From: Dropbear ® 21/12/2000 11:47:11 Subject: Photon Exchanges in QM post id: 191194 I read that the repulsive force of like charge is explain in quantum field terms by the exchanging of photons between the two replusive (chuckle) objects - for example two electrons. If that is correct - could we meausre these photons somehow as we bring two material objects together? Why is there not a flash of EM raditation released when two objects are brought together? From: Stephen Bosi 21/12/2000 12:43:39 Subject: re: Photon Exchanges in QM post id: 191227 These photons are what we call "Virtual photons". This means that they appear out of nowhere, momentarily violating the conservation of energy principle, but then disappear again before we have time to accurately measure the energy of the system. They are in-principle impossible to observe directly, but we can measure their effect, namely the electromagnetic force. From: Leslie ® 21/12/2000 12:47:03 Subject: re: Photon Exchanges in QM post id: 191233 if its a virtual photo than would it be real in any scense. could it be perhaps a shadow of the original one like a future eco From: Stephen Bosi 21/12/2000 12:49:48 Subject: re: Photon Exchanges in QM post id: 191234 P.S. You CAN feel their effect when two charged objects approach - it is the mutual attraction/repulsion the two objects experience. The reason you don't see a "flash" of light is because that would mean the virtual photons would become real photons, which would permanently violate the conservation of energy principle. (So don't write back saying you have just thought of a way of getting useful energy from nowhere.) From: Stephen Bosi 21/12/2000 12:52:00 Subject: re: Photon Exchanges in QM post id: 191237 A virtual photon is "real" in the sense that it has a measurable effect, but it does not and cannot have an independently observable existence. From: Leslie ® 21/12/2000 12:52:14 Subject: re: Photon Exchanges in QM post id: 191238 so you can get an elctrostatic reaction/measurement from a virtual photon? From: Stephen Bosi 21/12/2000 12:53:47 Subject: re: Photon Exchanges in QM post id: 191239 Electrostatic fields ARE the virtual photons. From: Leslie ® 21/12/2000 12:56:15 Subject: re: Photon Exchanges in QM post id: 191240 feel free to correct me but electrostatic is one of the four fundimental forces so wouldn't this in turn be violating that law? From: Leslie ® 21/12/2000 12:59:48 Subject: re: Photon Exchanges in QM post id: 191244 what i don't get is its there but it can't be observed but it can be measured so how can get any sort of information if nothing is there unless something is there which in turn violates the energy conservation law? From: Stephen Bosi 21/12/2000 13:17:06 Subject: re: Photon Exchanges in QM post id: 191252 There is a principle called "Heisenberg's Uncertainty principle". It says you cannot know the position and velocity of a particle with complete certainty at the same time. If you are certain of position, then you must know nothing about velocity and vice versa. Einstein showed that this also implied that you cannot know a particles energy and time with complete certainty at the same time. Without getting mathematical, he showed that this implied that over very short periods of time, there must be large uncertainty in energy - in other words, for short periods of time you could never be sure that the conservation of energy principle wasn't violated. The Japanese physicist Yukawa suggested that because of this, virtual particles could arise and that exchange of certain virtual particles might be resposible for the "strong nuclear force". It turns out that almost certainly all the fundamental forces are due to exchange of virtual particles. At this stage the graviton (responsible for you guessed it) is only hypothetical. From: Stephen Bosi 21/12/2000 13:27:13 Subject: re: Photon Exchanges in QM post id: 191262 P.S. In the end, after times longer than the lifetimes of these vitual particles, any virtual particles will disappear again the conservation of energy principle is obeyed. AND remember, you CAN'T quickly store the energy from virtual particles and save it up for tomorrow. The virtual energy can't be made "REAL". We tend to use the word "REAL" to mean particles whose presence is measurable over timescales long enough that the conservation of energy is meaningful. Virtual particles are not figments of our imagination and "real" particles, real. It's just that virtual particles don't behave the same way as everyday objects that we can feel and touch and we like to call real. Virtual particles are "real" in the sense that they have a