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More Quantum Strangeness: Particles Separated From Their Properties
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.
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.
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.
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.
This is optimization error in the compiler of our simulated universe.
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?
This was on here 6 months ago when the preprint hit arxiv.
Your explanation of Heisenberg with the inability to observe is incorrect. That's a RESULT of Heisenberg.
Heisenberg's Principle comes out of the wave/particle duality. To localize a particle, you have to add waves of differing frequency to its wave function (ala Fourier). The more you localize it, the more waves of higher frequency you add. Momentum is derived from the wave frequency. Therefore, when you localize a particle, you are increasing the uncertainty of the momentum (by adding more and more higher frequency waves).
This is the argument that Heisenberg used (yes, I've read his book).
General Relativity: Space-time tells matter where to go; Matter tells space-time what shape to be.
Follow up to my own post.
The fact that you cannot measure the momentum and location of a particle exactly is NOT a limitation imposed by measuring apparatus. The fact is that a quantum particle HAS no exact momentum and location, as a result of its wave function.
General Relativity: Space-time tells matter where to go; Matter tells space-time what shape to be.
Particles can't really be two places at once.
And here you are completely wrong. Finiteness of the universe disagrees.
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
You are wrong again. Stop. Double slit experiment has been duplicated using *individual photons*. Yes, one photon fired at detector at a time. ONE. No more, just ONE. After waiting sufficiently long, interference pattern was produced on the detector. The photon appears to have interfered with itself.
http://www.animations.physics....
IAAPhysicist. Parent isn't correct. I advise you to not worry too much about what is "real" and accept that physics looks for simple models which match our experiences. You need to think abstractly, and assume less. For example, everyone grows up with some intuition of what an object is, and then project that notion into realms where they don't apply. The letters on this webpage, for example.... These are black objects which move up and down when you scroll the page. Or, is it really the white spaces between the letters which are the real objects, and the black is just void? Actually both are wrong, and the "reality" is that your monitor is doing certain things, depending on how deep you want to look.
When physicists talk about a particle, they are talking about the smallest step in the amplitude of the fluctuation in some field or combination of fields. A fluctuation doesn't have to be purely one kind of field; for example, a phonon is made out of collective motions of atoms, and polaritons are sort of some mix of photon and phonon. These could be considered particles (but not fundamental particles). This isn't the only way to think about a particle (since it's all just a model anyways), but it is more accurate than billiard balls.
Heisenburg uncertainty principle exists because you are trying to pinpoint a fluctuation in fields which occupy all space.
Parent's description of the double slit experiment is fully wrong. Electrons do not interfere with some build up of electrons. Electrons interfere with themselves, because the fluctuation (which is the electron) exists in the full region between the source and screen. The interference pattern is the same no matter how slowly (in terms of electron rate) you fire the electrons, so build up is not a concern. A similar interference pattern exists in photons and neutrons as well, which aren't charged.