146 research outputs found
``Faster than Light'' Photons in Gravitational Fields -- Causality, Anomalies and Horizons
A number of general issues relating to superluminal photon propagation in
gravitational fields are explored. The possibility of superluminal, yet causal,
photon propagation arises because of Equivalence Principle violating
interactions induced by vacuum polarisation in QED in curved spacetime. Two
general theorems are presented: first, a polarisation sum rule which relates
the polarisation averaged velocity shift to the matter energy-momentum tensor
and second, a `horizon theorem' which ensures that the geometric event horizon
for black hole spacetimes remains a true horizon for real photon propagation in
QED. A comparision is made with the equivalent results for electromagnetic
birefringence and possible connections between superluminal photon propagation,
causality and the conformal anomaly are exposed.Comment: 15 pages, Plain Te
Automatically generating Feynman rules for improved lattice field theories
Deriving the Feynman rules for lattice perturbation theory from actions and
operators is complicated, especially when improvement terms are present. This
physically important task is, however, suitable for automation. We describe a
flexible algorithm for generating Feynman rules for a wide range of lattice
field theories including gluons, relativistic fermions and heavy quarks. We
also present an efficient implementation of this in a freely available,
multi-platform programming language (\python), optimised to deal with a wide
class of lattice field theories
A Local Effective Action for Photon-Gravity Interactions
Quantum phenomena such as vacuum polarisation in curved spacetime induce
interactions between photons and gravity with quite striking consequences,
including the violation of the strong equivalence principle and the apparent
prediction of `superluminal' photon propagation. These quantum interactions can
be encoded in an effective action. In this paper, we extend previous results on
the effective action for QED in curved spacetime due to Barvinsky, Vilkovisky
and others and present a new, local effective action valid to all orders in a
derivative expansion, as required for a full analysis of the quantum theory of
high-frequency photon propagation in gravitational fields.Comment: 22 pages, 9 figures, harvmac Te
`Faster than light' photons and rotating black holes
The effective action for QED in curved spacetime includes equivalence
principle violating interactions between the electromagnetic field and the
spacetime curvature. These interactions admit the possibility of superluminal
yet causal photon propagation in gravitational fields. In this paper, we extend
our analysis of photon propagation in gravitational backgrounds to the Kerr
spacetime describing a rotating black hole. The results support two general
theorems -- a polarisation sum rule and a `horizon theorem'. The implications
for the stationary limit surface bounding the ergosphere are also discussed.Comment: Plain TeX, 12 pages, 1 figur
Faster than Light Photons in Gravitational Fields II - Dispersion and Vacuum Polarisation
Vacuum polarisation in QED in a background gravitational field induces
interactions which effectively violate the strong equivalence principle and
affect the propagation of light. In the low frequency limit, Drummond and
Hathrell have shown that this mechanism leads to superluminal photon
velocities. To confront this phenomenon with causality, however, it is
necessary to extend the calculation of the phase velocity \vp(\w) to high
frequencies, since it is \vp(\infty) which determines the characteristics of
the effective wave equation and thus the causal structure. In this paper, we
use a recently constructed expression, valid to all orders in a derivative
expansion, for the effective action of QED in curved spacetime to determine the
frequency dependence of the phase velocity and investigate whether superluminal
velocities indeed persist in the high frequency limit.Comment: 27 pages, 7 figures, TeX with harvma
Leptogenesis and gravity: Baryon asymmetry without decays
A popular class of theories attributes the matter-antimatter asymmetry of the
Universe to CP-violating decays of super-heavy BSM particles in the Early
Universe. Recently, we discovered a new source of leptogenesis in these models,
namely that the same Yukawa phases which provide the CP violation for decays,
combined with curved-spacetime loop effects, lead to an entirely new
gravitational mechanism for generating an asymmetry, driven by the expansion of
the Universe and independent of the departure of the heavy particles from
equilibrium. In this Letter, we build on previous work by analysing the full
Boltzmann equation, exploring the full parameter space of the theory and
studying the time-evolution of the asymmetry. Remarkably, we find regions of
parameter space where decays play no part at all, and where the baryon
asymmetry of the Universe is determined solely by gravitational effects.Comment: Journal version, published in Phys. Lett.
Superluminality and UV Completion
The idea that the existence of a consistent UV completion satisfying the
fundamental axioms of local quantum field theory or string theory may impose
positivity constraints on the couplings of the leading irrelevant operators in
a low-energy effective field theory is critically discussed. Violation of these
constraints implies superluminal propagation, in the sense that the
low-frequency limit of the phase velocity exceeds . It is
explained why causality is related not to but to the
high-frequency limit and how these are related by the
Kramers-Kronig dispersion relation, depending on the sign of the imaginary part
of the refractive index \Ima n(\w) which is normally assumed positive.
Superluminal propagation and its relation to UV completion is investigated in
detail in three theories: QED in a background electromagnetic field, where the
full dispersion relation for n(\w) is evaluated numerically for the first
time and the role of the null energy condition T_{\m\n}k^\m k^\n \ge 0 is
highlighted; QED in a background gravitational field, where examples of
superluminal low-frequency phase velocities arise in violation of the
positivity constraints; and light propagation in coupled laser-atom
\L-systems exhibiting Raman gain lines with \Ima n(\w) < 0. The possibility
that a negative \Ima n(\w) must occur in quantum field theories involving
gravity to avoid causality violation, and the implications for the relation of
IR effective field theories to their UV completion, are carefully analysed.Comment: 42 pages, 14 figure
Comments on "Note on varying speed of light theories"
In a recent note Ellis criticizes varying speed of light theories on the
grounds of a number of foundational issues. His reflections provide us with an
opportunity to clarify some fundamental matters pertaining to these theories
Light propagation in non-trivial QED vacua
Within the framework of effective action QED, we derive the light cone
condition for homogeneous non-trivial QED vacua in the geometric optics
approximation. Our result generalizes the ``unified formula'' suggested by
Latorre, Pascual and Tarrach and allows for the calculation of velocity shifts
and refractive indices for soft photons travelling through these vacua.
Furthermore, we clarify the connection between the light velocity shift and the
scale anomaly. This study motivates the introduction of a so-called effective
action charge that characterizes the velocity modifying properties of the
vacuum. Several applications are given concerning vacuum modifications caused
by, e.g., strong fields, Casimir systems and high temperature.Comment: 13 pages, REVTeX, 3 figures, to appear in Phys. Rev.
Asymptotic solutions to the Gross-Pitaevskii gain equation: Growth of a Bose-Einstein condensate
We give an asymptotic analytic solution for the generic atom-laser system with gain in a D-dimensional trap, and show that this has a non-Thomas-Fermi behavior. The effect is due to Bose-enhanced condensate growth, which creates a local-density maximum and a corresponding outward momentum component. In addition, the solution predicts amplified center-of-mass oscillations, leading to enhanced center-of-mass temperature
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