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Casimir Effect

  1. Jul 25, 2005 #1
    Ummm...I don't know if this is the best section for this... but... it is said that a negetive energy can be created through the quantum theory via the Casimir Effect. Here is what I know as fact. In 1948, a Dutch physicist known as Henrik Casimir discovered how this negative energy can be acquired. Take two large neutrally charged metal plates, and place them in a vacuum. There is no force between the two plates, as they are neutral. (Neglecting gravity) However, due to the Heisenberg Uncertainty Principle, the vacuum is filled with trillions upon trillions of particles and antiparticles, appearing and disappearing back into the vacuum. These particles are so quick to appear and disappear they are on the most part unobservable. Therefore they supposedly dont violate any laws of physics. (I personnally have a hard time swallowing that they dont violate the law of conservation of mass and energy) Anyway, these particles create a net attraction between the two plates. Casimir precidicted that this negative energy was measurable. People viewed this with extreme skepticism back in the day. But in 1958, physicist M. J. Sparnaay observed this effect in a controlled laboratory experiment, and it would match Casimir's predictions. For me, it sounds rather...iffy. Is there other info someone knows? Do you people think it is a bunch of bunk? Feedback is appriciated. Thanks.
     
  2. jcsd
  3. Jul 25, 2005 #2
    No.

    The casimir effect is well understood to be a 'real' consequence of 'virtual' actions.
     
  4. Jul 25, 2005 #3
    Ahum, negative energy ??? Please elaborate ?


    Yes they DO violate energy conservation. But that is allowed for a certain amount of time thanks to the uncertainty principle. This is one of the weird behaviours of quantum field theory (at least to our classical eyes). Read my journal on this since i have debated this issue many times: Read the second entry on this page:
    Virtual Particles
    Most certainly not, the Casimir effect proves the existence of the virtual particle sea that is directly responsible for this effect.
    The Casimir Effect

    regards
    marlon
     
    Last edited: Jul 25, 2005
  5. Jul 25, 2005 #4

    Chronos

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    For a classical analogy, look up the accoustical casimir effect. The principle is very similar, and quite interesting.
     
  6. Jul 25, 2005 #5
    Here is the citation for the Casimir paper:

    Casimir, H.B., ``On the attraction between two perfectly conducting plates'', K. Ned. Akad.\ Wetensch. Proc., 51, 793-795, (1948).

    A good summary of Casimir effect by Trang Nguyen and its use in modern technology is at this link--see summary below:
    http://www.hep.caltech.edu/~phys199/lectures/lect5_6_cas.pdf

    Of interest to me is the statement by Dr. Nguyen that Casimir effect can be either attractive or repulsive depending on geometry of interacting entities--

    would not this lead to theoretical possibility of gravity (attractive) and antigravity (repulsive) interactions between two entities having different quantum wavefunction shapes ?

    Casimir Effect and Vacuum Fluctuations
    Trang T. Nguyen
    Department of Physics and Astronomy,
    Ohio University,
    Spring 2003 ​
    Conclusion
    The Casimir effect has become the subject of diverse studies of general physical interest in variety of fields. It is equally interesting and important for Quantum Field Theory, Condensed Matter Physics, Gravitation, Astrophysics and cosmology, Atomic physics and Mathematical Physics. Currently the Casimir effect has been advanced as a new powerful test for hypothetical long-range interactions, including corrections to Newtonian gravitational law at small distances, predicted by the unified gauge theories, supersymmetry, supergravity and string theory. It is also gaining in technological importance in vital applications such as in nanoelectromechanical devices. Precision measurements of the Casimir force have led to an excellent agreement between the experiments by means of atomic force microscope and theory.
     
  7. Jul 25, 2005 #6
    Thanks a bunch people, you all gave me more then I ever intended.

    Oh and btw, when you asked me to elaborate on "negative energy", your guess is as good as mine, that was what the energy gained by the Casimir Effect was described as. Hey, I think Im not doing that bad for a sophmore in high school. :rofl:
     
  8. Aug 1, 2005 #7
    the total energy of the free vaccum if taken to be 0 then still we can have creation of matter without violating the thermodynamics.
    0=+1-1
    ie. if we have a particle and antiparticle the energy is 0 and will be 0.
     
  9. Aug 1, 2005 #8
    But there IS violation of energy-conservation going on. That is the whole point of being 'virtual' and that is also why these virtual pairs are a typical quantum-fields-thing. Beware, both the quantum as well as the fields part are essential here to be able to talk about virtual particles. Quantum because of the HUP, fields because of the particles being excitations of fields. If you only had the quantum part, you'd be talking about virtual states that are the transition between an initial and a final state.

    marlon
     
  10. Aug 2, 2005 #9
    But I though these particles can happen "occur" in any environment, not just an absolute vacuum, even witha vacuum, there is still radiation passing through it. (At least by any vacuum able to be created on Earth) If radiation passes through, then it affects these particles, if not, then it violates the 3rd law of thermodynamics. Not to mention what if a particle and an antiparticle are given the chance to collide, that creates more radiation in itself on an extremely small scale.
     
  11. Aug 2, 2005 #10
    I'm really not an expert on virtual particles so someone please comment on the following assertions:

    1) A virtual particle pair does not posess a net energy. If they did, then the
    anhiliation would produce radiation, and it does not. Therefore, energy
    conservation is not violated regardless of the time duration of the particle pair.

