# What is the difference between a quantum and photon?

1. ### Florence.

1
I'm a little confused with quanta and photons, they seem to be used interchangeably. Could someone clear this up for me? That would appreciated Thanks!

2. ### Born2bwire

1,776
A photon is a quanta, but not all quanta are photons.

A quantum is just a discrete "thing." We are not referring to a precise property or object, just referring to the fact that in quantum mechanics (which unfortunately for this discussion uses the word in its name) the systems are now discretized in respect to some of their characteristics as opposed to being continuous as they normally would for their classical counterparts.

A photon is a quantum of electromagnetic energy. That is, instead of allowing electromagnetic energy being a continuous value from 0 to whatnot, it now can only exist in discrete amounts called photons. Each photon has an energy of \hbar \omega. The total electromagnetic energy in a system (ignoring the whole vacuum energy stuff, not needed for this discussion and we can always renormalize it to zero) is the sum of the energies of all the photons in the system.