system
Given the following post, respond as if you're commenting on Reddit:
human
Physics question:  Photon wavelength is longer for lower energy photons.  Is there a limit on how low the energy of a photon can be?

Radio waves and microwaves are typically what people talk about with "low energy" photons.  As the energy gets lower, the wavelength gets longer - can it be a mile, a thousand miles, a light year?  At some point longer and longer wavelengths don't make sense, the photons effectively no longer oscillate.  So is there a limit, a wavelength too long to get photons?
gpt
There's no theoretical limit. There are obviously practical limits. Once the energy of a photon gets so small that it's no longer definitely contained within the observable universe, it becomes a philosophical question as to whether the photon exists at all.

Different military applications use radio frequencies in the ELF band, which correspond to wavelengths several times larger than the Earth.