We can see through walls now. We don't have to depend on transparency to electromagnetic radiation along the line of sight between us and an event. Distance, obscuring dust, whatever, do not matter; we can detect phenomena formerly hidden from us. And as the equipment becomes more sensitive, we may be able to study other events besides galaxy-shaking explosions like black hole collisions. We may soon be able to eavesdrop on the collapsing cores of supernovae, study the supermassives in the cores of galaxies, maybe even listen to the ripples in space-time of the big bang itself.

The ancient Polynesian seafarers could count and time the rolls and pitches of their canoes and feel the wave patterns generated by weather systems thousands of miles away. This allowed them to steer a straight course even when the sky was overcast. They could detect the presence of land, its distance and direction, by sensing the diffraction of wave trains around islands far over the horizon. It is said they avoided the continents because of the confused and distorted reflections of the seas near large land masses. They sensed that was too close to the edges of their known universe, an infinite sea dotted with scattered archipelagoes.

And their radar was passive, there was no need to give away their presence.