2,189 research outputs found
A Modern Approach to Superradiance
In this paper, we provide a simple and modern discussion of rotational
superradiance based on quantum field theory. We work with an effective theory
valid at scales much larger than the size of the spinning object responsible
for superradiance. Within this framework, the probability of absorption by an
object at rest completely determines the superradiant amplification rate when
that same object is spinning. We first discuss in detail superradiant
scattering of spin 0 particles with orbital angular momentum , and then
extend our analysis to higher values of orbital angular momentum and spin.
Along the way, we provide a simple derivation of vacuum friction---a "quantum
torque" acting on spinning objects in empty space. Our results apply not only
to black holes but to arbitrary spinning objects. We also discuss superradiant
instability due to formation of bound states and, as an illustration, we
calculate the instability rate for bound states with massive spin 1
particles. For a black hole with mass and angular velocity , we
find when the particle's Compton wavelength
is much greater than the size of the spinning object. This rate is
parametrically much larger than the instability rate for spin 0 particles,
which scales like . This enhanced instability rate can be
used to constrain the existence of ultralight particles beyond the Standard
Model.Comment: 39 pages (v2 contains many added details and corrects an error in v1.
In particular, the instability rates for leading vector bound states are
computed exactly in the large Compton wavelength limit.
Kaplan's Sloppy Thinker and the Demonstrative Origine of Indeicals
In this paper we give some suggestions from etymology on the contrast between Kaplanâs direct reference theory and a neo-Fregean view on indexicals. After a short summary of the philosophical debate on indexicals (§1), we use some remarks about the hidden presence of a demonstrative root in all indexicals to derive some provisional doubts concerning Kaplanâs criticism of what he calls âsloppy thinkerâ (§2). To support those doubts, we will summarise some etymological data on the derivation of the so-called âpure indexicalsâ from an original demonstrative root (§ 3). The aim of the paper is to consider etymological data as providing evidence for alternative theories of language and fostering new directions in linguistic and philosophical research on specific topics
UV completion without symmetry restoration
We show that it is not possible to UV-complete certain low-energy effective
theories with spontaneously broken space-time symmetries by embedding them into
linear sigma models, that is, by adding "radial" modes and restoring the broken
symmetries. When such a UV completion is not possible, one can still raise the
cutoff up to arbitrarily higher energies by adding fields that transform
non-linearly under the broken symmetries, that is, new Goldstone bosons.
However, this (partial) UV completion does not necessarily restore any of the
broken symmetries. We illustrate this point by considering a concrete example
in which a combination of space-time and internal symmetries is broken down to
a diagonal subgroup. Along the way, we clarify a recently proposed
interpretation of inverse Higgs constraints as gauge-fixing conditions.Comment: 6 page
Effective string theory for vortex lines in fluids and superfluids
We discuss the effective string theory of vortex lines in ordinary fluids and
low-temperature superfluids, by describing the bulk fluid flow in terms of a
two-form field to which vortex lines can couple. We derive the most general
low-energy effective Lagrangian that is compatible with (spontaneously broken)
Poincare invariance and worldsheet reparameterization invariance. This
generalizes the effective action developed by Lund and Regge and by Endlich and
Nicolis. By applying standard field-theoretical techniques, we show that
certain low-energy coupling constants -- most notably the string tension --
exhibit RG running already at the classical level. We discuss applications of
our techniques to the study of Kelvin waves, vortex rings, and the coupling to
bulk sound modes.Comment: 62 pages, 6 figure
Radiation of scalar modes and the classical double copy
The double copy procedure relates gauge and gravity theories through
color-kinematics replacements and holds for both scattering amplitudes and in
classical contexts. Moreover, it has been shown that there is a web of theories
whose scattering amplitudes are related through operations that exchange color
and kinematic factors. In this paper, we generalize and extend this procedure
by showing that the classical perturbative double copy of pions corresponds to
special Galileons. We consider point-particles coupled to the relevant scalar
fields, and find the leading and next to leading order radiation amplitudes. By
considering couplings motivated by those that would arise from extracting the
longitudinal modes of the gauge and gravity theories, we are able to map the
non-linear sigma model radiation to that of the special Galileon. We also
construct the single copy by mapping the bi-adjoint scalar radiation to the
non-linear sigma model radiation through generalized color-kinematics
replacements.Comment: 30 pages, 5 figure
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