The Bad Astronomer periodically makes posts that show just how cool some astro phenomenon or astro observational capability can be. In keeping with this idea, I find this paper to be just damned impressive. (Apologies for the subscription-only link.) The investigators at Oxford University have one of the best and fanciest transmission electron microscopes (TEM) in the world. In TEM, a highly focused (on the atomic scale!) beam of electrons is fired through a very thin (under 100 nm thick) sample, and the transmitted electrons are analyzed as the beam is scanned over the sample surface. By using very clever electron optics techniques (aberration correction) and the right choice of samples, the investigators have been able to watch the motion of single atoms and few-atom clusters (of praesodymium, which has a big atomic number and therefore interacts strongly with the electron beam) within a carbon nanotube. They can study the formation of 1d crystals this way. Very impressive imaging tool. I want one :-)
Sunday, January 30, 2011
Saturday, January 29, 2011
No, I'm not talking about Peter Woit's website or Wolfgang Pauli. Instead, I mean this article, which shows that Allstate Insurance apparently thinks that it's meaningful to look at car accident risk as a function of the astrological sign of the driver. Astrology? A major company using astrology? We're supposed to believe that there is a statistically meaningful correlation between the time of the year you're born and your driving ability? This is why there is a crying need for math and science literacy.
Posted by Douglas Natelson at 1:55 PM
Saturday, January 22, 2011
I'm starting to know how Phil Plait must feel every time he has to write yet another article about how Betelgeuse is not about to explode. (Though my readership is about 0.01% of the Bad Astronomer's)
Once again, there is a claim receiving attention from various media sources (here, here, here) that someone has demonstrated some gadget that produces so much "excess heat" that the conjectured source of the energy is some kind of nuclear reaction taking place in a condensed matter environment. This time, it's two Italian researchers, and they have demonstrated (in some very restricted way, more on this below) a device that they say uses a reaction involving nickel and ordinary hydrogen. The claim is that for a steady state input power of 400 watts, they can produce around 12 kW steady state of power in the form of heat. The device when running supposedly takes in room temperature water at some rate and outputs dry steam, and doing the enthalpy balance and water flow rate is how one gets the 12 kW figure. Crucially, the claim is that this whole process only consumes a tiny amount of hydrogen (far too little for some kind of chemical combustion to be the source of all the heat). The conjectured nuclear reaction is some pathway from 62Ni + p -> 63Cu. No big radiation produced, though of course the demo doesn't really allow proper measurements. Don't even bother reading the would-be theoretical "explanation" - it's ridiculously bad physics, and completely beside the point. What's really of interest is the experimental question.
As always in these cases, there are HUGE problems with all of this. The would-be paper is "published" in an online journal run by one of the claimants. The claimants won't let independent people examine the apparatus. They also don't do the completely obvious demonstration - setting up a version that runs in closed cycle (that is, take some of that 12 kW worth of steam flow, and generate the 400 W of electrical power needed to keep the apparatus running, and just let the system run continuously). If the process really is nuclear in origin, and the hydrogen accounting is correct, it should be possible to run such a system continuously for months or longer. The claimants say that they've been using a 10 kW version of such a unit to heat a factory in Italy for the past year, but they conveniently don't show that to anyone.
The burden of proof is on these people - if they've really done this, the world will beat a path to their door, and that would be great. I'm not buying my nickel futures yet, however. Once again there will be people out there who claim that evil scientists are suppressing these unorthodox geniuses; this is such a ridiculous mischaracterization of science that it still ticks me off every time I read it. Of course I wish this were a genuine discovery - it would be world-changing and reveal enormous new physics. However, so far no version of this kind of low energy nuclear reaction business has passed the bar of reasonable reproducibility in controlled circumstances. (See here for a past discussion concerning the palladium variety and its reproducibility. Read the comments there before posting angrily below that I don't understand the situation, or that I haven't looked at this, or that I'm otherwise hugely ignorant on the subject.) That's not the establishment being oppressive, it's the way good science works. Extraordinary claims require extraordinary evidence. The self-sustaining demo I described above with independent verification and measurements would go a long way. I'm not holding my breath.
