Condensed concepts

Einstein considered that quantum mechanics must only be an approximate theory which was derivable from a "classical" theory which did not have the same philosophical problems.  I n a 1949 essay, Reply to Criticisms published in response to the essays in  Albert Einstein: Philosopher-Scientist he wrote ...Within the framework of statistical quantum theory there is no such thing as a complete description of the individual system. ....The attempt to conceive the quantum-theoretical description as the complete description of the individual systems leads to unnatural theoretical interpretations, which become immediately unnecessary if one accepts the interpretation that the description refers to ensembles of systems and not to individual systems. .... For if the statistical quantum theory does not pretend to describe the individual system (and its development in time) completely, it appears unavoidable to look elsewhere for a complete description of the individual system; in do

I find it interesting that there are some physicists who won't even entertain this question. I remember raising it in a "round table" discussion at a conference and people just laughed and did not want to engage with the question. Hence, it is nice that last year Science published a short piece, Is Quantum Theory Exact ? by Stephen Adler and Angelo Bassi.  They discuss a physical collapse  model called the continuous spontaneous localization (CSL) model which involves adding noise terms to the Schrodinger equation in order to produce spontaneous wave function collapse on "macroscopic" scales. It should be stressed that this is a radical proposal involving fundamental new "forces" in the universe. They discuss physical bounds from known experiments for the parameters in the model. Unfortunately, there are several significant issues that the short article does not mention.  Most of the dynamical equations and corresponding experimental signatures that

If you are applying for something it is really worth proof reading your application a couple of times and getting someone else to as well. This is a nice and helpful thing that students and postdocs can do for each other. Don't rely on your busy supervisor. At times I have to review large numbers of applications (jobs, grants, Ph.D proposals, or papers to referee ...). One thing I have noticed that quickly irritates me and some of my senior colleagues is the number of typos or incomplete information in many applications. Maybe it should not matter, but it does have an effect on how people perceive your application. Also if you are reapplying (or resubmitting) do NOT assume that people will remember the last version and why they asked you to revise for re-submission or re-application. It really helps your case if people can see that you have taken on board the feedback given. Ignoring it can be the kiss of death.... Again, you may not like this or agree with it. But, that is the way

Previously I asked a few fundamental questions about quantum theory . One was: Why doesn't decoherence solve the quantum measurement problem? This is a subtle question with subtle answers. I have hesitated on posting on this because the more I read the less sure I am of what the answer is. Basically, it seems there are a few key (distinct but related) aspects to the problem: how does a measurement convert a coherent state undergoing unitary dynamics to a "classical" mixed state for which we can talk about probabilities of outcomes? why is the outcome of an individual measurement is definite for the "pointer states" of the measuring apparatus? can one derive of the Born rule which gives the probability of a particular outcome? It seems that decoherence only solves the first problem, but not the last two. An accessible brief summary is given in a Book Review by Anton Zeilinger . He states: Fullerenes .... showing quantum interference

Should Newton's laws, the laws of thermodynamics, and Ohm's law all be called Laws ? Phil Nelson gives a nice summary of The Character of Physical Law by Feynman in Biological Physics , Section 5.4. The common features of physical Laws (with a capital L) are: They have a very great degree of generality. Although they are general, they need not be, and generally cannot be, exact. They are intrinsically mathematical in their expression. Out of the simplicity of a Law, there always emerge subtle, unexpected, and true conclusions revealed by mathematical analysis.

Seth Olsen brought to my attention an Editorial in the American Chemical Society journal Chemical Biology Deep Impact: Scientific Evaluation by the Numbers . It discusses some of the problems associated with the metric Impact Factors for journals: Acta Crystallographica—Section A had an impact factor of 2.0 in 2008, which vaulted up to 49.9 in 2009 .... What is even more remarkable is that this rise can be predominantly attributed to citations to a single article published in 2008.... Nature calculated that 25% of published articles contributed to 89% of the journal’s 2005 impact factor. While on the subject of research metrics the Wikipedia page on the h-index is worth reading.

