Condensed concepts

Yesterday my wife and I did our latest kids science demo gig, at a holidays kid club at our church. My son has encouraged us to come up with some new demonstrations since some of the kids have already seen some of the old favourites, such as Elephants toothpaste , coke and mentos rocket , and a few others  here . At first I was pretty excited when I saw this Youtube video of an LED powered by a lemon . Watch it and see what you think. I even got some LEDs to tried and do it. At the end of the post I tell the rest of the story. We settled on a few demos with dry ice [solid carbon dioxide]. The unique feature is that at atmospheric pressure the solid does not melt [become liquid] but sublimates [becomes vapour]. This is because in the phase diagram the pressure of the triple point [5 atm] is above atmospheric pressure. Here are some of the demonstrations. Many of them rely on the simple fact that the volume of one gram of vapour is of the order of five hundred times larger than th

I got a referee report for a paper submitted to PhysChemChemPhys that looks at the quantum entanglement of electronic and nuclear degrees of freedom in molecules. The paper goes beyond the calculations considered here , and explores subtle issues about how entanglement may or may not be related to the breakdown of the Born Oppenheimer approximation. One referee asked a good but basic question, "Why is the von Neumann entropy the appropriate measure of entanglement to consider here?" Here is my answer. I think experts could do better and so I welcome suggestions. The von Neumann entropy is widely accepted as the best measure of quantum entanglement for pure quantum states defined on bipartite systems, such as that considered here. This is because the von Neumann entropy satisfies certain desired criteria, including vanishing for separable states, monotonicity (it does not increase under local operations or classical communication between the subsystems), additivity, con

On the one hand I think it is very important that people and institutions should be accountable for their actions. Human nature is such that if people are not accountable they will often choose to act in selfish ways that lead at best to mediocrity and at worst to corruption, exploitation, and waste of precious resources (financial, human, and environmental). On the other hand, at some level you need to trust people and give them freedom to get on with their job. Too much regulation and oversight can be dehumanising, discouraging, stifle initiative, and also waste resources. For example, the fact that at most universities half of the staff are administrative should be a serious concern. Consider the following contrasting situations. The levels of accountability and trust are wide ranging. Sepp Blatter thinks that FIFA should be self-regulating and left to get its own house in order. CEO's of mining companies will claim that they will "do the right thing" when

I am often sprucing [Aussie slang for promoting] Dynamical Mean Field Theory (DMFT), and particularly how it captures many quantitative details of charge transport and bad metals in organic charge transfer salts. However, it is always good and important to be transparent about the limitations of any theory, particularly one that you are enthusiastic about. There is a nice paper Repulsive versus attractive Hubbard model: Transport properties and spin-lattice relaxation rate  Rok Žitko, Žiga Osolin, and Peter Jeglič The authors use DMFT to calculate various spectral functions using the numerical renormalisation group (NRG) as the impurity solver. This is probably, the most reliable method, at least for low temperatures. There is a lot I found interesting in the paper. But for now I just want to focus on one result in the paper: the temperature dependence of the NMR relaxation rate, 1/T_1. 1/(T_1 T) is proportional to the slope of the local spin fluctuation spectral function

Over the past few years I have been watching with interest a number of postdocs transitioning from academia to industry. From my very limited experience, reading, and some discussions I think there are two obstacles that need to be faced head on. Both involve wrong perceptions and misunderstandings. 1. Industry is not interested in me because my highly specialised technical skills are not relevant. This is true. Industry could not care less about spinons, lattice gauge theory, Bell's theorem, synchrotrons, femtosecond spectroscopy, cosmology, .... However, industry is VERY interested in some of the skills you do have and probably take for grant : problem solving, critical thinking, analyse complex data, ability to learn new technical skills quite, write and debug large computer code, not being scared of big data, technical communication [written and verbal], work in teams.... Furthermore, it is important to appreciate that the vast majority of people, including many with MBAs,

Here I consider quantum many-body theory, as it spans from quantum field theory to nuclear physics to ultra cold atomic gases to solid state physics to quantum chemistry. Here is my tentative list of the ten most important key concepts. n.b. I am not concerned with key techniques (e.g. path integrals, renormalisation group, Green's functions, Feynman diagrams, imaginary time (KMS), numerical methods....). That is a separate topic. Unfortunately, in most many-body theory texts the concepts are lost in the midst of all the technical details. Furthermore, concepts are what experimentalists need to know and understand. 1. Emergence This is the overarching concept that underscores almost all the others. Reality is stratified and at each length, energy, and time scale distinctly new phenomena, interactions and entities can emerge. More is different. 2. Effective Hamiltonians At each strata there is some Hamiltonian that describes the "particles" and interactions between

