6 years ago
Earth's "First Contact" May be Ghost Signals from Long-Vanished ET Civilizations
The human species has been transmitting radio waves for only about 80 years, so our radio waves cover less than 0.001 percent of the Milky Way. Electromagnetic signals (blue circles shown below) from alien civilizations will continue traveling through the Milky Way even after the aliens are gone. The appearance of a doughnut hole represents when a civilization dies out.
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