1,446 research outputs found
Common Causes and The Direction of Causation
Is the common cause principle merely one of a set of useful heuristics for discovering causal relations, or is it rather a piece of heavy duty metaphysics, capable of grounding the direction of causation itself? Since the principle was introduced in Reichenbachâs groundbreaking work The Direction of Time (1956), there have been a series of attempts to pursue the latter programâto take the probabilistic relationships constitutive of the principle of the common cause and use them to ground the direction of causation. These attempts have not all explicitly appealed to the principle as originally formulated; it has also appeared in the guise of independence conditions, counterfactual overdetermination, and, in the causal modelling literature, as the causal markov condition. In this paper, I identify a set of difficulties for grounding the asymmetry of causation on the principle and its descendents. The first difficulty, concerning what I call the vertical placement of causation, consists of a tension between considerations that drive towards the macroscopic scale, and considerations that drive towards the microscopic scaleâthe worry is that these considerations cannot both be comfortably accommodated. The second difficulty consists of a novel potential counterexample to the principle based on the familiar Einstein Podolsky Rosen (EPR) cases in quantum mechanics
Gamma-ray signatures of annihilation to charged leptons in dark matter substructure
Due to their higher concentrations and small internal velocities, Milky Way
subhalos can be at least as important as the smooth halo in accounting for the
GeV positron excess via dark matter annihilation. After showing how this can be
achieved in various scenarios, including in Sommerfeld models, we demonstrate
that, in this case, the diffuse inverse-Compton emission resulting from
electrons and positrons produced in substructure leads to a nearly-isotropic
signal close to the level of the isotropic GeV gamma-ray background seen by
Fermi. Moreover, we show that HESS cosmic-ray electron measurements can be used
to constrain multi-TeV internal bremsstrahlung gamma rays arising from
annihilation to charged leptons.Comment: 8 pages, 4 figures; minor updates to match published versio
An Unexpectedly Swift Rise in the Gamma-ray Burst Rate
The association of long gamma-ray bursts with supernovae naturally suggests
that the cosmic GRB rate should trace the star formation history. Finding
otherwise would provide important clues concerning these rare, curious
phenomena. Using a new estimate of Swift GRB energetics to construct a sample
of 36 luminous GRBs with redshifts in the range z=0-4, we find evidence of
enhanced evolution in the GRB rate, with ~4 times as many GRBs observed at z~4
than expected from star formation measurements. This direct and empirical
demonstration of needed additional evolution is a new result. It is consistent
with theoretical expectations from metallicity effects, but other causes remain
possible, and we consider them systematically.Comment: 4 pages, 4 figures; minor changes to agree with published versio
Spiking Neurons Learning Phase Delays
Time differences between the two ears are an important cue for animals to azimuthally locate a sound source. The first binaural brainstem nucleus, in mammals the medial superior olive, is generally believed to perform the necessary computations. Its cells are sensitive to variations of interaural time differences of about 10 Îźs. The classical explanation of such a neuronal time-difference tuning is based on the physical concept of delay lines. Recent data, however, are inconsistent with a temporal delay and rather favor a phase delay. By means of a biophysical model we show how spike-timing-dependent synaptic learning explains precise interplay of excitation and inhibition and, hence, accounts for a physical realization of a phase delay
Droplet spreading and pinning on heterogeneous substrates
The contact angle of a fluid droplet on an heterogeneous surface is analysed
using the statistical dynamics of the spreading contact line. The statistical
properties of the final droplet radius and contact angle are obtained through
applications of depinning transitions of contact lines with non-local
elasticity and features of pinning-depinning dynamics. Such properties not only
depend on disorder strength and surface details, but also on the droplet volume
and disorder correlation length. Deviations from Wenzel or Cassie/Baxter
behaviour are particularly apparent in the case of small droplet volumes and
small contact angles.Comment: accepted to Phys. Rev. E, 5 figure
The BBaRTS Healthy Teeth Behaviour Change Programme for preventing dental caries in primary school children: study protocol for a cluster randomised controlled trial
Open Access
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the Creative Commons license, and indicate if changes were madeThis trial is supported by an unrestricted grant from GlaxoSmithKlin
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