This continues to be a very busy time, but I wanted to point out two preprints that caught my eye this week. Their subjects are completely disparate, but they stand out as essentially reviews written in a much more conversational tone than the usual literature.
The first is this preprint about chirality-induced spin selectivity, a subject that I've mentioned before on this blog. There is now an extensive body of evidence (of varying quality) that there is a connection between structural chirality of molecules and their interactions with the spin angular momentum of electrons. This includes monolayers of chiral molecules leading to net spin polarization of photoemitted electrons (here), a lot of electronic transport experiments involving chiral molecules and magnetic electrodes that seem to show spin-dependent transmission that is absent with achiral molecules, and even a chirality dependence of molecular adsorption kinetics on magnetic surfaces (here). The preprint is a provocative discussion of the topic and possible mechanisms, and the importance of precision in the description of the various phenomena.
On a completely different topic, this preprint is a fun discussion about quantum gravity (!) and how condensed matter ideas of "the vacuum" can lead to insights about how quantum mechanics and gravity might need to play together. One fun bit early on is a discussion of something I like to point out to my undergrad stat mech students: A single hydrogen atom in a very very large box will apparently (if the usual stat mech formalism of partition functions is valid) be spontaneously ionized, even when the box and atom are at temperatures faaaaaar below the energy scale for ionization. This is discussed nicely in this 1966 article in the Journal of Chemical Education. Anyway, I thought this was an interesting discussion from three condensed matter theorists.
Interesting to see a "cm take" on quantum gravity, will take a closer look. But emphasizing the notion of a vacuum is not an obvious way forward to me. One of the only theories of quantum gravity (in 2D) that works is Liouville CFT, and afaik it does not have a conventional vacuum state. The "crisis" in strings and holography is that there are serious _technical_ difficulties to formulating even first-quantized strings on an intrinsically dynamical background like de Sitter space. It makes me wonder if the biggest issue is the need to formulate an intrinsically non-equilibrium theory. Even for quantum point particles we do not have many examples of fully quantum dynamics captured far from equilibrium. Putting that together with the need to eliminate gauge degrees of freedom (time-like components of the metric tensor or string coordinates) makes this especially nightmarish.
ReplyDeleteThe preprint reminds me of Volovik's "The Universe in a Helium Droplet" which shows how all the parts of the standard model (and even gravity) can plausibly emerge from a quantum liquid.
ReplyDeleteAnon@8:55, yes, they actually allude to Volovik's body of work on this. "In spite of attempts [45 - Volovik Rep Prog Phys 2001] to make mappings between 3He superfluid, with its 18-component order parameter, and a theory in which the standard model is added to the 10-component metric field, these two systems are very different, and we shall see that this analogy fails."
ReplyDeleteThe preprint says it has predictions that differ from QM even in the nonrelativistic regime, but when I went to that section it only said the prediction differs by a global phase--isn't that unobservable or am I misinterpreting equation 44? They also provide no examples of predicted curves or numbers that differ from the QM predictions. One would expect them to provide such as a basic summary of why this approach is worthwhile. (I have not looked at the original papers where they develop this approach; maybe they are in there. But they still should provide this in their summary.)
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