As it is known in Physics, two point-size charges (say two electrons) interact with each other through em forces. How this is happening? do they exchange photons? And if so, then do they exchange energy (carried by photons) at all time or in other words their interaction keeps on going forever?
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$\begingroup$ Possible duplicate of Where do the photons mediating the electromagnetic force come from? $\endgroup$– sammy gerbilCommented Sep 18, 2018 at 0:59
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$\begingroup$ See also The exchange of photons gives rise to the electromagnetic force and How is the EM force exchanged over long distances? $\endgroup$– sammy gerbilCommented Sep 18, 2018 at 1:06
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$\begingroup$ See also How do electrons and photons interact? $\endgroup$– Alfred CentauriCommented Sep 18, 2018 at 3:14
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$\begingroup$ Possible duplicate of If electrons are waves, how do they repel each other? $\endgroup$– John RennieCommented Sep 18, 2018 at 5:36
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2 Answers
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According to quantum electrodynamics, the charged particles exchange virtual photons. These are not the same as real photons and the terminology has often caused confusion. In particular, one should not think of these virtual "photons" as streams of actual particles going from one charge to the other. The interaction definitely goes on forever, but this is not the same as some kind of eternal absorption and emission event.
Heuristically, the charged particles cause ripples in the EM field, which influence each other and cause the particles to interact. Some energy is also stored in the electromagnetic field, so the charges do exchange some of their energy with the field, but again, it is not helpful to think of this as some process happening over and over.
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$\begingroup$ How reasonable is someone to describe a real particle interaction by means of 'virtual' energy exchange? $\endgroup$ Commented Sep 17, 2018 at 21:43
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$\begingroup$ Well, very reasonable: within the same framework of quantum electrodynamics, the magnetic moment of the electron was calculated to spectacular agreement with experiment, so something about it should be right. The first line of my second paragraph will probably do as a description of the exchange of virtual particles;a virtual particle can be seen as a deformation of a field, which is not free to propagate around like a real particle (hence, the name 'virtual particle' is bad). Note that 'virtual' does not mean 'non-existent' here. $\endgroup$– Stijn B.Commented Sep 17, 2018 at 21:58
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$\begingroup$ At first, I might agree with the usage of the term 'virtual' to describe the em interactions as 'existent' and 'virtual'. On the other hand, let consider the case of a non accelerating electron. That is supposed to create around it a stationary electric field. Which, in my conception means 'something' is sourcing out of the elementary charge and keeps on going to source out in order to sustain the e-field. Thus, the main (I hope reasonable) question is since the established e-field contains energy, where this energy comes from and is it upper bounded , at a certain time ? $\endgroup$ Commented Sep 17, 2018 at 22:18
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$\begingroup$ @SteveTheGrk The above answer is mostly correct, but to make it more clear, the interactions and exchanges of energy and momentum are not virtual, but always real. Two electrons interact, as if they exchange a photon, except this photon does not exist. "To exist" means "to move in time". Virtual particles don't move in time. They "are" at the moment of time of the interaction. Since energy is what moves in time, the energy of a virtual photon is off and governed simply by the overall energy conservation. Note that an exchange of real photon is prohibited by the conservation laws. $\endgroup$ Commented Sep 17, 2018 at 23:40
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$\begingroup$ @SteveTheGrk On this prohibition, see: physics.stackexchange.com/q/428964 $\endgroup$ Commented Sep 17, 2018 at 23:41
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Thinking in terms of quantum fields interactions is very fundamental and more intutive than thinking in terms of particle interactions.
Two electrons interaction:
The vacuum is filled with electron and EM field fluctuating at zero vacuum expectation value(also other quantum fields except Higgs).These two electrons are nothing but localized excitations of elctron field at there respective spacetime points. This electron field due to these localized excitations create disturbances in the EM field, causing excitations in the EMF. These excitations in the EMF then travell to the opposite point to transfer the momenta & energy to the localized excitations of electron field, causing the overall effect of electron-electron repulsion.
Because the intermediate particles("photons" in this case) have no overall existence, we say these particles to be "Virtual Particles" (i.e, "virtual photon" in this case).
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$\begingroup$ "Thinking in terms of quantum fields is more intuitive" - Not really, as is evident by this answer. "The vacuum is filled with quantum field" - This is an urban legend with no physical meaning, an artifact of mathematics filled with problems like infinities. A quantum field defines the probability of detecting a particle. Particles exist, they are observables; quantum fields do not exist and cannot be observed. They are a mathematical formalism fo predict how a particle would behave. A charge is the ability to exchange energy, momentum, and spin with another charge. The rest is just math. $\endgroup$ Commented Sep 17, 2018 at 23:55
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$\begingroup$ @safesphere your going totally wrong. There are proofs of quantum fields like Lamb shift, Casimier effect also, QED is the most precise theory ever made. If you don't believe, go tell Dirac for fields and Feynman for normalizing and subtracting Infinitities $\endgroup$ Commented Sep 18, 2018 at 2:32
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$\begingroup$ These "proofs" are just popular misinterpretations. See the following link for example. Also I didn't say the math was wrong, so your point is not taken. What you are missing is that quantum fields are the fields of probability, not real force fields. There is no question if they exist or not, because mathematical probabilities are not a part of the physical reality. On the Casimir effect, see this: ac.els-cdn.com/S0370269316304567/… $\endgroup$ Commented Sep 18, 2018 at 3:01
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$\begingroup$ @safesphere I already know the rel.Vander-waals explanation for the casimier force. But that is a different matter altogether! But my point of giving this as an example has not worked as i can see. So, you can believe in anything you want but reality doesn't change by one's beliefs. PLEASE no more discussion on this topic. Thanks. $\endgroup$ Commented Sep 18, 2018 at 4:16