Energy. In classical physics, energy means the ability to do work, which is defined as exerting a force over a distance. This definition, however, doesn't provide much of a picture, so in classical physics, energy is a rather abstract concept. In QFT, on the other hand, the energy of a quantum is represented by oscillations in its field. In fact, Planck's famous relationship between energy and frequency of oscillation {see Chap. 3} is a direct consequence of the equations of QFT. In our color analogy, we might say that the oscillations cause the color to "shimmer", and the faster the shimmer, the greater the energy of the field.
( Rodney A. Brooks )
[ Fields of Color: The theory ]
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