwards engineering" nature of SQM. An effect is observed, and we ascribe a particle to mediate it, but without the capacity for observation implicit in its nature... that's what I mean by a mathematical approximation.

    Hell, the Dirac Sea is an approximation of the current understanding of vacuum fluctuations/polarization. As for Dirac, I don't know personally, but he was a prety itense Logical Positivist... I doubt he believed that he was doing more than creating an intermediary description. That is of course, just a guess on my part.
     
  11. Apr 18, 2010 #10

    SpectraCat

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    And here I thought you were an Instrumentalist! One wouldn't know it from the passion with which you are arguing a mathematical position. :wink: All joking aside, *something* is holding those electrons in their (if you will allow me a rhetorical anachronism) orbits. Call them "virtual photons", or "vacuum fluctuations", or "spaghetti monsters", or whatever, they are *physical*. As I said before, in order for the EM force to be consistent with relativity, there have to be some sort of "messenger particles" exchanged between the protons and electrons which travel at c (or slower). I guess I see your point that the *specific* entities described as virtual photons within QED are mathematical in nature, I was speaking more about the phenomenological entities that must exist within a relativistically consistent framework of physical laws.

    Now, perhaps when the REALLY smart people get quantum gravity sorted out, we will see that there was never really any need for virtual particles, and that there is another, more general theory that can explain how the EM force (and all forces) can be relativistically consistent without them. Until then, it seems more reasonable to me to believe that there are some physically real entities carrying the EM force between interacting charged particles, whether or not they are correctly described by QED ... I am not picky about what to call them.
     
  12. Apr 19, 2010 #11
    Hey now, I only took on "Instrumentalist" so I could stay on the fence! :biggrin: AS for the rest, you got my point completely, and I agree with you about SOMETHING to remove the spooky from the action at a distance. The only reason I do get picky (other than the fact that I can be a royal ***), is that the term is losing its meaning by becoming this catch-all, and frankly, people seem confused enough with what's real and not about QM. That said, I guess it beats referring to them as "The stuff that keeps electrons with their nucleons, at least until we find a proper name for it." Compared to that "Virtual Particle" is a single sweet chord. Ah well, all that really matters is that people agree on the essential points.

    Given that we seem to agree on the physics... there has to be a better way to describe them, don't you think? Calling them Virtual Mediators perhaps... anything other than yet another mental conjuration of Newtonian billiard balls. What do you think Cat, am I just being overly picky? (serious here, not baiting or sarcastic)
     
  13. Apr 19, 2010 #12
    With all due respect wiz-kids, this is my thread even if I'm just a lowly mortal... so... can someone kindly comment on or set me straight on my last post (T-1:38 AM)? Or, is someone did and I missed it, kindly point it out.

    Also:
    " but as far as I know, there is no definitive proof that can distinguish the Casimir effect from van der Waals forces."

    It looks like Casimir effect (which says that the plates moved together because of virtual particle pairs coming and going... is that right?) is now in question due to the fact that something called the van der Waals forces would/could also move the plates together? Is this more or less correct?

    Thanks,
    Rusty
     
  14. Apr 19, 2010 #13

    SpectraCat

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    The standard interpretation of the Casimir effect is that the two very closely spaced metal plates form a cavity, which restricts the wavelengths of the virtual particles that can possible "fit" between the plates. No such effect exists for the outer surface of the plates, and so there is a net pressure on the external surfaces, which pushes the plates closer together by a measurable amount.

    The issue is, in order to measure the effect, the plates have to be so close together that the van der Waals (or dispersion) forces are non negligible. The forces arise from the interactions of instantaneous dipoles created by small, random fluctuations in the electron densities in materials, and are always attractive. Thus, measurements of the Casimir effect must be able to show that the attractive force measured is greater than what would be expected from vdW forces alone, and this has not been done to my knowledge.

    EDIT: The wikipedia page on the Casimir effect is pretty good, and much of it is written in fairly non-technical language, and so may be accessible for you.
     
  15. Apr 19, 2010 #14
    Hell, the picture of the Gecko is fairly illustrative.

    @rrw4rusty: I'm sorry if this strayed from your purpose. You're right that this is your thread, and I'm sorry if I was one who made assumptions which were upsetting or derailed matters.
     
  16. Apr 21, 2010 #15
    SpectraCat,
    Thanks for getting back to me! Given that virtual particle pairs are suppose to appear within solids as well as gasses (atmosphere) and vacuums, given that the vacuum energy forced the plates together suggests that the energy is not completely the same... otherwise wouldn't the plates be transparent to the energy? Or, is this wrong? Or, might the energy levels be different (would that give the same results)?

    Also, are there any predictions (proved or unproved) on the amount of energy produced by the virtual particle pairs?

    Thanks,
    Rusty
     
  17. Apr 21, 2010 #16
    The Casimir effect arise whatever the media between the plates is. As long as it can be described by a dielectric function… (the particular dielectric function fir the vacuum is 1 for all frequencies) See the famous article by Lifgarbagez in 1956. :smile:
     
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