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More Quantum Strangeness: Particles Separated From Their Properties 144

Posted by Soulskill
from the superposition-of-cat-metaphors dept.
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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  • Strange? (Score:5, Interesting)

    by jandersen (462034) on Wednesday July 30, 2014 @03:36PM (#47568973)

    I'm getting a little bit tired of the never ending fascination with QM 'weirdness', because it seems to me that it tries to see everything as 'weird' simply because it is 'quantum', with the danger that that it makes people blind to what might be explainable by more intuitive means.

    In this case I think we see an illustration of the fact that the notion of a particle as a mathematical point in space - something with zero dimensions - is an abstraction; an approximation that works well enough because we can't in that much detail any way, and it makes the equations so much easier. We have always known, somewhere, that this is not true - things like the mysterious wavefunction that mysteriously collapses as soon as we measure it is a big hint, I would say. As explanations go, that one has always sounded a bit strained - hopefully we will be able to handle the maths of a better model in the not too remote future.

    A more likely scenario, in my view, is that what we call particles is something more distributed in space, and that somewhere in that 'distributed particle' we can explain how a particle can travel through several paths at once. I mean, it isn't even an altogether new observation - the famous electron diffraction experiment shows something similar.

  • by mx+b (2078162) on Wednesday July 30, 2014 @03:58PM (#47569205)

    I have never been a fan of the quantum "weirdness" either. Everyone gets caught up in the Copenhagen interpretation and Schroedingers' cat and all, and ignores a simpler explanation. I think you may be on the right track with zero dimensions not being realistic -- and I believe that is the hypothesis of string theory actually, to model objects as 1d strings instead of 0d points -- but even that I think is overlooking something easier.

    The Heisenburg uncertainty principle illustrates the true nature, I think. We cannot measure position and momentum simultaneously. Why? Because on the scale of electrons, those electrons are very small and lightweight and can get jumbled around. We have to do something to measure speed. For cars, we can measure speed by bouncing light ways off them (radar guns). But try a light beam on an electron -- at that size, the electron can feel the full force of the electric field of the light wave, and gets moved out of the way. A car is so huge compared to a beam of light, that we don't affect a car when we measure its speed, but we DO affect the electron. So either we can use the light to find where it was (and knock it around so we're not sure what speed it was going), or we can use the light waves to get an accurate reading of how fast it was going, but now we've knocked the electron somewhere so we're less sure where it is now.

    Particles can't really be two places at once. But since we're knocking things around with our light beam, we can't say for sure where it is now -- so we instead talk in terms of probabilities of where the electron is, rather than saying matter-of-factly where it is. This is what quantum mechanics does, it calculates probabilities that the electron is in a certain place, probability it was going a certain speed, etc.

    The double slit experiment mentioned by another poster shows this is the correct interpretation too. As you can see from the photos on Wikipedia, when single particles are allowed thru, we see only single points on the detector. It is only when a flood of electrons are allowed that we see an interference pattern similar to that of a wave. Seems pretty weird!! But is it really? In actuality, as our detector reads electrons, it is knocking them around a little (think of billiard balls bouncing around, off of the detector). As electrons build up, the electric field also builds up in the area between the slits and detector. That electric field is so small that our instruments can't really detect it -- but it IS strong enough to again, knock around electrons. That slight push from the build-up electrons onto the electrons coming thru the slit means they get pushed away from the center, away from the build up, and then they settle down at the outer fringes of the build up. Naturally that means there's some gaps at play here, and so we observe it to be a wave interference pattern. This all happens so fast that it seems instantaneous too. But nothing particularly magical going on -- just the rules of forces mean that electrons get knocked around A LOT, even for imperceptible forces on the human scale (or scale of our equipment).

    Other physicists have argued for this interpretation. I know, [citation needed], but I'm drawing a blank who. I want to say Ed Witten but not sure. In any case, I know there have been proponents of this interpretation rather than the "weird" Copenhagen interpretation. But hey, people couldn't make TV shows about how quantum strangeness leads to time traveling thru the multiverse if we did away with it.

  • by Zalbik (308903) on Wednesday July 30, 2014 @05:25PM (#47569955)

    Particles can't really be two places at once. But since we're knocking things around with our light beam, we can't say for sure where it is now -- so we instead talk in terms of probabilities of where the electron is, rather than saying matter-of-factly where it is. This is what quantum mechanics does, it calculates probabilities that the electron is in a certain place, probability it was going a certain speed, etc.

    As others have mentioned, you are missing a couple of fundamental points of the double-slit experiement.

    1) The pattern observed has nothing to do with the photons being hard to measure (classically photons are sent through the slits),
    The pattern produced is exactly the interference pattern expected if light were actually a wave. The peaks and troughs of the two waves cancel each other out which results in the dark bands. Dual peaks or dual troughs reinforce each other, resulting in bright bands.

    2) If this was a result of electric field build up and the "detector knocking particles around a bit", then it should also happen for a single slit (it doesn't). It also should not occur for photons (electrically neutral), but it does.

    3) "when single particles are allowed thru, we see only single points on the detector"

    This is incorrect, and the weirdest thing about the experiment. If two slits are opened, and particles are sent through one at a time, there is still the same interference pattern created. Individual particles behave as if they do not have a fixed location, but only a probability of existing at a specific location.

    Heisenberg's principle is a result of quantum mechanics and wave-particle duality, not the cause.

Facts are stubborn, but statistics are more pliable.

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