ELI5: Is there any way of seeing someone without them seeing YOU

If you define 'can see' very loosely as 'photons can get from here to there,' then the answer is no, at least at short distances and low velocities. The physics of light propagation are symmetrical, in the sense that if there's a path from here to there, then there is also a path from there to here.

There are exceptions, if you're prepared to talk about who sees whom when. You might see light from a distant star which reaches you unobstructed, but light from you gets blocked on the return journey.

You can use diffusion to make an image indistinct, even if light is getting from place to place. Lots of 'privacy windows' in e.g. hotel bathrooms use tricks like that. So it might depend on exactly what you mean by see.

/r/explainlikeimfive Thread