Morgan Freeman: Quantum theory works, even though it shouldn't and perhaps the ultimate proof of just how unsettling quantum mechanics can be is something called the double slit experiment. It will make you question whether reality exists at all. This simple configuration shoots particles of light called photons, one at a time, through two tiny slits in a screen.
Anton Zeilinger: There's a laser which produces light. This light is attenuated such that only one photon at a time emerges. These photons pass through a two slit assembly, and then we have a camera which registers the pattern behind the two slit assembly. So, what we see is that the photons arrive one by one on the screen -- some here, some there -- and it looks pretty random.
Morgan Freeman: Since the photons travel one by one -- some through this slit, some through that slit -- you would expect them to leave a pattern of two stripes on the wall, and you would be wrong. They mysteriously create a band of stripes. This is what you would expect to see if a constant beam of light shined through the two slits. It would spread across the wall like a wave.
So, how can single bullet like particles of light create a wave pattern? This could only happen if the particles go through both slits at the same time. In other words, the particle is in two places at once but strangest of all, is what happens when you put detectors next to the slits. When the photons are being watched, the wave pattern disappears. Take away the detectors, and the wave pattern comes back.
This suggests that we can change the way reality behaves just by looking at it. Does this mean that reality itself is not real?
Anton Zeilinger: The modern answer is that the path taken by the photon is not an element of reality. We are not allowed to talk about the photon passing through this or this slit, neither are we allowed to say that the photons pass through both slits. All this kind of language is not applicable.There may be small errors in this transcript.