This month clearly has been a black hole month. Not only did we see the first (highly processed) image of a black hole, LIGO detected five potential events in April after starting their new run. What’s potentially exciting is the fact that not only is there a candidate event for a neutron star – neutron star merger (i.a. enabling independent measures of the universe’s expansion rate) but also the first BH-NS merger which apparently is an even more stringent test bed for General Relativity than BH-BH mergers (here’s Nature and physicsworld).
There’s also the new issue of the Cern Courier out with an interesting piece on precision physics in particle physics, this time on a muon decay that would be unobservable in the Standard Model (I love Chad Orzel’s argumentation for this orthography: “capital S, capital M, no matter what my copyeditors try to claim” – heck, yeah!).
It also does a pretty good job at explaining lepton-flavour conservation and why it is intriguing to search for its violation:
Lepton-flavour conservation is a mainstay of every introductory particle-physics course, yet it is merely a so-called accidental symmetry of the Standard Model (SM). Unlike gauge symmetries, it arises because only massless left-handed neutrinos are included in the model. The corresponding mass and interaction terms of the Lagrangian can therefore be simultaneously diagonalised, which means that interactions always conserve lepton flavour. This is not the case in the quark sector, and as a result quark flavour is not conserved in weak interactions. Since lepton flavour is not considered to be a fundamental symmetry, most extensions of the SM predict its violation at a level that could be observed by state-of-the-art experiments.
Finally, there is an absolutely awesome series by John Baez on PhysicsForum. He goes through all the big physics theories and shows how the assumption of a continuum of spacetime leads to mathematical problems and how e.g. the introduction of Quantum Mechanics ameliorates some of these Newtonian problems but leads to others. Although plebs like me have to skip all the maths parts it’s still very informative.