measureable effect. They just behave so differently from the familiar that we talk about them using weird language. From: Chris (Avatar) 21/12/2000 13:33:24 Subject: re: Photon Exchanges in QM post id: 191269 If I can just add to what Stephen's saying here: We used to describe the repulsion between two like charged objects solely by means of an electromagnetic field. Each charged particle has a field, and when the particles approach each other the fields overlap more and more strongly, resulting in the repulsion. This is essentially a wave description of the electromagnetic field, formalised by James Maxwell in the 1870s. Then along came Einstein's paper on the photoelectric effect, and quantum theory, and people began to realise that in the quantum world you could describe waves also as particles, and particles also as waves. So we should be able to describe this electromagnetic field between two electrons in terms of particles as well as by waves. In other words we should be able to say that electron A emits a stream of particles which electron B absorbs, and the result of this mechanism must be exactly the same as when we describe the situation with a wave description. So the photon is born. When two particles are interacting electromagnetically they emit and absorb virtual photons. When a particle emits electromagnetic radiaiton (eg gamma rays, x-rays, light, etc) the particle is emitting real photons. In this case we're talking exclusively of virtual photons. In the 60s a guy named Richard Feynman came up with a clear simple way of describing how two electrons interact with each other electromagnetically by exchanging virtual photons. His description became embodied in the theory of Quantum electro-dynamics and became the blueprint for describing quantum field interactions (for the weak force and the strong force at least - gravity doesn't want to play Feynman's game yet). I hope this helps! Chris From: Leslie ® 21/12/2000 13:37:21 Subject: re: Photon Exchanges in QM post id: 191273 would this mean that most things could have a virtual copy of its self? From: Karl Kruszelnicki (Dr Karl) 21/12/2000 13:39:37 Subject: re: Photon Exchanges in QM post id: 191278 Dear Stephen, That is probably the best explanation of "virtual" that I have ever come across. Thanks. karl From: Leslie ® 21/12/2000 13:42:07 Subject: re: Photon Exchanges in QM post id: 191280 excuse my ignorence but what does QM stand for? From: schmeardo ® 21/12/2000 16:08:06 Subject: re: Photon Exchanges in QM post id: 191355 how does one electron detect the presence of another in order to exchange a force through emission/absorbtion of a photon? From: Robert ® 21/12/2000 16:12:09 Subject: re: Photon Exchanges in QM post id: 191361 QM = Quantum Mechanics From: Daniel (illuminati) ® 21/12/2000 16:20:55 Subject: re: Photon Exchanges in QM post id: 191369 also, Leslie, you'll see GR (general relativity) and SR (special relativity) a fair bit around here From: Stephen Bosi 21/12/2000 16:58:01 Subject: re: Photon Exchanges in QM post id: 191398 how does one electron detect the presence of another in order to exchange a force through emission/absorbtion of a photon? If someone throws a ball at you, you get a bit of a kick from it when you catch it because the ball carries some momentum. Similarly, the momentum carried by the virtual photons gives the particles little kicks which results in a force between them. From: Stephen Bosi 21/12/2000 17:01:15 Subject: re: Photon Exchanges in QM post id: 191402 Oops schmeardo, I misread your question. How do the particles detect the presence of the other to know when to exchange? Each particle is surrounded by a cloud of virtual photons (in fact they are surrounded by all sorts of virtual particles). When the two clouds overlap, then they can start exchanging. From: John Devers ® 21/12/2000 17:16:20 Subject: re: Photon Exchanges in QM post id: 191413 How does a Firmi surface relate (or not relate) to this again? as virtual particles can be holes in this surface. From: Dropbear ® 21/12/2000 17:44:31 Subject: re: Photon Exchanges in QM post id: 191434 many thanks Stephen! As Dr K has said, your explanations are brilliant. Thanks also (of course) to Chris, who once again puts in a pearler From: Paul H. 21/12/2000 17:53:44 Subject: re: Photon Exchanges in QM post id: 191442 Dropbear, Did you get the book "E=MC^2 - A Biography"? Does it mention Oliver Heaviside? From: Dropbear ® 21/12/2000 17:55:16 Subject: re: Photon Exchanges in QM post id: 191444 Dropbear, Did you get the book "E=MC^2 - A Biography"? Does it mention Oliver Heaviside? Paul, I got and read and finished the book! It