Quantum Electrodynamics (QED) and Virtual Photons
 
Max Planck
Max Planck

The union of relativity, electromagnetism and quantum mechanics is called Quantum Electrodynamics or QED. If, for example, two electrons interact via Coulomb's law then according to QED the electrons exchange a photon that 'carries' the electromagnetic force from one electron to the other. A photon is emitted by one electron and absorbed by the other. These photons are examples of virtual particles and are not like ordinary observable photons since they are not limited by energy or momentum conservation. To see why, recall the Heisenberg Uncertainty Principle. The principle can be written in terms of energy and time:

deltaE deltat >= h/(2 delta)

This mathematical statement says that the uncertainty in the measurement of energy of a particle multiplied by the uncertainty of the time in which it can be measured, is greater than or equal to Planck's constant divided by 2. So over a very short time period, the uncertainty in the energy deltaE can be very large.

In particle physics, quantum particles can be thought of as being surrounded by clouds of virtual particles which are emitted and usually reabsorbed by the parent particles. These virtual particles have lifetimes that depend on the uncertainty principle. The more massive and energetic a particle is, then the more brief its lifetime and the smaller its range.

In QED, the electromagnetic interaction between two charged particles (such as two electrons) is understood to be due to the exchange of virtual photons. The virtual photon exists only as an unobservable intermediate state and for this reason particle physicists call them virtual.

The idea that force between particles can be mediated by a carrier particle is a very important concept in particle physics but QED had serious theoretical difficulties. Heisenberg and Pauli noticed that some calculations that involved the interaction of the electron with itself yielded nonsensical results associated with the self-energy of the particle. The self-energy arises due to (in the case of an electron) the fact that the electron has an electric charge and therefore has an electric field associated with it. The strength of the electric field is inversely proportional to the square of the distance from its source (the electron), and because the electron is at zero distance from itself, then the strength of the interaction should be infinite. This difficulty gave rise to infinite values of energy and mass in the quantum calculations making them meaningless. It can be shown that exactly equivalent problems arise with other interactions such as gravity and the strong force. As a result, the full development of QED was held back until a way was found to deal with these infinities some twenty years later by using a mathematical technique called renormalisation.


Links

Brief description of Quantum Electrodynamics
Quantum Electrodynamics and photons
About QED and virtual photons
Biography of Max Planck