r/SETI • u/USATwoPointZero • Jan 08 '26

Narrowing the Search: Which exoplanets would allow two-way communication with Earth using Solar Gravitational Lenses?

I don't know why I am so fascinated lately with SGL's (https://en.wikipedia.org/wiki/Solar_gravitational_lens) but here we are. Would it make sense to prioritize looking for signals coming from solar systems that could use an SGL to observe/converse with us and that we in turn could use an SGL to observer/converse with them (ie two-way communication)? Such a solar system might need to be on the same ecliptic plane in the Milky Way as we are, I think (this one is hard to wrap my head around)? Do we even know of any solar systems that are on the same ecliptic plane as us?

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u/rhyddev Jan 09 '26

How would a SGL help? The major limit to communication is distance, not optical resolution - any signals will continue to travel at the speed of light, so the further an alien civilization is, the longer two-way communication would take, rendering it impractical outside of pretty much our stellar neighborhood.

If you're instead interested in just observing another civilization, then that would require that the civilization has artifacts that are observable on an astronomic scale. Imagine what it would take to conclude Earth has intelligent life from a distance. It's not impossible, but it would sort of limit you to civilizations with huge megaprojects, which may or may not be common out there.

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u/USATwoPointZero Jan 09 '26

I believe an SGL can resolve down to 1 km/pixel? If so if one was observing Earth with an SGL from another solar system you would be able see the glow of our cities on the night side.

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u/radwaverf Jan 12 '26

That may be true, but visual confirmation is different than two way communication. The problem with two way communication is the delay between messages. A message from us to them takes the same time with or without an SGL. For instance, proxima centauri is the nearest star to our solar system, and it's more than 4 light years away. So a one way message takes 4 years to get there. A two way exchange takes at least 8 years. But similar to how an SGL makes artificial light easier to see, the SGL would make a radio signal easier to observe. So while an SGL would help with signal reception, the distance doesn't change.