didnt understand casimir effect well

Discussion in 'Physics & Math' started by rohIT, Nov 21, 2012.

  1. rohIT Registered Senior Member

    i guess casimir effect speaks of particle pairs formin n annihilating all the time at pico scales.
    what i did not understand is that how is energy conserved here?
    everytime particle pairs are produced, some energy is "borrowed". fine.
    but when they annihilate, shouldnt photons be emitted n energy increased?
    same thing in hawking radiation also where he speaks of hawking radiation.
    particle antiparticle pairs are produced, one of which enters the black hole n causes annihilation while the other adds to the mass of the universe.
    plz enlighten me

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  3. Syne Sine qua non Valued Senior Member

    Positive and negative virtual particles balance each other so that there is no net "borrowed" energy. As long as they annihilate with each other there is also no extra, unconserved energy. Hawking radiation is a special case where one of a pair of particles is trapped by a black hole, making it unable to annihilate with its opposite.

    The limited wavelength virtual particles between the plates in the Casimir Effect can still annihilate with their opposites. The effect is only due to the gap limiting the particle wavelengths to a subset, requiring fewer particle between them.
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  5. Crunchy Cat F-in' *meow* baby!!! Valued Senior Member

    Nope. The annihilation you are thinking of is probably matter-antimatter annhilation which does release energy. This is different. Virtual particles are positive energy and negative energy particle pairs. In other words, one particle has energy and the other particle has an energy debt. When energy comes together with an equal debt of energy, they simply cancel each other out. That cancellation is what is being referred to as annihilation here.

    With hawking radiation, a virtual particle pair forms right on the inside of the event horizon and the positive energy particle is boosted via quantum tunneling to the other side of the event horizon. That particle is now free to go and live a full life in the universe while the negative energy particle (the debt) goes right to the center of the black hole and subtracts energy from it.
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  7. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    I disagree with both answers given in post 2 & 3. Energy does not need to be conserved if the violation is very short in time. The product of Delta E and Delta T is constrained. One can be large if the other is small.

    When particle & anti particle spring into existence the Delta E is > 0. In the most common case of electron and positron each had 0.511Mev so Delta E is very briefly 1.022Mev. (Actually slightly less as the value is 0.5109.... and known with the same precision as the mass of the electron is know but I am also neglecting any KE they had.)

    If a pre-existing (for long time) electron and positron do annihilate, then yes there are two Gama ray photons produced, (traveling in nearly opposite directions, to conserve the original total momentum as well as energy) These Gama rays travel at the speed of light, and have well defined energies. I.e. a Fourier analysis of them would show that they have many cycles as almost a "pure frequency" to have well defined energy.

    Many cycles, means these gama have considerable length.* I am just guessing but probably they are thus several meters long so in some classical sense, it took considerable time (Much more "production time" than the Delta T associated with the Delta E violation of 1.022Mev in the case of spontaneous creation from vacuum of an electron positron pair) to produce them. Thus, there are no Gama rays produced when the spontaneous from vacuum pair mutually annihilate. - No long term energy added to the universe "from nothing." Long term, conservation of energy is a "rock solid" concept.

    Now, about Casmir Effect:
    This is best understood with an EM wave model. Wave lengths longer than twice the separation between two conducting plates can not exist in the space between the plates. This is just like micro-wave guides / pipes can only conduct wave lengths shorter than their "cut off" wave length. Longer wave lengths in both cases are "shorted out."

    Now all free space can have wave lengths in it. This potential means that "virtual wavelengths" do exist in it. I think that the virtual energy associated with these virtual wavelengths does give rise to the "zero point energy" but I don´t know much about this. These waves, like all waves do have momentum associated with them. In any case the principled absence of some wave lengths between the plates means there is more momentum pressing on the outside surface of each plate than on the inside surface. So there is a very weak, but observable, force trying to push the plates closer together which we call the Casmir effect.

    * Most people think of photons of all frequencies / wave lengths as being little balls or something like that, but that is a very wrong concept. I have measured the length of some "sodium D" photons and found them to be ~30 cm long. They came from a moderately high pressure lamp, so were "pressure broadened" - not very sharp spectral lines so relative short photons.

    The source of the Northen Lights green line was not known for many years as they are from a "forbidden transition" which takes a very long (by atomic radiation time scales) to produce. They have very many cycles and are many meters long. I don´t know how one could measure the length of gama ray. For optical wave length one uses a two path interferometer and slowly increase the difference of travel distance to the screen via the two paths. As each photon only interferes with itself**, when this distance difference is greater than the photon´s length the interference pattern is destroyed. (It is 50% faded out when that difference is half its length, etc.)

