Back to something people were discussing yesterday --
If you want to create radiation with a tremendously long wavelength, you can do it by moving a charge at a tremendously low frequency.
You can get a very very long wavelength if you are willing to move a charge in a sine wave with a frequency of 1 cycle per billion years. You can even do that inside any cavity you want, if you have a way to accelerate your charge the way you want it. In a small cavity it can't go very *fast* in one direction for a billion years, so somebody a billion years and a billion light-years away would need very…