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.
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.
Re:Can we dumb it down some more? (Score:5, Informative)
Dupe? (Score:5, Informative)
Re:Limits of Measurement (Score:5, Informative)
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).
Re:Can we dumb it down some more? (Score:4, Informative)
From its very beginning, quantum theory has been revealing extraordinary and counter-intuitive phenomena, such as wave-particle duality, Schrodinger cats and quantum non-locality. Another paradoxical phenomenon found within the framework of quantum mechanics is the ‘quantum Cheshire Cat’: if a quantum system is subject to a certain pre- and post-selection, it can behave as if a particle and its property are spatially separated. It has been suggested to employ weak measurements in order to explore the Cheshire Cat’s nature. Here we report an experiment in which we send neutrons through a perfect silicon crystal interferometer and perform weak measurements to probe the location of the particle and its magnetic moment. The experimental results suggest that the system behaves as if the neutrons go through one beam path, while their magnetic moment travels along the other.
Re:Limits of Measurement (Score:5, Informative)
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.
Re:Limits of Measurement (Score:5, Informative)
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.... [unsw.edu.au]
Re:Limits of Measurement (Score:5, Informative)
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.
Re:Limits of Measurement (Score:4, Informative)
I understand the mathematics involved in Fourier analysis, but that is the mathematics -- is the electron ACTUALLY doing that, or was that simply a mathematical/logical proof that correlates highly with what we see?
ISTM your question is meaningless. The best we have to offer on what the electron is ACTUALLY doing is with mathematics that correlates highly with what we see. I don't know what it means for there to be an actuality beyond that.
Even your question/remarks on the "correct conceptual framework" seems to miss the mark. The best we have there is the simplest mathematics that correlates highly with what we see.
All of this mathematical physics has its root in formulas that were derived based on data collected in labs, ..
Actually, a very big part of the theory is predicting new and unexpected results that have not been seen in the lab yet. Another big part is when the same mathematics can describe different phenomenon that were previously thought to be unrelated. Lee Smolin provides an excellent description of how this all works in his book The Trouble with Physics. I highly recommend it.