Condensed concepts

At the Quantum Theory and Nature of Reality conference on Tuesday night there was an interesting panel discussion with George Ellis , S ir John Polkinghorne , and Sir Roger Penrose. I was surprised and disappointed that Sir Roger Penrose re-iterated the same argument in his book The Emperors New Mind : quantum gravity, the collapse of the wavefunction, and consciousness must all be related. Furthermore, he stood by Hameroff's proposal that a particular type of microtubules are the component with quantum coherence. He mentioned how excited he was about unpublished experimental results of a Japanese group that claim to have measured an electrical resistance of  one ohm for microtubules which is comparable to the resistance of the leads. [He seemed to be hinting this implied they were superconducting!]. Why am I so skeptical? See the following: The Penrose-Hameroff Orchestrated Objective-Reduction Proposal for Human Consciousness is Not Biologically Feasible , published in Physic

The following important question arises about Adler's proposal for the emergence of quantum field theory from an underlying statistical mechanics of bosonic and fermionic fields. It appears to be a "hidden variables" theory. Shouldn't it obey Bell's theorem? When asked this yesterday after my talk I said I thought that it does not because the "hidden variables" are fermionic. However, this is not the correct answer. It is because the underlying theory is nonlocal. Here is an extract (page 96 in the preprint version ) of Adler's book: we note that there is a natural hierarchy of matrix structures leading from the underlying trace dynamics, to the emergent effective complex quantum field theory, to the classical limit. In the underlying theory, the matrices xr are of completely general structure. No commutation properties of the xr are assumed at the trace dynamics level, and since all degrees of freedom communicate with one another, the dynamics

Today Simon Benjamin (Oxford) gave a really nice talk,  Will we succeed in creating entanglement in macroscopic quantum systems? Can it exist in living systems? Do living systems already harness quantum mechanics in a "non-trivial" way? How do we define "non-trivial"? A good criteria is whether biologists and chemists need quantum physicists to understand the phenomena of interest? Specifically, Simon focused on recent theoretical  work  relating to the fascinating question of how migratory birds navigate (discussed in a previous post ). Specifically, they did a detailed analysis of decoherence and entanglement in the radical pair model for magnetoreception (where the small magnetic field of the earth causes a different decay rate for singlet and triplet channels). One thing I learnt is that a key to making this model work is that  the different spins experience a different spin anisotropy associated with the hyperfine interaction. (How can we justify this asymm

I just finished preparing my talk, Is emergence the nature of physical reality?  for the Polkinghorne birthday conference. I have really enjoyed thinking about these issues and learnt a lot.

John Bell had a list of BAD works which have been mentioned several times today. In his article, Against Measurement, Bell said: "For the good books known to me are not much concerned with physical precision. This is clear already from their vocabulary. Here are some words which, however legitimate and necessary in application, have no place in a formulation with any pretension to physical precision: system, apparatus, environment, microscopic, macroscopic, reversible, irreversible, observable,  information,  measurement. .... On this list of bad words from good books, the worst of all is 'measurement'." Chris Timpson  (Oxford, Philosophy) gave a nice talk, Information: more trouble than its worth , where he roundly and rightly criticised approaches to solving the quantum measurement problem which claim it is just an issue of "information" and the quantum theory is "just about information". His D. Phil thesis in on the arxiv  and is forthcoming

I am now in Oxford for the Quantum Theory and Nature of Reality conference. In their white paper, Andreas Doring and Chris Isham write about Quantum Field Theory (QFT): Notwithstanding the success of standard quantum theory in atomic, molecular and solid-state physics, there are good reasons for wanting to see beyond it. For example, relativistic QFT is still plagued with ultra-violet divergences, and even free fields encounter the self-energy problem. No matter how sophisticated the renormalisation procedure that is adopted, the fact remains that relativistic QFT is fundamentally flawed.   This ultra-violet problem is clearly linked to the continuum model for space and time... I may be missing their point but I would like to offer a possible alternative perspective: the divergence problems with renormalisation are not a problem with QFT but rather merely reflect the stratified nature of reality (discussed in my white paper ) i.e. there is a hierarchy of energy scales and with each the

