Alderson drive

The Alderson drive, named after Jet Propulsion Laboratory scientist Dan Alderson, is a fictional device that enables instantaneous interstellar transportation. It is featured in the CoDominium series of science fiction novels by Jerry Pournelle, including the Mote series by Larry Niven and Jerry Pournelle. Dan Alderson helped Pournelle work out the notional science behind the drive, and how it should work to be a useful plot device.

Overview

The Alderson drive is not, strictly, a faster-than-light drive: it can more nearly be likened to a device able to use a form of wormhole, whose entry and exit 'Alderson points' are at the ends of an 'Alderson tramline'. Alderson points are difficult to find. Alderson tramlines form between points of [[equipotential] thermonuclear flux located near stars. Not all star pairs form Alderson tramlines, and not all those tramlines which do form are large enough to take a spaceship. This means that in order to travel between star systems, it is frequently necessary to carry out a series of Alderson jumps interspersed with periods of travel in normal space between them. Alderson tramlines, when they form, form instantaneously, and travel between them appears to take no elapsed time. However, sentient beings who travel by Alderson drive experience "jump shock", a temporary period of extreme disorientation immediately following a jump between Alderson points. Computers are affected for an even longer period of time, making it difficult to automate a spacecraft after a jump. Space crafts are thus vulnerable to attack until their occupants recover from jump shock.

At the beginning of The Mote in God's Eye, only one tramline leads to Mote system. Its inner end is well above the plane of the local ecliptic and its outer end appears inside the photosphere of a red giant star. The Mote civilisations had long been able to construct an Alderson-type drive but, because they had no Langston Field technology, their many attempts to use the drive always failed: their unshielded ships making a jump were burnt up by the red giant's photosphere. Then the battlecruiser INSS MacArthur arrived in their system, leading to the events in the novel and its sequel.

The science

In modern physics, four forces are known: gravity, electromagnetism, nuclear weak, and nuclear strong. The fictional science behind the Alderson drive assumes a fifth force, generated by thermonuclear reactions. The force has little effect in our universe and is barely detectable.

In their CoDominum stories, Niven and Pournelle posit a second universe in point-to-point congruence with our own, the continuum univese, which 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. 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. 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: "tramlines".

The universe can be imagined as a thin, very flat rubber sheet. If heavy rocks of different weights are dropped onto it, the rocks will distort the sheet, making little cone-shaped (more or less) dimples. If two rocks are put 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. Stars can imagined 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. The effect can be important during a space battle.

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).

Technical details

Travel by Alderson drive consists of getting to the proper Alderson point and turning on the drive. Energy is used and 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 do move, but you instantaneously reappear in the spot where you started. That's all there is to the drive, but it dictates the structure of an interstellar civilization.

To begin with, 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 (as you could in future history series such as Beam Piper's and Gordon Dickson's). To reach a given planet, one must enter that system at one of the Alderson points and travel across its stellar system at sub-light speeds. As transit between Alderson points is instantaneous (the books suggest a difference of scientific opinion as to whether transit time takes very little time or no time at all), the problem of travel between star systems is reduced to the problem of travel within star systems.

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.

The drive's limits mean that uninteresting stellar systems won't be explored. There are too many of them. They may be used as crossingpoints if the stars are conveniently placed, but stars not along a travel route may never be visited.

Astrogation with the Alderson Drive

Interplanetary travel is still no small feat, even for a culture as advanced as the CoDominium and its successors. The location of an Alderson point is dictated by the balance of the fundamental forces, which for a Sol-like star leads to points that can be several hundred million kilometers apart by direct line--even farther when orbital paths are taken into account. Even at constant acceleration at high Gs, crossing a system can take weeks. As a result, it is frequently faster to make two, three, or even four jumps to other systems with points in close proximity than to completely cross a single system. Military ships usually travel under constant acceleration, which saves time, but consumes vast amounts of fuel. Commercial ships generally make use of efficient transfer orbits which can take several times as long.

As Alderson points can be difficult to find, even when one knows where the point in question is, human astrogators bring their ships to as near a complete stop as they can, enabling them to precisely determine their position before jumping. This requires a ship approaching a point to decelerate, increasing travel time between points. A possible alternative, discussed in The Gripping Hand is to reach the point at speed and activate the drive at precisely the right time. Human astrogators apparently lack the requisite instincts and precision, but Motie engineers are capable of this. Even allowing for the time saved by not decelerating on approach, as the ship will probably have to change the direction, requiring significantly more acceleration upon arrival in the destination system, not decelerating would result in very little time saved in all but the most fortuitous alignments of Alderson points.

See also

• Jumpdrive
• Hyperdrive