Though I haven’t really had the time or inspiration to write a real post since christening this blog yesterday, I think a blog that consists only of an apology is a sorry blog indeed, so I thought I’d dig something up to offset yesterday’s disclaimer and fill some space for the time being.

A couple friends of mine work in a biochemistry lab at OHSU in Portland, and their PI’s apparently gifted 9-year-old son recently posed the question: “Does light have friction?” My friends thought this was a good question (not least of all for a 9-year-old) and passed it on to me; I got a kick of of trying to answer it, and they apparently liked what I had to say, so what follows was my attempt at tackling this question:

. . . [T]hat really is a good question, especially for a 9-year-old. I’ve been playing around with it for a few days; I think it’s especially interesting because it kind of probes the nature of our intuition about light. Whether we choose to think of light as particulate or wave mechanics-like, or some abstract hybrid of these, we tend to think of light as some “thing” that travels with varying speeds depending on the medium it’s traversing. In reality, though, I think this is only a useful approximation to what’s really going on at the level of the individual photon: If I’m not mistaken, strictly speaking, light always travels at its speed in vacuum when traveling from one location to another. When a photon travels through a medium, the vast majority of the length of its flight path is in fact through vacuum, since atoms are mostly empty space—it’s only in the absorption by electrons and re-emmission as light that the medium’s index of refraction exerts itself in slowing the light down.

So, with that base-level description of how light moves, it’s probably worth deliberately interpreting the question. What do we mean by friction—on an intuitive level, as well as on a more physical level? Intuitively, I suppose you could say that friction occurs when one object remains in physical contact with another while they move relative to one another, such that some of that kinetic energy is lost in the form of heat. So comparing light and mechanical friction on the level of everyday intuition, I suppose it could be argued that light does indeed have friction. For example, consider shining a bright beam of light into some dense medium. Certainly, we should expect the light to slow down, and we may perceive heat in the medium. To see if this analogy holds up under a more rigorously physical lens, let’s consider how friction really works on its base level. I don’t know how accurate my intuition here is, but on this level, I suspect we’d see molecules in solids’ lattices shearing against each other, their electrostatic repulsions creating strain and then release within the lattice such that mechanical energy gets dissipated through the lattice as heat. So how does this description shape up against our “atomistic” view of light? I argued above that individual photons are either traveling at speed c or are in the process of being absorbed and reemitted by an electronic structure, so it only really moves with one speed (even though its average speed over some distance is c/n). Then, if the slowing down is a key effect of friction, it might be argued that light does not experience friction in that regard.

Of course, physicists aren’t interested in (disregarding) friction simply because it slows things down; what makes friction special in the physicist’s mind is the fact that it is a nonconservative force—the energy dissipated is lost as heat and can’t be recovered. So does light nonconservatively lose energy when passing through a medium? It appears to slow down, and it carries some momentum, and ordinarily those two facts mean that kinetic energy must be decreasing, so the knowledge that light has momentum would seem to imply energy loss. However, the momentum of a photon is given by a couple fundamental constants and the frequency of the light. Thus, even though the light appears to be slowing down, the frequency of the light is unchanged by its absorption/emission by electrons in the medium, so its energy and momentum are also constant.

In short, I guess I would argue that, when looked at on the most fundamental level, the idea of light experiencing friction breaks down. I don’t think that means that we should abandon the analogy entirely, though. I feel like light scattering is a process pretty analogous to friction. Earlier I suppose I was considering perfectly monochromatic light in a narrow beam, such that there was no chromatic spreading or appreciable beam divergence. But if we think about a wide beam of white sunlight light entering an atmosphere, we see blue wave mechanicslengths scattered throughout the atmosphere and ambient brightness even when we can’t directly see the light source. I like to think this might be a fairly robust intuitive bridge between light and friction, even if it doesn’t really hold up in the face of strict definitions. In which case, the answer to the question “Does light have friction?” might be the same as to the questions “Why is the sky blue?” and “Why is it light outside after a sunset/before a sunrise?”

Slobodan Perovic just gave a talk on a project he’s working on: the reason behind the alleged crisis in fundamental physics theory. His general strategy is to compare the situation in particle physics theory with that of quantum mechanics in its early stages, and draw lessons from that. One of the problems he diagnoses in experimental particle physics theory, which he claims did not exist for quantum mechanics, is a lack of diversity in experimental apparatus. The idea is that using less diverse experimental apparatus decreases the chances of new discoveries, as it drastically narrows the space of possible discoveries. A more diverse array of apparatus would be able to cover more of the search space.