This week BIPH3001 is reading Life in the slow lane: The Low Reynolds-Number World, chapter 5 in Biological Physics: Energy, Information, Life, by Phil Nelson. He begins with the following great quote Nobody is silly enough to think that an elephant will only fall under gravity if its genes tell it to do so, but the same underlying error can easily be made in less obvious circumstances. So [we must] distinguish between how much behavior, and what part, has a genetic origin, and how much comes solely because an organism lives in the physical universe and is therefore bound by physical laws. – Ian Stewart, Life’s Other Secret As with each chapter Nelson begins with a Biological question and a Physical idea : Biological question: Why do bacteria swim differently from fish? Physical idea: The equations of motion appropriate to the nanoworld behave differently under time reversal from those of the macroworld. Figure 5.1 is a picture showing the peculiar character of laminar flow characte

I have started reading (with Seth Olsen and Elvis Shoko) a beautiful Reviews of Modern Physics article by Roald Hoffmann, A chemical and theoretical way to look at bonding on surfaces. A key idea he discusses is that chemisorption is a compromise: metal-absorbate bonding is accomplished at the expense of bonding within the absorbed molecule and within the metal. One question I am interested in is: How do strong electronic correlations modify the picture Hoffmann presents using molecular orbitals and bands?

I have been asked to give a talk at a Symposium on this topic. All the speakers are meant to come up with one BIG question. Later, I will write some of the questions other speakers have proposed later. I am struggling to come up with my question. I want it to be focussed and ultimately addressable by experiment in the next decade. Here are a few possibilities: Is there some scale (energy, length, and time) on which the linear superposition principle breaks down? Is the quantum-classical "transition" a crossover or a phase transition? What are the distinct signatures of quantum complexity? Which interpretations of quantum theory are not experimentally falsifiable? Are there any biological processes which require/involve quantum coherence beyond the nanometer length scale and picosecond time scale? Are there any experiments which should cause one to abandon a philosophical position of critical realism? Why doesn't the decoherence interpretation solve the quantum measurement

I am helping teach BIPH3001 Frontiers in Biophysics which is running as a reading course (with a blog ) based on Philip Nelson's brilliant text, Biological Physics: Energy, Information, and Life. This week we are looking a chapter 4, Random Walks, Friction, an Diffusion. Here a few highlights. As I have noted previously he has great section headings which often summarise the main point. 4.1.2 Random walks lead to diffusive behavior 4.1.3 The diffusion law is model independent 4.1.4 Friction is quantitatively related to diffusion Einstein showed that friction * diffusion constant = kB T This is the first example of a fluctuation-dissipation relation. 4.2. Excursion: Einstein's role Einstein saw that the fluctuation-dissipation relation gave a falsifiable quantitative hypothesis. The experiments he proposed and Perrin performed (his actual data is in the Figure above) finally convinced people that atoms really did exist. 4.3.1 The conformation of polymers The diffusion constan

Here is a copy of the slides from my lecture tonight.

I am working on my public lecture for tomorrow night, " van der Waals: his legacy 100 years later ," for the UQ Physics Museum, marking the centenary of his Nobel Prize. Reading his Prize speech the following quote is particularly interesting: in all my studies I was quite convinced of the real existence of molecules, that I never regarded them as a figment of my imagination, nor even as mere centres of force effects. I considered them to be the actual bodies... . We do not know the nature of a molecule consisting of a single chemical atom. It would be premature to seek to answer this question but to admit this ignorance in no way impairs the belief in its real existence. When I began my studies I had the feeling that I was almost alone in holding that view. And when, as occurred already in my 1873 treatise, I determined their number in one gram-mol, their size and the nature of their action, I was strengthened in my opinion, yet still there often arose within me the questi

Previously I posted about the importance of written reports before and after weekly meetings with Ph.D students and postdocs. I think it is worth re-reading. One thing students sometimes struggle with is "But, I dont have anything to report. I have been working on problem X and made no progress." I think this is when a detailed and specific report is even more important. The difficult process of writing down the specifics of what you have been trying to do and why it has not worked can really focus your attention and clarify the way forward. Even if you don't see it your supervisor may. But, if they don't have the concrete specifics it is hard to help.

