6,935 research outputs found
Collisional Formation and Modeling of Asteroid Families
In the last decade, thanks to the development of sophisticated numerical
codes, major breakthroughs have been achieved in our understanding of the
formation of asteroid families by catastrophic disruption of large parent
bodies. In this review, we describe numerical simulations of asteroid
collisions that reproduced the main properties of families, accounting for both
the fragmentation of an asteroid at the time of impact and the subsequent
gravitational interactions of the generated fragments. The simulations
demonstrate that the catastrophic disruption of bodies larger than a few
hundred meters in diameter leads to the formation of large aggregates due to
gravitational reaccumulation of smaller fragments, which helps explain the
presence of large members within asteroid families. Thus, for the first time,
numerical simulations successfully reproduced the sizes and ejection velocities
of members of representative families. Moreover, the simulations provide
constraints on the family dynamical histories and on the possible internal
structure of family members and their parent bodies.Comment: Chapter to appear in the (University of Arizona Press) Space Science
Series Book: Asteroids I
The Science of Galaxy Formation
Our knowledge of the Universe remains discovery-led: in the absence of
adequate physics-based theory, interpretation of new results requires a
scientific methodology. Commonly, scientific progress in astrophysics is
motivated by the empirical success of the "Copernican Principle", that the
simplest and most objective analysis of observation leads to progress. A
complementary approach tests the prediction of models against observation. In
practise, astrophysics has few real theories, and has little control over what
we can observe. Compromise is unavoidable. Advances in understanding complex
non-linear situations, such as galaxy formation, require that models attempt to
isolate key physical properties, rather than trying to reproduce complexity. A
specific example is discussed, where substantial progress in fundamental
physics could be made with an ambitious approach to modelling: simulating the
spectrum of perturbations on small scales.Comment: paper at IAU256, The Galaxy Disk in Cosmological Context, Copenhagen,
2008 eds J. Andersen, J. Bland-Hawthorn & B. Nordstro
Efficient simulation scheme for a class of quantum optics experiments with non-negative Wigner representation
We provide a scheme for efficient simulation of a broad class of quantum
optics experiments. Our efficient simulation extends the continuous variable
Gottesman-Knill theorem to a large class of non-Gaussian mixed states, thereby
identifying that these non-Gaussian states are not an enabling resource for
exponential quantum speed-up. Our results also provide an operationally
motivated interpretation of negativity as non-classicality. We apply our scheme
to the case of noisy single-photon-added-thermal-states to show that this class
admits states with positive Wigner function but negative P -function that are
not useful resource states for quantum computation.Comment: 14 pages, 1 figur
Reconciling long-term cultural diversity and short-term collective social behavior
An outstanding open problem is whether collective social phenomena occurring
over short timescales can systematically reduce cultural heterogeneity in the
long run, and whether offline and online human interactions contribute
differently to the process. Theoretical models suggest that short-term
collective behavior and long-term cultural diversity are mutually excluding,
since they require very different levels of social influence. The latter
jointly depends on two factors: the topology of the underlying social network
and the overlap between individuals in multidimensional cultural space.
However, while the empirical properties of social networks are well understood,
little is known about the large-scale organization of real societies in
cultural space, so that random input specifications are necessarily used in
models. Here we use a large dataset to perform a high-dimensional analysis of
the scientific beliefs of thousands of Europeans. We find that inter-opinion
correlations determine a nontrivial ultrametric hierarchy of individuals in
cultural space, a result unaccessible to one-dimensional analyses and in
striking contrast with random assumptions. When empirical data are used as
inputs in models, we find that ultrametricity has strong and counterintuitive
effects, especially in the extreme case of long-range online-like interactions
bypassing social ties. On short time-scales, it strongly facilitates a
symmetry-breaking phase transition triggering coordinated social behavior. On
long time-scales, it severely suppresses cultural convergence by restricting it
within disjoint groups. We therefore find that, remarkably, the empirical
distribution of individuals in cultural space appears to optimize the
coexistence of short-term collective behavior and long-term cultural diversity,
which can be realized simultaneously for the same moderate level of mutual
influence
Thermodynamics of firms' growth
The distribution of firms' growth and firms' sizes is a topic under intense
scrutiny. In this paper we show that a thermodynamic model based on the Maximum
Entropy Principle, with dynamical prior information, can be constructed that
adequately describes the dynamics and distribution of firms' growth. Our
theoretical framework is tested against a comprehensive data-base of Spanish
firms, which covers to a very large extent Spain's economic activity with a
total of 1,155,142 firms evolving along a full decade. We show that the
empirical exponent of Pareto's law, a rule often observed in the rank
distribution of large-size firms, is explained by the capacity of the economic
system for creating/destroying firms, and can be used to measure the health of
a capitalist-based economy. Indeed, our model predicts that when the exponent
is larger that 1, creation of firms is favored; when it is smaller that 1,
destruction of firms is favored instead; and when it equals 1 (matching Zipf's
law), the system is in a full macroeconomic equilibrium, entailing "free"
creation and/or destruction of firms. For medium and smaller firm-sizes, the
dynamical regime changes; the whole distribution can no longer be fitted to a
single simple analytic form and numerical prediction is required. Our model
constitutes the basis of a full predictive framework for the economic evolution
of an ensemble of firms that can be potentially used to develop simulations and
test hypothetical scenarios, as economic crisis or the response to specific
policy measures
Range separation: The divide between local structures and field theories
This work presents parallel histories of the development of two modern
theories of condensed matter: the theory of electron structure in quantum
mechanics, and the theory of liquid structure in statistical mechanics.
Comparison shows that key revelations in both are not only remarkably similar,
but even follow along a common thread of controversy that marks progress from
antiquity through to the present. This theme appears as a creative tension
between two competing philosophies, that of short range structure (atomistic
models) on the one hand, and long range structure (continuum or density
functional models) on the other. The timeline and technical content are
designed to build up a set of key relations as guideposts for using density
functional theories together with atomistic simulation.Comment: Expanded version of a 30 minute talk delivered at the 2018 TSRC
workshop on Ions in Solution, to appear in the March, 2019 issue of
Substantia (https://riviste.fupress.net/index.php/subs/index
Evaluating the role of quantitative modeling in language evolution
Models are a flourishing and indispensable area of research in language evolution. Here we highlight critical issues in using and interpreting models, and suggest viable approaches. First, contrasting models can explain the same data and similar modelling techniques can lead to diverging conclusions. This should act as a reminder to use the extreme malleability of modelling parsimoniously when interpreting results. Second, quantitative techniques similar to those used in modelling language evolution have proven themselves inadequate in other disciplines. Cross-disciplinary fertilization is crucial to avoid mistakes which have previously occurred in other areas. Finally, experimental validation is necessary both to sharpen models' hypotheses, and to support their conclusions. Our belief is that models should be interpreted as quantitative demonstrations of logical possibilities, rather than as direct sources of evidence. Only an integration of theoretical principles, quantitative proofs and empirical validation can allow research in the evolution of language to progress
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