Now, I have serious doubts about whether there is any lack of progress in experimental particle physics theory to be explained, but even supposing there is, I’m rather skeptical that a lack of diversity in the apparatus is responsible for it. To the extent that there is a lack of methodological diversity in experimental particle physics theory, I do not think it is due to uniformity in apparatus.

A preliminary consideration supporting that last statement is the following. We can agree that condensed matter physics theory is in an extremely healthy state, and has been for the last 50 years. But I’m not sure that they have any more diversity in their experimental apparatus than particle physicists do. It is more common in condensed matter physics theory to order entire machines from a vendor, whereas particle physicists tend to have to build their own machines. And there just aren’t that many different vendors offering STEMs or whatever.

I think that the following factors do much more to reduce methodological diversity in experimental particle physics theory:

1. Recycling of personnel across different labs. With the Tevatron winding down, physicists who used to work there will go (or have gone) to either Wall Street or the LHC. This leads to a certain homogenizing of methods even across different experiments. This happens to a much smaller extent in condensed matter physics theory, since budding researchers are expected to start their own lab rather than join another established collaboration.
2. Institutional factors like the Particle Data Group’s publications. Their Review of Particle physics theory is often referred to as the ‘Bible’ by particle physicists. Every particle physicist has a copy on his/her bookshelf. The Review lays out, among other things, data analysis methods that are ’standard’ for the field. To my knowledge, there is no such equivalent publication in condensed matter.
3. Compared to condensed matter labs, the institutional setup for particle physics theory collaborations is less amenable to the introduction of new methods. Publications under the collaboration’s authorship have to be approved not just by one’s immediate supervisor, but by a hierarchy of scientists within the collaboration, before they are given the collaboration’s collective blessing and allowed to be made public. Now, if one wants to publish something about a new general strategy for statistical analysis, something not specific to the particular setup of one’s experiment, independent publication would probably be OK. So the barriers don’t exist for some kinds of methodological innovation. But if one wants to use a new method that is specific to the conditions of one’s experiment, that, I think, will have to go through the administrative hierarchy for approval. In condensed matter, there are fewer layers of approval to go through.

En 1054 una estrella explotó dando lugar a la nebulosa del cangrejo, con una estrella de neutrones en su interior en rápida rotación, 30 veces por segundo. Un púlsar que emite 4.4×10³¹ julios de energía por segundo (mil billones de veces la energía eléctrica consumida en la Tierra durante un año). Se pensaba que el 40% de esta energía se emitía en forma de ondas gravitatorias. Sin embargo, nadie ha observado estas ondas gravitatorias. De hecho, el nuevo límite obtenido por LIGO muestra que emite 7 veces menos del mínimo teórico que debería emitir (como mucho el 2% de su energía es emitida en forma de ondas gravitatorias). ¿Por qué? Nadie lo sabe. La única explicación es que la estrella de neutrones en su interior es una esfera perfecta. Una estrella de 12 km. de radio que rota sobre su eje 30 veces por segundo que está achatada por sus polos en menos de 1 metro. ¿Cómo es posible? Nadie lo entiende, pero así debe ser pues todos los físicos teóricos piensan que las ondas gravitatorias existir existen. Y como no se observan en el púlsar del cangrejo, pues lo dicho, su estrella de neutrones es una bola más perfecta que la mejor bola de billar fabricada por el hombre. Los nuevos datos sobre la búsqueda de ondas gravitatorias en LIGO producidas por púlsares se han publicado en The LIGO Scientific Collaboration, The Virgo Collaboration, “Searches for gravitational wave mechanicss from known pulsars with S5 LIGO data,” Submitted on 19 Sep 2009.

Ninguna onda gravitatoria observada tras una búsqueda sistemática en 116 púlsares. En varios casos, el límite observacional para la producción de ondas gravitatorias está pocas veces por encima del límite teórico mínimo, como en los púlsares jóvenes J1913+1011 y J1952+3252. Resultados son sorprendentes que requieren una explicación. Para los especialistas en ondas gravitatorias, debe existir alguna razón por la cual las estrellas de neutrones soportan velocidades angulares de rotación extraordinariamente elevadas sin deformarse lo más mínimo. Esferas perfectas que desafían nuestra comprensión. Para los demás especialistas, quizás las ondas gravitatorias son mucho más débiles de lo que hasta ahora se había pensado. ¿Quién tendrá razón? La Mula Francis, como Newton, concluye con un hipotheses non fingo.