    ** Many things true of photons are impossible for humans to understand (except via math). For example, EACH photon does travel via both widely separated paths of the interferometer. So speaking as humans are prone, one can say that when the path difference is greater than the photon length, the head of the long path traveling part arrives too late to interfere with even its tail part going the short path, so then there is zero interference.
    Last edited by a moderator: Nov 22, 2012
  8. Syne Sine qua non Valued Senior Member

    You can disagree all you like, but there is no measurable violation (time-energy uncertainty), so your objection is moot.

    We are really using the quantum-mechanical approximation method known as perturbation theory. In perturbation theory, systems can go through intermediate "virtual states" that normally have energies different from that of the initial and final states. This is because of another uncertainty principle, which relates time and energy.

    In the pictured example, we consider an intermediate state with a virtual photon in it. It isn't classically possible for a charged particle to just emit a photon and remain unchanged (except for recoil) itself. The state with the photon in it has too much energy, assuming conservation of momentum. However, since the intermediate state lasts only a short time, the state's energy becomes uncertain, and it can actually have the same energy as the initial and final states. This allows the system to pass through this state with some probability without violating energy conservation.

    Some descriptions of this phenomenon instead say that the energy of the system becomes uncertain for a short period of time, that energy is somehow "borrowed" for a brief interval. This is just another way of talking about the same mathematics. However, it obscures the fact that all this talk of virtual states is just an approximation to quantum mechanics, in which energy is conserved at all times. The way I've described it also corresponds to the usual way of talking about Feynman diagrams, in which energy is conserved, but virtual particles can carry amounts of energy not normally allowed by the laws of motion.
    - http://math.ucr.edu/home/baez/physics/Quantum/virtual_particles.html
  9. OnlyMe Valued Senior Member

    Billy T,

    Excellent post on our functional understanding of the Casimir effect. I hope those reading the thread pay attention to the transition in the first part of your post discussing particle/anti-particle pairs and the later part describing the Casimir effect as a function of "zero point energy" and EM radiation or photons.
  10. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Yes I think the more correct, but still wrong, way to classically speak of this math in terms humans can relate to is to say that very briefly 1.022Mev has been borrowed.

    It is much the same with Hawking radiation. It is correctly described ONLY in the math, which very few can even follow. There is no need to even mention the story many tell in terms humans can relate to about one member of an electron - positron pair formed very near the no escape surface being captured. Hawkings does not so speak. He speaks of the Black hole radiating as a black body very little energy until it becomes very small. This POV is much to be preferred* to the one you used as it explains the loss of mass of the black hole, instead of the false increase of mass energy the capture of many halfs of particle pairs, but it too is not fully correct - only the math is.

    *And not only for that reason (avoiding the false prediction that black hole grow by capture of one member of a virtual pair) but the black body radiation POV, (which Hawkins himself uses when not correctly limiting himself to just the math) is to be preferred as it also avoid the false prediction that large black holes, with larger event horizon surfaces would change mass more rapidly than small one do because they have more surface capturing members of virtual particle pairs.

    I don´t object to trying to build models humans can relate to but at least they should not make two false predictions as your post 3 model of Hawkings radiation does.
    Last edited by a moderator: Nov 22, 2012
  11. OnlyMe Valued Senior Member

    The virtual particles involved in the discussion you are citing are not the same as those involved in the Casimir effect. Whether you addresses the Casimir effect from QED (virtual photons) or SED which treats the ZPF as real, you are not dealing with the particle interactions and virtual particles used to fill in the gaps in your reference above.
  12. Crunchy Cat F-in' *meow* baby!!! Valued Senior Member

    Post 3 doesn't assert anything about energy conservation so I am not sure what you are disagreeing with; however, the rest of your quoted assertions within the context of virtual particle annihilation are incorrect. This is why.

    An electron and positron are antiparticles (that is they have the same mass and opposite charge). When they collide they release their energy. A virtual electron and positron pair, while antiparticles, are different. One particle in the pair has positive energy (not charge but energy). The other particle in the pair has negative energy (it's literally an energy debt... like an IOU). There is no known method of pre-determining which particle in the virtual particle pair has positive energy and which has negative energy; however, for the sake of demonstration we'll say the virtual electron has positive energy and the virtual positron has negative energy. When they collide, they dont' release their energy. Instead they cancel out (like adding +1 and -1). This is what happens when energy and an energy debt collide.
  13. RealityCheck Banned Banned

    Hi Billy T.

    Reading that bit evoked the suggestions made by some in the "Alternative Theories" section about the "Luminiferous Aether" as being a sort of background vrtual wave-field 'aether' which supports light propagation via serial induction of those pre-existing virtual waves encountered along the path of travel. Can you or anyone else specifically explain the difference and or reasons why such a 'virtual photonic field' (that already occurs in all empty space as you mention) could NOT be considered effectively AS THE 'Luminiferous aether' which real photons propagate along/by, as speculated by some in the alternative theories section?

