L'esperimento più bello di sempre e la figura di Akira Tonomura.

Circa due mesi fa è venuto a mancare Akira Tonomura (25 aprile 1942 - 2 maggio 2012), un fisico giapponese, conosciuto per il suo contributo allo sviluppo dell'olografia elettronica e la verifica sperimentale dell'effetto Aharonov-Bohm (un effetto straordinario di cui vorrei parlarvi prima o poi). In questo post voglio però concentrami su un'altro interessante esperimento condotto da Tonomura; si tratta della versione moderna dell'esperimento condotto dal gruppo bolognese composto da Pier Giorgio Merli, Gian Franco Missiroli e Giulio Pozzi nel 1974, in quello che è stato definito, secondo un sondaggio promosso dalla rivista Physics World nel 2002, il più bell’esperimento di fisica di sempre. 

Le Scienze su Peter Higgs

Link

Quando le misure contano: effetto Zenone quantistico e anomalia GSI.

In quest'articolo uscito su ArXiv, gli autori Francesco Giacosa e Giuseppe Pagliara interpretano l'anomalia GSI, di cui ho accennato in questo post, utilizzando solamente la Meccanica Quantistica (MQ). Gli autori fanno infatti notare che nell'ambito della Meccanica Quantistica non relativistica la legge del decadimento esponenziale è solo un'approssimazione, per quanto molto buona, che vale per tempi lunghi al seguito della "preparazione" dello stato instabile. Osservando invece il sistema subito dopo la preparazione si possono indurre delle deviazioni dalla legge del decadimento esponenziale a causa del cosiddetto effetto Zenone quantistico, una rivisitazione in termini quantistici di un particolare paradosso, chiamato appunto paradosso di Zenone. 

On motivations of pure research

Here is one of the most convincing arguments against detractors of LHC; from the web-comic SMBC
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Colliding planets and disappearing dust.

The collapse of a molecular cloud sets the starting point for the star and planet formation theory. For low mass objects (lower than 3 solar masses) there are (good?) chances that the final product, after few million years, will be similar to our current solar system.

Let's consider from now on a Sun-like star formation scenario. Molecular clouds usually rotate, during their collapse the angular momentum must be conserved. This leads to the formation of an accretion disk around the star: the so called protoplanetary disk. The disk is composed of both dust and gas, as the natal molecular cloud was. The presence of the dust (solid particles with sizes ranging from few nanometers to millimeters) is actually crucial for the planet formation. Processes like coagulation causes  dust growth, until a planetesimal (kilometer sized object) is formed. Subsequent mass accretion on these big bodies, in less than few million years, leads to planet formation.

Depending on their distance from the central star the planetesimals can then still grow until they accrete an icy envelope (far away), a lot of gas (a bit closer), or just keep growing to the size of the Earth (very close to the star). After this stage, the gas in these system should be gone, either accreted from the forming giant planets (see Jupiter), from the star or photo-evaporated by the stellar radiation. After 10 million years we should  then be left with something similar to our solar system, with little amount of free gas around and maybe some dust leftovers.

Let's see now how astronomers observe the stages listed above. To do so we need to define the SED, which stands for: Spectral Energy Distribution. It is a simple plot where one has in the y-axis the flux or luminosity emitted from these objects and in the x-axis the wavelength at which that radiation is emitted. Let's have a look at this figure:

Evolution of an SED. Class 0: only the dust respons to the obscured star is visible. Class I: the star looks for its way out. Class II: star+disk system. Class III: star only? Maybe a planetary system, with dust leftovers (debris disk).

This is the timeline of what happens.

Do you understand the "Higgs Boson"?

Se avete acceso il computer o la televisione nei giorni scorsi, avrete sicuramente appreso dai diversi organi di stampa e di informazione che il 4 Luglio è stato dato l'annuncio della scoperta, al CERN di Ginevra, di una risonanza che ha tutte le carte in regola per essere interpreta come il famoso e mancante bosone di Higgs. Preferisco sorvolare in questo post sulla serietà delle notizie riportate e sulle competenze dei giornalisti, che sembra abbiano fatto a gara per confondere le idee alla gente sull'importanza di questa scoperta. Se comunque vi siete persi la notizia della conferenza potete trovare una descrizione di cosa è stato riportato dagli esperimenti ATLAS e CMS in questo post.

Fabiola Giannotti, portavoce di ATLAS, abbraccia Peter Higgs, teorizzatore della famosa particella, al termine della conferenza del 4 Luglio.

Tale scoperta ha un impatto notevole perché il bosone di Higgs riveste un ruolo di primaria importanza nel cosiddetto Modello Standard delle particelle elementari, un modello teorico che descrive tre delle quattro interazioni fondamentali: la forza elettromagnetica, la forza debole e la forza forte, con la sola esclusione della forza di gravità. In questo post vorrei cercare di approfondire meglio cosa gli scienziati intendono quando dicono: "il bosone di Higgs è il responsabile della massa delle particelle" (immagino abbiate sentito spesso questa frase nei giorni seguenti la scoperta!) e spiegare perchè tale particella sia fondamentale nel MS, talmente tanto che negli ultimi venti anni ha concentrato su di sé gli sforzi di migliaia di fisici sperimentali in tutto il mondo.