Posted by Douglas Natelson at 3:13 PM
Tuesday, January 18, 2011
Here are a number of links that may be of interest:
Back in December, Steven Blau at Physics Today wrote an interesting blog post about the arrogance of physicists. For some reason I just came across this today. Prof. Stone's comment on the post is, I think, right on the mark, and reminds me of this xkcd comic.
Here is a series of four blog posts (one, two, three, four) from Mike Mayberry at Intel, to give you a sense of some of the research directions they're pursuing as we near the possible end of scaling for conventional Si-based FETs. Very interesting stuff on the challenges of integrating other materials (like III-V compound semiconductors) with Si.
Veering into humor, here is a video made by Adam Ruben, whom I know through the alumni network of the Princeton Band. It's called "The Grad Student Rap", and it's part of the promotion for his book, Surviving Your Stupid, Stupid Decision to go to Graduate School.
Posted by Douglas Natelson at 8:29 PM
Thursday, January 13, 2011
Having read something about this online, I had to see for myself. Take a look at this paper. One of the 2008 Nobel laureates for medicine is the lead author, and he claims that simply having certain kinds of DNA in water (1) creates electromagnetic waves at very low frequencies, like 7 Hz; (2) those waves are sufficiently strong that a simple pickup coil of copper wire can be used to detect them inductively; and (3) somehow those waves continue to self-propagate in a weird way so that repeated dilution of the solution preserves the "imprint" of those waves. Wow. The science here is so unbelievably bad, it's hard to imagine that this is serious. A pick-up coil?! No serious discussion of the magnitude of the effect, and whether it's even remotely credible that detectable inductive signals could be produced? Silly numerology demonstrating a complete lack of understanding of quantum mechanics? Impressive. Can we make a deal? Medicine laureates won't make crazy, misinformed claims about physics (which then naturally get picked up by the media, who love to report "the controversy", as if there is no such thing as a right or wrong answer to a scientific question), and physics laureates won't make crazy, misinformed claims about biology. Please?
Posted by Douglas Natelson at 10:28 PM
Yesterday I received a very nice and welcome email from a faculty member who had been one of my best classroom instructors in graduate school. This email was, effectively, a reply to an email that I had sent him regarding Stanford's graduate physics curriculum. The amusing bit is that I had sent him that email 14 years ago, when I was a senior grad student representative to Stanford's physics graduate committee. At the time, there had been ongoing discussions about what topics should be in the first-year graduate curriculum, particularly the "mechanics" sequence, and my opinion had been asked for. It's interesting to look back now as a faculty member at what I'd suggested at the time. Here are the bullet point topics I'd suggested. Remember that Stanford is on the quarter system, meaning that there are three ten-week quarters during the regular academic year.
For "Mechanics of Particles" (basically graduate mechanics and dynamics), I'd said:
- Brief review of variational calculus
- Lagrangians and Hamiltonians, action principle
- Canonical transformations, phase space
- Symmetries and conservation laws (Noether's thm?)
- Normal modes, harmonic oscillator review
- Rigid body motion (numerical work?)
- Orbital mechanics review
- Classical perturbation theory (w/ orbits, rigid body dynamics, anharmonic oscillator)
- Action-angle variables
- Poisson brackets, symplectic structure (*definitions of 1-forms, tangent spaces, tangent bundles?)
