It's a very busy time, so no lengthy content, but here are a few neat things I came across this week.
- A new PRL came out this week that seems to have a possible* analytic solution to Hilbert's 18th problem, about the density of random close-packed spheres in 2D and 3D. This is a physics problem because it's closely related to the idea of jamming and the onset of mechanical rigidity of a collection of solid objects. (*I say possible only because I don't know any details about any subtle constraints in the statement of the problem.)
- The Kasevich group at Stanford has an atom interferometric experiment that they claim is a gravitational analog of the Aharonov-Bohm effect. This is a cool experiment, where there is a shift in the quantum phase of propagating atomic clouds due to the local gravitational potential caused by a nearby massive object. (Phase goes like the argument of \(\exp(-i S(x(t))/\hbar)\), where the action can include a term related to the gravitational potential, \(m \times \Phi_{G}(x(t)\).) At a quick read, though, I don't see how this is really analogous to the AB effect. In the AB case, there is a relative phase due to magnetic flux enclosed by the interfering paths even when the magnetic field is arbitrarily small at the actual location of the path. I need to read this more closely, or perhaps someone can explain in the comments.
- A colleague pointed out to me this great review article all about charge shot noise in mesoscopic electronic systems.
- Speaking of gravity, there has been interest in recent years about "warp drives", geometries of space-time allowed by general relativity that seem to permit superluminal travel for an observer in some particular region. One main objection to these has been that past proposed incarnations violate various energy conditions in GR - requiring enormous quantities "negative matter", for example, which does not seem to exist. Interestingly, people have been working on normal-matter-only ideas for these, and making some progress as in this preprint. Exercises like this can be really important for illuminating subtle issues with GR, just like worrying about "fast light" experiments can make us refine arguments about causality and signaling.
- Thomas Wong from Creighton University has a free textbook (link on that page) to teach about quantum computing, where the assumed starting math knowledge is trig. It looks very accessible!
- People recommended two other books to me recently that I have not yet had time to read. The Alchemy of Us is a materials-and-people focused book from Ainissa Ramirez, and Sticky: The Secret Science of Surfaces is all about surfaces and friction, by Laurie Winkless. Gotta make time for these once the semester craziness is better in hand....
7 comments:
Today's offering from Sabine Hossenfelder is "Are warp drives science now?", for which there's a transcript here, http://backreaction.blogspot.com/2022/01/are-warp-drives-science-now.html
Her skepticism can be seen in her last two sentences, "At present those papers basically say if you throw out stuff that way, then the space-ship will go that way because momentum is conserved. And that is probably correct, but it’s not exactly a new idea."
Although quite interesting, that PRL on sphere packing is concerned with *random* packing, so it does address the Hilbert problem which does not concern itself primarily with randomness.
Typo, meaning to write "it does*not* address the Hilbert problem"
I would say that the PRL on random close packing is incomplete (see https://arxiv.org/abs/2201.07629 for a recent comment). The use of the PY relation to "close" the problem is problematic since the PY relation is a solution to a Gaussian theory and as such can not capture nonlinear behavior like that which presumably accompanies the onset of rigidity.
Twitter spaces can be a new platform for tech discussions. Hope you get on it someday.
Anon, I am on Twitter. @NanoscaleViews.
See here on the Zaccone paper-Comment on "Explicit Analytical Solution for Random Close Packing in d=2 and d=3" arXiv:2201.07629
Abstract:
A recent letter titled "Explicit Analytical Solution for Random Close Packing in d=2 and d=3" published in Physical Review Letters proposes a first-principle computation of the random close packing (RCP) density in spatial dimensions d=2 and d=3. This problem has a long history of such proposals, but none capture the full picture. This paper, in particular, once generalized to all d fails to describe the known behavior of jammed systems in d>4, thus suggesting that the low-dimensional agreement is largely fortuitous.
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