[UprightCreature]

In my experience the biggest problem people have when trying to understand these kind of experiments is not really understanding what it means to observe something.

People feel like, based on everyday interactions with the macroscopic world, that they can passively observe things without disturbing or interacting with them. In reality this isn't the case in either the macro or quantum world. When you see a person in a telescope you are detecting many photons that are being emitted or bouncing off of them. You couldn’t see the person if these photons were not interacting with the person.

This has already been said in this thread, but, you can not observe a photon without interacting with it. That interaction fundamentally changes the quantum state of the photon.

To further complicate matters you can have a quantum system of entangled particles where the state of one particle depends on the state of the other particle. By observing one of the entangled particles you know something about the other particle in the system, thus collapsing the wave function of both particles. Without explaining in great detail this is the fundamental concept of observing a photon but not developing the film... that is never observing the particle that was entangled with the photon.

It turns out that even though at first glance people think you can use this to send information back in time or faster than light, you actually can't.