[GlibMonkeyDeath]

>Earth's television and radio broadcasts would only be detectable at distances up to 0.3 light-years, less than 1/10 the distance to the nearest star.

This is what I always thought was one of the major problems - unless an alien civilization is directly targeting Earth, we would never detect them. And our EM emissions are too weak to be detected by other aliens unless they stumble near us for some reason.

[WithinReason]

Not so:

They found that airport radar systems, which sweep the skies for airplanes, send out a combined radio signal of 2×1015 watts. That’s enough for telescopes comparable to the Green Bank Telescope in West Virginia to pick up as far as 200 light-years away. [0]

200 ly is about 100k stars

[0]: https://earthsky.org/space/airport-radar-could-signal-earth-...

[GlibMonkeyDeath]

Huh - very interesting! Now I am going to have to calculate this for myself!

Seems that (r_d/(theta_e z))^2Pt_p ~ 100 hv (~100 photons at 3 GHz; a single 3GHz photon is about 2e-24 J) I'll drop this to 1 photon below.

Where

r_d = detector radius,

theta_e = diffraction angle of source,

z is detection distance,

P = 2e15 W (peak power of source, according to article) and t_p is pulse duration (say, ~5e-9 s).

Now theta_e ~ wavelength/(emitter radius), let's say the detector radius r_d ~ emitter radius = r ~ 3 meters for simplicity. Lambda = l = 3e8/3e9 m ~ 0.1 m) E_p = Pt_p ~ 2e155e-9 ~ 1e7 J (!)

(r^2/l)^2(1e7 J)/(1002e-24 J) = z^2, so z ~ \sqrt(100 m*(1e29)) ~ 3e15 m

But one light year is ~1e15 m. If I assume they have single-photon sensitivity in the microwave regime, then I can boost this result to ~30 ly. So about 1 order of magnitude less than their paper, but that is for a single photon of a single radar pulse. If you sum over many pulses from all the different radars that could be emitting over the integration time then, yeah, I guess 200 ly isn't ridiculous.

Curious what the cosmic microwave background is compared to this though...

[venusenvy47]

The signals will get there in up to 200 years from now.