Sciences, par Claude Aslangul Dans la leçon 22-3 de ses fameuses Feynman Lectures on Physics ( le grand physicien obtient l’impédance d’un réseau infini de composants où alternent condensateurs et inductances purs. A sa manière inimitable, avec très peu de calculs comme toujours, Feynman obtient un résultat qui, à basse fréquence, exhibe de façon surprenante une impédance donnant lieu à des effets dissipatifs alors que le circuit ne comporte aucune résistance électrique. N’en restant pas là, évidemment, Feynman donne toutefois une interprétation physique à ce résultat d’apparence paradoxale, et passe à la leçon suivante. Où est donc le faux-pas de Feynman ? La réponse repose sur l’analyse de l’existence de la limite, une question que les physiciens ont trop souvent tendance à négliger. Ici les choses se présentent comme suit. On aurait pu s’attendre à ce que les composées successives de l’application possèdent de nouveaux points-fixes. Feynman se serait donc trompé ?
You Know How This Experiment Ends, But You Should Watch It Anyway Kinja is in read-only mode. We are working to restore service. First off, I have to marvel at how incredible this is to watch and admit that it made me too gawk, giggle, and grin like a child. I can't wait to show people this. That said, does anyone else wonder how much it had to cost to spend the 3 hours pumping out the air, setting up, and conducting this experiment that everyone know the result of? (Again, not wanting to rain on anyone's parade cause I'm damn glad they did it. Flagged It cost the electricity to run the pumps for a few hours. Also I'm sure that the BBC payed a fee and the staff probably enjoyed goofing off for half the day.
le chat de Schrödinger La physique quantique 4 ou Le chat mort-vivant de Schrödinger L'autrichien Erwin Schrödinger (1887-1961) est l'un des pères de la physique quantique. Le paradoxe du chat Imaginons le dispositif suivant: Un pauvre chat est enfermé dans une boîte pourvue d'un hublot.Dans un coin de la boîte, un atome d'uranium radioactif et un détecteur conçu pour ne fonctionner qu'une minute (par exemple). Eh bien figurez-vous que la physique quantique a un doute: elle vous dira que le chat, AVANT observation, est vivant ET mort à la fois! CAT'ION le chat quantique superposé ! Par exemple, nous avons vu que l'électron, étant donné sa nature ondulatoire, peut être localisé tout autour du noyau d'un atome; il est présent simultanément à plusieurs endroits, et cela AVANT qu'il ne se soit observé. On comprend mieux dès lors le paradoxe du chat: L'état "superposé" de l'atome U devrait se transmettre à notre félin macroscopique et le transformer en un horrible mort-vivant! Affinons l'expérience.
Xaonon: Hawking Radiation Calculator Black holes were so named because they were once thought to give off no radiation whatsoever, but Stephen Hawking showed that this is not the case. Black holes do produce radiation, with an intensity inversely proportional to the square of their mass. Since most black holes are thought to form from collapsed stars and are very massive, they give off very little radiation. However, a smaller hole (on the order of only a few billion tons) would radiate a great deal more, making it an excellent power source for an advanced civilization. The series of input forms below (Javascript source) calculates various useful characteristics of a black hole and its emissions. The radius of a Schwarzchild black hole of mass M is As per [Hawking 1974], the thermodynamic temperature of such a hole is with brackets delimiting constants of multiplication. making the Hawking radiation luminosity at least At a distance r from a hole with mass M, the incident radiation flux is therefore And since dE = dM c2, where
Why is the sky Blue? [Physics FAQ] - [Copyright] Original by Philip Gibbs May 1997. A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight. The white light from the sun is a mixture of all colours of the rainbow. Tyndall Effect The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859. This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering—after Lord Rayleigh, who studied it in more detail a few years later. Dust or Molecules? Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere. Why not violet? Response curves for the three types of cone in the human eye Sunsets Blue Haze and Blue Moon
Multiloop Feynman integrals Multiloop Feynman integrals appear when quantum-field amplitudes are constructed within perturbation theory. They are integrals over so-called loop momenta. Feynman has invented their graph-theoretical interpretation. (The term `Feynman integral' is sometimes used also for path integrals.) Feynman integrals are usually complicated objects even in a one-loop approximation, so that the number of loops equal to two is already considered big. Introduction In perturbation theory, any quantum field model is characterized by a Lagrangian, which is represented as a sum of a free-field part and an interaction part, Amplitudes of the model, e.g. The basic building block of the Feynman integrals is the propagator that enters the relation Here is the Feynman propagator of the field of type denotes the time-ordered product and the colons denote a normal product of the free fields. where is the corresponding mass, is a polynomial and or (for the gluon propagator in the general covariant gauge).
