From SMBC

http://www.smbc-comics.com/

Neutrini, muoni e fattori g da non trascurare...

Il 25-26 Aprile si è tenuto un workshop della collaborazione del Fermilab (qui le slides) per la riconfigurazione dell’insieme dei rivelatori nella miniera di Homestake (miniera che ci è familiare dalla vicenda dei neutrini solari che ho descritto in un post precedente) dal nome LBNE (Long-Baseline Neutrino Experiment). In tempo di crisi economica mondiale lo scopo del workshop è stato più gestionale che altro, dal momento che i partecipanti hanno aiutato a valutare le migliori opzioni per fare buona scienza con la minor quantità di denaro.

Dalle parole del vicedirettore del Fermilab Young-Kee Kim, in riferimento agli esperimenti previsti, sembra che tutto proceda per il meglio:"We're in very good shape for the next 10 years, for neutrino and muon programs." 

Higgs Pain? Take a Preon!

Alzi la mano chi conosce i preoni!! Cosa sono i preoni? Suvvia tutti li conosciamo, sono le ulteriori particelle elementari che pare costituiscono, udite udite, i quark e i leptoni… va bene che ormai siamo abituati ad una fisica che imita lo stile delle mastrioske russe però forse ci stiamo spingendo troppo in là. Ma accantoniamo i giudizi soggettivi di stile e facciamo un piccola presentazione. Il modello fu proposto dai fisici Jogesh Pati e Abdus Salam (noto per il contributo alla teoria elettrodebole e che insieme a Sheldon L. Glashow e Steven Weimberg vinse il premio Nobel per la fisica nel 1979, fu inoltre uno dei fondatori dell’ICTP International Centre for Theoretical Physics a cui ora è dedicato) nel 1974 con un articolo sulla rivista Physical Review (http://prd.aps.org/abstract/PRD/v10/i1/p275_1), articolo poi susseguito da molti altri contributi tra i quali quello di Terazawa, Harari e Ne’eman.

Neutrini: storie di anomalie!!

Iniziamo questa avventura nella fisica proprio con il contest “Paper of the Day”, ma dal momento che non si è stabilita una data precisa per l’inizio, mi sono preso la libertà di controllare nell'archivo di "http://arxiv.org/"  in un intorno della data dell’inizio del blog. La mia attenzione è stata catturata da due articoli tra loro correlati, il tema legante è l'oscillazione di neutrini. Il primo "Resolving the LSND anomaly by neutrino diffraction" (http://arxiv.org/pdf/1109.3105.pdf ) e il secondo "Light Sterile Neutrinos: A White Paper" (http://arxiv.org/pdf/1204.5379.pdf). Il legante di questi articoli è il fenomeno dell’oscillazione dei neutrini e di qualche loro anomalia (tranquilli sto già pensando di inserire qualche ulteriore post per chiarire tutti gli aspetti che trascurerò in questo blog ma per ora mi devo accontentare di instillare la vostra curiosità), fenomeno teorizzato da un fisico italiano Pontercorvo, per interpretare un rompicapo che assillava i fisici, ed in particolare Ray Davis e John N. Bahcall negli anni ’60. Tale rompicapo riguardava il flusso dei neutrini provenienti dal Sole. Innanzitutto la rivelazione di tali neutrini permetteva di confermare il fatto che all’interno del Sole avvenivano dei processi nucleari, e in particolare la fusione di  4 nuclei di idrogeno in uno di elio, che passa per un decadimento beta inverso con la conseguente produzione di neutrini elettronici. Tali modelli teorici erano ben formulati e l’esponente principale era appunto il fisico John Bachall che  insieme all'astrofisico Ray Davis propose l’esperimento Homestake (chiamato così perchè situato nella miniera d’oro di Homestake nel Sud Dakota) per verificare tale ipotesi. Il risultato dell’esperimento fu positivo, i neutrini venivano prodotti nel sole confermando l’esistenza delle reazioni nucleari… ma .. c’è un ma, i neutrini rivelati erano meno della metà di quelli previsti. Si invocarono errori sperimentali o errori del modello teorico (che tuttavia fondava le sue solide basi sulla relazione tra la luminosità solare e il flusso di neutrini) ma tutti controlli successivi mostravano sia che l’esperimento funzionava bene e sia che il modello solare teorico era solido. Per cui si iniziò a pensare in maniera seria all’ipotesi di Pontecorvo: in maniera approssimativa possiamo dire che  i neutrini vengono prodotti come autostati del sapore quindi come neutrini elettronici, muonici o tauonici, ma essi si propagano come autostati dell’energia, se quindi i diversi neutrini hanno diversa massa si viene a creare una interfererenza che conferisce un carattere oscillatorio e permettere di mescolare tra lori i neutrini di diverso sapore. 

Back to my second office

I am far to be an expert on the subject. I cannot really judge the scientific relevance of this talk. I would like to hear some opinions from people that work in the field or at least are closer than me on these topics.

Did you know? Lorentz VS Lorenz

This is embarrassing.... but I'm positive i'm not the only one who didn't know this, so let me share it:

Just a couple of days ago, I discovered that what I used to call "Lorentz" gauge condition

 \[\nabla_\mu A^\mu=0\]

is actually the "Lorenz" condition, named after Ludvig Lorenz and not after Hendrik Lorentz, who "only" gave his name to Lorentz transformations.

                               

                                              Hendrik Lorentz

Ludvig Lorenz

In my partial defense, I can advocate two motivations:

1) Even Google tries to cheat you:

2) One of the textbooks I used to study field theory [Ryder L. H. - Quantum Field Theory] made the same error!

