402 research outputs found
Teaching statistical physics by thinking about models and algorithms
We discuss several ways of illustrating fundamental concepts in statistical
and thermal physics by considering various models and algorithms. We emphasize
the importance of replacing students' incomplete mental images by models that
are physically accurate. In some cases it is sufficient to discuss the results
of an algorithm or the behavior of a model rather than having students write a
program.Comment: 21 pages, 4 figures, submitted to the American Journal of Physic
Charged Condensation
We consider Bose-Einstein condensation of massive electrically charged
scalars in a uniform background of charged fermions. We focus on the case when
the scalar condensate screens the background charge, while the net charge of
the system resides on its boundary surface. A distinctive signature of this
substance is that the photon acquires a Lorentz-violating mass in the bulk of
the condensate. Due to this mass, the transverse and longitudinal gauge modes
propagate with different group velocities. We give qualitative arguments that
at high enough densities and low temperatures a charged system of electrons and
helium-4 nuclei, if held together by laboratory devices or by force of gravity,
can form such a substance. We briefly discuss possible manifestations of the
charged condensate in compact astrophysical objects.Comment: 1+11 pages; v2: comments and reference added, PLB versio
Trauma und Integration - eine gesellschaftliche Herausforderung
Wie sich die deutsche Gesellschaft zu traumatisierten Geflüchteten verhält und welche Möglichkeiten der Verarbeitung sie ihnen bietet, ist extrem wichtig – sowohl für die individuelle Situation der Ankommenden als auch für das gesellschaftliche Zusammenleben. Die Konzepte des Kollektiven Traumas und der transgenerationalen Traumaweitergabe zeigen: Traumatisierungen sind nicht nur psychische Verletzungen auf individueller Ebene, sondern ein kollektiv relevantes Phänomen. Daher muss bei der gesellschaftlichen Debatte über Integration das Thema Trauma mitgedacht werden. Dabei geht es nicht nur um Therapiemöglichkeiten, sondern auch um politische und soziale Faktoren
Modelling the Pioneer anomaly as modified inertia
This paper proposes an explanation for the Pioneer anomaly: an unexplained
Sunward acceleration of 8.74 +/- 1.33 x 10^-10 m s^-2 seen in the behaviour of
the Pioneer probes. Two hypotheses are made: (1) Inertia is a reaction to Unruh
radiation and (2) this reaction is weaker for low accelerations because some
wavelengths in the Unruh spectrum do not fit within a limiting scale (twice the
Hubble distance) and are disallowed: a process similar to the Casimir effect.
When these ideas are used to model the Pioneer crafts' trajectories there is a
slight reduction in their inertial mass, causing an anomalous Sunward
acceleration of 6.9 +/- 3.5 x 10^-10 m s^-2 which agrees within error bars with
the observed Pioneer anomaly beyond 10 AU from the Sun. This new scheme is
appealingly simple and does not require adjustable parameters. However, it also
predicts an anomaly within 10 AU of the Sun, which has not been observed.
Various observational tests for the idea are proposed.Comment: 15 pages, 2 bw figures, accepted by MNRAS 19th December 200
Quantum mechanical description of Stern-Gerlach experiments
The motion of neutral particles with magnetic moments in an inhomogeneous
magnetic field is described in a quantum mechanical framework. The validity of
the semi-classical approximations which are generally used to describe these
phenomena is discussed. Approximate expressions for the evolution operator are
derived and compared to the exact calculations. Focusing and spin-flip
phenomena are predicted. The reliability of Stern-Gerlach experiments to
measure spin projections is assessed in this framework.Comment: 12 pages, 7 eps figures included, revtex, submitted to PR
Mg impurity in helium droplets
Within the diffusion Monte Carlo approach, we have determined the structure of isotopically pure and mixed helium droplets doped with one magnesium atom. For pure (4)He clusters, our results confirm those of Mella et al. [J. Chem. Phys. 123, 054328 (2005)] that the impurity experiences a transition from a surface to a bulk location as the number of helium atoms in the droplet increases. Contrarily, for pure (3)He clusters Mg resides in the bulk of the droplet due to the smaller surface tension of this isotope. Results for mixed droplets are presented. We have also obtained the absorption spectrum of Mg around the 3s3p (1)P(1) ← 3s(2) (1)S(0) transition
Derivation of the Planck Spectrum for Relativistic Classical Scalar Radiation from Thermal Equilibrium in an Accelerating Frame
The Planck spectrum of thermal scalar radiation is derived suggestively
within classical physics by the use of an accelerating coordinate frame. The
derivation has an analogue in Boltzmann's derivation of the Maxwell velocity
distribution for thermal particle velocities by considering the thermal
equilibrium of noninteracting particles in a uniform gravitational field. For
the case of radiation, the gravitational field is provided by the acceleration
of a Rindler frame through Minkowski spacetime. Classical zero-point radiation
and relativistic physics enter in an essential way in the derivation which is
based upon the behavior of free radiation fields and the assumption that the
field correlation functions contain but a single correlation time in thermal
equilibrium. The work has connections with the thermal effects of acceleration
found in relativistic quantum field theory.Comment: 23 page
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