Roll the in the idea we'll communicate with them with messages encoded in radio waves, and they'll understand, and it sounds hopelessly optimistic.
Earth is home to just one species who've developed wireless communication, and we don't broadcast across astronomical distances if we can help it these days.
It is very probable that electromagnetic waves are what you start with.
My point is that this limits our search somewhat to just interstellar species, and that's a crazy high bar!
The most advanced species we know (us) is still confined to one planet. We'd be difficult to impossible to detect from Alpha Centauri, let alone Andromeda.
If someone sent you raw radio data, what's it modulated by? Frequency? Phase? Amplitude? A combination of the three? Is it multiplexed with other data? And then, if you magically figure that out, it's not even in a language you speak? If it's even a spoken language!
Good luck haha.
Communicating like this must be what it's like being an LLM trying to understand your training data.
Perhaps we just missed the signals?
Let's ponder some timescales. The Earth has been around ~4.5 billion years, life began a half billion to a billion years later. It wasn't until about 2 million years ago that the species that was to evolve into modern humans started to use fire. That's 3.498 billion years or so of life before what we might considered civilization.
Humans first started using radio about 125 years ago. An instant in cosmic timescales. Powerful broadcast radio signals first showed up in 1930s. We are already starting to transition to lower power point-to-point wireless transmission. That means our radio waves are become far less detectable at stellar distances. Give it another 25 or 50 years and humans might well have gone silent, for all practical purposes, to hypothetical aliens.
To have another advanced civilization arise both within detection distance of Earth and in an overlapping instant of time where we might detect each other? The odds are unfathomably tiny.
Furthermore it also makes no sense to just scream in all directions so you will use highly directed microwave beam or laser. If you are not in the path of such communication, you will hear silence.
For example, once upon a time "tool use" was considered a unique capability of humans, and from that was derived definitions of intelligence, etc. Well, turns out lots of other animals use tools, at least under that original, somewhat simplistic definition. But what's the significance of that fact--that other animals use tools? Does it mean they're also intelligent, just like humans? People have argued as much, but the "tool use" test was basically pulled out of someone's butt to begin with; the fact we all agree other animals use tools, now, doesn't actually imply much of anything.
People draw conclusions where none can be drawn. And people establish presumptions and definitions to fit their preferred conclusions.
In furtherance of my point. We struggle enough with interpersonal communication. Communication with intelligent animals, of which the examples are many and diverse, has to this point been unmanageable.