A quantum of electromagnetic energy. The photon is the gauge boson responsible for carrying the electromagnetic force.

Making Photons

A photon is emitted when an atom dumps some of its energy. An excited atom, one in a high energy state, has an electron in a higher energy orbit than normal. Nothing likes having more energy than it needs, so the electron will drop to a lower energy state. The difference in energy between the two levels is emitted as electromagnetic radiation, a photon.

How the atom gets in the excited state in the first place can be through many mechanisms. A commonly exploited example is electronic excitation in the cathode ray tube. Electrons accelerated in the tube strike the phosphor on the front of the screen, exciting the electrons in the phosphor atoms to high energy states. They then decay back to lower energy states, emitting a photon on the way. We see these photons.

The same thing can happen in the nucleus of the atom. If one of the nucleons is in a high energy state it can drop to a more desirable low energy state by emitting the excess energy as a photon. The energies involved in a nucleus are orders of magnitude greater than those for the electrons around the nucleus, so the photon has much more energy and we label it gamma radiation.

Wave or Particle?

Well, both. The wave-particle duality, as it's known. All matter, not just light is both a wave and a particle. Most of the time light is a wave, an electromagnetic wave. It is only when it interacts with something that is assumes a particle-like nature. For example, when the light travels towards your eye it is as a wave. Then when the light falls on your retina it can only deposit energy in distinct lumps, or quanta - so it appears particulate. This is true for all matter, just it is more noticeable for light.

You can neatly explain the double-slit experiment 'paradox' by thinking of light in this way. The light remains as a wave during its passage through both the slits and a standing wave pattern is set up on the other side. This standing wave is the quantum mechanical wave function, which directly relates to the probability of observing a photon at a given position. It is only when the light interacts with the screen that it appears as a particle. A full quanta of energy must be deposited.

Photon Mass

Mass is a confusingly misused term. We all know Einstein's rather famous mass-energy equivalence, E=mc², and it's fairly obvious that photons have energy, so they must have mass, right? Well, no. The correct way to interpret E=mc² is to use it define the energy of an object when it is not moving, or rest energy, E0, in terms of a fixed quantity, it's mass. This is sometimes emphasised by calling this value the rest mass, but this isn't helpful, a particle only has one mass, it may have variable energy depending on how fast it is travelling, but mass is constant.

When a particle is moving the total energy is given correctly by, E² = m²c4 + p²c², where p is momentum. You can see if the object is at rest then p²c²=0, and the equation reduces back to E0=mc². In the case of a massless, but moving, particle then it reduces to E=pc. This means that a particle can have energy without mass. You can't stop photons so they always have momentum.

Theoretically, if photons did have mass we would see deviations from the Coulomb inverse square law. It is photons that transfer the electromagnetic force, they are gauge bosons. If they are massless then they can have infinite range and the 1/r² law holds true, if they have mass, they become limited in their range so the 1/r² rule will not hold anymore. Experimental tests for photon mass concentrate on finding such deviations. The upper limit for photon mass so far stands at 3x10-27 eV, which is about 10-46 kg.

Other stuff

The photon is a boson. This means that many photons can exist with the same energy states at the same time in the same place. Not all particles are like this, the Fermions exclude each other from being in the same state - electrons for example, otherwise atoms would collapse. Because the photons can have the same state it is possible to superimpose many of them with the same energy in the same place, this is basically what a laser is.