- Chaos, nonlinear dynamics, ergodicity
- Brief review of Einstein summation convention
- Special relativity w/ Einstein summation convention, space-time diagrams
For "Continuum mechanics" (fairly unique, I now realize - many departments offer no such course), my suggestions reflected my undergrad engineering background to some degree. I now realize that what I list below is considerably too much for a 10 week course:
- Mechanics of solids:
+ Continuum mechanics version of Hooke's law; stress, strain, tension, compression, shear, bulk modulus, a few numbers about strength of materials, Young's modulus, shear modulus
+ Lagrangian/Hamiltonian densities, more variational calculus
+ *Flexure of beams, bending moments, areal moments of inertia (why I-beams are stiffer than rods of the same cross-sectional area)
+ *Torsion of members, polar "moments of inertia"
+ *Dynamics of beams: the wave equation, longitudinal and transverse sound, natural frequencies of cantilevers
+ Acoustics, idea of acoustic impedance and mismatch
- Fluid statics
+ Hydrostatics, Archimedes' principle, buoyancy
+ *Surface tension, capillary action, wetting
- Fluid mechanics
+ Euler and Lagrange pictures
+ "Convective derivatives", transport of momentum and energy
+ The energy equation, the momentum equation, the continuity equation, the Navier-Stokes equation
+ Inviscid, incompressible flow:
- Bernoulli's Eqn.
- Potential theory
- *Vorticity, circulation, Magnus' law, "lift"
+ Viscous, incompressible flow:
- Definition of viscosity, comparison w/ shear modulus, definition of Newtonian fluid
- Stoke's law
- Intro to dimensional analysis, Reynolds' number
- Laminar flow, parabolic velocity profile in a round pipe
- Turbulent flow, mention engineering approach to these problems (Moody chart, friction factor, Bernoulli w/ losses)
- Froud number, hydraulic jumps (example of a "shock" discontinuity that you can demonstrate in a sink)
+ Compressible flow
- Mention of shockwaves, scaling
For "Statistical Mechanics", the main challenge was dealing with the divergent backgrounds of incoming students - some people had very strong undergrad preparation in statistical and thermal physics, others much less so. This is an issue in graduate quantum mechanics to an even greater degree. Now that I've taught undergrad stat mech several times, I think what I listed below could use some additional advanced topics:
- Definition of entropy, why it's a logIt was definitely interesting to me to see how my thinking on this stuff has evolved now that I have to teach it.
- The equal prob. postulate/ergodic thm.
- The Boltzmann factor and the partition fn., Fermi and Dirac distributions
- *Mention of Feynman diagram methods, saddle-point integration to get Z in complicated systems
- The canonical and grand canonical ensembles, the chemical potential
- "Natural" variables, Legendre transforms, thermodynamic potentials, *the idea of a constrained maximization of S, the Maxwell relations, the "thermodynamic square"
+ Ideal classical
+ Van der Waals, virial coefficients
+ Fermi gas at zero and finite T
+ Ideal Bose gas, BEC, phonons & photons *(incl. laser discussion!)
- Liquids - diagrammatic methods of treating interactions?
+ Concept of long-range order
- *Correlation functions, *connection w/ susceptibilities
- *Correlations and fluctuations, *how they're measured!
- Theories of phase transitions
+ Concept of order parameter
+ Ginsberg-Landau theory, diff. betw. 1st and 2nd order, extensions to include fluctuations
+ 1st order: Van der Waals reprise, Clausius-Clapeyron
+ Mean-field theory, example of magnetism
+ Ising model in 1-d
+ Renormalization group to solve Ising model, critical behavior, correlation length ideas
+ *Boltzmann equation
+ *Noise in transport: fluctuation/dissipation thm
Posted by Douglas Natelson at 10:42 AM
Sunday, January 09, 2011
While I don't do any research on the subject myself, over the last few years I've become more interested in the origins of friction, a subject about which almost no physics progress was made between from around 1650 to 1950. Since the development of the tools of surface science (ultrahigh vacuum, for example) and scanned probe microscopy, however, people have learned much about where friction comes from.
We all have an intuitive grasp of what friction is, and in freshman physics (or even high school), we learn that we can model friction as a (shear) force between two surfaces as they slide (or attempt to slide) relative to one another. That force is modeled as proportional to the normal force between the surfaces, with the surface-dependent friction coefficient as the proportionality constant. The force is further traditionally modeled as being independent of the contact area between the two surfaces, and independent of the relative speeds of the two surfaces (except for the distinction between static friction - with no relative motion - and kinetic or sliding friction). That approach does a very good job at describing many many experiments on friction between macroscopic objects.
