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Humanity just very short time have instruments to hear sky. Visual observations exists much longer, but they are limited to just our galaxy, Milky way (more distant sources are not bright enough to see them with naked eye). And even most obvious mask ideas (and development), which we see on our current sci-tech stages, very fast move spectrum to infrared, which is near impossible to observe via thin atmosphere. And even now, I hear about developments of laser cooling, which could emit extreme amounts of heat into some direction, where nobody will see it. For example, now Earth communications conversed to Ultra high frequencies and very high gain antennas (3G, 4G, 5G, emit only very narrow beam to receiver and are very short range), and to fiber-optic communications, which emit just infrared from heat. Also, Ultra high frequencies very quickly absorbed by moisture, so also effectively converted to heat infrared. So, right question should be, how much time industrial civilization emit observable to others emissions. And imagine, what could alien civilization learn about Earth from locations, where clouds are rare, as others they could not hear :) |