How are photons being watched in the double slit experiment? What exactly does being observed mean, as it is obviously changes the state of the photon somehow - it must be depriving the photon of something or emitting something that interacts with the photon.
Usually the photon itself is absorbed, changing the state of some locus on the absorber.
For example, if the detector is a CCD, the photon is absorbed thus changing the state of charge carriers in the cell. Or the absorber could be a photographic film, which would change state of its emulsion in the neighborhood of the molecule which absorbed the photon.
So, in most cases the photon is deprived of its own existence.
I don't know which detectors are usually used for the detection of photons. But there's also a double slit experiment with electrons, i'll explain some stuff about that. You might be interested in particle detectors such as cloud, bubble or spark chambers. The detection is done as a result of the ionizing effect of the passing particle. You could see an 'observation' as some kind of physical interaction (collision, ionization, electromagnetic forces,etc...). Note that electrons interact with air (else a cloud chamber wouldn't work), so the experiment must be done in a vaccuum chamber in order for an interference pattern to appear. To make the interference pattern collapse, you could put tiny cloud chambers at the slits for example. For photons, a vaccuum chamber is not needed, since they pass through air without interacting with it (after all, air is transparent).