    2) According to Hawking, if one of a virtual particle pair "falls into" a black hole,
    the other particle becomes "real" and persistent. The one that fell in had the
    effect of converying some negative energy density into the black hole, thus
    shrinking it.
     
  12. Aug 3, 2005 #11
    Yes they do posses a net energy, otherwise energy conservation would NOT be violated. And yes they can influence the physics of oher objects being present. Just look at the nature of the vacuum polarization process. Keep in mind that such virtual particles can arise from EM radiation. In the vacuum, this EM radiation, strong force, weak force,.. are NON ZERO. this is the clue of it all, together with the HUP allowing to violate energy conservation.

    Err, can't a real particle have negative energy ? YES IT CAN. So what makes you so sure that the negative particle is absorbed into the black hole.

    Besides, you are looking at it the wrong way. The created virtual pair can become real if there is enough energy available (provided by the black hole) to make it real, to give it a legitimate reason to exist, if you will

    marlon
     
  13. Aug 3, 2005 #12
    Last edited: Aug 3, 2005
  14. Aug 3, 2005 #13
    Can we say the Casimir-force is a consequence of virtual particles? I thought it is a consequence of the electromagnetic vacuum fluctuation. If we calculate the lowest energy state of a harmonic oscillation we get 1/2*h(bar)omega. And a virtual photon has energy E=h(bar)omega. So, what's wrong here? I mean the lowest state of harmonic oscillator is defined in that way, that there are no photons present.

    Or are there two ways to explain the Casimir-effect?
     
    Last edited: Aug 3, 2005
  15. Aug 3, 2005 #14
    The Casimir effect proves the existence of virtual particles, so the answer to your question is YES.

    Also, yes, virtual particles come from the EM vaccuum fluctuations, but also from all of the other forces like the strong force ...

    marlon
     
  16. Aug 3, 2005 #15
    If the Casimir-Force is a consequence of virtual particles and electromagnetic vacuum fluctuation, why we just derive the force by sum over the different wave vectors k of the harmonic oscillator in ground state in and outside the plates? Is the em-vacuum fluctuation the same as the virtual particles?
     
  17. Aug 3, 2005 #16
    Of course they exist and influence other objects around them. This
    is not in dispute. But why is there no energy released on their anhilation?

    The simple explanation is that the paricle pair posesss no net energy.
    That way there's nothing to release on anhilation.

    I'm not sure. Stephen Hawking is sure- I'm wondering why as well.
     
    Last edited: Aug 3, 2005
  18. Aug 3, 2005 #17
    Listen, HUP says that during a short period of time d(t) a particle with energy d(E) can exist in that way that d(E)d(t)=h(bar)/2 is sadisfied. This particle can acutally be a virtual photon. During its lifetime it can make following process: gamma->e+e-. With real particle this process is not possible in free space. Virtual particles can make this process. And remember, if this proces occur the virtual positron can be seen as a particle going backwards in time with negative momentum. Thus if the photn has momentum k and the electron momentum e then the positron has momentum e-k. The virtual electron and positron pair and also the virtual photon at begining have all a exact defined momentum and thus (position-momentum uncertainty) can be everywhere in space. The virtual e- and e+ can annihilate (remember their position is not well known) to a virtual photon. The virtual photon just gives its energy back to the "vacuum" (energy-time HUP).

    The virtual photon and all virtual particles are off mass shell. They don't obey Einstein's momentum-energy relation: E^2=m^2c^4+p^2c^2. Thus virtual photons (always having mass = 0) can have energy u and momentum k such that u^2>p^2c^2.
     
  19. Aug 4, 2005 #18
    I appreciate this Kruger. But couldn't it be said that this is bookeeping? If energy
    is briefly "borrowed and returned" in such a way that it can never be seen as energy
    (i.e. it is impossible to detect the energy of the virtual photon) then I question
    whether energy was "borrowed" in the first place.

    Let me make a classical analogy. In the case of total internal reflection, one can say
    that beyond the critical angle the angle of refraction becomes complex and therefore
    there is refraction into a complex space. This is mathematically correct. But it would
    be incorrect to interpret it as a physical space. In the physical space, the
    energy become non-propogating and hovers near the interface decaying exponentially
    away from it.

    Could it not be that the energy "borrowed" by a virtual particle is of this nature?
    Sure it has units of energy but isn't possibly a form of "evenecsence"- physically real
    to be sure but NOT to be physically interpeted on the basis of the terminology or units
    involved?
     
    Last edited: Aug 4, 2005
  20. Sep 13, 2005 #19
    Here are some questions for you?

    If you took a particle with mass and a spin of 1/2 and had the ability to instantly subtract the particle to 1 dimensional space, Would this particle still have spin?

    Next, Would the particle that now was reduced to 1 dimension still be called by the same name?

    Now the same questions but with increasing dimensions with the exception of the dimension you already concluded the particle to exist in.

    I thought that I would bend your minds for a bit.

    Gerald L. Blakley
     
  21. Sep 13, 2005 #20
    I have a question I need answered after reading about Hawking radiation. Is it correct to think virtual particles can be made 'real' if you apply a force on them to pull them apart? So if I had a large enough electric field, I could in theory pull a virtual electron and positron apart before they annihilated?
     
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