is BRILLIANT... It mentioned a heck of a lot of people, but Oliver doesnt ring a bell! But maybe I wasn't paying attention :-) From: Paul H. 21/12/2000 18:11:24 Subject: re: Photon Exchanges in QM post id: 191455 >>But maybe I wasn't paying attention :-) Thanks anyway... (sniff). ;-) From: John Devers ® 22/12/2000 7:54:30 Subject: re: Photon Exchanges in QM post id: 191887 Stephen and Chris, there is also the reactions of (excitons) virtual particles with electrons that emit a real photon when the electron collapses the virtual particle so another third rule would be when particles and virtual particles are interacting electromagnetically they emit real photons. Is there a virtual photon emmission and absorbtion description in this reaction as well? You mention that virtual energy cannot be made real or stored and decaying lifetimes of virtual particles. Does the reaction between an exciton and an electron constitute "made real"? Does a polariton or virtual isotope constitute "made real"? Doesn't a Josephson junction hold a virtual particle in one place without it decaying or is the virtual particle being renewed all the time? There are names for virtual particles like exciton and polariton these are the ones that form inside semiconductors.. Is there a name or names for various virtual photons because when you use a general term like virtual particle for me it gets a bit confusing? Don't scientists often describe mesons as virtual particles which makes another one? There is also the virtual particles that are expanding our universe, how should I differenciate between these, do I call them excitons, mesons or virtual photons? From: John Devers ® 22/12/2000 13:40:54 Subject: re: Photon Exchanges in QM post id: 192040 Chris, just got my mail in and did a bit more reading, now I've got some more virtual particles that need names, great pics of them though, what is Lorentze microscopy? We now have giant vortices and antivortices in superconductors. It seems that when you induce 3 vortices into a square you get a spontaneous symmetrically induced vortex anti vortex pair to preserve symmetry. From: Stephen Bosi 22/12/2000 14:39:45 Subject: re: Photon Exchanges in QM post id: 192088 John Devers I think you are confusing "quasi-particles" with "virtual particles". Quasi-particles refer to situations where a composite of particles or particles in a special excited state of a system can be treated as though they are an individual particle. Quasi particles can be real in the sense that they can persist and can have real energy etc. I guess one can even have "virtual quasi-particles". Try getting your head around that one. From: John Devers ® 23/12/2000 0:11:01 Subject: re: Photon Exchanges in QM post id: 192350 Thanks Stephen, I'm starting to get the idea. I am still puzzled by the use of quasi particle and virtual particles though, below I have listed some quotes about each from Nature a peer reviewed journal. It seems to me that there are a lot of things descrbed as virtual and a lot as quasi, though there seems to be lots of varieties of virtual particles, quasi particles seem to me to be a particular type of virtual particle. How can a quasiparticles be replaced by a distinct collective excitation if it is an excitation? Could you refine the differences between the two concepts quasiparticle and virtual particle just a bit more for me please? From: John Devers ® 23/12/2000 0:11:35 Subject: re: Photon Exchanges in QM post id: 192352 virtual particles 1 Because of the uncertainty principle the 'vacuum' contains virtual particles that can, like the molecules in an insulator, arrange themselves to partially screen an inserted charge. 2 co-tunnelling via a virtual state 3 simply understood in terms of virtual d-wave pair formation 4 virtual phase slips need to be considered 5 case the damping, which is still proportional to the normal conductance, is also virtual. 6 They produce virtual photons of very short wavelength cloud of short-lived virtual gluons 7 virtual quark-antiquark pairs produced by quantum fluctuations in the gluon field 8 if the proton and neutron are assumed to contain virtual pions 9 Both groups see an excess of virtual down-antiquarks over up-antiquarks 10 the only CP-violating effects occur through the small, indirect influence of virtual heavy particles from the third family 11 by pictures known as Feynman graphs that follow the trails of virtual particles in space and time 12 Virtual particles and the very rare case of Kplus meson decay 13 fluctuates through a loop including heavy 'virtual' particles before materializing into api(u quark and d (down) antiquark) and neutrinos 14 their virtual analogues arise only rarely as quantum fluctuations in the vacuum. 