25,158 research outputs found
Self-Healing Tile Sets
Biology provides the synthetic chemist with a tantalizing and frustrating challenge:
to create complex objects, defined from the molecular scale up to meters,
that construct themselves from elementary components, and perhaps
even reproduce themselves. This is the challenge of bottom-up fabrication.
The most compelling answer to this challenge was formulated in the early
1980s by Ned Seeman, who realized that the information carried by DNA
strands provides a means to program molecular self-assembly, with potential
applications including DNA scaffolds for crystallography [19] or for molecular
electronic circuits [15]. This insight opened the doors to engineering with the
rich set of phenomena available in nucleic acid chemistry [20]
A first experimental test of de Broglie-Bohm theory against standard quantum mechanics
De Broglie - Bohm (dBB) theory is a deterministic theory, built for
reproducing almost all Quantum Mechanics (QM) predictions, where position plays
the role of a hidden variable. It was recently shown that different coincidence
patterns are predicted by QM and dBB when a double slit experiment is realised
under specific conditions and, therefore, an experiment can test the two
theories. In this letter we present the first realisation of such a double slit
experiment by using correlated photons produced in type I Parametric Down
Conversion. Our results confirm QM contradicting dBB predictions
How crystals that sense and respond to their environments could evolve
An enduring mystery in biology is how a physical entity simple enough to have arisen spontaneously could have evolved into the complex life seen on Earth today. Cairns-Smith has proposed that life might have originated in clays which stored genomes consisting of an arrangement of crystal monomers that was replicated during growth. While a clay genome is simple enough to have conceivably arisen spontaneously, it is not obvious how it might have produced more complex forms as a result of evolution. Here, we examine this possibility in the tile assembly model, a generalized model of crystal growth that has been used to study the self-assembly of DNA tiles. We describe hypothetical crystals for which evolution of complex crystal sequences is driven by the scarceness of resources required for growth. We show how, under certain circumstances, crystal growth that performs computation can predict which resources are abundant. In such cases, crystals executing programs that make these predictions most accurately will grow fastest. Since crystals can perform universal computation, the complexity of computation that can be used to optimize growth is unbounded. To the extent that lessons derived from the tile assembly model might be applicable to mineral crystals, our results suggest that resource scarcity could conceivably have provided the evolutionary pressures necessary to produce complex clay genomes that sense and respond to changes in their environment
Rheotaxis facilitates upstream navigation of mammalian sperm cells
A major puzzle in biology is how mammalian sperm determine and maintain the
correct swimming direction during the various phases of the sexual reproduction
process. Whilst chemotaxis is assumed to dominate in the immediate vicinity of
the ovum, it is unclear which biochemical or physical cues guide spermatozoa on
their long journey towards the egg cell. Currently debated mechanisms range
from peristaltic pumping to temperature sensing (thermotaxis) and direct
response to fluid flow variations (rheotaxis), but little is known
quantitatively about their relative importance. Here, we report the first
quantitative experimental study of mammalian sperm rheotaxis. Using
microfluidic devices, we investigate systematically the swimming behavior of
human and bull sperm over the whole range of physiologically relevant shear
rates and viscosities. Our measurements show that the interplay of fluid shear,
steric surface-interactions and chirality of the flagellar beat leads to a
stable upstream spiraling motion of sperm cells, thus providing a generic and
robust rectification mechanism to support mammalian fertilisation. To
rationalise these findings, we identify a minimal mathematical model that is
capable of describing quantitatively the experimental observations. The
combined experimental and theoretical evidence supports the hypothesis that the
shape and beat patterns of mammalian sperm cells have evolved to optimally
exploit rheotaxis for long-distance navigation.Comment: 18 pages, 4 figures, supplementary information available at
eLifesciences.or
Is there a tendency for the rate of profit to fall? Econometric evidence for the U.S. economy, 1948-2007
The law of the tendential fall in the rate of profit has been at the center of theoretical and empirical debates within Marxian political economy ever since the publication of Volume III of Capital. An important limitation of this literature is the absence of a comprehensive econometric analysis of the behaviour of the rate of profit. In this paper, we attempt to fill this lacuna in two ways. First, we investigate the time series properties of the profit rate series. The evidence suggests that the rate of profit behaves like a random walk and exhibits "long waves" interestingly correlated with major epochs of U.S. economic history. In the second part, we test Marx's law of the tendential fall in the rate of profit with a novel econometric model that explicitly accounts for the counter-tendencies. We find evidence of a long-run downward trend in the general profit rate for the US economy for the period 1948-2007. JEL Categories: B51, C22, E11falling rate of profit, Marxian political economy, time series analysis, unit roots.
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A Monte Carlo model checker for probabilistic LTL with numerical constraints
We define the syntax and semantics of a new temporal logic called probabilistic LTL with numerical constraints (PLTLc).
We introduce an efficient model checker for PLTLc properties. The efficiency of the model checker is through approximation
using Monte Carlo sampling of finite paths through the model’s state space (simulation outputs) and parallel model checking
of the paths. Our model checking method can be applied to any model producing quantitative output – continuous or
stochastic, including those with complex dynamics and those with an infinite state space. Furthermore, our offline approach
allows the analysis of observed (real-life) behaviour traces. We find in this paper that PLTLc properties with constraints
over free variables can replace full model checking experiments, resulting in a significant gain in efficiency. This overcomes
one disadvantage of model checking experiments which is that the complexity depends on system granularity and number of
variables, and quickly becomes infeasible. We focus on models of biochemical networks, and specifically in this paper on
intracellular signalling pathways; however our method can be applied to a wide range of biological as well as technical
systems and their models. Our work contributes to the emerging field of synthetic biology by proposing a rigourous approach
for the structured formal engineering of biological systems
A biphotons double slit experiment
In this paper we present a double slit experiment where two undistinguishable
photons produced by type I PDC are sent each to a well defined slit. Data about
the diffraction and interference patterns for coincidences are presented and
discussed. An analysis of these data allows a first test of standard quantum
mechanics against de Broglie-Bohm theory
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