Colour

The three original quarks are ``up'' (u), ``down'' (d) and of course ``strange'' (s). Each is a spin- fermion but it took some time to understand how three similar quarks could coexist in the same state within a baryon. (The extension of the Pauli exclusion principle forbids this.) The resolution of this dilemma was to propose (and later believe) that each quark comes in three different complementary `` colours'' (call them red, green and blue) that have to be combined to make the composite particle (meson or baryon) colourless (white) just the way the three colours on a TV monitor must all be lit up at once to produce a white ``pixel.'' Of course, we have no idea what colour is - it certainly has nothing whatsoever to do with the wavelengths of visible light! - but by now you should be comfortably disconnected from the world of empirical personal experience, so the fact that the metaphor of colour gives us a handy way of getting right answers should suffice.

Using this quark model with gluon exchange [gluons are colour changers, they convert a quark from one colour to another when emitted or absorbed] in a fashion exactly analogous to , theorists are now able to accurately describe much of the structure of hadrons, thereby rescuing perturbation theory from the ashes of strong interactions, where it failed miserably. The new theory inevitably became known as Quantum ChromoDynamics (or ) by analogy with except with colour (Greek chromos) in place of electric charge.