If you are wondering about the Lorenz condition, check this out.

The importance of being hardest

Here we are. The launch of the NuSTAR satellite, initially scheduled for PI day this year (03/14), will happen around June 13th. The reason for this slip was an update of the software in the Pegasus rocket that will launch the satellite.
This "cheap" (a Small Explorer mission, that cost less than 200 million dollars) telescope will be launched in space inside a Pegasus rocket and extend its 10-meter long deployable mast to provide images of the sky in the so-called hard X-rays, meaning X-rays at higher energies than it was possible with the current X-ray imaging satellites.
Is it a sort of a worthless "My satellite is harder than yours" competition stealing money from taxpayers?
'Course not.

From xkcd

Why or how?

I am not sure that we all realized the sutble change that happened around the time we started using mathematics to describe reality. It is very "inter-esting" to suddenly realize that physics is mostly about "how". If the universe works as a clock, with a precise and never-failing rigorous mechanism behind, there is one unambiguous answer to the "how" question.The "why" is very intrincate to answer. Bodies fall, we know how, we can precisely determine the position at all times from initial conditions. To know why they fall...well, I think Feynman does it better than anyone could, watch him in action

 

...and yet, wouldn't we all agree that our insights all occur when trying to understand the "why" part of the question?

Survey finds no hint of dark matter near Solar System

Link to the original

Out of contest

Today's Gerard 't Hooft's paper. The incipit reads:

In modern science, real numbers play such a fundamental role that it is difficult to imagine a world without real numbers. Nevertheless, one may suspect that real numbers are nothing but a human invention. By chance, humanity discovered over 2000 years ago that our world can be understood very accurately if we phraze its laws and its symmetries by manipulating real numbers, not only using addition and multiplication, but also subtraction and division, and later of course also the extremely rich mathematical machinery beyond that, manipulations that do not work so well for integers alone, or even more limited quantities such as Boolean variables.

Now imagine that, in contrast to these appearances, the real world, at its most fundamental level, were not based on real numbers at all. We here consider systems where only the integers describe what happens at a deeper level. Can one understand why our world appears to be based on real numbers?

Isn't this enough to continue reading?

Paper of the Day (gr-qc)

http://arxiv.org/abs/1204.4524

During the weekend I though it could have been a good experiment having a small "journal club" on papers submitted daily on the arXiv (i'll write something on arXiv one of these days). I cannot promise this blog-review will be very regular, but i'll do my best for the gr-qc and hep-th sections. Other contributors are in charge for other sections.

Our idea is to pick selected papers and describe them very briefly and informally. This brings me to the next problem: selecting the papers.

ArXivwise, Monday is usually quite a boring day. Papers appearing on Mondays are those submitted during the weekend and this explains why there are (on average) less papers. Well, not today!

Today the choice was quite hard (and not only because Frank Wilczek's "A Long View of Particle Physics"). Anyway my choice goes to Kent Yagi's paper above, which (or probably also because) is closely related to some projects I'm recently involved to.

Yagi is interested in constraining "alternative theories of gravity", i.e. theories that modify General Relativity (GR) in some regime. Typically, alternative theories differ from GR in  the description of the dynamics of black holes, neutron stars and other compact objects, but they are conceived in such a way that they pass Solar System tests. I'll postpone the motivation to investigate these theories and my personal interest for them to a future post, for the time being let me just say that it is very hard to modify GR in a way that is compatible with current and past experiment.

In brief, the paper above discuss: (i) a way to put very stringent constraints (in fact, much more stringent than those coming from Solar System experiments) to a particular class of theories that modify GR in the strong-curvature regime and (ii) that, despite these strong constraints, these theories "could" explain an unexpected astrophysical result: apparently, the orbital separation of some binary system (i.e. the distance between two compact objects orbiting each other) decreases in time faster than what is predicted in GR. That the distance decreases is well-known: it is due to the emission of gravitational waves (GWs) from the system. The fact that the this decrease is faster than expected, points to some poor understanding of the astrophysical processes involved in the process or to a more copious emission of GWs than what expected in GR. A quite common feature of modified gravities is that they predict a more efficient GW emission but (iii) the results of this paper would be very difficult to obtain by considering a alternative theory other than what Yagi considers. This should come as a surprise, given the ridiculously large number of proposed alternatives but, on the other hand, it makes the results more interesting. 

As the author stresses all over the paper, the cause of this discrepancy is most probably of astrophysical origin. However, it's intriguing to observe that current observations do not rule possible deviations from Einstein's theory at astrophysical level and, most importantly, that these corrections may eventually play a role in explaining the dynamics of compact stars and black holes [i'm sure the astro-contributors won't agree with me here :)]

[Note added in proof: next posts will be shorter!]

Happy Bday, Rita!

Nobel laureate Rita Levi-Montalcini, today 103 years old

here is her Nobel lecture's video

MUST read: The Crisis of Big Science by Steven Weinberg

"Physicists can point to technological spin-offs from high-energy physics, ranging from synchotron radiation to the World Wide Web. For promoting invention, big science in this sense is the technological equivalent of war, and it doesn’t kill anyone. But spin-offs can’t be promised in advance."

Read it all: [The Crisis of Big Science by Steven Weinberg | The New York Review of Books]

Steven Weinberg (on the right) together with Sheldon Glashow and Abdus Salam
receiving the 1979 Nobel Prize for Physics

Alheira

Nunca comer alheira ao almoço, se tiver trabalhar mais tarde

A classic, but always good basis to start with

Uyuni, Bolivia. Taken from here

No excuses: time to start.

Here is where TGR was born in April 2012