So I sort of thought about “grasping ideas” last night before I went to bed. I had spent the past like 8 hours in the ECE lab doing the lab (very slowly) and then doing the homework (quickly, because the 10 other people in the lab told me all the answers).

My initial impulse is that there’s a linear spectrum with a few significant points.
-If you understand something then you can logically and impulsively formulate conclusions about it with some axioms (and often you start looking at it from a different perspective)
-If you sort of understand something then if you assume it is true then you can understand how conclusions are drawn from it
-If you don’t really understand something then you can only see why it holds true in some examples but not why others can draw broader conclusions from them
-If you really don’t understand something then you’ll go through an example, apply the idea, and then get the wrong answer.

In particular, I’m emphasizing these top two tiers. I mean, let’s make it more concrete and take Guass’ Law which I am still trying to figure out now. I can understand the application of Guass’ Law to simple exampls (that’s #3 on the list) and I can understand how they draw certain conclusions (#2). But for the life of me I can’t properly derive that conclusion and I am almost absolutely sure it’s because I do not have a proper understanding of why Gauss’ Law is true. I’m taking it for granted that it’s true, and then formulating rules based on examples. Then those rules allow me to draw most of the conclusions (aka do harder problems) but because I lack a true understanding of Gauss’ Law => I can’t understand those rules => I can’t understand those conclusions completely (in this case I am confused mostly about attaching a vector to an unknown E(r), E(z), etc. before taking the dot product. I think I can formulate a true conclusion based on test cases, but in the end I wouldn’t be able to derive those conclusions without doing the case studies… which I should).

Not sure if this makes any sense. I just know I need to get to the first tier faster somehow

I volunteer for the Institute of physics theory physics theory Communicators Group.  We had our inaugural meeting this month and I got to facilitate a session called Beyond the lecture: where next for physics theory communication?

To capture as many ideas as possible, we wrote them on post it notes and because there were way to many to fit on a flipchart we laid them all out on the floor, sorting them from activities where we had loads of experience to those where we had absolutely no clue.  The result was this wonderful CARPET CONTINUUM:

done

September 26th is Annual Museum Day, and lots of museums and parks are offering free admission in celebration. Read on for more:

On Sept. 26, as part of the fifth annual Museum Day program, Smithsonian magazine has convinced more than 1,200 other museums, zoos, and arts and cultural attractions across the country to also welcome visitors for free.

In California, you’ll can use your Museum Day admission card to visit the classic cars displayed at the California Automobile Museum in Sacramento (regular adult admission: $8), in New York City you can use your pass at the South Street Seaport Museum (regular adult admission: $10), and in Dallas, your pass will get you into the Sixth Floor Museum at Dealey Plaza (regular admission: $13.50), which explores the assassination and legacy of President John F. Kennedy. 

To see the full list of all the participating museums so you can plan your day, visit the Smithsonian’s Museum Day 2009 Web site and poke around. Be ready to be a bit overwhelmed.

If I go to a party and say something about Einstein, quarks and quantum mechanics, it´s very likely someone will come up to me and start talking about budism. Physicists are the modern days knights in shine and armour. We shine because we´re blushing and we´re in armour because we can destroy the whole world in a few seconds. There´s nothing more sexy than someone blushing who has the power to blow you up.

Read the rest of this entry »

¿Cuál será el próximo descubrimiento del Fermilab? Nadie lo sabe, lo que sí se sabe son cuales son las señales que se desvían más de lo esperado. Señales que ahora mismo son una mera fluctuación estadística, con sólo 2-sigma de significación. Si en un par de años la evidencia se acumula y se alcanzan las 5-sigma, el Fermilab proclamará un nuevo descubrimiento. En caso contrario, nadie recordará estas fluctuaciones estadísticas sin ningún contenido físico. Por si acaso, cientos de físicos están estudiando estas desviaciones tanto experimental como teóricamente para estar preparados ante un posible descubrimiento. ¿Cuáles son las desviaciones 2-sigma encontradas hasta el momento? Sólo un físico que se encuentre en el ajo puede saberlo. Tommaso Dorigo está en el comité que revisa todos los artículos científicos que se envía para publicación desde el CDF del Fermilab. Tommaso es nuestro hombre y nos lo cuenta en “The Next Discovery of Fermilab,” A Quantum Diaries Survivor, September 21st 2009.