Why is it so central in Quantum Field Theory? Has it been observed in solid state physics? At the Journal Club of Condensed Matter, Patrick Lee has a very nice and helpful commentary, Observation of the chiral anomaly in solids , that highlights a  preprint , based on a recent talk by Phuan Ong at the APS March meeting. Later I hope to write more about the solid state physics, earlier work of which I have mentioned before. I just want to highlight a beautiful and succinct paragraph from the preprint that explains why the chiral anomaly is so important in quantum field theory. A bit of topological physics fell into quantum field theory (QFT) in the late 1960s [8–11]. The charged pions π ± are remarkably long-lived mesons (lifetimes τ ∼ 2 . 3 × 10 − 8 s) because, being the lightest hadron, they can only decay by the weak interaction into muons and neutrinos via the processes π − → μ − + ν ̄ μ and π + → μ + + ν μ . Mysteriously, the neutral pion π 0 decays much more quickly (b

There are several important career transitions that the fortunate few need to adapt to. Getting a potentially "permanent" job. (e.g., Assistant Professor). Getting Tenure. Getting promoted to full Professor. Previously I wrote my survival and sanity guide to young faculty. My general observation is that people don't adapt well to these transitions. In particularly, they tend to just continue to operate in the same mode. Unlike in North America, in Australia promotion to full Professor is not the natural trajectory. Thirty years ago, there was virtually no promotion. You had to get a separately advertised "Chair". Sometimes there was only one per department and they were the default department head. Most people ended their careers as Senior Lecturers or Readers. Now there is no limit on the number of Professors, but only a few will attain it. [Also in Australia, tenure no longer strictly exists, but that is another story ]. Some will run themselves ragg

Last friday we had Peter Harrison give the Physics colloquium on "The Progress of Science and the Decline of Religion?" He is a historian, who prior to coming back to UQ, held a chair at Oxford, and in 2011 gave The Gifford Lectures , which were recently published. He is probably best known for arguing that changing approaches to Biblical interpretation, associated with the Reformation [moving away from an emphasis on allegorical interpretations towards more literal and historical interpretations] changed peoples conception of "nature" and had a significant influence on the development of modern science. Peter is director of the Centre for the History of European Discourses at UQ and attracts many stimulating and distinguished seminar speakers, some of whom I have blogged about before. One issue Peter addressed head on is the "conflict thesis"  which claims that science and religion have always been in conflict and particularly that religion has imp

Here are some of the emotions I experience when I have to review a bunch of research grant applications. This has become accentuated because I now have to review some applications outside physics and chemistry. Guilt. I don't spend as much time on each application as perhaps I should. I don't read every word, cross check details, learn the necessary science, find an expert, ... Guilt . I do sometimes look at metrics. But, in my defense this is very coarse-grained. Most of the time it tells me little or what I can guess already. However, occasionally I think it does help. For example, someone who is decades past their Ph.D and with only single digit citations on their hundred plus papers. Or, someone with a few recent papers with 50-100 citations, may mean something. Frustration. That I have to assess non-competitive applications. Why did this person waste all the time preparing an application. Did any senior colleague advise against it? Or does the applicant live in a d

Nandan Pakhira and I just finished a paper Shear viscosity of strongly interacting fermionic quantum fluids Eighty years ago Eyring proposed that the shear viscosity of a liquid, η, has a quantum limit η larger than n hbar where n is the density of the fluid. Using holographic duality and the AdS/CFT correspondence in string theory Kovtun, Son, and Starinets (KSS) conjectured a universal bound η/s ≥ hbar/4πk_B for the ratio between the shear viscosity and the entropy density, s. Using Dynamical Mean-Field Theory (DMFT) we calculate the shear viscosity and entropy density for an fermion fluid described by a single band Hubbard model at half filling. Our calculated shear viscosity as a function of temperature is compared with experimental data for liquid 3He. At low temperature the shear viscosity is found to be well above the quantum limit and is proportional to the characteristic Fermi liquid 1/T^2 dependence, where T is the temperature. With increasing temperature and interaction

On monday I went to two interesting talks at UQ  by  Eric Mazur . The slides are available here. For reasons I discuss at the end I found the talks both inspiring and discouraging. The first talk, "Flat Space, Deep Learning" described a new course Mazur developed at Harvard, AP50, a calculus based introductory physics course for engineers. The whole goal is get students to "own their own learning". There are no lectures, just 2 three hour class sessions. The "exams" are non-traditional. Students don't do regular labs, but rather four team projects. Students have to read the relevant part of the text before class and annotate an online version with questions and answers to other students questions. Central to the course is extensive use of the Learning Catalytics software, developed by Mazur, King, and Lukoff, and subsequently sold to Pearson. The second talk, "Teaching Physics, Conservation Laws First" was an infomercial for Mazur

Politicians are famous for not answering the question. A colleague recently told me how in a Q and A session with some university managers it was amazing the ability that they had to not answer the question. Undergraduate students in the humanities often write essays that don't answer the question being asked. If at a seminar a nervous young graduate student misunderstands a question and struggles to answer it that is understandable. Similar things sometimes happen in job interviews. However, this post is about something different, that is not excusable. Unfortunately, I am in the thick of reviewing a bunch of research grant proposals, from a range of fields. Here is my rant. Previously, I posted  about how the question "What are your major contributions to a research field?" gets irrelevant answers involving grants, job offers, journal impact factors, and citations, .... I have now encountered a few more, paraphrased below. What scientific hypothesis is this p