What's Up With That: How a Swinging Pendulum Proves the Earth Rotates Once upon a time, you were probably on an elementary school field trip at a science museum or an observatory. Just before lunch, your teacher had the class stand in a circle around an enormous weight suspended on a string, and watch it swing back and forth, back and forth. The teacher (or maybe a tour guide) explained that if you watched the pendulum for long enough, it would seem to alter its course, swinging in a slightly different direction. And that this somehow proved the Earth was rotating beneath your feet. Now that you’re older, you’ll occasionally think back on that pendulum and wonder how it could have proved anything. This famous experiment, now found in museums around the world, was first demonstrated in 1851. After an hour, the line the pin drew in the sand intersected with the first line at an angle of roughly 11.25 degrees, which is exactly what Foucault had predicted. So how does this all work? Click to Open Overlay Gallery Imagine the same setup at the equator.
La physique des particules Après les astrophysiciens qui étudièrent l′action des rayons cosmique sur la haute atmosphère pour y trouver des neutrinos. Le perfectionnement des accélérateurs de particules permit d'étudier un grand nombre de réactions nucléaires et de découvrir de nouvelles particules instables. Dans les années 1950, les particules prolifèrent: on en comptait alors plus de 400. Pour s'y retrouver, les physiciens ont essayé de classer ces nouvelles particules dont la durée de vie est inférieure á 10-10 secondes Classification Les physiciens parmi toutes les particules firent la distinction entre particules de matière et les autres puis séparèrent les particules de matière en en fonction de leur poids : léger, moyen et lourd : Particules de matière Les leptons (leptos = léger) sont des particules élémentaires constituées d′aucune autres particules dont : Les Hadrons parmi les quels on classe : Les Quarks : Désintégration du neutron Mais pourquoi faire simple Si l′on peut faire compliqué.
ilnuovosaggiatore.sif 1 Introduzione La radiazione elettromagnetica trasporta essenzialmente quattro tipi distinti di informazione: direzione, tempo, energia e polarizzazione. Le prime tre proprietà della luce sono familiari a tutti noi, e le corrispondenti tecniche osservative (imaging, timing e spettroscopia) sono ormai strumenti di routine per lo studio degli oggetti celesti a tutte le lunghezze d’onda. Gli sviluppi che queste tre branche dell’astronomia X hanno vissuto, sulla scorta dei progressi tecnologici dell’ultimo mezzo secolo, sono niente meno che spettacolari ed è sufficiente, e.g., uno sguardo anche distratto ad una delle ormai celebri immagini di Chandra (come quella mostrata in fig. 1) per avere un’idea del grado di sensibilità e del livello di dettaglio raggiunti. La polarizzazione è una proprietà della luce comparativamente più complessa, che ha a che vedere con la direzione di oscillazione del campo elettrico associato alla luce stessa. Se la risposta azimutale ha la forma tipica
The Egg in the Bottle Trick The Standard "Right-Side-Up" Version Using a pair of scissors, cut a strip of paper about 8" x 1." Carefully use a match or lighter to light the strip of paper at one end and drop it into the large-mouthed bottle. Get the Egg Out of the Bottle! Want to do the experiment again? The "Eggsclusive" Upside-Down Twist Carefully push two or three small birthday candles into the narrower end of a hardboiled egg. Hmmm... Carefully fill the balloon with water so the balloon is about the size of a tennis ball. In the traditional version of the Egg in the Bottle experiment, the burning piece of paper heats the molecules of air in the bottle and causes the molecules to move far away from each other. In the Upside-Down Twist, the science is the same as the traditional Egg in the Bottle trick, but the whole thing is just inverted.
