3,277 research outputs found
Zitterbewegung is not an observable
It has recently been claimed that Zitterbewegung has been observed. However,
we argue that it is not an observable and that the authors' observations must
be reinterpreted
Note on the derivative of the hyperbolic cotangent
In a letter to Nature (Ford G W and O'Connell R F 1996 Nature 380 113) we
presented a formula for the derivative of the hyperbolic cotangent that differs
from the standard one in the literature by an additional term proportional to
the Dirac delta function. Since our letter was necessarily brief, shortly after
its appearance we prepared a more extensive unpublished note giving a detailed
explanation of our argument. Since this note has been referenced in a recent
article (Estrada R and Fulling S A 2002 J. Phys. A: Math. Gen. 35 3079) we
think it appropriate that it now appear in print. We have made no alteration to
the original note
Human life histories: primate trade-offs, grandmothering socioecology, and the fossil record
Journal ArticleHuman life histories differ from those of other animals in several striking ways. Recently Smith and Tompkins (1995, p. 258) highlighted the combination of "slow" and "fast" features of human lives. Our period of juvenile dependency is unusually long, our age at first reproduction is late, and we have the maximum life span of the terrestrial animals. Yet we wean babies relatively early, and we space births closely. We also have (midlife) menopause. Smith and Tompkins predicted that the evolution of our life cycles would be explained by a combination of developments in life history theory with increasingly sophisticated techniques for extracting information from the fossil record. Their prudent guess was that "no new sunburst theory-in which all human characteristics are drawn from one adaptive shift is - likely" to emerge (1995, p. 274)
A Note on Frame Dragging
The measurement of spin effects in general relativity has recently taken
centre stage with the successfully launched Gravity Probe B experiment coming
toward an end, coupled with recently reported measurements using laser ranging.
Many accounts of these experiments have been in terms of frame-dragging. We
point out that this terminology has given rise to much confusion and that a
better description is in terms of spin-orbit and spin-spin effects. In
particular, we point out that the de Sitter precession (which has been mesured
to a high accuracy) is also a frame-dragging effect and provides an accurate
benchmark measurement of spin-orbit effects which GPB needs to emulate
The PHASES Differential Astrometry Data Archive. I. Measurements and Description
The Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES)
monitored 51 sub-arcsecond binary systems to determine precision binary orbits,
study the geometries of triple and quadruple star systems, and discover
previously unknown faint astrometric companions as small as giant planets.
PHASES measurements made with the Palomar Testbed Interferometer (PTI) from
2002 until PTI ceased normal operations in late 2008 are presented. Infrared
differential photometry of several PHASES targets were measured with Keck
Adaptive Optics and are presented.Comment: 33 pages emulateapj, Accepted to A
Laplace transform of spherical Bessel functions
We provide a simple analytic formula in terms of elementary functions for the
Laplace transform j_{l}(p) of the spherical Bessel function than that appearing
in the literature, and we show that any such integral transform is a polynomial
of order l in the variable p with constant coefficients for the first l-1
powers, and with an inverse tangent function of argument 1/p as the coefficient
of the power l. We apply this formula for the Laplace transform of the memory
function related to the Langevin equation in a one-dimensional Debye model.Comment: 5 pages LATEX, no figures. Accepted 2002, Physica Script
The Quantum Langevin Equation
The macroscopic description of a quantum particle with passive dissipation and moving in an arbitrary external potential is formulated in terms of the generalized Langevin equation. The coupling with the heat bath corresponds to two terms: a mean force characterized by a memory function μ(t) and an operator-valued random force. Explicit expressions are given for the correlation of the random force. The random force is never Markovian. It is shown that μ̃(z), the Fourier transform of the memory function, is a positive real function, analytic in the upper half plane and with Re{μ̃(ω + i0^+)} a positive distribution on the real axis. This form is then derived for the independent-oscillator (IO) model of a heat bath. It is shown that the most general quantum Langevin equation can be realized by this simple model. A critical comparison is made with a number of other models which have appeared in the literature
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