5 ms·

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

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 2mo 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)

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 2mo 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)