On this weeks Horizon they showed the 'Twin Slit' experiment. In this experiment, they have two tiny slits, which they then fire photos of light at. Imagine firing bullets through two neighbouring windows, with the pattern on the wall across the road, you'd expect to see two columns of marks. Do the same with two slits and fire single photons from a laser though, and they suggest you get three groupings? Stranger still they continued to suggest if you then look at the individual photos as they approach the slits to see what's happening, the phenomenon stops happening? Now my brain has trouble with this, so I'm wondering if someone can clarify it. Is it really three groupings? Or more than that in a traditional interference pattern? The suggestion is, there's some sort of interference (even though there's only ONE particle) so you don't get just the two expected groupings. If this is the case, when ONE particle is fired, is just one detected in the groupings, or even more strangely, are TWO detected in the groupings on the other sides of the slits? And there suggestion, just looking at the experiment somehow changed it? How can that be? Surely the 'look' must be actually affecting the experiment itself?