15 The reason is that the QMC Green's function also involves processes like those in , where a virtual phonon is created followed by the decay of this phonon 16 Each virtual slit region is selected by the frequency of a weak radio wave field 17 The nature of this foamy vacuum may be visualized by imagining processes that include the pair creation of virtual black holes 18 producing a virtual substance with the mass of diffuse silicon, but the chemical, physical and electrical properties of some new, hybrid material. 19 Hideki Yukawa's wonderful paper of 1935 in which he introduced the idea of virtual quanta, proposed the existence of a heavy nuclear force quantum From: John Devers ® 23/12/2000 0:12:11 Subject: re: Photon Exchanges in QM post id: 192353 quasi particles 1 The many-body theory of interacting electrons in solids establishes the existence of elementary excitations, named quasi-particles, which show a one-to-one correspondence with non-interacting electrons. But this so-called Fermi liquid approach breaks down spectacularly in one-dimensional metals. In this situation, which is described by the Luttinger liquid formalism, the quasiparticles are replaced by distinct collective excitations involving spin and charge, called spinons and holons, 2 (Transport can involve either electrons or quasi-particles, such as polaron and soliton carriers.) 3 An effective attraction between pairs of electrons, or more precisely of fermion quasiparticles near the Fermi surface 4 a part of the complex interaction between the quasiparticles can depend on the relative orientation of the spins 5 almost identical, or quasi-equivalent, sites 6 The redshift of the quasi-stellar radio source 3C279. 7 it stalls between a radius of 100 and 200 km into a quasi-stationary accretion shock 8 is principle of quasi-equivalence was formulated by Caspar and Klug when they considered how polio virus 9 a class of inorganic materials, the quasicrystals 10 True quasicrystals can probably also be described as icosahedral clusters 11 as if a bullet shatters a billiard ball but in the process makes a quasi-elastic collision 12 if an electron has a quasi-elastic scatter on a quark 13 is proportional to the charge of the carriers to establish fractional quantization of quasiparticles in the fractional quantum Hall effect 14 Above this value, carriers can enter the niobium electrodes as quasi-particles with a single electron charge 15 fundamental interactions mediated by composite entities known as 'quasi particles' 16 movement of air masses or by quasi-isentropic transport from the extratropical stratosphere. 17 these clusters grow quasi-one-dimensionally with lengths between 18 (the self-organized structure termed topological doping with locally quasi-one-dimensional electronic character) 19 which can be described by BoseEinstein condensation of Cooper pairs of quasi-particles having both spin and orbital angular momentum 20 correspond most closely to quasi-equilibrium conditions in which the mean number of excitons in the dot is an integer. 21 where we assume a quasi-equilibrium for the electron/hole system. From: John Devers ® 27/12/2000 14:57:07 Subject: re: Photon Exchanges in QM post id: 193529 Chris if your still around could you clear up some of my questions here? From: Chris (Avatar) 27/12/2000 15:16:54 Subject: re: Photon Exchanges in QM post id: 193536 Sure John. Can you rephrase for me please? From: John Devers ® 27/12/2000 15:22:53 Subject: re: Photon Exchanges in QM post id: 193538 Not sure which one you want me to rephrase so I'll take it step by step. Are the virtual particles that expand space virtual photons? From: Chris (Avatar) 27/12/2000 15:24:25 Subject: re: Photon Exchanges in QM post id: 193539 If you're talking about the expansion of the universe, that expansion is not occasioned by any virtual particles. From: paul w ® 27/12/2000 15:24:29 Subject: re: Photon Exchanges in QM post id: 193541 in order for us to understand the things in our part of the universe and beyond we need to have a frame of reference. that frame of reference contains our sun (sol) now we can base everything we learn according to this frame of reference, our frame of reference contains items which have become familiar to us and we know what they are. we also know what an orange is and we know that even if it is in a solid concrete box that when it was put it was orange in colour. now it might turn black later on but this is not what we are discussing. this is a bold statement (considering the previous