OK, good question. How do we know that the electromagnetic wave that travels through a copper or other metal conductor is comprised of photons? There are several clues. One clue is that all electromagnetic waves are comprised of photons, not just EM waves in the visible light spectrum. All radio waves, from ELF to VLF to SHF to UHF are also electromagnetic waves and are comprised of photons. Same for more energetic EM waves like X-rays and Gamma waves. They are all photons but with different energies. Satellite communications - also photons. Another clue is that we know there is a 1/4 second up and down delay associated with satellite communications for satellites in synchronous orbit, which most are. That delay of 1/4 second is due to the speed of photons which we know to be constant in a vacuum of 186,000 miles per second. The up and down distance for satellite communications is about 48,000 miles. That explains the 1/4 second delay. Are you with me? Then assume the audio signal is an electromagnetic wave. There is a lot of evidence for that especially that the audio signal travels at some large fraction of the speed of light in audio cable. We also know how fast audio signals travel through the copper cable and or optical fiber based telephone system. It’s also a large fraction of the speed of light.
Even if you want to accept the theory that much of the audio signal travels through the dielectric and outside the wire per se, you’re still stuck with the conclusion that the audio signal is an electromagnetic wave and therefore comprised of photons. Obviously photons can travel through solid materials since visible light can travel through air and water, those are obvious examples. Other obvious examples are RFI traveling through concrete and glass and your microwave-based mobile device’s capability to operate inside a house or apartment building or shopping center.
re the Double Slit experiment - the famous Double Slit experiment demonstrated that LIGHT, i.e., photons, acts as both particles and waves. The visible light photons don’t pass through the solid material, only through the slits. The interference pattern that showed up on the other side of the Double Slit experiment was proof that the individual particles of photons fired through the slits acted like waves. They interfered with each other. Of course we know now that all particles act as waves also, electrons, neutrons, even a bowling ball would exhibit wave properties - if you could build the proper experiment. By the way, the Single Slit Experiment also produces an interference pattern. What?! 😬
Al, you’re not familiar with the Double Slit experiment? Shirley you jest. 😁
