We Can’t Read the Signs
Now, it’s totally possible that the signs of ETIs are all around us, but we just can’t see them. Either we’re too stupid to notice, or we still need to develop our technologies to detect the signals. According to the current SETI approach, we should... be listening for radio signatures. But a civilization far more advanced than our own might be using a different technique entirely. They could be signaling with lasers, for example. Lasers are good because they’re tightly focused beams with excellent informational bandwidth. They’re also able to penetrate our galaxy’s dusty interstellar medium.
11 of the Weirdest Solutions to the Fermi Paradox
Or, ETIs could use “calling cards” by exploiting the transmit method of detection (e.G. By constructing a massive perfectly geometrical structure, like a triangle or a square, and put in orbit around their host star).
And and as Stephen Webb has pointed out, there’s also the potential for electromagnetic signals, gravitational signals, particle signals, tachyon signals, or something completely beyond our understanding of physics. It’s also quite possible that they are in fact using radio signals, but we don’t know which frequency to tune into (the EM spectrum is extremely broad). More conceptually, we may eventually find a message buried in a place where we least expect it — like within the code of our DNA