We know that the faster you move, the slower your time is perceived from an external observer and the space along the direction of the movement shrinks.
Photons, having no mass, move at the speed of light c (in the vacuum) and for them the space has no meaning because it's collapsed into one single point (edit: better say into a single plane, because only the direction along the path collapses). Actually, they don't "move", just "are" at the same time in all places along their path.
In the same way, for them time has no sense. Because they don't "move" they take 0 time to reach their destination.
This seems to be odds we the causality principle, but of course I'm wrong. I try to explain it with a simple example. Let say a photon is emitted from a very far star millions of years ago and I receive it in my eye now. It means its path was started on that star and ended in my retina.
It took millions of years, for us, and 0 seconds from its point of view. So, I apologize for the stupid question, but how it was able to "know" that my retina will be there? If I was few cm apart, perhaps it would have continued for other million of years until it will eventually reach something other. But for it, the time is still 0.
How this could be explained?
And, we know the speed of light is less than c in other media. But it's not clear to me if from the point of view of a photon changes something. For it, the space and time are still 0? As an external observer I can say it still moves as fast as it can. From its point of view it's always in all points of its path.
Is it correct?