Gravity drive

The gravity drive, first invented in the 24th Century, and subsequently improved incrementally throughout the millennia, is the preeminent mode of FTL travel available to human civilization. The drive's name comes from how it relies on gravity wells and frame dragging to allow it to make jumps across long distances of space by connecting to other gravity wells. As such, a gravity drive will only work in proximity to a massive object in space, and the destination point must also be near a massive object.

The total distance that a gravity drive can allow a spacecraft to jump is dependent on the mass of the object at the departure point and, to a lesser extent, the mass of the object at the arrival point. Jumps within a solar system are possible by using planets. Jumps to different star systems within a galaxy can be achieved by using giant planets and stars, with larger stars and stellar black holes permitting longer jumps across a galaxy. Intergalactic jumps can only be achieved by using a galaxy's central supermassive black hole to connect to another galaxy's supermassive black hole. Given a large enough departure mass, it is possible to achieve an intergalactic jump to other massive objects in a distant galaxy besides its central black hole, but targeting the central black hole is always the safest and easiest method of galaxy hopping.

Because the departure mass is more important for determining jump distance, there are jump asymmetries that exist between two galaxies. For instance, it may be possible to jump from a galaxy with a very large central black hole to a distant galaxy with a smaller central black hole, but the reverse jump may not be possible. Vast leaps across uncharted space are dangerous, difficult, and unadvised. Because distant galaxies are viewed from the departure point at a time thousands or millions of years in the past, the layout of the destination will have changed, and the target will have moved. For this reason, chaining along galaxies has been the method by which human civilization has spread, with a galaxy closer to a distant target providing better information about its current state than a galaxy that is too distant. Precision in mass determinations also diminishes with distance, leaving open the possibility of being stranded after a jump if the destination galaxy does not have any concentration of mass high enough to enable a return journey.

Throughout the millennia, several colonization missions have ended with the colonists being stranded in remote galaxies. Furthermore, many dwarf galaxies remain uncolonized, as they offer no suitable agglomeration of mass to power a return jump to any nearby giant galaxies. Some religious sects have deliberately traveled to dwarf galaxies in the past to set up their own, isolated societies.

Mass Lanes

The filament structure of galaxy superclusters creates lanes through which space can be explored and colonized. These filaments run along the intergalactic voids, creating sheets around large bubbles of mostly empty space. Mass lanes, furthermore, prohibit the build of of dark energy that pervades the voids, the presence of which further limits the jump distances achievable by gravity drives. For this reason, paths that penetrate into the scant few galaxies amidst the intergalactic voids are few and far between, and routes around voids to other clusters rich with mass are always being sought after by major trading companies. Even two class 10 black holes of the maximum theoretical mass, with a rating of approximately 10.69897, would not permit a jump all the way across a void.

Jump Mechanics

Gravity drives take time to power up. The time to initiate a jump is inversely proportional to the mass of both the departure and arrival points, and can range from seconds to days. The distance does not factor into the time to prepare for a jump. A jump creates gravitational waves originating from both the departure and arrival points, seconds to minutes previous to the jump, again inversely proportional to the masses of the jump anchors, and the gravitational wave signatures from gravity drives are unmistakeable as such. Thus, there is always a precursor warning that a ship is about to arrive or depart, and the gravitational wave signatures for departures and arrivals are different. The magnitude of the gravitational wave disruption caused by a jump is directly proportional to the distance jumped.

Once a jump is initiated, the time to complete the jump is precisely the time it takes a photon moving in the medium of the object's departure point to cross the longest dimension of the object that is jumping. For a typical human vessel or structure, this can range from a few hundred nanoseconds to a few microseconds. The energy required to power a jump is proportional to the square of the time, meaning it becomes increasingly difficult to move very large objects with gravity drive technology. While some fringe scientists believe it is possible to turn black holes themselves into gravity drives, mainstream science holds that gravity anchors used to form a gravity drive bridge cannot move across the bridge so created.