Grahame- You’re right that electromagnetic waves, whether viewed classically or in terms of quantized photons, are not affected by static electrical or magnetic fields. They have no charge. Nevertheless, they do exert electrical and magnetic forces on charged particles and magnetic particles. Viewed classically, they consist of nothing but electrical and magnetic fields propagating through space, so it’s entirely appropriate to call them electromagnetic waves.
Mike W.
and for some thoughts on the relation between the classical and quantum views of electromagnetic waves:
Photons are little units of light -- they are the original "quanta" of quantum mechanics. Their existence was hypothesized to explain the details of the photoelectric effect -- photons with enough energy can knock electrons out of materials. Since then, photons have been found to play a central role in the explanations of many physical phenomena, from explaining how much heat is radiated by hot objects to modern quantum cryptography.
But on to your question! The classical model of electricity and magnetism makes use of the ideas of electric and magnetic fields. Maxwell’s equations describe how these fields behave, and the Lorentz force equation, which describes how the fields push and pull charged particles and magnets. A prediction of Maxwell’s equations is that there are waves in the electromagnetic field which travel at the speed of light. These waves were identified with light by the experiments of Hertz and others. We therefore have two very different ideas for how light works -- as waves in the electric and magnetic fields, and as motion of particles -- photons. This pair of explanations is called "wave-particle duality" and is a recurring theme of quantum mechanics. Depending on the experimental situation, light acts as a wave or as a particle (but never both simultaneously).
Weirder still -- static electric and magnetic fields also exhibit wave-particle duality. The collision of a charged particle with another (repulsive or attractive) can be modeled as the exchange of photons and you get the same answer as if you had calculated everything with just the classical fields (in the limit that the classical calculation applies -- slow incoming particles). The quantum calculation involving the exchange of photons is more accurate in describing actual collisions at higher energies.
And now the really weird part. You might ask -- "Photons travel in straight lines at the speed of light. Why doesn’t the electric field of a charged object just zoom away at the speed of light?" It turns out that the photons which make up a static electric or magnetic field are "virtual" -- their energy and momentum doesn’t satisfy the relationship for "real" photons -- E=p*c (E is energy, p=momentum, and c is the speed of light). The virtual photons are constantly emitted and reabsorbed. A charged object with an electric (and possibly also a magnetic) field is surrounded by an entourage of photons, constantly being emitted and reabsorbed.
Photons, real and virtual, are emitted and absorbed by charged particles, even though they are not charged themselves. They only interact with charged particles, and not with each other. That’s why photons don’t interact with magnetic fields -- the photons which make up the magnetic field are not charged so other photons cannot interact with them.
Technical p.s.: photons have entourages of electrons (and other stuff) around them, and so photons can interact with other photons by interacting with this cloud of charged stuff. The effect is so small it hasn’t been observed yet for low-energy photons. Very high-energy photons produced in particle accelerators may collide with themselves readily.
Tom
(published on 10/22/2007)
