Is it true that matter behaves differently according to how it is observed?
I watched a physics documentary called 'What The Bleep! Down the Rabbit Hole' that showed an experiment involving a photon gun and a sort of backboard that could capture the imprint of the photon as it collided with it. The photon particle first had to pass a through an obstacle with multiple openings. The scientists could see how the photons moved through space (which is apparently very mysterious and illogical) and could see the pattern that was created on the backboard after firing the gun repeatedly. However, when they set up a highly sensitive camera to record everything close up, they discovered that the pattern created on the backboard changed completely (i.e. the photons behaved as they would logically expected them to). I assume that they took the possibility of equipment or some other sort of interference into account.
Yes and no. Forces acting upon matter in the macroscopic world follow Classical Mechanics, or Newtonian, physics, where matter acts as we would expect it to, whether we are observing it or not. However, in the microscopic (atomic and subatomic, actually) world, Newtonian concepts of how matter exists, and for that matter, what matter is, begin to break down -- the study of photons in the 1890's began the shift in physics from Newtonian to Quantum Mechanics. What the Bleep! (which I've watched a few times) explores some of the implications of Quantum Physics. In the dual slit experiment, which you describe, photons act sometimes like matter, and sometimes like energy, or in other words, sometimes like a particle, and sometimes like a wave. In the macroscopic world, the particle properties of matter predominate; conversely, as you go down into the microscopic world, the wave properties of matter predominate. Photons act like both wave and particle, and in this dual slit experiment, it appears photons act like one or the other if they are being observed or not. This isn't a paradox -- what this experiment does is force us to consider what it means for things to change state merely by observation. Philosophy and Physics start to overlap here.