    Just curious as to what similarities/differences may be sorted out about those alternative speculations in the light of what you have pointed out above. Thanks. Cheers!

    Well differentiated, CC.

    I myself have since many years differentiated the two processes as "Annihilation" and "Negation".

    I use "Annihilation" to imply "mutual deconstruction" of the existing energy form into another form.

    Whereas "Negation" is used to imply "rebalancing" and hence 'neutralization' of the "transient imbalance" energy back to the "balanced" ground state of the quantum vacuum. Much as you have done by saying the energy debt is 'repaid' and the net result is 'zero' rather than continued energy forms as would be the case if 'annihilation' merely transformed energy not 'paid it back' to the unbalanced 'energy debt' state initially created in the vacuum by the virtual energy fluctuation.

    I agree with your characterization. Subtle but important difference. Cheers!
  14. OnlyMe Valued Senior Member

    Cite a credible reference for this fantasy. There is no positive vs negative energy, in the sense that it appears you are describing.

    Particles and anti-particles whether they are real or virtual, do not represent positive and negative energies.

    And yes there is perhaps the occurrence where the virtual particles of transition may be described in a manner that seems as though there were some "negative" energy or energy debt, but those "virtual" transitions exist outside of time. Virtual particles have not even a brief half life, that is at least to some extent why they are "virtual".
  15. OnlyMe Valued Senior Member

    There are two discussions involving two uses of virtual particles (or virtual waves/wavelengths) going on. Those referenced by Billy T above are the zero-point-fluctuations of vacuum energy.., they are virtual photons theirselves and could in no way then be a medium for the propagation of photons or EM waves....

    The junk you reference from Alt Theories is just that junk...
  16. RealityCheck Banned Banned

    Thanks for your response, OM. Much appreciated.

    But...if these virtual photons actually arise and revert back to the ground state, then they must exist 'for a time', or the 'event' of 'fluctuation' would not occur, and hence the virtual photons would be imaginary to start with. Is that what you imply, that they are imaginary 'artifacts' of maths/theory and nothing more? If not, then such a ubiquitous 'virtual sea' of different virtual wavelengths (which Billy T pointed out) should have some 'background' effect of some sort on the processes of the quantum vacuum at any quantum level like photonic propagation/processes 'for real' (just as their 'absence' within Casimir plates has an observable real effect on macro scale plates).

    The further question/speculation I raised is based on what Billy T has pointed out, as I made clear in my quoted excerpt. Nothing else was implied/involved

    I merely asked for the actual explanation based on that understanding, and not on some other perspective which I have not offered.

    So, can you explain it in that context as to why such a virtual sea of virtual photons cannot have any relation to the speculative 'luminiferous aether' of the alternative theories section? I only want to arrive at some clear and decisive 'why yes' or 'why no' before I read anything further on this aspect, that's all.

    Thanks. Back later.
    Last edited: Nov 23, 2012
  17. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    This quote, from post 3 and repeated above is what I disagree with:

    " Virtual particles are positive energy and negative energy particle pairs. In other words, one particle has energy and the other particle has an energy debt."

    Here is why:
    Both the electron and the positron have the same mass. Ergo, by E = mC^2 they each have positive energy, which happens to be 0.511Mev. I don´t even understand your concept of "negative energy" - as that cannot even exist.

    Energy is the always positive, as by definition it is the ability to do work; although "potential energy" which is not really an energy, can have the zero value defined at an arbitrary point. For example when point far from sun is the zero of gravitational potential, the earth has negative potential, but when the sun is chosen as the zero level, then the Earth has positive potential. Same is true of other potentials. For example if the central post of a "D Cell" battery were assigned zero potential then the case has negative potential of about -1.5Volts. Note that volts is NOT a unity energy. Never is potential an energy.

    "Potential" should always be a noun and never used as an adjective if the common false confusion that potential is an energy is to be avoided.

    You may note that I have dropped the word "energy" after speaking of "potential" as potential, as the name implies, is not energy, but the difference between different energy systems either one of which can be said to be at zero potential.

    The rest mass energy, mC^2 is always positive as negative mass does not exist. Every particle, say a neutron, (or an electron or a positron) has POSITIVE rest mass energy.
    Last edited by a moderator: Nov 23, 2012
  18. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    Luminiferous aether was postualted to exist and many believed it must as they could not under stand waves with no medium of propagation. We now know that photons do not propagate along, in or by stresses in Luminiferous aether which does not exist. It was so natural an idea to man that killing it was hard.

