The Alderson Drive is a device that enables instantaneous interstellar transportation in the CoDominium book series of science fiction novels by Jerry Pournelle.
Traditionally there were four known forces in modern physics: two subnuclear forces responsible respectively for alpha and beta decay; electromagnetism, which includes light; and gravity. Eventually, scientists lead by physicist Dan Alderson at the California Institute of Technology detected a fifth force. This "Alderson force" is generated by thermonuclear reactions.
The force has little effect in our universe; in fact, it is barely detectable. Simultaneously with the discovery of the fifth force, was the discovery of a second universe in point-to-point congruence with our own. The "continuum univese" differs from the one we're used to in that there are no known quantum effects there. Within that universe particles may travel as fast as they can be accelerated; and the fifth force exists to accelerate them.
You can get from one universe to another. For every construct in our universe there can be created a "correspondence particle" in the continuum universe. In order for your construct to go into and emerge from the continuum universe without change you must have some complex machinery to hold everything together and prevent your ship—and crew—from being disorganized into elementary particles.
Correspondence particles can be boosted to speeds faster than light: in fact, to speeds nearly infinite as we measure them. Of course they cannot emerge into our universe at such speeds: they have to lose their energy to emerge at all. More on that in a moment.
There are severe conditions to entering and leaving the continuum universe. To emerge from the continuum universe you must exit with precisely the same potential energy (measured in terms of the fifth force, not gravity) as you entered. You must also have zero kinetic energy relative to a complex set of coordinates that we won't discuss here.
The fifth force is created by thermonuclear reactions: generally, that is, in stars. You may travel by using it, but only along precisely defined lines of equipotential flux: tramways or tramlines.
Imagine the universe as a thin rubber sheet, very flat. Now drop heavy rocks of different weights onto it. The rocks will distort the sheet, making little cone-shaped (more or less) dimples. Now put two rocks reasonably close together: the dimples will intersect in a valley. The intersection will have a "pass," a region higher than the low points where the rocks (stars) lie, but lower than the general level of the rubber sheet.
The route from one star to another through that "pass" is the tramline. Possible tramlines lie between each two stars, but they don't always exist, because when you add third and fourth stars to the system they may interfere, so there is no unique gradient line. If this seems confusing, don't spend a lot of time worrying about it; we'll get to the effects of all this in a moment.
You may also imagine stars to be like hills; move another star close and the hills will intersect. Again, from summit to summit there will be one and only one line that preserves the maximum potential energy for that level. Release a marble on one hill and it will roll down, across the saddle, and up the side of the other. That too is a tramline effect. It's generally easier to think of the system as valleys rather than hills, because to travel from star to star you have to get over that "hump" between the two. The fifth force provides the energy for that.
You enter from the quantum universe. When you travel in the continuum universe you continually lose kinetic energy; it "leaks." This can be detected in our universe as photons. To get from the quantum to the continuum universe you must supply power, and this is available only in quantum terms. When you do this you turn yourself into a correspondence particle; go across the tramline; and come out at the point on the other side where your potential energy is equal to what you entered with, plus zero kinetic energy (in terms of the fifth force and complex reference axes).
Travel by Alderson Drive consists of getting to the proper Alderson Point and turning on the Drive. Energy is used. You vanish, to reappear in an immeasurably short time at the Alderson Point in another star system some several light-years away. If you haven't done everything right, or aren't at the Alderson Point, you turn on your Drive and a lot of energy vanishes. You don't move (In fact you do move, but you instantaneously reappear in the spot where you started.)
The Drive works only from point to point across interstellar distances. Once in a star system you must rely on reaction drives to get around. There is no magic way from, say, Saturn to Earth: you have to slog across.
Thus space battles are possible, and you can't escape battle by vanishing into "hyperspace." To reach a given planet you must travel across its stellar system, and you must enter that system at one of the Alderson Points. There won't be more than five or six possible points of entry, and there may only be one.
Star systems and planets can be thought of as continents and islands, then, and Alderson Points as narrow sea gates such as Suez, Gibraltar, Panama, Malay Straits, etc. To carry the analogy further, there's telegraph but no radio: the fastest message between star systems is one carried by a ship, but within star systems messages go much faster than the ships.