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How about using gravity instead of radio waves? Gravity has detectable effects over great distances, and we currently have sensors that can detect and map fluctuations in gravity, so if you had a d...
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#1: Attribution notice removed
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System
·
2021-03-30T23:53:13Z (over 3 years ago)
Source: https://worldbuilding.stackexchange.com/a/11799 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/