10 ms·

>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

>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 2mo ago
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-existence-to-aliens/ https://earthsky.org/space/airport-radar-could-signal-earth-...
- venusenvy47 2mo ago
  The signals will get there in up to 200 years from now.
- GlibMonkeyDeath 2mo ago
  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...
  - GlibMonkeyDeath 2mo ago
    Replying to myself here, but three things: 1) the receiver arrays are about 50x 6m diameter (3m radius) antennas (https://www.seti.org/projects/ata/ https://www.seti.org/projects/ata/) So that helps about 7x. 2) The 2e15 W/10 MJ/pulse has to be all the radars summed together already, so that is all the power available. Take these two points together, the single photon detection limit is indeed about 200 ly. Third: if we assume a 1 Hz detection bandwidth, I get ~200 photons/s for the CMB @ 3 GHz. I can't find the original paper (just the abstract) so they must assume 100-200 s integration time or so to get to an SNR of 1. So 200 ly is on the very edge of possibility - the example curves were calculated for 10x closer (~30 ly), as I would expect. Still, a lot further than a fractional light year!
    - M95D 2mo ago
      Consider military radars, Cernobyl woodpecker [1]. [1] https://en.wikipedia.org/wiki/Duga_radar https://en.wikipedia.org/wiki/Duga_radar
      - GlibMonkeyDeath 1mo ago
        I am learning a lot about radar :) This radar is using 1 MJ, 3 millisecond pulses at ~3-30 MHz (not ~5 ns pulses at ~3 GHz), so peak power is a far cry from the 2e15 W in the article (~3e8 W.) That's why I think the 2e15 W in the article has to be peak power of all radars summed together. The average power used by all human activity is something like 20 TW (20e12 W, https://ourworldindata.org/energy-production-consumption https://ourworldindata.org/energy-production-consumption)

WithinReason 2mo ago

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-existence-to-aliens/ https://earthsky.org/space/airport-radar-could-signal-earth-...

venusenvy47 2mo ago
The signals will get there in up to 200 years from now.
GlibMonkeyDeath 2mo ago
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...
- GlibMonkeyDeath 2mo ago
  Replying to myself here, but three things: 1) the receiver arrays are about 50x 6m diameter (3m radius) antennas (https://www.seti.org/projects/ata/ https://www.seti.org/projects/ata/) So that helps about 7x. 2) The 2e15 W/10 MJ/pulse has to be all the radars summed together already, so that is all the power available. Take these two points together, the single photon detection limit is indeed about 200 ly. Third: if we assume a 1 Hz detection bandwidth, I get ~200 photons/s for the CMB @ 3 GHz. I can't find the original paper (just the abstract) so they must assume 100-200 s integration time or so to get to an SNR of 1. So 200 ly is on the very edge of possibility - the example curves were calculated for 10x closer (~30 ly), as I would expect. Still, a lot further than a fractional light year!
  - M95D 2mo ago
    Consider military radars, Cernobyl woodpecker [1]. [1] https://en.wikipedia.org/wiki/Duga_radar https://en.wikipedia.org/wiki/Duga_radar
    - GlibMonkeyDeath 1mo ago
      I am learning a lot about radar :) This radar is using 1 MJ, 3 millisecond pulses at ~3-30 MHz (not ~5 ns pulses at ~3 GHz), so peak power is a far cry from the 2e15 W in the article (~3e8 W.) That's why I think the 2e15 W in the article has to be peak power of all radars summed together. The average power used by all human activity is something like 20 TW (20e12 W, https://ourworldindata.org/energy-production-consumption https://ourworldindata.org/energy-production-consumption)