The problem is, as many famous scientists (e.g., Coulomb) discovered, it's very difficult to come up with a microscopic model of the interaction between surfaces that has these properties. One of the essential difficulties is rather deep: friction has to result in real dissipation. Energy has to be transferred from macroscopic degrees of freedom (the motion of a hockey puck relative to the ice) into microscopic degrees of freedom (the relative vibrational motions of the atoms in the hockey puck, and similar motions of the atoms in the ice - heat, in short.). That transfer of energy from macroscopic coordinates to microscopic motions or coordinates is irreversible in the same sense that the motion of water in a pond is irreversible after a stone is tossed in. (Yes, it's physically conceivable from the point of view of Newton's laws that all the little bits of water at the edge of the pond could jiggle just right so as to send coordinated ripples inward toward the center of the pond, spitting the stone back out. However, that's incredibly unlikely, given all of the possible microscopic states of the water, so from the standpoint of macroscopic thermodynamics, the water rippling process is irreversible.)
There has been some beautiful work on friction at the nanoscale, and much of it has focused on chemical interactions between surfaces, as well as vibrations (phonons) as the relevant microscopic degrees of freedom. However, in the case of metals, there are other excitations where the energy could end up: electrons! That's one defining characteristic of a metal, the existence of possible electronic excitations of (almost) arbitrarily low energy. How can you tell if the energy is ending up in the electrons? Well, you'd really like to do an experiment where none of the vibrational properties are changed, but that allows you to compare between with-electrons and without-electrons. Amazingly, it is possible to do something close to that by working with a metal that is superconducting! Above the superconducting transition temperature, Tc, the metal has plenty of low energy electronic excitations. Below Tc, however, in the superconducting state, electronic excitations are forbidden below some threshold energy (this "gap" in the excitation spectrum is one key reason why superconductors have no electrical resistance). In this new paper (sorry about not having an arxiv version to link), the investigators have demonstrated that the (noncontact) friction between a metal tip and a niobium film drops dramatically once the niobium becomes superconducting. This argues that electronic dissipation is responsible for much of the friction in this case (in the normal state). I should point out that previous work with lead films had hinted at similar physics. The new experiment is very clear and benefits from technique developments in the meantime.
Posted by Douglas Natelson at 8:49 PM
Thursday, January 06, 2011
Happy new year! I want to write a little about what physicists call spin-orbit interactions. It turns out that there is a deep connection between electric and magnetic fields that can be made somewhat obvious by considering a thought experiment. (For a great discussion of this, see the textbook by Purcell.) Imagine a line of stationary positive charges. From our perspective (at rest relative to the line of charges), there is no current, so one should see an electric field pointed radially outward from the line of charges, and a positive charge placed next to the line of charges should respond accordingly, being pushed radially outward. Now consider viewing this from a reference frame moving parallel to the line of charges. From our point of view in that frame, we see a current, and therefore there should be a magnetic field associated with that current (as well as an electric field from the net positive charge). In special relativity, one can figure out how electric and magnetic fields transform into and out of each other when changing reference frames.
This shift of point of view is the way that spin-orbit coupling is usually explained in undergrad quantum mechanics. Consider a hydrogen atom. The electron zipping around the proton has a spin degree of freedom, and a corresponding magnetic moment. From the point of view of the (classically) moving electron, the proton is essentially a current producing a magnetic field, which will tend to align the electron magnetic moment. This couples the spin of the electron to the orbital motion of the electron; hence the name "spin-orbit coupling"; and it is technically a relativistic effect which tends to be bigger in heavier atoms.
Why should you care? Well, spin-orbit coupling can be important in solids, too, since one can think of their electronic states as being built out of atomic orbitals. As ZapperZ points out, a recent paper shows that these kinds of relativistic corrections are not necessarily tiny in ordinary, everyday solids. In fact, it appears that it is essential to worry about such relativistic effects in order to understand why the electrochemical redox potentials of an ordinary car battery are what they are!
Posted by Douglas Natelson at 10:09 PM