Afshar's experiment is controversial because the author says he refuted the principle of complementarity. This states that two descriptions, the wave and corpuscular, are needed to understand the quantum world, but are complementary: when it's one the other is not valid, and vice versa. This conclusion can be questioned as discussed below. Tuesday, April 7, 2009
The Molecular Structure Of Clf2
Afshar's experiment is controversial because the author says he refuted the principle of complementarity. This states that two descriptions, the wave and corpuscular, are needed to understand the quantum world, but are complementary: when it's one the other is not valid, and vice versa. This conclusion can be questioned as discussed below. 
In this experiment we know that if both slits are open it shows the wave behavior in the form of interference on the photographic plate to the right. If however we are able to determine the beginning and somehow the way that a particle took (slot S1 or S2), then the board does not appear to be interference and shows us corpuscular behavior. Now imagine that after the slots first put a grid. This grid is such that openings correspond to the interference peaks. This means that for the case of the two open slots in the grid does not attenuate the signal no end to the photographic plate as it is letting through the correct photon peaks. Returning to the experiment
double ranuda usual, above the grille. Consider now that we slots behind a lens, which acts as follows. When only slot 1 is open, the lens sends the photons that pass through it to the detector 1. When only the slot 2 is open, the lens sends the photons that pass through it to the detector 2.
Now imagine that both things together, the grid behind the lens after slots and the slots.
This
Afshar's experiment. The result is that there is no signal attenuation in the photographic plate. That is, there is interference between waves passing through each of the slots. Asfhar concluded that these two things happen at once: there is interference due to the lack of attenuation and there is a determination of the path of each photon due to the use of the lens. This is the principle of complementarity, as in a single experiment, the photons have wave and particle characteristics shown.
However, this conclusion need not be correct. What seems quite beyond doubt is that there is interference. But that does not seem clear is that the lens actually is determining the way in the case of two open slots. What is proven is that the lens sends the photons that pass through the slot 1 to detector 1 for slot 2 being closed, and vice versa. Conclude from this that there is a determination of the road for two open slots for an extrapolation is probably unacceptable.
To illustrate consider an electron in a superposition of the projection of its spin on the z axis ( is in the state mentioned say anything about the position during the trip. We can not ask which slit the electron passed since the system is not in an eigenstate of the basis on which we project.
The difference in this experiment is that the final projection of the particle state is not about all possible vertical positions on a photographic plate, but as Ruth Kastner (in
Why the Afshar Experiment Does Not Refute Complementarity
) calls "slit-basis"
In the double-slit experiment, the system prepared in a particular state can be projected on all possible positions on the vertical axis using a measurement of position with a photographic plate. In this case, using lenses, the system can be designed only on L 'and U'. However, these two basic states do not correlate one to one with the states L and U.
More information:
http://en.wikipedia.org/wiki/Afshar_experiment
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