On Friday we had a nice clear and stimulating physics colloquium, Turbulent times in quantum physics from Brian Anderson. What are unique characteristics of turbulence ? A beautiful video of a dragon fly in fluid flow was shown to illustrate this. 1. continuous flow 2. unpredictable flow details 3. eddy formation, interaction 4. rapid mixing 5. energy input at one length scale and energy dissipation at another length scale. The latter is described in a landmark paper from 1941 by Kolmorgorov . He used dimensional analysis to show that the kinetic energy spectrum E(k) ~ k^-5/3 where k is the wave vector. A superfluid has no viscosity. But turbulence is still possible. Feynman suggested in 1955 that this could arise as a disordered tangle of vortices. Three features of quantum turbulence 1. dynamics is described by a quantum dynamical equation (e.g., a non-linear Schrodinger equation) rather than the Navier-Stokes equation. 2. Kolmogorov scaling (this was observed in 1998) 3. disordere

On Complex Matters, Steve Simon has a post claiming that two public lectures given by Shankar at Aspen are the "best physics lectures ever". I have not watched them yet but would be curious to here what others think. The lectures are on quantum physics and relativity.

Quantum oscillations such as the de Haas - van Alphen and Shubnikov - de Haas oscillations are generally viewed as a distinct signature of a well-defined Fermi surface. These oscillations have proven to be a powerful probe of the electronic properties of metals: in elemental metals they have provided information about the geometry of the Fermi surface, effective masses, and scattering times. This information is extracted by making use of the expressions of Lifshitz and Kosevich (LK) for the dependence of the amplitude of the oscillations on the temperature and magnetic field. They derived their results for non-interacting electrons in three dimensions, but have since been extended to two dimensions and quasi-two dimensions. Kartsovnik has given a nice review of how m agnetic oscillations have also provided significant information about layered organic metals. Most experiments are consistent with the LK form, but there are exceptions, e.g., in α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 and β ″

Ten years ago Phil Anderson wrote a really stimulating "Reference Frame" article, Brainwashed by Feynman? in Physics Today. Anderson gives several concrete examples to stress the importance of non-perturbative effects in quantum many-body theory, whether it is the standard model of particle physics or molecular bonding. Bound states are a prime example of a non-perturbative effect. He also mentions the highly non-trivial (and counter-intuitive) case of the chiral anomaly where the whole Fermi sea matters. Anderson cites Roman Jackiw's Dirac Prize lecture as a beautiful exposition of the relevant physics. As an aside I think the title is a bit unfair to Feynman. Feynman would have been the last person to "brainwash" anyone and he certainly appreciated the importance of non-perturbative effects. The problem is really people who are enamoured by the success of Feynman diagrams (in their appropriate context) and then apply the formalism when one can't

In Seattle, I had a really interesting and helpful discussion with Charlie Campbell about doped rare earth oxides. Cerium oxides have attracted a lot of industrial attention because they have an amazing ability to reversibly release and uptake oxygen. [Just like hemoglobin in your blood!]. Hence, along with many others I thought this was a fundamental issue about pure cerium oxide. However, it turns out all the industrial materials (such as solid oxide fuel cells) are doped with transition metal ions. So the fundamental problem is the following: mixed alloys of ceria and zirconia (ZrO2) have this large uptake-release capacity; it is much larger than pure zirconia or pure ceria. This paper [which my Indian colleagues made me aware of when I visited Bangalore earlier this year] examines the corresponding question for titania-ceria alloys. [A paper on zirconia-ceria is here.] They find that in the alloys there is a significant relaxation of the oxygen sublattice. In particular four of

I just bought a copy of Quantum mechanics in chemistry by George Schatz and Mark Ratner (the Dover edition is only US$17 at Amazon). I wish I had bought a copy years ago. I knew of the books existence but assumed it was just another introductory book on quantum chemistry. However, the book is very different. There is significant focus on dynamics, condensed phase effects, and density matrices. Second quantisation is used extensively [something many theoretical chemists are not comfortable with but physicists are]. Much of the material is things the last few years I have struggled to learn myself or find where it is clearly written down. It has nice problems and chemical applications.

