>Even humanity’s (weak) radio emissions would be detectable from tens of light years away, and stronger emissions from much further.

That's not true. Non-directional radio transmissions (e.g. TV, broadcast radio) would not be distinguishable from cosmic background radiation at more than a light year or two away [0]. Highly directional radio emissions (e.g. Arecibo message) an order of magnitude more powerful than the strongest transmitters on Earth would only be visible at approximately 1000 light years away [1], and would only be perceptible if the detector were perfectly aligned with the transmission at the exact time it arrived.

[0] https://physics.stackexchange.com/a/245562

[1] https://arxiv.org/pdf/astro-ph/0610377.pdf

▲

voidUpdate 4 hours ago | parent | next [-]

This is my biggest issues with all of the messages we keep sending out to space. By the time it gets to its destination, it will basically be indistinguishable from noise

▲

bluGill 4 hours ago | parent [-]

That depends. If there is "someone" within 20 light years advanced enough to detect our signals we can establish communication and learn from each other - the 40 year round trip time means we can only ask long term questions, but just sending all of human knowledge, and them returning with their knowledge can be a big leg up for both (though sorting through all the things we already know will be a big effort). They may have solved fusion, while we are still 50 years away, meanwhile we have solved something else they are interested in but haven't solved yet.

20 light years is about the farthest useful communication can be established. The farther out things are the longer the round trip and thus the more likely we have already figured things out by the time we get their answer. It would still be interesting to get a response, but our (and we assume their) civilization is moving too fast for much knowledge sharing. Eventually with knowledge sharing you assume something is obvious that isn't and so you get another round trip. Watching an alien movie no matter who far away they are will be interesting (even if it is more a smell based or something that we don't think of)

There is no reason to think we will ever visit them, but we can do other things when they are close.

There are not many stars within 20 light years though. The Femi paradox doesn't exist at that distance, there just not enough stars to expect to find life that close.

▲

Alex-Programs 3 hours ago | parent [-]

Is there a reason we would need to coordinate on what to exchange rather than, say, beginning with encyclopedias and textbooks then moving to a constant stream of notable papers, news, discoveries, etc? What kind of bandwidth can you hit with a cooperating neighbour where improvements become civilisationally important? How many bytes (megabytes? Terabytes?) of meaningful new data does humanity produce per second? I suspect it's reasonably low.

	▲	bluGill 3 hours ago \| parent [-]
		Good question. My thought is similar to yours, but there is a lot of room for debate on what to send. They probably don't care about the roman empire like we do - but there are enough references in modern science that we need to send a summary just so they understand some things. We produce a lot of data, but most of it isn't meaningful. if this is a real situation I wouldn't be asked. So use salt

▲

isolli 5 hours ago | parent | prev [-]

Thanks, these rules of thumb are very useful.

When you say perfectly aligned, what kind of precision are we talking about? If we aimed a receiver at a nearby star, would we be able to achieve this kind of precision?