dialog) but sometimes its better to accept a truth and move beyond this fact. From: John Devers ® 27/12/2000 15:30:14 Subject: re: Photon Exchanges in QM post id: 193546 Next , is a hole (exciton) a virtual particle before it takes on an electron and then becomes a quasi particle? From: Chris (Avatar) 27/12/2000 15:34:23 Subject: re: Photon Exchanges in QM post id: 193551 No, in the parlance of the standard model, a virtual particle is a particle which pops into existance for a time dependant on its energy and the uncertainty principle. Spehpen Bosi has described the difference between virtual and real particles above very succinctly. An exciton is a hole in an electron sea which behaves like a particle - something different. From: John Devers ® 27/12/2000 15:42:24 Subject: re: Photon Exchanges in QM post id: 193557 If the repulsive force is described as virtual photon exchange how is the attractive force explained? What causes the exchange? why would they want to? ps. Stephen you did an excellent job, I've just got to wrap my mind around the implications From: Chris (Avatar) 27/12/2000 16:45:00 Subject: re: Photon Exchanges in QM post id: 193602 First a plug - in the post christmas sales try and get hold of Richard Feynman's book on QED - the man knows (and explains) how electrons exchange photons better than anyone. Having said that I'll give it a bash anyhow. First thing is to make sure you understand the concept of a virtual photon. We start from the uncertainty principle, which you've probably seen in this form: Dx · Dp ³ h` where the uncertainty in position and momentum is constrained to never be less than a constant. There is a corresponding relation for energy and time: DE · Dt ³ h` What this relation says is that the more precisely we measure either energy or time, the less precisely we may measure the other. Thus if I choose to measure time to the nearest second then I can be very precise in my measurement of energy (and vice versa). Now consider the case of the quantum vacuum. I can never know the energy in a given location precisely - I'm constrained by how precisely I measure time. As an example a photon of red light has an energy E. If I measure this energy precisely, then I may only measure time within the precision limits given by the uncertainty relation above: Dt ³ h` / DE Dt ³ 1.3*10-15s Now any more precision in either time or energy will mean less precision in the other. From this we can see that within the time limit Dt it is possible for a photon of red light to pop into existance, as long as it disappears again within the time uncertainty. Such a photon is a virtual photon. Now according to QED, an electron is constantly surrounded by virtual photons - this is how the quantum theory describes the electron's electromagnetic field. When another charged particle comes within range there is a chance that one or other electron's virtual photons will interact with the other electron, and this "chance" takes on the same form as the wave description of the e/m field (ie stronger at small r, weaker at large r). The interaction of the two electrons can't be characterised by a single photon event, which is the essence of QED. Rather there are many many possible convoluted interaction paths between two electrons as each electron interacts with itself and the other electron. However the genius of the theory is being able to take this tangled weave of quantum possibilities and describe it mathematically. In practice we use a Feynman diagram to show the interaction as a single event - one electron emits, the other absorbs - and this is enough for popular physics. The other important thing to remember is that the exchange of virtual photons (or other bosons in the other quantum theories) should not be treated as a classical conservation of momentum problem (eg billiard ball collision). Thus the interaction can be responsible for either repulsive or attractive actions. Hope this helps! Chris From: John Devers ® 27/12/2000 16:59:19 Subject: re: Photon Exchanges in QM post id: 193609 Isn't a hole (exciton) in the electron sea a measurment of E/M? Though a negative one. From: Chris (Avatar) 27/12/2000 17:13:18 Subject: re: Photon Exchanges in QM post id: 193626 No, read the definitions provided again. The exciton is not emitted by an electron to carry the e/m interaction, is it? From: John Devers ® 27/12/2000 17:21:45 Subject: re: Photon Exchanges in QM post id: 193635 Agreed, is it a space absent of virtual photons though? It emits a real photon when an elctron falls into it. From: Chris (Avatar) 27/12/2000 17:26:13 Subject: re: Photon Exchanges in QM post id: 193640 