Propagators and Virtual Particles
In the previous section we showed how a phase shift produces a potential difference that is propagated away, which can interact with another wave and hence produce a force. This also means that the phase shift causes a wave that transports energy and momentum away. So when a wave causes phase shifts all the time, it sends off energy all the time. This cannot be possible, since the wave would be losing energy all the time and soon would cease to exist.