Classical interacting fields: a wave cannot generate waves with a higher frequency
wave -->
___ ___
____/ \____/ \____ tense string
| | | | | | rubber bands
________________________ tense string
Above we have two interacting fields: two tense strings. The interaction happens through rubber bands which are tense and attached to both strings.
Intuitively, the interaction cannot produce waves which have a higher frequency than the input wave.
The frequency in quantum mechanics is associated with the particle energy. In a Feynman diagram we have an analogous rule: the energy of the particles coming out of an interaction cannot exceed the input energy.
Can a transient wave in classical interacting fields contain a higher frequency Fourier component? A transient wave is analogous to a virtual, off-shell, particle.
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