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

Don’t radio waves weaken proportionally to the square of the distance? No one would be able to detect them past a (relatively) small distance.

Omnidirectional source, yes.

However, beamed sources don't fall off that way.

A search for optical laser emission from Alpha Centauri AB - https://academic.oup.com/mnras/article/516/2/2938/6668809

> ... This search would have revealed optical laser light from the directions of Alpha Cen B if the laser had a power of at least 1.4–5.4 MW (depending on wavelength) and was positioned within the 1 arcsec field of view (projecting to 1.3 au), for a benchmark 10-m laser launcher

For comparison, with our measly human technology...

https://www.ukri.org/news/uk-science-facility-receives-85m-f...

> The Vulcan 20-20 laser is so named because it will generate a main laser beam with an energy output of 20 Petawatts (PW) alongside eight high energy beams with an output of up to 20 Kilojoules (KJ). This is a 20-fold increase in power which is expected to make it the most powerful laser in the world.

Or even five decades ago (TODAY!) ... https://en.wikipedia.org/wiki/Arecibo_message

> The entire message consisted of 1,679 binary digits, approximately 210 bytes, transmitted at a frequency of 2,380 MHz and modulated by shifting the frequency by 10 Hz, with a power of 450 kW.

https://www.seti.org/seti-institute/project/details/arecibo-...

> The broadcast was particularly powerful because it used Arecibo's megawatt transmitter attached to its 305 meter antenna. The latter concentrates the transmitter energy by beaming it into a very small patch of sky. The emission was equivalent to a 20 trillion watt omnidirectional broadcast, and would be detectable by a SETI experiment just about anywhere in the galaxy, assuming a receiving antenna similar in size to Arecibo's.

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.

Anywhere in the galaxy within the super narrow beam that the Arecibo antenna happened to cover at the time, at least.

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.

A perfectly parallel source wouldn't fall off with inverse square, but all real sources are not — and cannot be — perfectly parallel.

What you get from lasers is very high gain in the direction it is pointed in, but it's still subject to the inverse square law.

It's capable of being enough gain to be interesting, to be seen from a great distance.

If you engineer it so the gain is enough to outshine the rest of the parent galaxy in the direction it is pointed, then that's effectively good enough because the galaxy is also following inverse-square and you'll continue to outshine the parent galaxy even as you and it both get weaker, but it's still falling off inverse-square.

> 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?

I stand corrected on the inverse square.

I still hold that it would be possible to send and detect signals set with intention with not too much more advanced technology than what we have.

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.