Emit a form of energy that is slightly dispersed by particles suspended in the medium being searched. If you see a shadow, there's something out there absorbing the energy.

A static shadow may be hard to detect, due to fluid flow and density of the dispersing substance. A moving shadow should be easier to track, especially one that is moving quickly.

What do you think of this idea or comment?
(You can change your vote at any time)

I agree

No opinion

I disagree