24 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.
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The Effective Field Theory Approach to Fluid Dynamics
In this thesis we initiate a systematic study of fluid dynamics using the effective field theory (EFT) program. We consider the canonical quantization of an ordinary fluid in an attempt to discover if there is some kind of quantum mechanical inconsistency with ordinary fluids at zero temperature. The system exhibits a number of peculiarities associated with the vortex degrees of freedom. We also study the dynamics of a nearly incompressible fluid via (classical) effective field theory. In the kinematical regime corresponding to near incompressibility (small fluid velocities and accelerations), compressional modes are, by definition, difficult to excite, and can be dealt with perturbatively. We systematically outline the corresponding perturbative expansion, which can be thought of as an expansion in the ratio of fluid velocity and speed of sound. This perturbation theory allows us to compute many interesting quantities associated with sound-flow interactions. Additionally, we also improve on the so-called vortex filament model, by providing a local field theory describing the dynamics of vortex-line systems and their interaction with sound, to all orders in perturbation theory. Next, we develop a cosmological model where primordial inflation is driven by a 'solid'. The low energy EFT describing such a system is just a less symmetric version of the action of a fluid---it lacks the volume preserving diffeomorphism. The symmetry breaking pattern of this system differs drastically from that of standard inflationary models: time translations are unbroken. This prevents our model from fitting into the standard effective field theory description of adiabatic perturbations, with crucial consequences for the dynamics of cosmological perturbations. And finally, we introduce dissipative effects in the effective field theory of hydrodynamics. We do this in a model-independent fashion by coupling the long-distance degrees of freedom explicitly kept in the effective field theory to a generic sector that "lives in the fluid'', which corresponds physically to the microscopic constituents of the fluid. At linear order in perturbations, the symmetries, the derivative expansion, and the assumption that this microscopic sector is thermalized, allow us to characterize the leading dissipative effects at low frequencies via three parameters only, which correspond to bulk viscosity, shear viscosity, and---in the presence of a conserved charge---heat conduction. Using our methods we re-derive the Kubo relations for these transport coefficients
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
General coordinate invariance in quantum many-body systems
We extend the notion of general coordinate invariance to many-body, not
necessarily relativistic, systems. As an application, we investigate
nonrelativistic general covariance in Galilei-invariant systems. The peculiar
transformation rules for the background metric and gauge fields, first
introduced by Son and Wingate in 2005 and refined in subsequent works, follow
naturally from our framework. Our approach makes it clear that Galilei or
Poincare symmetry is by no means a necessary prerequisite for making the theory
invariant under coordinate diffeomorphisms. General covariance merely expresses
the freedom to choose spacetime coordinates at will, whereas the true, physical
symmetries of the system can be separately implemented as "internal" symmetries
within the vielbein formalism. A systematic way to implement such symmetries is
provided by the coset construction. We illustrate this point by applying our
formalism to nonrelativistic s-wave superfluids.Comment: 14 pages; v2: minor update with additional references and
acknowledgments, version to appear in Phys. Rev.
An effective formalism for testing extensions to General Relativity with gravitational waves
The recent direct observation of gravitational waves (GW) from merging black
holes opens up the possibility of exploring the theory of gravity in the strong
regime at an unprecedented level. It is therefore interesting to explore which
extensions to General Relativity (GR) could be detected. We construct an
Effective Field Theory (EFT) satisfying the following requirements. It is
testable with GW observations; it is consistent with other experiments,
including short distance tests of GR; it agrees with widely accepted principles
of physics, such as locality, causality and unitarity; and it does not involve
new light degrees of freedom. The most general theory satisfying these
requirements corresponds to adding to the GR Lagrangian operators constructed
out of powers of the Riemann tensor, suppressed by a scale comparable to the
curvature of the observed merging binaries. The presence of these operators
modifies the gravitational potential between the compact objects, as well as
their effective mass and current quadrupoles, ultimately correcting the
waveform of the emitted GW.Comment: v1: 43+16 pages, 11 figures, 2 tables; v2: minor corrections; v3:
minor corrections, JHEP published versio
(Re-)Inventing the Relativistic Wheel: Gravity, Cosets, and Spinning Objects
Space-time symmetries are a crucial ingredient of any theoretical model in
physics. Unlike internal symmetries, which may or may not be gauged and/or
spontaneously broken, space-time symmetries do not admit any ambiguity: they
are gauged by gravity, and any conceivable physical system (other than the
vacuum) is bound to break at least some of them. Motivated by this observation,
we study how to couple gravity with the Goldstone fields that non-linearly
realize spontaneously broken space-time symmetries. This can be done in
complete generality by weakly gauging the Poincare symmetry group in the
context of the coset construction. To illustrate the power of this method, we
consider three kinds of physical systems coupled to gravity: superfluids,
relativistic membranes embedded in a higher dimensional space, and rotating
point-like objects. This last system is of particular importance as it can be
used to model spinning astrophysical objects like neutron stars and black
holes. Our approach provides a systematic and unambiguous parametrization of
the degrees of freedom of these systems.Comment: 30 page