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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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  • by Animats ( 122034 ) on Wednesday July 30, 2014 @03:33PM (#47568933) Homepage

    That's a nice result. It's in accord with theory. It doesn't match human intuition based on large-scale objects, but it's the way the universe really works. The theory in this area is well understood; Feynman's "QED" has a good overview.

    Ever since the double-slit experiment [], it's been clear that this stuff is real. Over the last few decades, more of the weirder predictions of quantum electrodynamic theory have been confirmed experimentally. This is another predicted event confirmed. Nice work.

  • by enharmonix ( 988983 ) <> on Wednesday July 30, 2014 @03:51PM (#47569117)
    I'm not exactly sure I followed what happened, and I read the dumbed down version. I don't see how this isn't an extreme case of superposition, but I'm not clear on what they did. They split a stream of neutrons into an upper beam with spin going forward and a lower beam with spin going backward. They did stuff to the lower beam that didn't happen to the upper beam? And it keeps mentioning recombining the beams but I didn't quite catch what profound result that had. Can somebody who follows this please explain it?
  • by Anonymous Coward on Wednesday July 30, 2014 @04:33PM (#47569465)

    What you're describing is incorrect. The particle *actually* behaves as if it is two places at once - including things like interfering with itself.

    However, if you want an interpretation that seems more "intuitively correct" than the Copenhagen interpretation, I like Cramer's transactional interpretation of quantum mechanics []. It avoids any "magic" and sticks with a single universe; it does, however, introduce zero mass transaction particles going at the speed of light backwards in time. Assuming relativity as true, this is fine, because at the speed of light time is compressed into nothing, so going backwards or forwards makes no real difference (as there is no change.)

  • by DamnOregonian ( 963763 ) on Wednesday July 30, 2014 @11:27PM (#47571957)
    Multiple electrons sent serially.

    The interference pattern emerges in spite of a conga line of electrically unconnected electrons, sent one at a time at a double-slit interface to the detector. Leptons, not bosons. Things with *rest mass*. Volume. Real shit, not just light, is *actually* a wave-function.

    It's the most fucking bizarre thing in the Universe that I'm aware of, and upon learning of the single-electron version of the experiment, I finally realized that what we perceive of the universe isn't anything close to what it really is. We are little circles in a flat universe trying to perceive spheres passing through our planes of perception, or something that our evolved senses have similarly not equipped us to grok.
  • by Anonymous Coward on Thursday July 31, 2014 @05:55AM (#47572939)

    No, this is not just quantum superposition.

    Did you even consider the possibility that you might not have as deep a grasp of quantum physics as these scientists?

Never buy from a rich salesman. -- Goldenstern