Initial assumptions : 1. An unpolarised photon has 50% chance of being transmitted through a linear polariser. 2. When a photon meets a polarizer we subject it to an observation. 3. The effect being that we have forced it to collapse from being in the superposition of the vertical and horizontal states to ONE of them. 4. The energy of the transmitted photon is still the same as before it hit the polarizer. My Question 2 photons are created at some common source , so they have the same energy(frequency). Let one photon go through a linear polariser . And let another go unpolarised. Both go to some to some distant place , not linked to our reference frame. There the two photons arrive and somebody unknown to us wants to determine which of these photons has bee subjected to a polariser . How can they determine this, without any knowledge of what path they took? Is it possible , and how? What does polarization actually do to a photon that we can say , its different from before it went through a linear polariser in this instance?