Interstellar Travel w/o Magic 2

In the previous
post I suggested that a giant linear accelerator in our solar system couldn’t
propel a manned spaceship to relativistic speeds because even accelerators
thousands of miles long would still have to crush the crew with impossible gee
forces to get to relativistic velocities. 

Well… that isn’t exactly true.  That is only true if the accelerator works by
pushing some portion of the starship, like an electromagnetic launcher would
push on magnets or a gun’s gas pressure would push on the rear of the
projectile.  If, however, you could build
an accelerator that pushed on everything in it with the same acceleration then
even if the spaceship was accelerated at thousands of gees the crew and passengers
would feel like they were weightless (the same way that the crew of the ISS
feels weightless even though Earth’s gravity is constantly accelerating them
and the space station).  What could
produce a gravity-like acceleration on the spaceship which affected every
substance equally?  Well… gravity.  All you need to do is create a very intense
gravitational field down the length of the linear accelerator and let your
starship free-fall down the barrel.

Can you generate gravity fields of that sort without some “magic”
assumptions?  Yes, according to General Relativity
it appears that it can be done quite simply. 
Dr. Forward proposed in his 1962 paper “Guidelines
to Antigravity” the construction of a gravity generating machine based on
General Relativity using a toroid of rotating matter to create a gravitational
field normal to the axis of the toroid. 
Basically a “Forward Coil” can generate gravity (beyond just the weight
of the coil) by spinning mass around the coil at high speed.  The Forward Coil gravity generator does not
require “negative mass” or any other theoretical but unobserved phenomenon to
operate, as do some other Relativity-based propulsion schemes.  Unfortunately, this is where the difference
between “simple” and “easy” become very important.  While the design of a Forward Coil gravity
generator is not complex, the density and speed of the matter that must be accelerated
around the torus in order to get usable gravity fields is astronomical.  Forward proposed that the machine may require
the use of super-cold tetraneutrons as the working fluid.  So, while there do not appear to be any
physical limitations to generating artificial gravity fields, making it “just
an engineering problem”… the engineering difficulty of collecting a large
amount of neutron-star material and then building a plumbing system to pump it
around a circuit at relativistic velocities is likely beyond even an early interstellar
civilization unless some more elegant, less brute-force solution to generating artificial
gravity forces can be found.

Fortunately, there is some evidence that there is a more efficient,
elegant way to generate artificial gravity fields.  Quite the opposite of the Forward Coil, this
method has experimental evidence of operation but no working theory.

There have been a number of sporadic reports of experiments
having gravity shielding effects or producing gravity-like fields, these
usually involve rapidly
rotating objects, superconductors,
or (and especially) rapidly
rotating superconductors.  While attempts
to develop a theory on special interactions between gravity and superconductors
goes back to at least DeWitt’s
attempt in 1966, there is yet no good theory explaining these occasional
observations.  Unfortunately, these
observations are usually near the limits of measurement and difficult to reproduce.  Sometimes they even occur by accident and the
exact conditions that created the observed gravity or gravity-like effect are
not well understood.   One of the most
dramatic claims of observed superconductors generating a gravity-like force
comes from Podkletnov.  Unfortunately, some non-technical issues
complicated replication of this work (including dismissal of the experimental
work due to distrust by some of the theories used to explain it and Podkletnov’s
own limited collaboration, perhaps relating to intellectual property issues or
perhaps merely international and security obstacles).  Lorincz and Tajmar did a very thorough
experimental investigation into Podkletnov’s work recently, and have not been able to
reproduce it.  Absence of evidence is
not, however, evidence of absence.  Also,
Tajmar’s null-result from his careful attempts to replicate one of Podkletnov’s results,  does not address the many other
experiments that show odd behaviors regarding superconductors and gravity-like
forces or mass anomalies.  So, it seems
like there are pestering hints in both theory and experiments that
superconductors could provide some lever for affecting gravity or generating gravity-like
forces far more economically than the Forward Coil.  We just don’t have a firm grip on that lever
yet.  If we could, then one particularly
handsome and kind gentleman and his brilliant colleague have already provided a
blueprint for how to turn it into a propulsion system.

If that doesn’t work, it may be still possible using diamagnetism.  Even nonmagnetic materials are
repelled by a magnetic force.  If a
capsule could be created to contain the crew (and pets, livestock, plant
samples, and anything else sensitive to very high gee forces) that consisted
only of material that was not para- or ferro-magnetic, then it might be
possible for the entire thing and the crew to be accelerated at high gee by a
magnetic field without causing any concentrated forces anywhere in them or the
system.  It would feel, to the passengers
and cargo, as if the entire vessel were free-falling in a gravity-like way at
high gees.  This capsule could then rendezvous
with the rest of the starship that had been accelerated at higher gee forces
without them, and transfer to the spaceship.  But that probably wouldn’t make as dramatic a
sequence in a sci-fi epic.