He is suggesting that advanced civilizations would use gravitational waves to communicate because they are hard to detect. This summary of Dark Forest helps ex

He is suggesting that advanced civilizations would use gravitational waves to communicate because they are hard to detect. This summary of Dark Forest helps explain why a civilization would want to conceal their communications[0]. [0] https://www.quora.com/What-is-the-Dark-Forest-Theory-of-the-cosmos-which-is-a-response-to-the-Fermi-Paradox https://www.quora.com/What-is-the-Dark-Forest-Theory-of-the-...