Hydrogen bonds are incredibly important for understanding the properties of water and for a wide range of biomolecular structures and processes. Without them you would be dead! Unlike most chemical bonds, hydrogen bonds can range significantly in length, strength, and vibrational frequency. The graph below from a Science paper The Quantum structure of the Intermolecular Proton bond. shows how the vibrational frequency of an asymmetric A-O-H...O-B bond varies with the proton affinity difference between A and B. Note the substantial softening as things get more symmetrical. The figure below from another paper shows how the frequency softening is correlated with the bond length. I am particularly interested in this because I want to know these effects depend on breakdown of the Born-Oppenheimer approximation the partially covalent character of hydrogen bonds and whether a simple model Hamiltonian with two diabatic states and one vibrational mode  treated exactly (see this earlier p

Why are animals the size they are? Are giant ants physically possible? No. This comes up in the banter in an episode The Wheaton Recurrence of The Big Bang Theory which I watched on the Brisbane-Singapore flight last night. A post on The Big Blog Theory discusses the science. Basically, weight increases with volume while function increases with area. Astute observers (esp. Australians may recognize the cliffs in the photo above as the coastline near Bondi Beach). The sculpture above is described here.

I just re-read most of Tony Leggett's review article, Testing the limits of quantum mechanics: motivation, state of play, prospects.  He considers that different interpretations of quantum mechanics broadly fall into three classes. 1. The statistical interpretration. This claims that quantum state amplitudes (i.e. wavefunctions) have no reality but are merely a calculational device to calculate the probabilities of the outcome of measurements. Questions about whether a cat is dead or alive before it is measured are ruled to be not "meaningful." The most "trenchant" advocate is Leslie Ballentine. 2. The orthodox interpretation. QM amplitudes are "real" at the microscopic level but effectively not real (or at least not relevant) at the macroscopic level. This is claimed to be because of decoherence one can "For All Practical Purposes" (FAPP) never observe quantum interference between macroscopically distinct states. 3. The many-worlds

There is an interesting piece  on the New York Times web site which quotes numerous scientists slamming Bob Laughlin's recent article about climate change. I wish he would go back to doing regular science.... On a more positive note Laughlin has a worthwhile project  that is digitising Conrad Herring 's famous file card box. On his website he also has all the cartoons from his wonderful book, The Emergent Universe.

How can something become more "ordered" as the temperature increases? Are one-dimensional Ising-type models good for anything? How can you distinguish different models for a complex system? When and how do biomolecules exhibit collective effects? In Chapter 9 of  Nelson's Biological Physics  answers to these questions are illustrated by the Figure below which shows the temperature temperature of the fraction of a polypeptide chain that form an alpha-helix. The brief answers are 1. The fraction of the chain which forms alpha helices increases with increasing temperature. This is somewhat counter-intuitive. The reason it occurs is although the entropy of the chain decreases with increasing alpha helix content the entropy of the solvent decreases. Formation of alpha helices makes the solvent less ordered because formation of intra-chain hydrogen bonds mean there is less H-bonding of the solvent to the chain. 2. Yes. I remember solving the one-dimensional Ising model

This cartoon appears in “Decoherence and the Transition from Quantum to Classical – Revisited,” quant-ph/0306072  by W. Zurek [This is a “remodeling” of his influential 1991 Physics Today article].

Why and how do atoms chemically bond to surfaces? This can be described by the Newns-Anderson model Hamiltonian. A nice brief overview is the chapter on Hetergeneous Catalysis by Bligaard and Norskov, from the book Chemical Bonding at Surfaces and Interfaces.

Cooperative transitions in macromolecules is the title of Chapter 9 in Biological Physics by Phil Nelson and this weeks reading for BIPH3001. Biological question : Why aren’t proteins constantly disrupted by thermal fluctuations? The cartoons in cell biology books show proteins snapping crisply between definite conformations, as they carry out their jobs. Can a floppy chain of residues really behave in this way? Physical idea : Cooperativity sharpens the transitions of macromolecules and their assemblies. As usual the section headings are informative. 9.1 Elasticity models of polymers 9.1.1 Why physics works (when it does work) Here he gets to the heart of emergence and effective Hamiltonians introducing the important idea. When we study a system with a large number of locally interacting, identical constituents, on a far bigger scale than the size of the constituents, then we reap a huge simplification: Just a few effective degrees of freedom describe the system’s beha

Everyone should make sure they are familiar with the American Physical Society's guidelines: Authorship should be limited to those who have made a significant contribution to the concept, design, execution or interpretation of the research study. All those who have made significant contributions should be offered the opportunity to be listed as authors. Other individuals who have contributed to the study should be acknowledged, but not identified as authors.  Some reasons why someone should not be a co-author: All they did was obtain the funding for the project. Their only contribution is that they are the official supervisor of a student who is a co-author. They have not read the paper! Their only contribution is that their status in the field may help get the paper published. They are not confident the results are valid. In particular, they cannot claim the future option of saying, "Well I didn't work on that part. I always had my doubts about that part..."

