Quantum properties of long wavelength electromagnetic radiation How could we have known that Electromagnetic radiation is quantized if we only knew about long wavelength radiation? What are the 'quantum' properties shown by long wavelength electromagnetic radiation?
 A: It is difficult to answer by keeping the what-if approach of the original question. It is difficult to imagine the technological development necessary for an experimental investigation of QED at low frequencies without the accompanying understanding of the high-frequency regime.
However, it is exciting to note that many traditional limitations are progressively disappearing in recent years. For example, until a few years ago, one would have answered that the direct presence of photons at low frequency (radio waves) would have been undetectable. Nowadays, we have some experimental evidence of photons in the radio-frequency part of the electromagnetic spectrum. See, for instance, a recent measurement of the average occupancy of single-photon states in a radio-frequency resonator.
A: I will assume here that by "long wavelength" you mean longer than ~150 meters (i.e., in the AM radio band).
It is true that all EM radiation exists in quanta, each photon with energy E = (planck's constant x frequency). But in the AM radio regime, the energy per photon is so small that there is no experimental evidence at all that this electromagnetic radiation consists of quanta. RF transmitters and receivers that work with 150 meter wavelengths are designed, built, and operated with no considerations of quantization at all.
So if all we had to work with were AM radio technology, we would have no experimental evidence in hand that long wavelength electromagnetic radiation consists of individual photons, because its quantum properties are so small.
