16,252 research outputs found
Open Questions in Classical Gravity
We discuss some outstanding open questions regarding the validity and
uniqueness of the standard second order Newton-Einstein classical gravitational
theory. On the observational side we discuss the degree to which the realm of
validity of Newton's Law of Gravity can actually be extended to distances much
larger than the solar system distance scales on which the law was originally
established. On the theoretical side we identify some commonly accepted but
actually still open to question assumptions which go into the formulating of
the standard second order Einstein theory in the first place. In particular, we
show that while the familiar second order Poisson gravitational equation (and
accordingly its second order covariant Einstein generalization) may be
sufficient to yield Newton's Law of Gravity they are not in fact necessary. The
standard theory thus still awaits the identification of some principle which
would then make it necessary too. We show that current observational
information does not exclusively mandate the standard theory, and that the
conformal invariant fourth order theory of gravity considered recently by
Mannheim and Kazanas is also able to meet the constraints of data, and in fact
to do so without the need for any so far unobserved non-luminous or dark
matter.Comment: UCONN-93-1, plain TeX format, 22 pages (plus 7 figures - send
requests to [email protected]). To appear in a special issue of
Foundations of Physics honoring Professor Fritz Rohrlich on the occasion of
his retirement, L. P. Horwitz and A. van der Merwe Editors, Plenum Publishing
Company, N.Y., Fall 199
Coexistence of Quantum Theory and Special Relativity in signaling scenarios
The coexistence between Quantum Mechanics and Special Relativity is usually
formulated in terms of the no-signaling condition. Several authors have even
suggested that this condition should be included between the basic postulates
of Quantum Theory. However, there are several scenarios where signaling is, in
principle, possible: based on previous results and the analysis of the relation
between unitarity and signaling we present an example of a two-particle
interferometric arrangement for which the dynamics is, in principle, compatible
with superluminal transmission of information. This type of non-locality is not
in the line of Bell's theorem, but closer in spirit to the one-particle
acausality studied by Hegerfeldt and others. We analyze in this paper the
meaning of this non-locality and how to preserve the coexistence of the two
fundamental theories in this signaling scenario.Comment: See also the comment by G C Hegerfeldt in the online version of the
journal, including more reference
Vascular injury from an arterial closure device
onographic examinations are commonly used to assess groin access sites for complications after endovascular procedures. Along with a rise in the number of endovascular procedures, there has also been increased use of percutaneous arterial closure devices, which facilitate immediate hemostasis and earlier patient mobilization.1,2 Here we report the sonographic appearance of an injury related to the deployment of an arterial closure device
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