Dehumanized professors, wretched writers, hopeless romantics, all of us are searching for truth as the end of the world draws near. But even Tarkovsky can't help but feel like an impostor in the face of endless beauty and mystery.
Probably and apart "Andrei Rublev", this remains the most fascinating densely Tarkovsky film, despite the tangential with "Solaris", another magnificent film. In fact, the universe of science fiction is, with this filmmaker, an appeal of an existential and metaphysical combination (in a pictorial-physical form) of the usual dissonance between science and belief, being that an immanence of this one, both substance.
Besides to all the existentialist/philosophical messages I think the main concept for Tarkovsky is to find a compromise. The stalker is the compromise between the scientist and the writer: he strictly follows the rules, but he doesn't really knows how they work and why. It's the innate human contrast between what you really are and what you're told to be. Rationalism vs Art. Anyway there's no salvation for us.
The people who made this incredible film, one of the greatest films ever, all died as a result; that is, the ones who were in the radioactive zone: Tarkovsky, the 3 male actors who entered the "zone", the photographer, and assistant director. They all died a slow death from cancer within 17 years of making this film. You owe it to yourself to see the masterpiece that these people all gave their lives for.