Chapter 5 – Interactions

Virtual particles

Virtual particles can appear and disappear during an interaction. For example, a photon can spontaneously change to an electron and positron which then change back to a photon.

Total charge and spin are conserved at each step. That is why the creation of an electron in this example must the accompanied with the creation of a positron (anti-electron). Total energy at the end will be the same as total energy at the start but the uncertainty principle allows temporary variations in energy during the interaction.

Particles that only temporarily exist during an interaction are called virtual particles.

Uncertainty tells us that empty space cannot have exactly zero energy. A vacuum contains quantum fluctuations represented by virtual particles that continually appear and disappear. In this example an electron and positron spontaneously appear and disapear.

Confirming empty space is empty

There have been several experimental confirmations that virtual particles appear in empty space.

Magnetic moment of electrons

The original quantum formulae predicted the magnetic moment of an electron to be exactly 2.0.

In 1948, Kusch and Foley measured found it was about 0.1% too high

In the same year Julian Schwinger calculated that electrons emit and reabsorb a virtual photons, which slightly increases its magnetic moment. The magnetic moment is 2.00231930436092 which agrees with the value calculated using virtual particles to 1 part in 10 trillion – this is the most accurately tested theory in physics!

Lamb effect

In 1947 Willis Lamb found and precisely measured a slight difference in energy of two electron orbits in the hydrogen atom that classical quantum theory said should be identical

Hans Bethe showed that the difference was from the interaction of the electrons with the vacuum fluctuations