String theory relies on spacetime supersymmetry to guarantee the existence of stable vacua. In this review, we survey two features of non-supersymmetric strings that challenge both aspects: appearance of tachyons and worldsheet tadpoles. We describe how tachyons arise, how to characterize their presence in closed strings and in their orientifold projections, and how off-shell approaches can be used to tackle them. We then turn to tachyon-free, non-supersymmetric strings. After introducing the simplest ten-dimensional models, we address the additional issues raised by tadpoles and the spacetime consequences of their cancelation. Finally, we discuss recent attempts to explore the non-supersymmetric string landscape.
We all know that the first laser device was realized by Theodore Maiman at Hughes Labs in 1960. Less known is that the very first computer simulations of the relaxation oscillations displayed by Maiman’s laser were also performed in 1960 on a digital IBM 704 computer. The reason is that lasers and almost all photonic devices are described by nonlinear equations that are more often than not impossible to be solved analytically, i.e., on a piece of paper. Since then the development and applications of lasers and photonic devices have progressed hand in hand with computer simulations and numerical programming. In this review, we introduce and numerically solve the model equations for a variety of devices, lasers, lasers with modulated parameters, lasers with injection, Kerr resonators, saturable absorbers, and optical parametric oscillators. Using computer simulations, we demonstrate stability and instability of nonlinear solutions in these photonic devices via pitchfork, saddle-node, Hopf and Turing bifurcations; bistability, nonlinear oscillations, deterministic chaos, Turing patterns, conservative solitons; bright, dark and grey cavity solitons; frequency combs, spatial disorder, spatio-temporal chaos, defect-mediated turbulence and even rogue waves. There has been a one-to-one correspondence between computer simulations of all these nonlinear features and laboratory experiments with applications in ultrafast optical communications, optical memories, neural networks, frequency standards, optical clocks, future GPS, astronomy, and quantum technologies. All of this has been made possible by ’novel insights into spatio-temporal dynamics of lasers, nonlinear and quantum optical systems, achieved through the development and application of powerful techniques for small-scale computing’ (2011 Occhialini Medal and Prize of the Institute of Physics and Societa’ Italiana di Fisica).
It is often said that “granular matter is ubiquitous”. Many natural components and human products look and behave like grains: stones, debris, soils, on the one hand; food, pharmaceuticals, building materials, etc., on the other. However, the physics involved is still poorly understood due to its inherent difficulties. In fact, granular materials are an example of frictional, dissipative, nonlinear, out-of-equilibrium systems. One consequence is that they exhibit, under various circumstances, large and irregular fluctuations, finite size effects, and poor reproducibility (as everyone knows from trying to slowly pour sugar or coffee powder). This article summarizes some experimental results on the response of horizontal grain beds subjected to low rate shear stress. In this case, the response is often intermittent and irregular, the so-called stick–slip regime, and can only be described statistically. Small-scale experiments are the best way to collect the necessary large amount of data and, despite the difference in scale, can provide the basis for a better understanding of larger scale phenomena such as avalanches, landslides and earthquakes.
Silicon Photomultipliers (SiPMs) have emerged as leading photon detectors in experimental physics since their introduction in the late 1990s. With performance characteristics superior to those of traditional photodetectors, SiPMs exhibit up to 60% photon detection efficiency, rapid signal rise times, and resistance to magnetic fields. Their solid-state construction enables mass production, compactness, and high spatial resolution, facilitating their integration into a wide range of experimental setups. Although susceptible to radiation damage, mitigation strategies are being studied to allow their reliable operation even in environments with elevated radiation levels. SiPMs excel in detecting low levels of light, making them well suited for applications involving scintillation and Cherenkov light. Their ability to operate effectively at cryogenic temperatures allows the construction of a new class of multi-tons rare event search experiments such as Darkside-20k. Insensitivity to the magnetic field and mitigation of the radiation damages are making SiPMs well-suited to be used in accelerator driven physics such as Cherenkov light detectors for Particle IDentification (PID) in the future Electron Ion Collider (EIC).
Many cosmological observables derive from primordial vacuum fluctuations evolved to late times. These observables represent statistical draws from some underlying quantum or statistical field theoretic framework where infinities arise and require regularization. After subtraction, renormalization conditions must be imposed by measurements at some scale, mindful of scheme and background dependence. We review this process on backgrounds that transition from finite duration inflation to radiation domination, and show how in spite of the ubiquity of scaleless integrals, ultraviolet (UV) divergences can still be meaningfully extracted from quantities that nominally vanish when dimensionally regularized. In this way, one can contextualize calculations with hard cutoffs, distinguishing between UV and infrared (IR) scales corresponding to the beginning and end of inflation from UV and IR scales corresponding to the unknown completion of the theory and its observables. This distinction has significance as observable quantities cannot depend on the latter, although they will certainly depend on the former. One can also explicitly show the scheme independence of the coefficients of UV divergent logarithms. Furthermore, certain IR divergences are shown to be an artifact of the de Sitter limit and are cured for finite duration inflation. For gravitational wave observables, we stress the need to regularize stress tensors that do not presume a prior scale separation in their definition (as with the standard Isaacson form), deriving an improved stress tensor fit to purpose. We conclude by highlighting the inextricable connection between inferring N_eff bounds from vacuum tensor perturbations and the process of background renormalization.
