Recently, I read a article talking about the average paths of single photons in a double-slit experiment(http://scienceblogs.com/principles/2011/06/watching_photons_interfere_obs.php [Broken]). And I am a little bit confused with this experiment and the concept of Feynman's formulation of quantum theory in the double-slit experiment. The theory states that the particles, in this case photons, would take every paths from the source to the screen. And they would interference with themselves and causes the interference pattern. The theory also states that if we observe the path of any one of particles, the particle's would have only a single path and the interference pattern would be destroy. But in this experiment, the scientist did observe the path of the photons how come they can still obtain the paths of the photons without destroying the pattern. Any help or corrections would be great!!