Q & A: Magnetic field is made of photons

My question is--What is a magnetic field made of? I’ve read a lot of things about magnets and the fields they generate, and even that electrons themselves have magnetic fields around them, but I haven’t as of yet come across anything saying what the field itself is made of. It is matter so it has to be made of something. Is there a name for these "particles"? Or are they simply electrons themselves?
- Douglas (age 35)
Louisiana USA

Hi Douglas,

The electromagnetic interaction is mediated by the constant exchange of photons from one charged object to another. The magnetic field is really just a classical approximation to the photon-exchange picture. In a moving reference frame, a magnetic field appears instead as a combination of a magnetic field and an electric field, so electric and magnetic fields are made of the same "stuff" (photons).

Some electromagnetic interactions involve "real" photons with definite frequencies, energies, and momenta. Electrostatic and magnetic fields involve the exchange of "virtual" photons instead. Very close to an electron is a dense cloud of virtual photons which are constantly being emitted and re-absorbed by the electron. Some of these photons split into electron-positron pairs (or pairs of even heavier stuff), which recombine into photons which are re-absorbed by the original electron. These virtual particle loops screen the charge of the electron so that far away from an electron it appears as if it has less charge than close by.

Normally we wouldn't call any of these fields "matter", but it is true that the electric and magnetic fields which surround a charged object like an electron do store energy, and therefore have a rest mass, via E=mc^2 (in a reference frame in which the electron has no momentum).

Tom

(published on 10/22/2007)

Would it be possible to answer the question clearly using more elementary terms for elementary particles? Or, perhaps a diagram of particles as the cascade in various energy states and produce reciprocal forces or actions on other matter?
- JS BAIRD (age 67)
Davao City,Philippines

It sounds like what your asking for is a Feynman diagram to represent electromagnetic interactions. You can get that with a discussion on Wikipedia:
http://en.wikipedia.org/wiki/Virtual_particle#Virtual_particles_in_Feynman_diagrams.

Meanwhile, I'll take the opportunity to somewhat modify Tom's presentation. We routinely say things like "virtual photons ... are constantly being emitted and re-absorbed by the electron" but that isn't really what we mean.  Two particles that are interacting electromagnetically are indeed surrounded by a virtual photon cloud. However, in familiar cases (e.g. a hydrogen atom) that cloud is completely unchanging in time. Nothing at all is going on. The words about things fluctuating around are a rough way to convey one of the peculiar properties of quantum fields. The electric and magnetic fields have not only average values but also ranges of possible values around the average. That's what's so different from classical fields.  (It's just like the positions of quantum particles, which have ranges around the average position, unlike classical particles.) You can convey an image of that range by pretending that the fields are jumping around between the different possible values, just like you can pretend that a particle  is jumping around among the different positions in its cloud. However, the fields (including their spread of values) no more need to be jumping around than do the particles in space. In a nice stable atomic state, for example, nothing changes in time. The static range of possibilities turns into an actual range of outcomes only when the system interacts in particular ways with the bigger world outside.

Mike W.

(published on 04/28/11)