How can an electron distinguish between another electron and a positron? They use photons as exchange particles and photons are neutral, so how does it know to repel or attract?

# [Physics] How does charge work if photons are neutral

electromagnetismquantum-electrodynamics

#### Related Solutions

You have to realize that when we are speaking of photons, we are speaking of elementary particles and their interactions are dominated by quantum mechanics, not classical mechanics, and in addition special relativity is necessary to calculate anything about them.

In general, we know about elementary particles because we observe their traces in detectors for almost a hundred years. We never see an electron, or a proton in the way we see a particle of dust.

This is the most visual detector, a bubble chamber photo of electromagnetic events.

Here we see some electromagnetic events such as pair creation or materialization of high energy photon into an electron-positron pair (green tracks), the Compton effect (red tracks), the emission of electromagnetic radiation by accelerating charges (violet tracks) (bremsstrahlung) and the knock-on electrons or delta ray (blue tracks)

Now lets see about your questions:

1) How did we arrive at "electrons exchange virtual photons and that's the cause of the electromagnetic force between them" from merely observing electrons absorbing or emitting photons?

That is not the way we arrived at this conclusion. A very large number of controlled scatterings, which is what this picture shows, of electrons on matter have been studied over the years and the theoretical framework of calculating the probability of the scatter and the angular distributions has been very well developed for years. This involves mathematics which cannot be handwaved. To start with, the crossection of an electron scattering on an electron can be written in a series of convoluted integrals which can be pictorially represented by Feynman diagrams. In those Feynman diagrams, the propagators of the interaction between the incoming and outgoing particles can be thought as virtual photons because they carry the quantum numbers of the photon but are off mass shell. So it is a convenient mathematical identification which defines virtual photons.

Anything between the incoming vertices and the outgoing vertices is virtual, and their reality depends on the correct representation of the quantum numbers for the exchanged particle, in this case photon quantum numbers.

2) If electrons throw photons at each other doesn't that mean that they should only scatter (repel)? If that is so why do magnets attract?

Virtual photons are not like balls, they are off mass shell, they are useful a mathematical construct .There is an interesting analog though where two boats throwing balls at each other represent the repulsive forces, and boomerangs the attractive.

Do these two phenomena happen here on earth naturally (no LHC or other particle accelerators), in the upper atmosphere, or only in the deep dark space?

There exist cosmic rays of all energies, the cosmic accelerator, and elementary particles were first seen in emulsions exposed to cosmic rays in high altitudes, for example the pion was thus discovered. So any process seen in accelerators can be found if looking hard enough in cosmic rays. Accelerators allow detail and exact measurements of crossections and branching ratios etc. because of the high statistics possible.

This is the phenomenon of electrostatic induction.

Since "neutral" objects are made out of many positive and negative charges in equal measure, some of which can move, the presence of an electric field from a charged object *will* move these charges, and result in a region of *opposite (to the object creating the field) charge* where the neutral object is nearest to the charged object, and this will indeed result in an attraction between the formerly neutral object and the charged object.

Therefore, you cannot conclude from the attraction of two conducting objects that they must have the opposite charge - one of them may well be uncharged.

## Best Answer

Well, one quick answer, if you want to answer this at the level of QED, is that there are two Feynman diagrams (one where an electron scatters off of a photon, which then scatters off of a positron; another where the electron and positron annihilate, and then the photon decays into an electron and positron) describing $e^{+} + e^{-} \rightarrow e^{+} + e^{-}$, while there is only one that describes $e^{-} + e^{-} \rightarrow e^{-} + e^{-}$ (electron scatters off of a photon, which scatters off of another electron). The short of it is that you can't consider just QED vertices--you have to look at entire Feynman diagrams of processes, and at least know how many of them there are.

Of course, another answer is that, simply, the classical theory distinguishes these two processes--Maxwell's equations plus the Lorentz force law tell me that one process is attractive, and the other is repulsive. Naïvely, one would expect that the quantum process, by land large, would mirror the quantum process.