The size polydispersity of carbon nanotubes (CNTs) and their dispersion in the matrix are factors that strongly influence the conductivity characteristics of CNT-polymer nanocomposites. Our experiments on polydisperse CNT-SU8 materials and the theoretical modelling hint at a simple, yet comprehensive, understanding of these factors and of the role they have in the conductivity behaviour of the composite.
Sometimes, interactions between particles make things simpler. Here, I show that the percolating network of hard spherical particles with a short connectivity range has a dendritic, tree-like structure, which allows for a closed form solution for the percolation threshold. I derive an analytic expression of the percolation threshold which becomes increasingly accurate as the connectivity range diminishes. In principle, the tree-ansatz approach could be extended to describe percolation also in systems of anisotropic hard objects like, e.g., rod and platelet particles.
Area Coverage of Expanding E.T. Signals in the Galaxy: SETI and Drake's N
Claudio Grimaldi, Geoffrey W. Marcy, Nathalien K. Tellis, and Frank Drake
The famous Drake equation estimates the number N of currently emitting civilizations from a product of probabilities that events, which are necessary for the development of communicative life in the galaxy, occur. Here we show that Drake's N coincides with the mean number of isotropic signals crossing Earth, at any moment. Our result implies therefore that N is, at least in principle, a quantifiable quantity, whose value could be assessed probabilistically by SETI experiments.
Welcome to my site!
I am currently a guest scientist at the Laboratory of Statistical Biophysics (LBS), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland.
A short CV can be found here. The list of publications is here or here.
Ongoing and future initiatives in the search for extraterrestrial intelligence (SETI) will explore the Galaxy on an unprecedented scale to find evidence of communicating civilizations beyond Earth. Here, we construct a Bayesian formulation of SETI to infer the posterior probability of the mean number of radio signals crossing Earth, given a positive or a null outcome of all-sky searches for nonnatural radio emissions. We show that not detecting signals within ∼40 kly from Earth is compatible with the absence in the entire Galaxy of detectable emitters of a wide range of radiated power. The discovery of even a single emission within ∼1 kly implies instead that over 100 signals typically cross our planet from the Milky Way.
Electrical transport in onion-like carbon-PMMA nanocompsites
C. Grimaldi, E. Kecsenovity, M. Majidian, V. L. Kuznetsov, A. Magrez, and L. Forro
Polymethyl-methacrylate (PMMA) filled with onion-like carbon (OLC)nanoparticles show electrical conductivities even higher than that of nanotube or graphene polymer composites at similar loadings. at low-temperatures, the conductivity follows a Coulomb gap regime at ambent pressure and an activated behavior at 2 GPa. We interpreted these results in terms of a non-trivial interplay between the morphology af the OLC aggregates and the electron hopping processes between OLC nanoparticles
Quantifying the information impact of future searches for exoplanetary biosignatures
The search for life on extrasolar worlds by way of spectro- scopic biosignature detection is among the most compelling scientific endeavors of the next decades. This article explores the implications of either discovering or ruling out the pres- ence of detectable biosignatures on planets within a few tens of light years from Earth, a distance within reach of future searches. Using a Bayesian methodology, we show that not detecting biosignatures in such sample volume would bring no added information about the galactic population of life-hosting exoplanets. Conversely, if life arose indepen- dently on other planets, even a single detection would imply exobiospheres to be more abundant than pulsars. Putative interstellar transfer of life through the panspermia mechanism may, however, significantly lower this estimate.
Probabilistic arguments about the existence of technological life beyond Earth traditionally refer to the Drake equation to draw possible estimates of the number of technologically advanced civilizations releasing, either intentionally or not, electromagnetic emissions in the Milky Way. Here, I introduce other indicators than Drake’s number ND to develop a demography of artificial emissions populating the Galaxy and provide a comprehensive overview of the values these new quantities can achieve as functions of the emission birthrates, longevities, and directionality
We predict that in three dimensional (3D) systems hosting a giant spin-orbit Rashba coupling, the dc electrical conductivity displays a strong anisotropic renormalization due exclusively to the Rashba interaction. We show that the electron velocity components orthogonal to the Rashba field are strongly renormalized, while the component parallel to the Rashba vector remains unaffected. Measurements of the conductivity anisotropy in bulk Rashba metals may therefore give a direct experimental assessment of the spin-orbit strength.
Feasibility of detecting interstellar panspermia in astrophysical environments
Claudio Grimaldi, Manasvi Lingam, and Amedeo Balbi
The proposition that life can spread from one planetary system to another (interstellar panspermia) has a long history, but this hypothesis is difficult to test through observations. We develop a mathematical model that takes parameters such as the microbial survival lifetime, the stellar velocity dispersion, and the dispersion of ejecta into account in order to assess the prospects for detecting interstellar panspermia. We show that the correlations between pairs of life-bearing planetary systems (embodied in the pair-distribution function from statistics) may serve as an effective diagnostic of interstellar panspermia, provided that the velocity dispersion of ejecta is greater than the stellar dispersion. We provide heuristic estimates of the model parameters for various astrophysical environments and conclude that open clusters and globular clusters appear to represent the best targets for assessing the viability of interstellar panspermia.
Signal coverage approach to the detection probability of hypothetical extraterrestrial emitters in the Milky Way
The lack of evidence for the existence of extraterrestrial life, even the simplest forms of microscopic life, makes it difficult to decide whether the search for extraterrestrial intelligence (SETI) is more a highrisk, high-payoff endeavor than a futile attempt.
The main unknown factor in SETI is the likelihood of detecting electromagnetic signals from possible galactic civilizations. Here, I derive the detection probability in terms of the probability that the Earth intersects a region of space covered by hypothetical extraterrestrial signals, without referring to particular hypothesis about the existence and the number of extraterrestrial emitters. I show that a universal bound sets an upper limit for detecting signals from hypothetical extraterrestrial civilizations in the galaxy. A surprising conclusion is that even if we assume that the Earth has a probability of 50% of being within a region covered by the signals, the mean number of potentially detectable emitters is less than one.
Inferring the rate of technosignatures from 60 yr of nondetection
For about the last 60 years the search for extraterrestrial intelligence has been monitoring the sky for evidence of remotely detectable technological life beyond Earth, with no positive results to date. While the lack of detection can be attributed to the highly incomplete sampling of the search space, technological emissions may be actually rare enough that we are living in a time when none cross the Earth. Here we derive the likelihood of the Earth not being crossed by signals for at least the last 60 years to infer upper bounds on their rate of emission. We find less than about one to five emissions generated per century with 95% credible level. This implies optimistic waiting times until the next crossing event of no less than 60-1800 years with a 50% probability. Our results provide a benchmark for assessing the lack of detection and may serve as a basis to form optimal strategies for the search for extraterrestrial intelligence.