A mental trick that helped me understand the concept of “an observer” in particle physics is to imagine it like playing billiards in a pitch black room.

In normal human-scale billiards, there are immense numbers of photons flying around bouncing off everything. The photon interactions are far too small to affect the path of a moving billiard ball, but we can detect them easily with our eyes. So we can use photons to passively observe the balls rolling around.

But when you’re trying to observe a photon itself… there are no tiny photon-equivalents flying around. It’s like playing billiards in pitch black: the only way to know which direction a ball is rolling is to touch it. And you can’t touch a rolling billiard ball without changing its path somehow. Likewise, you can’t “observe” a single photon without interacting with it in some way.