Radiation from a Superluminal Source

The sweep speed of an electron beam across the face of an oscilloscope can exceed the velocity of light, although of course the velocity of the electrons does not. Associated with this possibility there should be a kind of Cherenkov radiation, as if the oscilloscope trace were due to a charge moving with superluminal velocity

Circular Orbits Inside the Sphere of Death

A wheel or sphere rolling without slipping on the inside of a sphere in a uniform gravitational field can have stable circular orbits that lie wholly above the "equator", while a particle sliding freely cannot.Comment: Figures revised slightly since publicatio

Gaussian Laser Beams via Oblate Spheroidal Waves

We deduce a set of circularly polarized Gaussian laser beam modes via a separation-of-variables solution to the Helmholtz wave equation in oblate spheriodal coordinates. On transforming to cylindrical coordinates these become the well-known Gaussian-Laguerre modes.Comment: 15 pages, 4 figure

Can a "Hidden Variable'' Quantum Theory Evade the "No-Cloning" Theorem?

If YES, then we can look forward to physical realization of superluminal communication, as the original considerations of the ``no-cloning'' theorem were motivated in part as an explanation of why certain schemes for superluminal signaling cannot work. If NO, then it would seem that some aspects of the ``hidden'' variables must be ``intrinsically hidden'', i.e., ``unknowable'', such that ``hidden-variable'' theories belong more to the ``idealist'' than to the ``realist'' school of thought. I pose this question without proposing a definite answer. I am unaware of any commentary on this topic during these 23 years since the formulation of the ``no-cloning'' theorem, but I would be pleased to be enlightened by more knowledgeable readers

Hidden Momentum in a Coaxial Cable

As an illustration of the concept of "hidden" mechanical momentum, we consider a coaxial cable one end of which is connected to a battery and the other to a load resistor. The nonzero electromagnetic momentum of this configuration is balanced by the slight difference in the mechanical momenta of the conduction electrons in the inner and outer conductors.Comment: 4 pages, no figure

An Off-Axis Neutrino Beam

In this pedagogic note, set as problem for the reader, it is shown how the existence of a maximum energy of a neutrino that appears at a given angle to the direction of its parent pion leads to a useful enhancment of neutrino flux in an off-axis beam

The Laser-Driven Vacuum Photodiode

A vacuum photodiode driven by a pulsed laser can sustain an average current very similar to that of a continous diode, where the latter is described by Child's law

An Electrostatic Wave

In general, Maxwell's equations require that a wave of electric field be accompanied by a wave of magnetic field, and vice versa. However, it is possible to have a plane wave in a dielectric medium with electric field E parallel to the wave vector k (a longitudinal wave) with no time-dependent magnetic field provided the electric displacement D is zero. We give an example from plasma physics: the so-called Bernstein wave.Comment: 8 pages, no figure

Axicon Gaussian Laser Beams

We deduce the simplest form for an axicon Gaussian laser beam, i.e., one with radial polarization of the electric field.Comment: v2 adds one figure and one referenc

Magnetostatic Spin Waves

In general, Maxwell's equations require that a wave of magnetic field be accompanied by a wave of electric field, and vice versa. In magnetic media it is possible to have waves of magnetization with negligible electric field. We discuss an example of this based on ferromagnetic spin waves.Comment: 7 pages, no figure