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物理学一周有趣消息简报
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消息来自外网，我转载的。并且附上了一些国外网友的评论给大家看看。 1.Quantum field theory [1907.02497] Pedagogical Materials and Suggestions to Cure Misconceptions Connecting Special and General Relativity https://arxiv.org/abs/1907.02497 This short paper appeared on the arxiv earlier. It discusses the Einstein equivalence principle (EEP) and accelerated frames within special relativity, whose treatment is a common source of confusion in special relativity. The article then suggests some ways to help clarify common misconceptions. By Rhinosaurier While the authors are technically correct, I would say the professors that "failed" their quiz were historically correct. If you want to properly treat the dynamics of noninertial frames, i.e. do more than just calculate a proper time, you're going to need to know about covariant derivatives and connections. To the authors, SR is just the case where this connection happens to have vanishing curvature and hence no gravitational fields. But that's not how it played out historically. SR predates all this business with differential geometry in physics, or even the notion of the invariant interval. Historically it was introduced with plain algebra and simple physical postulates, and this simple formulation doesn't work for noninertial frames. To most people, SR is this theory, not the result of taking the full formulation of GR and setting the Riemann tensor to zero. If that's what the professors were thinking, they weren't wrong. BY kzhou7 2.Computational physics Calculating Pi recursively and using Monte Carlo Simulations | Love Letters to Pi - Experiments With Truth http://www.experimentswithtruth. ... love-letters-to-pi/ Engineers at Yokohama National University teleported data inside of a diamond, using polarized photons and quantum entanglement https://www.nature.com/articles/s42005-019-0158-0 WE NEED TO CONSTRUCT ADDITIONAL PYLON-- virokePen 这里有科普报道（不想看论文的话） Scientists teleport information inside a diamond https://www-upi-com.cdn.ampproje ... 29167/?amp_js_v=0.1 3.Scientists provide a rigorous quantum mechanical explanation of the concept of atomic oxidation number and solve a long-standing conundrum in the physics of ionic conductors, thus paving the way to accurate simulations of a broad class of materials https://www.eurekalert.org/pub_r ... /sisd-tpu062819.php ABSTRACT:According to the Green–Kubo theory of linear response, the conductivity of an electronically gapped liquid can be expressed in terms of the time correlations of the adiabatic charge flux, which is determined by the atomic velocities and Born effective charges. We show that topological quantization of adiabatic charge transport and gauge invariance of transport coefficients allow one to rigorously express the electrical conductivity of an insulating fluid in terms of integer-valued, scalar, and time-independent atomic oxidation numbers, instead of real-valued, tensor and time-dependent Born charges 评价： IMO it's a nice technical advance but not groundbreaking. (Note: this describes essentially all physics papers.) From my reading of the paper, it is probably mostly interesting to people doing DFT simulations of ionic fluids within the Green-Kubo framework. EDIT: Here is my summary of what they actually assign as the "oxidation number." In an ionic liquid, some electrons are tightly attached to a particular atom and others are more messily floating and exchanged around. Because of this messiness its a priori a bit tricky to say how many electrons a single atom has given up i.e. it's oxidation number. However, many-body quantities like the net electric polarization are well-defined. So here's what you do. Put your liquid in a box with periodic boundary conditions (so it's a three-torus and we can use topological arguments). Take the Born-Oppenheimer approximation, which freezes the atomic nuclei in space and makes them act as a static potential for the electrons. Now pick one of the atoms, and very slowly move it around in a loop, so that it returns to its starting configuration. As you do this, the gigantic messy many-electron cloud of your whole system will change in response. At each point in time of this dragging, compute the $\dd\mu$ of how much the dipole moment / polarization of the system is changing, then integrate these all up and divide by the size of your box: $(1/L) \int \dd \mu$. Under certain conditions (i.e. gapped g.s.) this final integrated quantity will be an integer -- and if this loop goes around an essential cycle of the torus it will not generically be zero. So we can assign this integer as the net charge / oxidation number of the atom that we dragged around. The key idea of topological quantization and adiabatic charge transport goes back to Thouless, which is what he won the Nobel for. -InfinityFlat At first glance, it seems very reminiscent of what happened with "the modern theory of polarization". The fact that materials are polarized (when you apply an external electric field, their bound atomic charge is separated slightly yielding dipoles whose induced electric field act against the applied field) have been central to electromagnetism since the mid 1800s. However, if one wanted to actually calculatethe polarizability of a material starting with quantum physics and computer simulation (i.e. "from first principles" or "ab initio"  , no one could actually figure out how to do it. The reason being that polarization doesn't seem to be uniquely defined so you can't just assign some unique number to a system.It was only in the... I believe... 1980s when things changed and polarization was re-visited in the rather heady framework of quantum geometric phase that a unique way of assigning it (or at least a unique way of defining changes in it) was established. After that it became possible to both predict the polarizability of a given material ab initio, but also to use ab initio methods to help engineer new materials with desired polarization properties. This work seems to have a lot of similarity to that historical affair, except with oxidation numbers. It seems to allow their prediction using ab initio methods which is very valuable if one can predict and explore their behaviour with the speed and flexibility of computer simulation. -----@DefsNotQualified4Dis Checkmate, chemists ----tuffcone 4.The Perils of Being Paul Ehrenfest, a Forgotten Physicist and Peerless Mentor https://thereader.mitpress.mit.e ... orgotten-physicist/ |
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