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More Quantum Strangeness: Particles Separated From Their Properties 144

Dupple sends word of new quantum mechanical research in which a neutron is sent along a different path from one of its characteristics. First, a neutron beam is split into two parts in a neutron interferometer. Then the spins of the two beams are shifted into different directions: The upper neutron beam has a spin parallel to the neutrons’ trajectory, the spin of the lower beam points into the opposite direction. After the two beams have been recombined, only those neutrons are chosen which have a spin parallel to their direction of motion. All the others are just ignored. ... These neutrons, which are found to have a spin parallel to its direction of motion, must clearly have travelled along the upper path — only there do the neutrons have this spin state. This can be shown in the experiment. If the lower beam is sent through a filter which absorbs some of the neutrons, then the number of the neutrons with spin parallel to their trajectory stays the same. If the upper beam is sent through a filter, than the number of these neutrons is reduced.

Things get tricky when the system is used to measure where the neutron spin is located: the spin can be slightly changed using a magnetic field. When the two beams are recombined appropriately, they can amplify or cancel each other. This is exactly what can be seen in the measurement, if the magnetic field is applied at the lower beam – but that is the path which the neutrons considered in the experiment are actually never supposed to take. A magnetic field applied to the upper beam, on the other hand, does not have any effect.
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More Quantum Strangeness: Particles Separated From Their Properties

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