L’étrange physique de monsieur Feynman Il y a 100 ans presque jour pour jour, naissait Richard Feynman, un physicien hors pair, au fonctionnement intellectuel iconoclaste que d’aucuns qualifièrent de génial. Richard Feynman a contribué à repenser la mécanique quantique, les forces fondamentales, il a eu l’intuition de l’ordinateur quantique, et fut dans le même temps un vulgarisateur exigeant, à la pensée complexe, parfois ardue mais qui gagna, certainement pas son originalité, le respect et la notoriété outre-Atlantique, où son parcours suscite aujourd’hui encore de nombreuses vocations. L’étrange physique de Monsieur Feynman, c’est le programme qui est le nôtre pour l’heure qui vient. Bienvenue dans La Méthode Scientifique. Et pour évoquer les travaux et la carrière de Richard Feynman, nous avons le plaisir de recevoir aujourd’hui Jean Zinn-Justin, physicien théoricien, conseiller scientifique au CEA et membre de l’Académie des Sciences, et Jean-Bernard Zuber, physicien théoricien, professeur émérite à Sorbonne-Universités.
Why is the sky blue? Blue skies make the heart soar and poets rush for quill and ink. The blue dome has subtle variety. Overhead it is darker - noticeably so from mountains or airplanes. Near the horizon it pales almost to white. The sun's light is a mix of violet, blues, greens through to reds. Air molecules, mostly nitrogen and oxygen, are 1000X smaller still. Air molecules individually scatter sunlight it into all directions. At the risk of disillusioning poets, it is not a pure blue. Why is the sky whiter near the horizon? Théorie des cordes Un article de Wikipédia, l'encyclopédie libre. Les niveaux de grossissements : monde macroscopique, monde moléculaire, monde atomique, monde subatomique, monde des cordes. La théorie des cordes est un domaine actif de recherche traitant de l'une des questions de la physique théorique : fournir une description de la gravité quantique c’est-à-dire l’unification de la mécanique quantique et de la théorie de la relativité générale. La principale particularité de la théorie des cordes est que son ambition ne s’arrête pas à cette réconciliation, mais qu’elle prétend réussir à unifier les quatre interactions élémentaires connues, on parle de théorie du tout. La théorie des cordes a obtenu des premiers résultats théoriques partiels. Présentation élémentaire du problème[modifier | modifier le code] Il reste que certains phénomènes nécessiteraient l'utilisation des deux théories. Hypothèses et prédictions[modifier | modifier le code] La théorie repose sur deux hypothèses : Le graviton, boson (c.
ilnuovosaggiatore.sif 1 Smart Windows Elettrocromiche Le smart windows consentono di progettare una “membrana adattiva”, in dialogo con l’ambiente circostante, puntando a massimizzare il comfort termico e visivo. Esse si basano sull’impiego di materiali e dispositivi cromogenici, ossia un’ampia gamma di materiali responsivi, o smart, capaci di cambiare caratteristiche cromatiche in ragione della variazione di uno stimolo esterno specifico. Il target dei sistemi cromogenici consiste nel controllo dinamico del fattore di trasmissione solare g (rapporto tra energia termica globalmente trasmessa dalla lastra e quella incidente). Secondo diversi ricercatori, tra cui Azens et al., il risparmio energetico derivante da una finestra elettrocromica supererebbe la produzione di energia elettrica che si otterrebbe applicando sulla medesima superficie un sistema fotovoltaico, di pari area. 2 Smart windows fotoelettrocromiche e fotovoltacromiche