I would have thought that the solid state of something as "simple" as elemental boron would be both well understood and "boring". However, that is not the case. There is a beautiful paper from Giulia Galli's group. A few things I learnt about beta-rhombohedral boron. Boron tends to form three centre two electron bonds . It contains 320 atoms per hexagonal unit cell! 6 out of 20 crystallographic sites are only partially occupied.  The defect formation energy is negative leading to macroscopic numbers of intrinsic defects. The occupancy of the defects can be described in terms of a generalised Ising spin model on an expanded Kagome lattice. This Ising model describes the geometrical frustration associated with the defects, and the residual entropy present at zero temperature. I am curious as to whether there is any need to take into account quantum fluctuations.

The Big Bang Theory now has 12 million viewers! In the first episode when Penny and Sheldon first met he showed off his whiteboard. Which he said had a spoof of the Born-Oppenheimer approximation at the bottom ( video) [I could not read it. Anyone know what it is?]. I also read an  interview  with UCLA physics professor David Saltzburg who is consultant to the show. He now writes a blog The Big Blog Theory  which explains the science which features in each show.

The data in the Figure above changed the face of medical research. It showed for the first time that disease can have a molecular basis. It was another great scientific achievement and bold discovery of Linus Pauling. The Figure is discussed in chapter 8 of Nelson's Biological Physics: Energy, Information, Life. He states: In a historic discovery, Linus Pauling and coauthors showed in 1949 that the red blood cells of sickle-cell patients contained a defective form of hemoglobin. Today we know that the defect lies in parts of hemoglobin called the β-globin chains, which differ from normal β-globin by the substitution of a single amino acid , from glutamic acid to valine in position six. This tiny change (β-globin has 146 amino acids in all) is enough to create a sticky ( hydrophobic ) patch on the molecular surface. The mutant molecules clump together, forming a solid fiber of fourteen interwound helical strands inside the red cell and giving it the sickle shape for which the dise

I am looking forward to the conference Quantum Physics and the Nature of Reality beginning in 10 days in Oxford. It is in honour of the 80th Birthday of Sir John Polkinghorne. The following have been proposed as The Oxford Questions which will be refined at the conference. 1. Are quantum states fact or knowledge, and how would you distinguish? (Stig Stenholm) 2. What does quantum physics teach us about the concept of physical reality? (Richard Healey) 3. What statements about quantum reality could be experimentally evaluated? 4. Are complex quantum systems best described in terms of constituents or relationships? 5. How does macroscopic classical behaviour emerge from microscopic quantum properties? (Gerard Milburn) 6. Why isn't nature more non-local? (Sandu Popescu) 7. Is realism compatible with true randomness? (Nicolas Gisin) 8. Is emergence the nature of physical reality? (Ross McKenzie) 9. Who (or what) is the quantum mechanical observer? (Alexei Grinbaum) 10. Can

The comic appears here . I thank Joel Gilmore for bringing it to my attention.

Many previous posts have discussed the Heisenberg model on the anisotropic triangular lattice which is relevant to the Mott insulating phase in a range of organic charge transfer salts and Cs2CuCl4. A recent paper suggested that CuNCN is also described by this model, based on DFT based [GGA + U] calculations for the measured crystal structure. They found J1/J2 ~ 0.8 meaning the system could have a spin liquid ground state. I found this quite exciting. However, last year some of the same authors suggested a different point of view here , based on a hypothetical crystal structure. I do not follow the arguments in the second paper. Experimentally they do not observe magnetic moments and that motivates looking at alternative crystal structures without antiferromagnetic interactions [at the Hartree-Fock level]. But a non-magnetic ground state is just what you expect for the relevant Heisenberg model in the parameter regime of the first paper. I think the material and these importan

