18,452 research outputs found
Causality of fluid dynamics for high-energy nuclear collisions
Dissipative relativistic fluid dynamics is not always causal and can favor
superluminal signal propagation under certain circumstances. On the other hand,
high-energy nuclear collisions have a microscopic description in terms of QCD
and are expected to follow the causality principle of special relativity. We
discuss under which conditions the fluid evolutions for a radial expansion are
hyperbolic and how the properties of the solutions are encoded in the
associated characteristic curves. The expansion dynamics is causal in
relativistic sense if the characteristic velocities are smaller than the speed
of light. We obtain a concrete inequality from this constraint and discuss how
it can be violated for certain initial conditions. We argue that causality
poses a bound to the applicability of relativistic fluid dynamics. }Comment: 23 pages, 13 figures; Added references, corrected typos, added
discussion as section 2, results unchange
Inconsistencies of Massive Charged Gravitating Higher Spins
We examine the causality and degrees of freedom (DoF) problems encountered by
charged, gravitating, massive higher spin fields. For spin s=3/2, making the
metric dynamical yields improved causality bounds. These involve only the mass,
the product eM_P of the charge and Planck mass and the cosmological constant
\Lambda. The bounds are themselves related to a gauge invariance of the
timelike component of the field equation at the onset of acausality. While
propagation is causal in arbitrary E/M backgrounds, the allowed mass ranges of
parameters are of Planck order. Generically, interacting spins s>3/2 are
subject to DoF violations as well as to acausality; the former must be overcome
before analysis of the latter can even begin. Here we review both difficulties
for charged s=2 and show that while a g-factor of 1/2 solves the DoF problem,
acausality persists for any g. Separately we establish that no s=2 theory --DoF
preserving or otherwise -- can be tree unitary.Comment: 25 pages, late
Loop quantum gravity corrections to gravitational wave dispersion
Cosmological tensor perturbations equations are derived for Hamiltonian
cosmology based on Ashtekar's formulation of general relativity, including
typical quantum gravity effects in the Hamiltonian constraint as they are
expected from loop quantum gravity. This translates to corrections of the
dispersion relation for gravitational waves. The main application here is the
preservation of causality which is shown to be realized due to the absence of
anomalies in the effective constraint algebra used.Comment: 27 page
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