# How long would it take for two civilizations in the same system to notice eachother [closed]

I have this hypothetical world that's a rocky planet about 5 times the size of the earth and its moon about the size of mars. Assuming each civilization starts at the same time and progresses about the same way earth has how long would it take for each civ to notice each other that's there's life on the other planet/moon.Would the moon civilization have a harder time noticing or the planet?

• Details needed: Distance between the planet and the moon? Thickness and composition of atmosphere for planet and moon? Definition of "5 times the size of Earth - do you mean 5x the mass or 5x the diameter? The title question and body seem to be asking different questions, one is how long to notice a civilisation, the other is how long to notice that there is life - which is it? Also note that unless both planets are being manipulated by an outside force, the odds that both will make key inventions/discoveries simultaneously (eg telescope) are effectively zero. Feb 23, 2022 at 1:16
• "How long would it take": What is the starting point? If we start counting from the moment the first australopithecine climbed down from a tree, it would take more than four million years. If we start counting from the moment an astronomer pointed a good telescope to the moon, it would take a few minutes. Feb 23, 2022 at 1:47
• Possible duplicate of At what stage would we have detected sentient/intelligent life on the moon? Feb 23, 2022 at 1:51
• A nice question, but you're going to get closed because it's unanswerable without you supplying a full timeline with a starting point that shows when particular scientific and technological benchmarks are passed (what inventions are available to each of them when, the telescope springs to mind here), you need to give your question a full rethink and then edit it. Feb 23, 2022 at 2:30
• "Well, I must admit that Ptolemy's 1500-year-old theory about those lights on the Moon seems to be correct. Thanks to these spiffy new 1600-era Dutch telescopes, we can indeed clearly see that some of the lights occur when their armies burn a city. There's a rumor that Ptolemy recycled the theory from astronomers perhaps several thousand years earlier, but those records have been lost." Feb 23, 2022 at 3:04