198 research outputs found
Composite Mediators and Lorentz Violation
We briefly review the history and current status of models of particle
interactions in which massless mediators are given, not by fundamental gauge
fields as in the Standard Model, but by composite degrees of freedom of
fermionic systems. Such models generally require the breaking of Lorentz
invariance. We describe schemes in which the photon and the graviton emerge as
Goldstone bosons from the breaking of Lorentz invariance, as well as
generalizations of the quantum Hall effect in which composite excitations yield
massless particles of all integer spins. While these schemes are of limited
interest for the photon (spin 1), in the case of the graviton (spin 2) they
offer a possible solution to the long-standing UV problem in quantum linear
gravity.Comment: 6 pages, no figures, revtex4. To appear in the proceedings of the
Third Meeting on CPT and Lorentz Symmetry, University of Indiana at
Bloomington, August 4-7, 2004. v2: Some typos fixed, explanation of why GR
evades the Weinberg-Witten theorem clarifie
An Elementary Treatment of the Reverse Sprinkler
We discuss the reverse sprinkler problem: How does a sprinkler turn when
submerged and made to suck in water? We propose a solution that requires only a
knowledge of mechanics and fluid dynamics at the introductory university level.
We argue that as the flow of water starts, the sprinkler briefly experiences a
torque that would make it turn toward the incoming water, while as the flow of
water ceases it briefly experiences a torque in the opposite direction. No
torque is expected when water is flowing steadily into it unless dissipative
effects, such as viscosity, are considered. Dissipative effects result in a
small torque that would cause the sprinkler arm to accelerate toward the
steadily incoming water. Our conclusions are discussed in light of an analysis
of forces, conservation of angular momentum, and the experimental results
reported by others. We review the conflicting published treatments of this
problem, some of which have been incorrect and many of which have introduced
complications that obscure the basic physics involved.Comment: 16 pages, 8 figures. The subject of this paper is often referred to
in the literature as the "Feynman sprinkler" or the "Feynman inverse
sprinkler." v2:references added, discussion of angular momentum conservation
clarified, section III expanded to include consideration of dissipative
phenomena such as viscosity. v3:minor corrections of style. To appear in the
American Journal of Physic
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