Michael Smith and I just finished a paper with the long title, Proposed measurements of the interlayer magnetoresistance of underdoped cuprate superconductors can distinguish closed pockets from open arcs in the Fermi surface. [Originally, we had a much shorter title but a journal editor proposed this one]. An outstanding question concerning the underdoped cuprate superconductors [pseudogap state] concerns the true nature of their Fermi surface which appear in STM and ARPES experiments as a set of disconnected arcs. Theoretical models have proposed two distinct possibilities: (1) each arc is the observable part of a partially-hidden closed pocket, and (2) each arc is open, truncated at its apparent ends. We show that measurements of the variation of the interlayer resistance with the direction of a magnetic field parallel to the layers can qualitatively distinguish closed pockets from open arcs. The first figure below shows the weak angular dependence of the field dependence o

A week ago, we had a very interesting Physics department colloquium by  Marcelo Gleiser  about his recent book,   A Tear at the Edge of Creation: A Radical New Vision for Life in an Imperfect Cosmos.   He discussed his growing disillusion with string theory and the search for a "theory of everything" which is based on the predominance of symmetry. He gave important examples of symmetry breaking in nature including CP violation in the electro-weak interactions [which because of the CPT theorem implies time reversal invariance] and the unique chirality of amino acids in proteins. Although it was a nice talk I thought it was all a bit sad to see someone who had become so enamoured with the propaganda of the reductionism in the high-energy physics community that it was painful when doubts emerged. Most of the points Gleiser was making seem to me to have been made long ago (in a more constructive sense) by Anderson in his 1972 " More is Different " article in Science

This week Elvis Shoko, Seth Olsen, and I read more of the beautiful review article, by Roald Hoffmann,  A chemical and theoretical way to look at bonding on surfaces. One of the key ideas I learnt was how chemical bonding between a metal surface and a molecule can have a completely different physical mechanism than between just molecules. The left side shows how the interaction between two molecules due to the two filled highest orbitals is repulsive.   In contrast, similar interactions between a molecular orbital and a filled metal orbital can be attractive because once the metal-molecule interaction is strong enough to push the anti-bonding orbital above the Fermi energy, there can be charge transfer to unoccupied metal states. The Figure above shows the energy of interaction as a function of the distance of the molecule from the surface. This also provides an understanding of the energy barrier to chemisorption of the molecule on the surface.

Today I gave a talk at the cake meeting about dynamical Jahn-Teller models.

Earlier this year there was a conference in Singapore celebrating the 8oth birthday of Murray Gell-Mann. He gave an interesting talk, Some Lessons from Sixty Years of Theorizing , which is a good read. Here are just a couple of extracts: things I wish someone had explained to me around 60 years ago.... The one I should like to emphasize particularly has to do with the frequently encountered need to go against certain received ideas. Sometimes these ideas are taken for granted all over the world and sometimes they prevail only in some broad region or in certain institutions. Often they have a negative character and they amount to prohibitions of thinking along certain lines. Now we know that most challenges to scientific orthodoxy are wrong and many are crank. Now and then, however, the only way to make progress is to defy one of those prohibitions that are uncritically accepted without good reason.... He goes on to give some good historical examples, e.g., opposition to continental d

I really like chapter 7, "Entropic forces" in Nelson's Biological Physics: Energy, Information, and Life, the reading for this week in BIPH3001 . Biological question : What keeps cells full of fluid? How can a membrane push fluid against a pressure gradient? Physical idea : Osmotic pressure is a simple example of an entropic force. You can learn a lot just from reading the great section headings 7.2.1 Equilibrium osmotic pressure obeys the ideal gas law. 7.2.2 Osmotic pressure creates a depletion force between large molecules 7.3.1 Osmotic forces arise from the rectification of Brownian motion 7.3.2 Osmotic flow is quantitatively related to forced permeation 7.4.1 Electrostatic interactions are crucial for proper cell functioning 7.4.3 Charged surfaces are surrounded by neutralizing ion clouds 7.4.4 The repulsion of like-charged surfaces arises from compressing their ion clouds 7.4.5 Oppositely charged surfaces attract by counterion release 7.5.1

Struggling to write your thesis or a paper or a research proposal? You are not alone. This morning I read the following account of one person's struggle. Writing was not a childhood dream of mine. I do not recall longing to write as a student. I wasn’t sure how to start. Over the following weeks I refined my plot outline and fleshed out my characters. One night I wrote “Chapter One” at the top of the first page of a legal pad; the novel ..... was finished three years later. The book didn’t sell, and I stuck with my day job, defending criminals, preparing wills and deeds and contracts. Still, something about writing made me spend large hours of my free time at my desk.  I had never worked so hard in my life, nor imagined that writing could be such an effort. It was more difficult than laying asphalt, and at times more frustrating than selling underwear. But it paid off. Eventually, I was able to leave the law and quit politics. Writing’s still the most difficult job I’ve ever ha