Stimulated by the arxiv posting of Ben Powell and my review on organic mott insulators Kai Schmidt kindly brought to my attention a really nice preprint, Effective spin model for the spin liquid phase of the Hubbard model on the triangular lattice , of work he did with Yang, Laeuchli, and Mila. This is a rather definitive study using a high powered perturbative continuous unitary transformation which allows them to derive an effective spin Hamiltonian in the Mott insulating phase. They find that as U/t decreases there is a first-order phase transition from the 120 degree Neel ordered phase to a spin liquid phase (no net magnetic moment) and large numbers of singlet excitations below the lowest lying triplet excitation. The Figure to the right shows the excitation spectrum as a function of U/t with red (empty) dots being magnetic (non -magnetic) excitations. The Neel phase is destroyed by higher order spin interactions such as ring exchange which become more important as t/U increases.

Sorry. This is about the TV show, The Big Bang Theory . I just flew on Air New Zealand and they showed two episodes I had not seen (we don't have TV) from Season 3. I thought The Creepy Candy Coating was one of the funniest ever. (Although it is disappointing there wasn't any real physics...) Here is one great scene .

When is it time to mothball Density functional theory? [sorry for the second pun..] Yesterday I had a nice meeting with Charlie Campbell and members of his group at the University of Washington. It was fascinating to see their lab (as a theorist it is always a reality check!) . Over the past decade they have developed several high resolution microcalorimeters which allow accurate determination of the binding energies of different atoms and molecules to specific surfaces. These results present a significant challenge/benchmark for electronic structure methods (such as density functional theory) which claim to be able to calculate accurately such quantities. The results are also of fundamental importance for understanding mechanisms of heterogeneous catalysis. I found results for adsorption of benzene and napthalene on Pt particularly interesting. They are summarised in the Figure above. DFT gets a binding energy which is too small by almost a factor of three. I discovered my friends J

Last year I helped run a reading course PHYS3170 Intermediate Biophysics where we made use of a course blog. Some of my posts related to the course can be seen here . This course has now morphed into BIPH3001 Frontiers in Biophysics. Again we are running it as a reading course due to low enrolments. The course blog has got off to a good start. Students have to post 2 items per week and comment on other student posts. It is worth looking at to see what is possible with this medium.

Here is the current version of the slides for a talk, "Charge redistribution near oxygen vacancies in cerium oxides", that I am giving tomorrow in the Chemistry Department at University of Washington. The main point of the talk is that the standard model of charge localisation (pictured above) is incorrect. A detailed discussion is contained in a review co-authored with Elvis Shoko and Michael Smith.

Ben Powell and I have just finished the first iteration of a review article invited by Reports of Progress in Physics, Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors . The article is rather ambitious in scope and so I am sure there are many ways it can be improved and so we welcome feedback. Regular readers of this blog may notice that some of the material in the review was "trialled" here. Some of the feedback I received was quite helpful in clarying and sharpening certain points.

I am visiting my in-laws and they subscribe to the Economist. There is an interesting and worrying article Expensive Iteration : A huge international fusion-reactor project faces funding difficulties . The article begins VIABLE nuclear fusion has been only 30 years away since the idea was first mooted in the 1950s. Its latest three-decade incarnation is ITER ,... It is worrying how the EU is considering diverting substantial amounts of other scientific funding towards ITER . The article ends it is far from clear whether the best way of countering this trend in energy funding is to plough yet more money into the fusion project, with its vested political interests, at the expense of less prominent scientific endeavours. It never ceases to amaze me how these big projects have such large cost over-runs and that the initial budgets were based on wishful thinking. Coincidentally, the same issue of The Economist has an article about the spiralling costs of the London Olympics.

In a previous post When a little makes a big difference I wrote how in frustrated quantum antiferromagnets small perturbations can have a big effect. A recent PRB (and an Editors Choice ) by Starykh, Katsura, and Balents makes a similar point in great detail, for the specific case of Cs2CuCl4, which as a first approximation can be modelled by a Heisenberg model on an anisotropic triangular lattice. Here is some of their abstract: First, we find that when the magnetic field is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. This is despite the fact that the interlayer exchange coupling in Cs 2 CuCl 4 is about an order of magnitude smaller than the weakest (diagonal) exchange in the triangular planes themselves. Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions , heretofore neglected, of order a few percent or less of the largest exchange couplings . Th