That is an interesting (and long!) question.
quite clear from your statement whether the "device" is then to be
ejected, or whether it is to be carried with the ship, so that it could
be repeatedly used. If you try to keep the whole device on board
without ejecting anything, then you can't accelerate the ship, because
that would violate Newton's third law. The total momentum of the ship
and device can't change. If you do eject some particles, then you're
describing a type of rocket drive which has been considered.
Anyway, the setup you describe in your first question is similar to a
setup actually used in particle accelerators at various laboratories
around the world, although it doesn't work in exactly the way you
describe. Instead of space ships, the objects that are accelerated are
electrons and protons and atoms that have extra electrons or are
missing some so that they are charged and can feel the forces of an
A negatively-charged object will be attracted to a
positively-charged object, but curiously enough, not if it is inside.
The reason for this is that the negative charges feel the combined
forces of all the positive charge on all sides of it, and they all
cancel each other out. The electric field inside of a long,
uniformly-charged tube is zero(!).
But don't lose hope just yet -- the field *outside* of the
positively-charged tube (and for a little bit inside near the edges,
but it disappears quickly) does in fact want to drag a negatively
charged object into the tube. These two features can be taken advantage
of to accelerate particles to very high energies and surmount one of
the main difficulties of charged-particle acceleration. The problem is
that if you want to give, say, an electron 100 million electron-volts
of energy, you have to drop it across the terminals of a 100-million
volt battery, of which there are none easily available.
The neat trick is to arrange a sequence of tubes and gaps between
the tubes in a straight line. Charge up the first tube to be positive
and allow an electron to accelerate into it (say, from a hot wire or an
electron gun like the one in a TV set). While the electron is traveling
through the tube, quickly reverse the charge on the tube so that it
becomes negative. The electron in the tube won't notice this because
the field inside is zero. Meanwhile, charge up the next tube in line to
be positive, so that the electron gets a push from the tube it leaves
and a pull towards the tube it approaches. When the electron is inside
the next tube, flip its polarity too. Then, to get the electron going
as fast as you like, just arrange as many tubes as you can afford. It
is easiest to connect adjacent tubes with a low-resistance wire and
make them oscillate in polarity with microwaves. That way, the maximum
voltage around is just the voltage between two adjacent tubes; nowhere
do 100 million volts need to be applied. In practice, these are
sometimes made out of superconducting rings with the tubes on the ends
("split-ring resonators"). See
for a description.
On the second question, some of the things you say don't sound
familiar. What is true is that if you have an intense magnetic field,
any electrical conductor entering it will form eddy currents. Those
make a magnetic field which tends to repel the object from the magnetic
field which was already there. So in some cases magnetic fields can be
shields against quickly moving conducting objects, like bullets.
Unfortunately, the field needed to do this for a bullet is much larger
than can easily be produced. Furthermore, such a field will turn any
loose iron object nearby into a deadly bullet. Of course if anyone
wanted to circumvent such a shield, they would only need to make
bullets that were poor conductors.
If the field oscillates as you say (this is easier to do
electrically than to mechanically spin the field-producing object on
its axis), then a large amount of power will be radiated at the
frequency of oscillation. The effect will be to produce a very powerful
radio station or microwave or visible light source, depending on the
frequency chosen. Some astrophysical objects (quasars, neutron stars,
black holes) may produce radiation powerful enough to stop a bullet,
but it would be far more likely just to melt the incoming bullet (or
vaporize it). Not only impractical, this object would probably melt or
vaporize everything else nearby, defeating the original purpose of
reducing damage from incoming bullets.
Tom and Mike
(republished on 07/19/06)