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 either end 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 between Alderson points in normal space. Alderson tramlines, when they form, form instantaneously, and travel along them appears to take no elapsed time. However, sentient beings who travel using an Alderson drive experience "jump shock", a temporary period of extreme disorientation immediately following an Alderson jump. 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.
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 warriors are capable of this. However, in most alignments of Alderson points, the ship will have to significantly change direction upon arrival in the destination system, possibly requiring significantly more acceleration than after a jump at rest and in most cases wasting time.