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Comment by spuz | Hacker News Reader

This paper seems to explore a question I've had for a while which is: What is the maximum distance an Alien civilisation could eventually detect life signatures from Earth assuming their detection technology matches ours? I know that our current radio signals extend 100 light years or so but at which distance could these eventually be detected?

I ask because I've read that in order for the SETI program to make a detection the signal would need to be much more powerful than the radio transmissions we currently produce.

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Our electromagnetic shell drops below the noise floor around 15 light years out for say 5 MW military radars. For coherent signal decoding into carrier wave and payload waveforms as opposed to being able to point to a signal and say, yep that's not natural, probably around 5 light years out. Some discussion here [1] to get you started.

Even assuming super duper alien technology that violates what we know of the physics of signals transmission that can pick up the faintest earliest human radio signals, our 200 light year shell is insignificantly sized against the backdrop of the galaxy at large [2].

Yes, if someone out there is broadcasting on radio frequencies, it would be the galactic equivalent of The Russian Woodpecker [3]. That's possibly a Kardashev Type II or at least a very high like 0.90 Type I level technological civilization accomplishment to put out artificial radio frequencies on the scale of naturally-produced ones.

There is lots of discussion on ideal frequencies to broadcast upon and look for [4]. There is even a discussion familiar to IT data protection folks that is the interstellar equivalent of a station wagon filled to the brim with tape hurtling down the freeway [5].

But the gist is we'd do better to reconfigure our economic north star upon moving the metric ass-tons of humans off Earth into habitats and working out how to move us around space faster and faster, than waiting for ET to visit us.

[1] https://www.quora.com/Radio-and-other-signals-have-been-leak...

[2] https://www.popularmechanics.com/space/news/a27934/galaxy-ma...

[3] https://en.wikipedia.org/wiki/Duga_radar

[4] https://www.cambridge.org/core/journals/international-journa...

[5] https://en.wikipedia.org/wiki/Interstellar_communication

I would guess advanced civs don't bother with radio and look instead for direct visual and spectrographic evidence.

We're very close to being able to look for spectrographic evidence and maybe 50-100 years from building a telescope that uses a planet's gravity well for optical enlargement.

Earth already looks quite distinctive spectrographically because of the free oxygen, water, and CO2. In the general case you look for chemistry that is far from equilibrium.

Hubble already has the sensitivity to analyse exoplanet spectra, and Webb can do much better.

But some way beyond that, I'd imagine it's possible to look for spectral technosignatures directly. Artificial lighting, heat sources, and unusual non-biosignature gas combinations would all be clues that life exists.

I'm also fascinated by Przybylski's Star, which must be one of the weirdest objects in the galaxy. It doesn't take much imagination to wonder if it's an artificial beacon.

https://en.wikipedia.org/wiki/Przybylski%27s_Star

To me it looks more like a radioactive waste dump than a beacon.

For baryonic-matter-based technological civilizations, above a certain technology level there is precious little to no such artifact as "waste", especially waste with energetic properties of any kind. Baryonic matter is vanishingly rare in the universe compared to the size of the universe and dark matter, so I wonder if someone there is dumping radioactive isotopes whether that is another SETI plateau like we hold out conjectures for about radio signals today transitioning to low-power digital signals, and one day we'll notice the spectral output changing.

With correct encoding you can detect signals way below the noise floor. Thats how GPS work, you detect a satellite signal orbiting at 400 km with an antenna the size of a chip.

The Curiosity rover communicates with Earth from Mars with an antenna the size of a soda can.

But the receiver has to know the encoding to be able to detect it. Maybe the aliens are using a different type of spectrum spreading technique that makes a lot of sense to them but is unknown to us.

May be true for radio signals, with the possible exception of OTH radar? But there are other technosignatures such as atmospheric composition that they may also be able to detect at substantial distances I think.

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