Anyway, what’s interesting about the experiment isn’t that light is being observed acting like a wave in transit and as a particle when it hits the detector - that happens all the time, and isn’t surprising at all. What’s surprising is that, if you can “follow” the light for the whole time - that is to say, you know which slit each photon through and where they’re going to end up - they should act like particles the whole time, and there should therefore be no interference pattern. To quote a standard textbook (Introductory Quantum Mechanics, by Richard Liboff):
“We conclude that if it is possible to observe which slit [photons] go through, their interference pattern is destroyed. In observing the position of the [photons], their wave quality (e.g., interference-producing mechanism) diminishes.”
(Liboff used electrons instead of photons as an example, but the principle is the same.)
This is determined by the Heisenberg uncertainty principle, which is very closely linked to Bohr’s idea of complementarity. This concept is poorly explained by the article - what it actually states, basically, is that if the photon exists within a well-defined locality of space, momentum will be ill-defined, and it will act like a particle. If it doesn’t exist within a well-defined locality, momentum can be defined more precisely and it acts more like a wave (although it will still always register on a detector in a particle-like way.) So, if you know which slit it goes through (locality defined), it should act like a particle - no interference pattern.
What Afshar has done is set up a double-slit experiment where, because a lens directs the photon streams to different detectors, he knows which photon came from where. So they should be behaving like particles the whole time. Then he plunks down a set of wires right where there would be interference nulls if the photons were being wave-like. If the photons are behaving as particles, like they should, photons should hit the wires, scatter, and the image at each lens should get all fuzzy, as it does with a non-interfering single slit. Instead, nothing much happens, which is what you’d expect to happen if the wires were in the null points of a wave interference pattern. This is a very interesting result.
Now, there’s an obvious problem with this, which was brought up by the article you link to. Putting a set of wires in like that is essentially creating a whole new set of slits after the first pair, which could be effectively destroying the “we know where each photon comes from” phenomenon. One way to test this might be with a single-photon experiment, which is probably what’s being referred to in the link in Gyan’s “update” post; if you fire single electrons at each slit, and every time they show up at the detector they’re supposed to even if the wires are there, that’s supporting evidence that the wires aren’t making things screwy (single photons should behave exactly as photon streams do, including with regard to interference patterns.)
So, it looks like this is a pretty interesting experiment, since it seems to contradict predictions made by Bohr, Heisenberg, and others which have stood up to a lot of tests and experimentation. I can understand why he might be excited about it. However, it’ll require a lot more investigation and experimentation to determine exactly what’s going on, and it hardly invalidates Einstein, Bohr, or quantum mechanics in general - but it might be a step towards correcting, amplifying, or even contradicting some of their work in areas which are still poorly understood.