Reference Jerry Pournelle's web site, Chaos Manor: http://www.jerrypournelle.com/mail/mail407.html#Dean
For my own SF writings I had given much thought to the properties of mass, inertia, and gravity, in order to come up with some plausible parameters for things like inertial damping devices.
I first realized how necessary inertial damping technology was sometime around 1977 or so, when Star Wars first hit the big screen. Knowing, as I did, that a body in motion will stay in motion unless its direction is changed, I reasoned that the X-wing, Y-wing, and TIE fighters had to have some kind of inertial dampers; otherwise the maneuvers they made in space were impossible.
I was 10 when Star Wars first came out. It must have been a bit later; perhaps around the time of The Empire Strikes Back, perhaps. After Star Wars, I got into science fiction and started reading all the greats: Heinlein, Clarke, Asimov, Niven, etc. Heinlein's works taught me to respect the laws of physics (through osmosis, it seems) and so, at some point in my pubescent years, it occurred to me that airplane-like flight in outer space demanded the use of inertial damping.
After that the notion languished. Around 1999 I started revisiting the SF universe I'd done so much work on in my high school years (you have a lot of free time when you're on the bottom of the social ladder, and don't have to worry about things like parties or dating) and decided to update it--make it all make sense, and jettison the juvenile stuff, and in general update the canon so I could stomach writing stories in that world again.
The problem of inertial damping has three basic facets:
1) In what way is inertia controllable? It is a property of mass, so you can't just wish it away; you have to have a route to manipulating it. Minimizing mass minimizes inertia, but for any practical spacecraft there is a minimum mass below which you cannot go.
2) It takes power to control inertia. The kinetic energy equation e=1/2mv^2 must be satisfied. You must be able to supply and store this energy; more on that in a moment.
3) "reference frame"--how do you control the inertial damping effect?
And a fourth problem, which is related--if you can control inertia, why can't you just build a reactionless thruster?
Okay, so let's tackle them!
How do you control inertia?
For the purposes of my stories, I thought of an inductive electrical circuit; the most simple example is a battery in series with a switch; across this assembly, a resistor in parallel with an inductive coil. When the switch is closed, it connects the battery to the circuit; when open, it isolates the battery.
With the battery connected, current will flow through both the resistor and the coil. The coil will, at first, allow no current to pass; the current which flows into the coil instead is converted into a magnetic field. As the magnetic field increases in strength, current gradually begins to flow through the coil, until the coil is saturated; the magnetic field reaches a steady state and the coil looks like a simple piece of wire to the battery.
If the switch is now thrown, isolating the battery from the circuit, something interesting happens: the coil becomes a current source. The magnetic field in the coil collapses, pumping electons into the wire. In short, the coil acts as a kind of flywheel. In a magical circuit with no resistance, the current would flow forever. (Of course, if we have a superconducting circuit, we don't need the inductor for this; a simple loop of wire is enough.)
I think the analogy is probably obvious: the coil resists changes in the electrical force acting on it, until a steady state is reached. This is like an object tending to remain in motion (or stationary) until a force acts on it.
Having thought all this through, I realized that if inertia was thought of as a kind of field, then it was inductive in nature.
So: an inertial damper must be able to create or modify this "inertial field". At the time I thought all this through, a theory of "electro-gravitation" was percolating around the world of physics, so it wasn't all that far-fetched, was it? If the electric force was unified with gravity, I reasoned, there might be a way here to make inertial damping technology a possibility. I don't know how, exactly, but then I don't really need to know how. The possibility is enough for the purposes of a science fiction story.
(I have had some thoughts about M-brane theory and the time-based property of gravitation, and the notion that physical forces may be dimensional properties, but trying to include them here would make an already long entry FAR too long.)
If I have an inertially damped ship, I must have a large resevoir of energy handy. If I want to accelerate a mass to a certain speed v, I must pump 1/2mv^2 joules into it, somehow. Currently we do this by burning fuel in an engine; on the ground we tend to turn that energy into waste heat when we want to stop, by using our brakes. In space, we have to expend delta-v, the same amount of delta-v we used to start moving.
With an inertial damper, though, the rules change.
My ship will move only so long as an acceleration or force is acting on it--and here things get quite interesting. If I turn on a thruster, then my thruster's impulse determines my speed. If my thruster throws out matter at 100 meters per second, then my ship will instantly begin moving at 100 meters per second. If my ship weighs 1000 kg, my inertial damper sucks 1/2 x 10^6 x 10,000 joules from whatever power source I've got it hooked to. If we assume it takes a second to work, that's five gigawatts of power that must come from somewhere. Not a trivial problem!
The instant I shut that thruster off, though, the inertial damper returns five gigawatts of power and I come to an absolute dead stop in space again. (I know! See below.)
I am ignoring, for the sake of this discussion, the thermodynamic considerations implied by generating and storing such massive amounts of power. There would be a considerable amount of waste heat that one would have to deal with!
What about the reference frame?
This one is actually the easiest problem to deal with. We all know that there is no "absolute stop" in space, because there's nothing to judge it against. For a practical inertial damper, "absolute stop" can be relative to the largest nearby mass: a planet, star, asteroid, or even a larger space ship or space station. This even makes sense when viewed from the standpoint of "real physics"; in most cases we define the Earth's surface as our reference point, and it works just fine.
I have not given much consideration about how relativistic speeds might change this; in my SF universe, most sublight travel takes place at non-relativistic speeds. And the few times I have shown vessels in deep space, I have purposely not dwelled on the ship's velocity in space. When you're light-years from the nearest star, and planning to make a hyperspace jump, the ship's residual velocity is unimportant, anyway. (And I purposely devised the physics of hyperspace travel such that the velocity is normalized to the destination star's velocity in its orbit around the galactic core.)
The fourth issue: if you can make an inertial damper, why can't you make a reactionless space drive?
The main problem that I see with this seems simple to define: which way do you point it?
Standing here on Earth, we instinctively know which way "forward" is. But if you're generating an inertial field using a machine rather than good old-fashioned "action/reaction", vintage Isaac Newton, which way do you point it? Does inertia have a polar property like magnetic fields do, or is it a scalar property with only magnitude defining it? If so, how does an object know to keep moving in the same direction?
Is it possible that inertia, like gravity, also changes the shape of space-time?
I think it must; I think that being able to control inertia implies control over gravity, or vice-versa: in my universe, artificial gravity, and anti-gravity, came before inertial control. If you can change the shape of space-time to modify gravity, then you damn well ought to be able to do it to modify inertia.
So: how does all this relate to the article linked from Jerry Pournelle's web site?
The referenced experiment, if repeatable--if the conclusions are valid--is analogous (as others mention) to Faraday's experiments in the 19th century. If this is so, we are on the threshold of understanding how to control gravity--and, possibly, inertia.