Quantum theory & harmonic overtones

Quantum theory asserts that the universe is digital. This is to say that if you look any phenomenon -- light, for example -- closely enough, you will find that it comes in discreet quantities: units, intervals, or layers. So what appears to be a smooth decrease in light when you turn a rheostat type light dimmer, is according to quantum theory, a series of very small jumps in light intensity.

For light, these discreet quantities are known as photons.
Harmonics are the discreet intervals present in all sounds. Picture a guitar string vibrating (see below): the center of the string bounces up and down after being plucked. This creates the primary note, which musicians call the tonic. At the same time however, each half of the string is vibrating, as if the center of the string were fixed in place! This creates the first harmonic overtone, which is exactly twice the frequency of the tonic -- one octave higher.

Just as with the whole string though, each half is also vibrating in halves: quarters of the whole string. It also vibrates in thirds, sixths, eighths, and so on. It is the difference in the intensity of these harmonic overtones that create the distinctive tone qualities of different instruments or voices.

In talking about quantum theory, keep in mind that the quantities or intervals involved are for the most part too small to be directly perceived. What are more commonly perceived are large scale quantum effects, like rainbows.