8,261 research outputs found
Comment on: Hawking Radiation from Ultrashort Laser Pulse Filaments
In a recent paper Belgiorno {\em et al} claimed to have observed the analog
of the Hawking effect because of the detection of radiation in a frequency
range in which what they called "phase horizons" existed. They created rapidly
moving pulses of light in a silica glass whose Kerr effect altered the
refractive index to create those horizons. Unfortunately, while the
observations are very interesting, the cause of the radiation is not
understood, and we feel it is not justified to call this a detection of the
Hawking effect in an analog system.Comment: 1 pag
On the origin of the particles in black hole evaporation
We present an analytic derivation of Hawking radiation for an arbitrary
(spatial) dispersion relation as a model for ultra-high energy
deviations from general covariance. It turns out that the Hawking temperature
is proportional to the product of the group and phase
velocities evaluated at the frequency of the outgoing radiation far
away, which suggests that Hawking radiation is basically a low-energy
phenomenon. Nevertheless, a group velocity growing too fast at ultra-short
distances would generate Hawking radiation at ultra-high energies
(``ultra-violet catastrophe'') and hence should not be a realistic model for
the microscopic structure of quantum gravity.Comment: 4 pages RevTe
On the Universality of the Hawking Effect
Addressing the question of whether the Hawking effect depends on degrees of
freedom at ultra-high (e.g., Planckian) energies/momenta, we propose three
rather general conditions on these degrees of freedom under which the Hawking
effect is reproduced to lowest order. As a generalization of Corley's results,
we present a rather general model based on non-linear dispersion relations
satisfying these conditions together with a derivation of the Hawking effect
for that model. However, we also demonstrate counter-examples, which do not
appear to be unphysical or artificial, displaying strong deviations from
Hawking's result. Therefore, whether real black holes emit Hawking radiation
remains an open question and could give non-trivial information about Planckian
physics.
PACS: 04.70.Dy, 04.62.+v, 04.60.-m, 04.20.Cv.Comment: 11 pages RevTeX, 6 figure
Self-subdiffusion in solutions of star-shaped crowders: non-monotonic effects of inter-particle interactions
We examine by extensive computer simulations the self-diffusion of
anisotropic star like particles in crowded two-dimensional solutions. We
investigate the implications of the area coverage fraction of the
crowders and the crowder-crowder adhesion properties on the regime of transient
anomalous diffusion. We systematically compute the mean squared displacement
(MSD) of the particles, their time averaged MSD, as well as the effective
diffusion coefficient. The diffusion appears ergodic in the limit of long
traces, such that the time averaged MSD converges towards the ensemble averaged
MSD and features a small residual amplitude spread of the time averaged MSD
from individual trajectories. At intermediate time scales we quantify the
anomalous diffusion in the system. Also, we show that the translational---but
not rotational---diffusivity of the particles is a non-monotonic function
of the attraction strength between them. Both diffusion coefficients decrease
as with the area fraction occupied by
the crowders. Our results might be applicable to rationalising the experimental
observations of non-Brownian diffusion for a number of standard macromolecular
crowders used in vitro to mimic the cytoplasmic conditions of living cells.Comment: 16 pages, 7 figure
Sensing viruses by mechanical tension of DNA in responsive hydrogels
The rapid worldwide spread of severe viral infections, often involving novel
modifications of viruses, poses major challenges to our health care systems.
This means that tools that can efficiently and specifically diagnose viruses
are much needed. To be relevant for a broad application in local health care
centers, such tools should be relatively cheap and easy to use. Here we discuss
the biophysical potential for the macroscopic detection of viruses based on the
induction of a mechanical stress in a bundle of pre-stretched DNA molecules
upon binding of viruses to the DNA. We show that the affinity of the DNA to the
charged virus surface induces a local melting of the double-helix into two
single-stranded DNA. This process effects a mechanical stress along the DNA
chains leading to an overall contraction of the DNA. Our results suggest that
when such DNA bundles are incorporated in a supporting matrix such as a
responsive hydrogel, the presence of viruses may indeed lead to a significant,
macroscopic mechanical deformation of the matrix. We discuss the biophysical
basis for this effect and characterize the physical properties of the
associated DNA melting transition. In particular, we reveal several scaling
relations between the relevant physical parameters of the system. We promote
this DNA-based assay for efficient and specific virus screening.Comment: 11 pages, 7 figures, supplementary material included in the source
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