Why quantum electrodynamics?

Question

Most people seem pretty much content with classical electromagnetic theory. And most of the applications use classical EM theory. Hence, I would like to know what was the necessity of Quantum Electrodynamics. Is it an advanced version of classical electrodynamics? Does it remove any flaws in classical electrodynamics? Are there any practical applications of QED?

Answer 1

Quantum electrodynamics is needed to describe Nature instead of classical electrodynamics because quantum phenomena are observed – and have been observed at least since 1900 – which prove that classical physics in general and classical electrodynamics in particular is incorrect as a description of Nature and a better theory is needed.

The quantum phenomena include the fact that the energy carried by frequency $f$ electromagnetic wave isn't continuous. Instead, it is an integer multiple of $E=hf$, the energy of a "photon". This may be seen (and was seen by Max Planck) e.g. by analyzing the black-body radiation whose total output is finite and not infinite as classical electrodynamics would predict. So the right theory (which turns out to be QED) must agree with classical electrodynamics whenever classical electrodynamics was tested but it must also be compatible with the existence of photons.

The interactions of the electromagnetic field with other charged particles which follow quantum mechanics – e.g. electrons orbiting the nuclei in the quantum way – make the transition to QED necessary, too. QED has lots of new implications relatively to classical electrodynamics – the existence of antiparticles (and the possilibity to create or annihilate pairs), corrections to Coulomb's law at short distances, and many many others. In all of them, it may be seen by experiments that classical electrodynamics is wrong and QED is right.

QED also helps to solve some puzzles of classical physics. Classical electrodynamics predicted that atoms would collapse in less than a picosecond as they emit electromagnetic waves. The quantum theory makes atoms in the ground state stable. As I mentioned, the ultraviolet catastrophe – the wrong classical prediction that heated bodies emit an infinite amount of energy per second – was fixed and made finite by QED, too. Also, the intrinsic self-interaction energy carried by a charged particle may be made finite, thanks to renormalization (and related effects and their interpretation), in QED.

So yes, QED is an "advanced" version of classical electrodynamics, it removes lots of flaws of classical electrodynamics, and it's needed pretty much for every single physical phenomenon in Nature that was unknown in the 19th century but known since the 20th century.