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Author Topic: The Afshar Experiment - Can someone explain why it is controversial?  (Read 3233 times)

Heronumber0

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I have just been reading about the Afshar experiment.  He says that it is a single experiment that shows that light can be detected both as a wave and as a particle. However, his finding has generated heated debate.  Something about the complementarity principle being violated ????

There seem to be three stages to the experiment:

Stage 1: Laser light through a single slit, a lens and mirrors direct the light to a photon detector



Stage 2:Wires are placed in the place where dark interference lines were seen from the first stage and lens and mirror direct light from one slit on to a photon detector



Stage 3:Wires are placed in the place where dark interference lines were seen from the first stage and lens and mirrors direct light from both slits on to a photon detector



Why all the controversy?

Source:

http://en.wikipedia.org/wiki/Afshar_experiment
« Last Edit: 05/08/2007 22:34:52 by Heronumber0 »


 

Offline JP

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I don't know the technical details, but the complementarity principle in quantum mechanics says that you cannot measure two "complementary" physical observables at the same time.  It's a fundamental part of quantum mechanics that was formulated early on (by Neils Bohr, I believe), which is why there's so much controversy.

  In an experiment like this with two holes through which light can pass, the photon can be made to choose a single hole (usually by closing one hole), in which case it acts like a particle being shot through that hole.  The photon can also be allowed to pass through both holes simultaneously, in which case it acts like a wave and generates interference patterns past the holes.  The wave and particle states of the photon are complementary, so the photon cannot be measured in both at the same time.  In practice, the complementarity principle says when when you look at the light coming out of two pinholes, you either see one or two bright spots corresponding to particle-photons passing through each pinhole (which is what the lens is doing in the first figure you posted), or you see a bunch of interference fringes corresponding to wave-photons, but never both in the same experiment.

What the Afshar experiment does is to start by allowing wave-photons to pass through the pinholes.  You can then look at the interference pattern produced.  It will have dark fringes which are associated with areas in which the waves completely cancel out.  If you place wires at those dark fringes, you don't block any of the light passing through, so if you measure the total light hitting the detectors, it's the same with or without the wires.  Now, if leave those wires in place and you have particle-photons passing through the pinholes (by blocking one, for example), they don't have that interference pattern, so the wires should block some of them: you measure less light passing through with the wires in place than without them (figure 2).

Afshar's setup (the last figure) uses a lens to form an image of both pinholes (therefore, he claims, measuring the particle nature of the photons).  However, his results show that the total light measured is the same with or without the wires in place.  This means when the light hits the wires, it is behaving like a wave.  In this way, he claims to have measured the photons as waves (they aren't blocked by the wires) and as particles (he can form images of each pinhole), which would violate complementarity.
 

Heronumber0

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Ah ha! Excellent explanation. Thank you. So the controversy is about single photons which cannot show both properties (wave and particle) in the same experiment.  Does this mean that this experiment just needs a different explanation?
 

Offline JP

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I would guess th
Does this mean that this experiment just needs a different explanation?

That's the point of debate. :)  According to Afshar's own interpretation, the results of the experiment don't violate any predictions of quantum mechanics aside from the complementarity principle for waves/particles, so there's no major problem if the experiment is valid (aside from having to throw out this long-held principle). 

However, there are also a handful of proposed ways (by highly qualified physicists) of explaining the experiment's results without violating complementarity, all of which are different, so...

???

 

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