47 research outputs found
Testing gravity to second post-Newtonian order: a field-theory approach
A new, field-theory-based framework for discussing and interpreting tests of
gravity, notably at the second post-Newtonian (2PN) level, is introduced.
Contrary to previous frameworks which attempted at parametrizing any
conceivable deviation from general relativity, we focus on the best motivated
class of models, in which gravity is mediated by a tensor field together with
one or several scalar fields. The 2PN approximation of these
"tensor-multi-scalar" theories is obtained thanks to a diagrammatic expansion
which allows us to compute the Lagrangian describing the motion of N bodies. In
contrast with previous studies which had to introduce many phenomenological
parameters, we find that the 2PN deviations from general relativity can be
fully described by only two new 2PN parameters, epsilon and zeta, beyond the
usual (Eddington) 1PN parameters beta and gamma. It follows from the basic
tenets of field theory, notably the absence of negative-energy excitations,
that (beta-1), epsilon and zeta (as well as any new parameter entering higher
post-Newtonian orders) must tend to zero with (gamma-1). It is also found that
epsilon and zeta do not enter the 2PN equations of motion of light. Therefore,
light-deflection or time-delay experiments cannot probe any theoretically
motivated 2PN deviation from general relativity, but they can give a clean
access to (gamma-1), which is of greatest significance as it measures the basic
coupling strength of matter to the scalar fields. Because of the importance of
self-gravity effects in neutron stars, binary-pulsar experiments are found to
constitute a unique testing ground for the 2PN structure of gravity. A
simplified analysis of four binary pulsars already leads to significant
constraints: |epsilon| < 7x10^-2, |zeta| < 6x10^-3.Comment: 63 pages, 11 figures.ps.tar.gz.uu, REVTeX 3.
Perturbative Approach to Higher Derivative Theories with Fermions
We extend the perturbative approach developed in an earlier work to deal with
Lagrangians which have arbitrary higher order time derivative terms for both
bosons and fermions. This approach enables us to find an effective Lagrangian
with only first time derivatives order by order in the coupling constant. As in
the pure bosonic case, to the first order, the quantized Hamiltonian is bounded
from below whenever the potential is. We show in the example of a single
complex fermion that higher derivative interactions result in an effective mass
and change of vacuum for the low energy modes. The supersymmetric
noncommutative Wess-Zumino model is considered as another example. We also
comment on the higher derivative terms in Witten's string field theory and the
effectiveness of level truncation.Comment: Latex, 21 pages, minor modification, ref. adde
Stochastic thermodynamics of holonomic systems
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