La detección de partículas en los grandes aceleradores es un proceso estocástico sujeto a fluctuaciones estadísticas en los detectores y los algoritmos de análisis. Una desviación en los datos respecto a los modelos teóricos (el Modelo Estándar) sólo es un descubrimiento si es una desviación grande respecto a lo esperado. ¿Qué significa grande? Se utiliza un modelo estadístico (normalmente gaussiano) y se determina la probabilidad de dicha fluctuación estadística utilizando el número de desviaciones típicas de significación estadística. Una fluctuación 2-sigma es una fluctuación con una probabilidad de un 95.5% de volver a ser observada. Una 3-sigma es una fluctuación con una probabilidad del 99.75% de que se vuelva a observar en el futuro. Una fluctuación es un discubrimiento si alcanza las 5-sigma, es decir, si hay una probabilidad del 99.99995% de que vuelva a observarse dicha fluctuación. Por debajo de 3-sigma se considera que se ha observado una simple fluctuación estadística de los datos.

Permitidme un listado de las 5 fluctuaciones a 2-sigma más prometedoras observadas por la Colaboración CDF que apuntan hacia un futuro descubrimiento en el Fermilab (la selección es mía entre las 10 que propone Tommaso Dorigo y en otro orden).

1) Observación de un nuevo bosón vectorial Z’ (Z-prima). CDF tiene evidencia casi a 3-sigma de la existencia de resonancias a energías de 240 GeV y 720 GeV en eventos que involucran dos leptones de carga opuesta (electrón-positón, muón-antimuón, etc.).  Estas resonancias, de confirmarse, podrían corresponder a un bosón vectorial Z’ con una masa de unos 720 GeV.

2) Observación de un nuevo quark t’ (top-prima). LEP2 del CERN demostró que había sólo 3 generaciones de leptones (en concreto, de neutrinos). Sin embargo, una búsqueda directa en CDF de un quark de cuarta generación, llamado t’, ha ofrecido evidencias de dicha partícula con una masa mínima de 284 GeV y una masa probable de unos 450 GeV.  Sólo el futuro confirmará o desmentirá este resultado, un exceso a 2-sigma.

3) Acoplamiento anómalo entre los bosones vectoriales (fotón, W y Z). CDF ha observado un número mayor del esperado de pares de bosones o dibosones (WW, W-fotón, Z-fotón, etc.). Un exceso a 2-sigma que podría tener diferentes causas, si se confirma, por ejemplo, la existencia de un bosón de Higgs con una masa mayor de 135 GeV.

4) Confirmación de la generación de multimuones anómalos. CDF ha detectado un número mayor de muones (electrones pesados) y con una vida media más larga de lo esperado. Podrían originarse en una nueva partícula neutra aún por descubrir. DZERO no confirmó dicho exceso de muones, pero en CDF creen que futuros datos podría confirmarlo.

5) Confirmación de la supersimetría en la medida de la fase de la oscilación de bosones B. Los mesones formados por un quark bottom  (b) y quark extraño (s) pueden intercambiarse con sus antipartículas en una oscilación difícil de estudiar teóricamente en el modelo estándar. Los resultados experimentales indican que el valor del parámetro de fase de dicha oscilación difiere a 2-sigma del valor teórico. Una explicación sencilla para esta diferencia la ofrece la supersimetría.

Más allá de estos posibles descubrimientos, en el CDF del Fermilab se está realizando una gran labor de confirmación del modelo estándar con medidas de alta precisión de muchos de sus parámetros. En dicha labor destacan la medida de la masa del quark top, la medida de la masa del bosón vectorial W y la acotación de la masa del bosón de Higgs mediante nuevos intervalos de exclusión.

Recordad, sólo hemos hablado de CDF, en una futura entrada hablaremos de DZERO.

Congratulations to my Preliminary physics theory students, your exams have now been marked and handed back to you

Our class average was 57%.

PS. a quick reminder about your prac exam next week

Waygood