Two galaxies aligned in a way where their gravity acts as a compound lens

The energy density drops off as inverse square law, but the photons go forever. Radio is just photons (light) so it goes forever until it interacts with something it hits. The expanding universe will stretch it's wavelength slightly however.

Sure, but the amount of photons as a percentage of the background radiation drops as a function of the distance. It's not all that far away in cosmic distances when any signal from Earth is millions of times less powerful than the noise level.

> amount of photons as a percentage of the background

That's what "density" means. (i.e. the amount of something per unit volume)

> noise level

A photon will travel thru space forever without losing energy, unless it hits something. What noise are you talking about?

Regular EM Radio waves are not photons. Photons have special configuration which prevents leaks into surrounding space, while regular radio waves are just waves.

> A photon will travel thru space forever without losing energy, unless it hits something. What noise are you talking about?

I'm talking about the https://en.wikipedia.org/wiki/Noise_floor, in particular the unavoidable receiver noise caused by the cosmic background radiation.

A single photon is not a viable communication signal, certainly not at interstellar distances. In practice you need to send out some sort of modulated beam. Even very narrow beams have nonzero dispersion, so the further you get the lower the signal energy will be at an antenna of a given size. So to get more energy you'd need a bigger antenna, but that in turn means receiving more of the background noise as well. In practice there is a minimal signal strength level at which it is still practical to receive the signal.

Long story short: A photon will go on forever (unless it hits something), but a radio signal rapidly spreads out so much that no realistic receiver will be able to recover it from out of the cosmic background noise.

Nope. Radio waves are made of photons. All EM waves are made of photons.

I didn't say sending single photons at a time is a viable communications mechanism. I said a photon will travel indefinitely, without losing any energy, until it interacts with something.

Interestingly, if you send out a single photon from a radio antenna not even the universe itself will have 'determined' which direction it even went until it DOES interact, because there would be a Quantum Mechanical superposition/indeterminacy similar to the famous slit-experiment, if you were dealing with one photon at a time.

So even the thought experiment itself is complex due to wave/particle duality.

Radio waves are not photons. Light beams are not photons.

Light beams (or similar sources of EM waves generated by individual electrons or nucleus) are made by photons. We can record individual photons.

Maybe, radio waves are made of photons, but nobody confirmed that yet, so I can safely say «no». If you can confirm that, Nobel prize is yours.

Are radio waves quantized? Of course, at Planck scale.

Is it possible to form a single 100kHz photon using a macro antenna? I hope for «yes», but I have no idea about «how».

Photons are EM-waves. Are photons made of photons?

Maybe check Wikipedia? Because it refutes you in the first sentence on the articles for "radio", "photon", and "light". You're just being pedantic about word definitions to play games with people.

Great, now add to that the fact that radio waves are an EM-wave too, and that answers your original confusion.

Maybe you should contribute something useful to discussion.

So, in your opinion, photons are EM-waves, which are made of photons, which are EM-waves, ad infinitum? Or you oppose this?

Please, say something useful.

The experiment (one of them, that I'm aware of) that cements wave-particle duality is that you can dial the energy of an emitter down until it's emitting one photon at a time and still detect interference in a double-slit experiment. This is impossible if the photons and waves are distinguishable phenomena.

Radio waves are photons; photons are quantum entities that have particle- and wavelike behavior simultaneously.

I did. I told you Radio waves are made of photons.

Saying that Radio waves are a particular frequency range of photons is not a tautology. The only one making up tautologies is you.