Nope, only happens in semiconductors. Only. Imagine you have a board full of holes and you fill all of them bar one with marbles. Now you can move any marble from its current position to the one hole, but this leaves a new gap. This gap behaves a little like an "anti-marble" - so with an exciton in a semiconductor. From: John Devers ® 27/12/2000 18:08:20 Subject: re: Photon Exchanges in QM post id: 193679 Is the exciton more a cross between a positron and a proton? in having the ability to have the electron orbit it or destroy each other. From: John Devers ® 02/01/2001 13:01:03 Subject: re: Photon Exchanges in QM post id: 197105 I still have some questions on the abitity to use this virtual energy. Virtual photons of very short wavelength and lifetime are used to take snapshots of the proton's interior. These virtual photons are made by scientists in the lab. If you can't make the energy real, how can scientists be currently using them for the above purpose? If no energy is able to be collected or used then what do they collect or use to make the detection? If you can store energy to make virtual photons doesn't it follow that you can get back some of energy from them and store it again, as you gave it to create virtual photons in the first place. From: Robert ® 02/01/2001 13:05:49 Subject: re: Photon Exchanges in QM post id: 197111 What about Hawking radiation? There's a virtual => real conversion there, isn't there? (sorry if I'm missing the point) From: John Devers ® 02/01/2001 16:27:58 Subject: re: Photon Exchanges in QM post id: 197236 Chris you said An exciton is a hole in an electron sea which behaves like a particle - something different and Now according to QED, an electron is constantly surrounded by virtual photons - this is how the quantum theory describes the electron's electromagnetic field. Now does this mean that an exciton is also a hole in a sea of virtual photons? From: Pete ® 02/01/2001 16:38:18 Subject: re: Photon Exchanges in QM post id: 197256 {Speculation = ON} I wouldn't think you could have a hole in a photon sea in that sense, because the Pauli exclusion principle doesn't apply to photons. I would guess that for a "hole" to have meaning, you need a specific set of locations that the "anti-holes" (particles) can occupy. I can conceptualise this for electrons, but not for photons. From: John Devers ® 08/02/2001 23:55:49 Subject: re: Photon Exchanges in QM post id: 225809 Chris and Stephen, I still have some problems with your ideas that virtual particles can't be made real. Let me explain. Particle antiparticle pairs are being created all the time in the vacuum of space, these are called virtual pairs, a virtual particle if it had enough energy would become a real proton or as some people know it an ionic hydrogen atom. According to GR theory these virtual pairs have no FoR so for the very short time that they exist, they have a velocity that is relative to the surrounding matter, this velocity is random according to GR and truely random velocity is plus infinity or minus infinity. This means that some virtual pairs would be travelling close to the speed of light, this means that to external observers the time the virtual pair lives for will seem much longer. During this time these virtual pairs could travel far enough and at enough speed to collide with something and become something detectable. So as you can see you may be wrong when you say that a virtual particle cannot be made real, if it does not have this random velocity and is still then you have to be supporting the ether theory and not GR? Stephen you said refering to virtual photons This means that they appear out of nowhere, momentarily violating the conservation of energy principle I was under the impression that they could not do this and popped out before they could violate the principle unless enough EM energy is transfered to them in some way and they become real? Stephen you said P.S. In the end, after times longer than the lifetimes of these vitual particles, any virtual particles will disappear again the conservation of energy principle is obeyed. AND remember, you CAN'T quickly store the energy from virtual particles and save it up for tomorrow. The virtual energy can't be made "REAL". I still cannot accept this without some GR or QM reason as to why this cannot happen. Stephen you said (in fact they are surrounded by all sorts of virtual particles) Could you give me your list of these names of these "all sorts"? Ps. the virtual pairs that had/have their random velocity greater than light speed might be known as tachyons. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.