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 spacecraft after a jump. Spacecraft 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 civilizations 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 universe, 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.

The Alderson Drive takes advantage of the premise that, for every particle here, a correspondence particle can be created in the continuum universe. Given machinery that can hold all of one's particles together, one can switch, for example, a spaceship to a corresponding spaceship in the continuum universe, travel instantaneously, and then switch back. The catch is that this can be done only between corresponding Alderson points, which have the same thermodynamic potential. To reach a given planet, one must enter its stellar system at an Alderson point and travel across it 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.

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 astronomical units apart by direct line – even farther when orbital paths are taken into account. Even at constant high acceleration, crossing a system can take weeks. As a result, it may be faster to make several jumps through other systems with points in close proximity than to cross a single system whose points are far apart. 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 their position is known in principle, human astrogators bring their ships to as near a complete stop as they can, enabling them to precisely determine their position before jumping. A ship approaching a point thus must 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. However, in all but the most fortuitous alignments of Alderson points, the ship will have to change direction upon arrival in the destination system, requiring significantly more acceleration than after a jump at rest and resulting in very little time saved overall.