Where does this leave us What is light We can describe very precisely what it does:

1. It has wave-like properties that give very exact answers according to the classical theory of interference.
2. It has particle-like properties that give very exact answers in collision processes between photons and particles.
3. It travels through a vacuum at the constant speed of light.
4. It has the same constant speed in a vacuum no matter how we look at it, be it in a laboratory frame of reference or from a space ship moving close to the speed of light.
5. The particles of light have no mass. . . .

To describe these properties quantitatively we need the theories of relativity and quantum electrodynamics, we need the theories of relativity and quantum mechanics. These are combined in a quite superb theory known as quantum electrodynamics, which can describe very beautifully all of these properties and is the most precise theory that has been tested in the whole of physics.

—Malcolm Longair in Light and Colour, an essay in Colour: Art & Science, edited by Trevor Lamb and Janine Bourriau, 1995.