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I would be interested in what people think of the following experiment.

Imagine a laser beam is split into two coherent beams A and B that are made to cross each other at right-angles. Beyond the point of intersection let us assume that there is a detector A in the path of beam A and detector B in the path of beam B.

In the region where the beams intersect there will be parallel diagonal planes of constructive and destructive interference separated by a wavelength.

Now let us block both of the beams A and B before they reach the intersection point.

Next we place a thin absorbing layer, whose thickness is much less than a wavelength, at the position of one of the diagonal planes of destructive interference.

Now we open each of the beams A or B alone and verify that the beam is absorbed by the thin absorbing layer so that neither detector A or B fires.

Now we open both of the beams A and B together.

The two beams now reach both sides of the diagonal absorber in such a way that there is destructive interference of their amplitudes in the plane of the absorber. Thus the probability of the absorber absorbing a photon is zero.

I believe now that the beams will "leap-frog" the absorber so that half the time detector A detects a photon and half the time detector B detects a photon.

How could we interpret such a result?

Let us assume detector A fires. By conservation of momentum we can track back along the path A that the photon must have taken up to the diagonal absorber.

But how did it get through the absorber?

I submit that the only way it got through was that, using David Deutsch's phrase, a "shadow" photon passed along path B at exactly the same time such that it destructively interfered with the path A photon allowing them both to avoid being absorbed.

That shadow photon must have been detected by detector B in a "parallel" world.

That parallel version of the experimenter makes the same argument, tracking back his shadow photon along path B, and thus reasons that another photon travelling along path A must have crossed the absorber at exactly the same time as his photon and must have been detected by detector A in a parallel world.

John

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# Simplified Afshar experiment: many-worlds interpretation

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