    To explain why Earths motion thur it was not drag producing required other postualtes about it. Then the fact that wave lengths in star light measured on Earth were the same at 6 month intervals*, required the assumption that there was a layer of luminiferous aether bound to the Earth. Stellar aberation experiments ruled that out. (crudely put: Star location don´t shift as if refracted by this layer.)

    In addition to lack of any observable effects, even the idea which you may be suggesting fails as photons (virtual waves) pass thru each other with no interactions, not even scattering.

    *Did not have shorter wave lengths when going up stream thru it, - sort a a Doppler shift.
  19. RealityCheck Banned Banned

    Duplication deleted. RC.
  20. RealityCheck Banned Banned

    Thanks for that reply, Billy T. Much appreciated.

    I understand from conventional theory that every 'feature' has a field' associated with it. The literature speaks of 'electro-magnetic field' permeating all of space, of which a photon is a mere 'excitation feature' that propagates along/by that pre-existing field.

    The obvious question arises, can the ubiquitous background of "virtual wavelengths" in this statement by you earlier...

    ...in any way be likened in any way to the speculated pre-existing 'luminiferous aether' which some postulate as a pre-existing 'quantum waves field' much as any other 'field' is used to support/explain the properties and behaviour of other 'particles/phenomena' having their own particular range of 'field waves/entities' etc etc?

    That is still my question. Can the background of your mentioned 'wavelengths' in any way support/determine the nature/behaviour/propagation etc characteristics of the photon/light as being a mobile 'field excitation' of that ubiquitous 'background field' of virtual wavelengths you mention? Further consideration/speculation: Such a 'field' may be like the superconductor/superfluid in behaviour....ie NO 'friction' and hence no 'drag' on the propagating 'excitation' (photon) itself? And hence why the field excitation feature continues forever in free 'flat' space motion unless and until it encounters another field excitation feature to interact with, whatever that other particular feature or field may involve?

    If not, can you explain precisely why such a possibility is definitely ruled out, in that context? Thanks.
  21. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

    I don´t know if it will help, but lets start with the idea that there must be some mechanism for everything. I.e. for example the attraction between a separated + & - charge seems to imply "action at a distance" which I think Einstein even called "ghostly." Thus we now believe that all that seems to be action at a distance is mediated by the exchange of virtual particles. These virtual particles don´t need any medium to propagate in - perhaps prefer vacuum for that. Many postulate that even gravity is effected by the exchange of gravitons, which if memory serves me, have spin 2. Others more modern still, don´t consider gravity to be a force, needing exchange of virtual particels, but a mass distortion effect upon space its self.

    I got my Ph.D. in physics before some or these ideas were wide spread, so do not know much more than this about them. I don´t really like what has happen to physic in the last few decades - it has become applied mathematics, which was never my strong suit.
  22. RealityCheck Banned Banned

    Thanks Billy T.

    Refreshing to talk to someone honest and thoughtful, irrespective of agreeing or disagreeing on something. Kudos.

    Perhaps 'vrtual' force exchange 'particles' do NOT actually move, but arise and revert back to ground state in situ? If so, then only 'real' persistent excitations (ie, real photons etc) are the only 'moving' particles in 'free space' conditions, thus making the overall background of in-situ oscillations of virtual wavelengths some sort of 'field' which supports the propagation of real particles (much as a roller conveyor system's rollers rotate in situ while the package runs along the conveyor 'field' of rollers)?

    Just a though to try and tease out what all those "luminiferous Aether' speculations in the alternative section are trying to convey. That is why your previous statement about the existence of quantum 'virtual' wavelengths in empty space got me trying to compare similaroties/differences with those speculations.

    Anyhow, just trying to get a real handle on the actual effective differences between such concepts in the context you put. Very helpful. Thanks again!

    If anyone else has any other 'take' on the comparisons/aspects asked about, please feel free to tell! Cheers!
  23. Crunchy Cat F-in' *meow* baby!!! Valued Senior Member

    There are two problems with your request. The first is that you want references for fantasy. I can't give a reference to that but I would suggest returning to the pseudoscience subforum and re-submitting your request there. The second is asking for a "credible" reference without any exemplification of what you consider "credible". I am not going to play subjective interpretation games with you. Instead, I will provide a couple of references that concern negative energy. What you choose to learn or not learn from them is up to you:

    University lecture on black hole evaporation due to negative energy from a virtual particle pair (see page 18/21).

    Nobel prize lecture by Erwin Shcodinger and Paul Dirac on Eletrons, positrons, and negative energy:

    Theoretical "sea" of negative energy (see Modern interpretation):

    Of course one particle in the virtual particle pair isn't made of negative energy. It's not like their net energy is zero or anything stupid like that... it's just particle and antiparticle pairs colliding and releasing little and somewhat painful puffs of energy all over you. /end sarcasm

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