Sparse Swedish Thoughts

• My talk is in a couple of hours, i should finish it instead of writing here
• Stockholm is just amazing: it looks like nobody is really rich and nobody is poor. On average everybody is wealthy enough and the standard deviation is tiny. I guess that's the secret of this country (plus the fact it's just 10 millions people)

"Blond girls... blond girls everywhere"

New insights on the matter-gravity coupling paradigm

Back in October 1915, Albert Einstein published his revolutionary theory of gravity, General Relativity. His first version of the equations named after him were actually not compatible with local energy conservation. The second version was modified accordingly and claims that geometry is proportional to matter content. The fundamental geometric object is the so called metric, which prescribes how to measure length and time intervals. As John A. Wheeler said, 'Matter tells space how to curve, and space tells matter how to move.' This is the tenor of the Einstein equations. Furthermore, the metric itself is dynamical and contributes to the energy, warping the spacetime. This is encoded in the nonlinear nature of the geometrical part of the Einstein equations.

Although Einstein's equations successfully describe a lot of astrophysical or cosmological phenomena, it is worth noting that there is something missing in the theory. For instance, galactic rotation curves can be understood only by postulating the existence of some weakly interacting, nonvisible dark matter. The birth of the universe requires an inflationary period, which in turn requires new matter content. In fact, the most successfully tested sector of Einstein's theory is vacuum, as most experiments are performed outside of matter.

On another hand, it seems legitimate to wonder why an intrinsically nonlinear theory should couple linearly to the matter content. This should be valid in some regime but there is no reason to impose it. Modified theories of gravity usually propose to change the geometrical description, generally leading to strong experimental constraints. Instead, the coupling between matter and geometry is most often left unchanged. A modification of this coupling deserves a comprehensive analysis, which is the purpose of our letter 'New insights on the matter-gravity coupling paradigm.'

We were working on a particular alternative theory, namely the Eddington-inspired-Born-Infeld theory, when we realized that it can be interpreted as a standard General Relativity model, but with a modified coupling. This model was brought up to date in 2010 by Banados and Ferreira, who showed that in some situations singularity problems can be avoided. Using the reinterpretation of the model, we put forward the mechanism behind the singularity avoidance and explicitly show the effect of the coupling modification.

Even better, we provide a relation between how matter affects the geometry and how we perceive matter. Indeed, if the coupling to geometry changes, we must interpret measurements in a different way. Otherwise one is led to the (probably false) conclusion that spacetime is curved by a matter source different from the one directly seen... This sounds familiar? It should! Indeed, modified couplings are yet quite unexplored, experimentally viable, and might provide new perspectives to currently unanswered questions, e.g., mimic the new kinds of matter proposed in cosmology.

arXiv link

The hunt ended. Let's eat the prey.

The discovery of a "resonance" with a mass of ~125 GeV that may well be the Higgs boson, the missing piece in the Standard Model (SM) puzzle, has finally been announced by the ATLAS and CMS collaborations at CERN.
The (experimental) hunt started in 1989 at CERN with the Large Electron-Positron (LEP) collider and ended in the same place, but with the Large Hadron Collider (LHC), 23 years later.
For those of you interested in details, slides from the presentations given by representatives of the collaborations can be found at http://indico.cern.ch/conferenceDisplay.py?confId=197461

What follows may not be technically entirely correct but it is meant for people not working in particle physics to have an idea of how results are produced. Still a basic understanding of statistics is required.
For consistency, only results from ATLAS will be shown, however the reader must be aware that very similar plots have been released by CMS.

Using network science to forecast the spread of emerging diseases

I know, I know, this is not about the Higgs boson, and some of you might think that is not even about physics. I leave this debate for later. For now stop thinking about lagrangians, 3+1+x dimensions, ghost's fields (forever), and have a look at what we do here at the Mobs (Laboratory for the Modeling of Biological and Socio-technical Systems) lab @Northeastern University.

Domani siete tutti invitati, offre il CERN! Piatto del giorno...il bosone di Higgs!

Mancano ormai meno di 24 ore all'attesissimo aggiornamento sullo stato della ricerca del bosone di Higgs da parte degli esperimenti ATLAS e CMS, situati presso il Large Hadron Collider (LHC) del CERN di Ginevra.

L'annuncio del seminario (il webcast per il pubblico sarà disponibile qui), avvenuto la settimana scorsa da parte del direttore generale del CERN, Rolf Heuer, e anticipato dai soliti “rumors” che accompagnano ormai immancabilmente gli annunci scientifici più importanti (si veda il caso dei neutrini superluminali), ha suscitato un interesse mediatico notevole, con siti e giornali che hanno fatto e fanno tuttora a gara per dare la notizia che il bosone di Higgs sia ormai stato trovato, affermando di avere come fonti scienziati interni alla collaborazione, che vogliono però sempre rimanere anonime.