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

Sprinkler Head Revisited: Momentum, Forces, and Flows in Machian Propulsion

Many experimenters, starting with Ernst Mach in 1883, have reported that if a device alternately sucks in and then expels a surrounding fluid, it moves in the same direction as if it only expelled fluid. This surprising phenomenon, which we call "Machian propulsion," is explained by conservation of momentum: the outflow efficiently transfers momentum away from the device and into the surrounding medium, while the inflow can do so only by viscous diffusion. However, many previous theoretical discussions have focused instead on the difference in the shapes of the outflow and the inflow. Whereas the argument based on conservation is straightforward and complete, the analysis of the shapes of the flows is more subtle and requires conservation in the first place. Our discussion covers three devices that have usually been treated separately: the reverse sprinkler (also called the inverse, or Feynman sprinkler), the putt-putt boat, and the aspirating cantilever. We then briefly mention some applications of Machian propulsion, ranging from microengineering to astrophysics.Comment: 11 pages, 9 figures. v4: Replaced to match version to appear in the European Journal of Physics. (Minor improvements to presentation, added hyperlinks for references.

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

Why do we observe a weak force? The hierarchy problem in the multiverse

Unless the scale of electroweak symmetry breaking is stabilized dynamically, most of the universes in a multiverse theory will lack an observable weak nuclear interaction. Such "weakless universes" could support intelligent life based on organic chemistry, as long as other parameters are properly adjusted. By taking into account the seemingly-unrelated flavor dynamics that address the hierarchy of quark masses and mixings, we show that such weakless (but hospitable) universes can be far more common than universes like ours. The gauge hierarchy problem therefore calls for a dynamical (rather than anthropic) solution.Comment: 5 pages, 1 figure. v2: Minor modifications, reference added. v3: Minor clarifications; matches published versio