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09/16/2020

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MadRocketSci

This doesn't really address your post directly. I saw that article that floated recently about another SETI survey of some small fraction of the sky turning up empty again.

It seems popular to assume that if our radio waves could have traveled 40LY in principle, that we're visible out to 40 LY, and that we could see aliens broadcasting like we do from such a distance. I think interstellar communication is more difficult, and necessarily more directional than most people assume.

I was doing some physics doodling to get some numbers back into my head: I recall we managed to laboriously drag a 100 bit-per-second signal out of the noise floor from the Gallileo probe at Jupiter. It was an S-band transmission, 20W, which amounts to 2,6E-24 W/m2 on average here at Earth. We had to coordinate several dishes with detectors cooled to 11K to obtain that signal, and we could only do it because we already knew Gallileo was there and where to point the dishes!

A 1GW omni-directional transmitter does roughly 100x worse at 5 LY, the approximate distance of our nearest-neighbor star. Aliens right next door couldn't pick up a massive omnidirectional transmission, nor could we. Practical interstellar communication *must* be directional.

With a 1km dish in the X-band (8-12GHz) we can get gains of something like 2E9, by creating a beam that has a half-angle of 4.57E-5 radians. A 1GW sender could then be heard at "gallileo levels" at 25000 ly, or 1/4 distance across the galaxy. A 1kW sender can be heard at 25 LY at those extreme-limit levels, but the beam would only be about twice as wide as the orbit of pluto at that distance. For economical transmission powers, ranges are in the local-stellar neighborhood and necessarily pointed at specific stars. No one else could intercept that beam unless along a direct line of sight.

Your optics don't have to be as extravagant in optical-laser wavelength ranges. An equivalent optic to that 1km dish for a 535nm laser is a 1.5cm lens: easily doable. You'd have to have a power of >800W to outshine the sun at a 1nm bandwidth, also doable with pulse lasers.

Anyway, I suppose what I'm saying is that it isn't too surprising that a radio survey is coming up empty: Doesn't indicate anything even if the galaxy is full of civilizations, other than that we're not on the "point to" list. (The massive time disparity between "has radios and lasers" and "is moderately intelligent" that you discuss here seems more important.)

MadRocketSci

Forgot to say: The Gallileo probe high gain antenna failed to deploy. Earth was trying to find the 20W omnidirectional low-gain antenna to talk to. It's amazing that they could do it at all.

Those big antenna dishes on planetary probes are not there for cosmetic reasons!

Cambias

Wow, you're following this thread closely! I hadn't even posted the link to Facebook yet.

Don't worry, I've planned a later post on whether or not we can actually detect alien civilizations, and how little we've actually looked for them.

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