A lot of the theoretical physics that make life in my SF universe possible came from that book, and having re-read it I understand how and why I made the decisions I made. Everything--from the conjecture behind the working of the hyperdrive, to the ideas governing how inertial damping works, and so on--was inspired by that book.
Before the late 1990s I had only the faintest notions of what, why, and how things worked. I had FTL drive because I needed it; it worked the way it did because I wanted some decent limitations so it wasn't a "poof, we're there" magic system. And the other ideas also were there because I wanted and needed them; there was no underlying logic to them.
Forward's book gave me enough information to ground my stuff in the fringes of theoretical physics. Until I read that book, I had never seen a non-mathematical digest of the various topics given in there; it's full of interesting concepts reduced to words for the average reader.
Re-reading the book gave me insight, as I said, into why I made the decisions I did when I completely revamped the canon for my SF universe when I decided to make an RPG setting of it. Surprisingly little of the physics had to be scrapped, though--I had been careful, before, with the laws of nature--and the stuff from Forward's book slipped neatly into place as if precision-machined.
But what caught my attention this time was Forward's discussion of the conservation laws: Conservation of Mass-Energy; Conservation of Angular Momentum; Conservation of Linear Momentum.
We all know, thanks to Einstein, that E=MC2. In his book, Forward mentioned that the direct conversion of momentum into energy might also yield scads of power.
Note that he did not mean in the sense of, for example, applying the brakes to your car. What he meant was the instant and complete converstion of momentum into energy, the way that matter and antimatter convert into energy: not changing the momentum into heat or noise via a mechanical process, but by unraveling it.
Inertia, I figure, is the gravitic equivalent of a magnetic field. If you move electrons, they generate a magnetic field. You have to push to get them started; in the absence of any resistance they'll keep on going, forever, until you push them to stop them.
When you push an object, it takes work to get it moving, but in the absence of friction or air resistance it'll keep on going forever until you apply a force to stop it. The work you expend on getting the object moving generates a field that we call inertia.
Forward seemed to indicate that if you could manipulate that inertial field to generate electricity, much the same way we now can manipulate magnetic fields to do so, it would make a prodigious amount of energy: energy equals momentum times the speed of light squared.
I'm not so sure. Mass contains a lot of energy because of how particles are constructed; I don't see how you could possibly extract that kind of energy from (say) a spinning gyroscope, particularly when you didn't put anything like that kind of energy into making it spin.
Of course, Forward knew the math and the physics a lot better than I do. There probably is something he couldn't convey with mere words; I do know that the concept of converting angular momentum into energy seems trivial at first glance--we do it all the time with nothing more sophisticated than magnetic coils--but at the same time I know that is not what he meant.
Problem is, it makes for perpetual motion machines if it works. You spin a flywheel with a motor, and use this magic angular-momentum-to-pure-energy convertor on it, producing electricity...you drive the motor with some of the energy and use the rest for whatever: instant free power, for however long the machine is capable of operating.
Then again, 100 years ago, people would have scoffed at the idea of pitchblende being used to make single bombs which can destroy whole cities.
Forward's book was good because it made me think about the properties of mass and gravity, and it led me to some interesting conclusions which I am now revisiting:
1) Like gravity, inertia must change the shape of space. "An object in motion tends to remain in motion" because it's got a four-dimensional divot in space-time in front of it and it's moving along a four-dimensional curve.
2) Unlike gravity, inertia is dependent on mass and velocity, so it's probably more correct to say that pushing an object moves its space-time divot forward of its center of mass. (By "forward" I mean "in the direction of travel".) When we apply a force to a mass, making it move, its divot shifts, and the shift in its divot won't change unless we apply another force. The fact that inertia is partly dependent on velocity somehow means the object doesn't accelerate due to the space-time curve, as it does in one due to gravity.
3) Magnetism also has gravity-like properties. If you place a ferrous object in a magnetic field, it moves along a four-dimensional curve to minimize its potential energy. Many non-ferrous materials have other properties such as diamagnetism or paramagnetism which may or may not be analogous to theoretical constructs such as negative mass.
4) Gravity doesn't seem to be polar, unlike electromagnetism. If it is, we have never observed it having polarity. But the "poles" may simply be too close together to be distinguishable, or there may be something else preventing us from noticing this property. Neither the strong nor weak force seem to be polar, either, yet there are plenty of references in modern physics to the "electroweak" force. (What is the relationship of electromagnetism to spin? Is the spin of the electron why we have "north" and "south" magnetic poles?) Nonetheless, if electromagnetism can be unified with a nonpolar force, why can't gravity?
5) Unifying gravity with electromagnetism would probably open the door to all kinds of neat stuff, like artificial gravity and flying cars and stuff. "Antigravity" is probably impossible, unless gravity is polar.
Overall I'm glad that the old noodle isn't being turned to mush while I work as a stockboy at Target--my brain appears to work, still, even though I haven't actually used it for much lately.