A nice model to discuss non-adiabatic effects, vibronic transitions, and the breakdown of the Born-Oppenheimer approximation is the two site Holstein model, where two coupled degenerate electronic states are each coupled to a vibrational model. The can be reduced to following Hamiltonian which is also sometimes called the E x beta Jahn-Teller problem. This model breaks the inversion symmetry Q to  -Q, but has a combined symmetry when this transformation is combined with mapping the "left" electronic state |l> to the "right" one |r>. (see the previous post about such combined symmetries). This allows one to make a unitary transformation due to Fulton-Gouterman that reduces the problem to two "vibrational" problems which can be solved as continued fractions. This is discussed by in a paper by Kongeter and Wagner  who calculate the spectrum below of eigenstates. p=+1 and -1 are the quantum numbers associated with the combined inversion symmetry. The

In the interesting physics colloquium that Marcelo Gleiser gave on friday at UQ he used the Escher print above to illustrate CP symmetry in a system which violates both C and P symmetry. [C is charge conjugation and P is parity]. I found the image here . Another example of breaking of a symmetry while conserving a combined symmetry concerns vibronic transitions in molecules. For molecules which have inversion symmetry selection rules suggest that electronic transitions that involve no change in parity should not be observed. However, they sometimes are because they are combined with a vibrational transition. Hence, vibronic [= vib(rational)-(elect)ronic] transitions. I think the first observed case of this may have been in benzene.

Yesterday I finished my White paper  for the conference, "Quantum physics and the nature of reality," to be held in honour of the 80th Birthday of Sir John Polkinghorne, later this month in Oxford. I welcome any comments on the paper. The abstract is below. Is emergence the nature of physical reality? The concept of emergence provides a unifying framework to both characterise and understand the nature of physical reality, which is intrinsically stratified. Emergence raises philosophical questions about what is fundamental, what is real, and the possible limits to our knowledge. Important emergent concepts, particularly predominant in condensed matter physics, are highlighted: spontaneous symmetry breaking, quasi-particles and effective interactions, universality and protectorates. Three alternative emergent perspectives on the quantum measurement problem are discussed. (i) The classical world emerges from the quantum world via decoherence. (ii) Quantum theory is not exa

"every decade or so, a grandiose theory comes along, bearing similar aspirations and often brandishing an ominous-sounding C-name. In the 1960s it was  cybernetics . In the 1970s it was  catastrophe theory . Then came  chaos theory  in the '80s and complexity theory in the '90s." Steven Strogatz ,  Sync : the emerging science of spontaneous order

Where do the following come from? Planck's constant. Dirac's canonical quantisation. (Identification of the quantum commutator with classical Poisson brackets). Locality of quantum field theory. The Born rule and definite outcomes of quantum measurements. Could they emerge from some underlying theory? Here is a brief summary of some of the key ideas in Quantum theory as an emergent phenomonen , by Stephen L. Adler. Helpful introductions are the slides [ hand-written viewgraphs! ] from a talk Adler gave in 2006 (see the summary slide below) and a review of the book by Philip Pearle. [There is also a draft of the book on the arXiv ]. This is an incredibly original and creative proposal. The starting point are "classical" dynamical variables pr, qr which are NxN matrices. Some bosonic and others are fermionic. They all obey Hamilton's equations of motion for an unspecified Hamiltonian H. Three quantities are conserved, H, the fermion number N, and the

Finding organic molecules with large nonlinear optical NLO polarisabilities is desirable for many applications. An important example is the imaging of biological cells using fluorescent protein chromophores. These images based on two photon microscopy using fluorescent protein chromophores. Understanding how the  NLO properties are related to the structure of the molecules is a fundamental question that has attracted considerable attention. In particular the NLO polarisability of conjugated dyes has been correlated with the amount of bond length alternation. Seth Olsen and I have just finished a paper  Bond Alternation, Polarizability and Resonance Detuning in Methine Dyes  which looks at these questions. Even though the molecules of interest can have a complex chemical structure it is possible to establish structure-property relations. This Figure shows how the bond length variations can be correlated with a single parameter which measures the deviation of the molecule from re