THE MOST REMARKABLE feature of the Q drive is that its performance is almost independent of the size of the payload. In theory, a standard Q drive could move a planet. In practice, radiation and tidal stress set limits, which may best be illustrated by comparing the two types of Q ship, rocks and boats.
Despite the universal designation "rocks," interstellar vessels are never fashioned from stony meteorite material, only from the nickel-iron variety. Asteroids of that type represent fragments of the cooled cores of differentiated planetesimals. Although originally homogeneous, they have been spalled from larger bodies by violent collisions; all of them now contain flaws and hidden fractures. The larger the rock, the greater the stress and the more numerous the flaws.
Because the Q drive works by pulling, not pushing, starships are in constant danger of falling apart. Ships as large as ten kilometers in diameter have been reported, hollowed out by generations of inhabitants into vast metallic cheeses. In the past, foolhardy souls attempted to convert even larger bodies, only to have their masterpieces fall apart along preexisting planes of weakness and swallow themselves. As tensional stress is greatest during acceleration and deceleration, Patrol regulations limit rocks to a velocity increment of one-half gee.
The Patrol also sets an absolute speed limit for rocks of 333 millicees, or one-third light speed, and less than that in certain areas of high dust content. The danger is radiation, for nowhere is space ever a perfect vacuum. Even at rest, a Q drive singularity will devour stray molecules, and when traveling at interstellar velocities, it sweeps up the galactic medium—gas and dust and stray cometary debris. Matter ripped apart by an infinite gravity gradient is converted to radiation, some of which escapes absorption. Depending on the speed of transit and the nature of the medium, frequencies from radio waves to hard gamma may be found in the resulting fireball. Gravitational redshift lengthens the wavelength of the radiation, which is why the first Q ships detected were for many years mistaken for quasars.
Thus the forward quasi mass not only provides the impetus to drive the ship, but also protects it from potentially disastrous impact. Several hundred meters of nickel-iron will suffice to shield the crew, but the rock itself corrodes at the molecular level, and in extreme cases may even melt. The main reason Q ships star-hop and shun very long runs is that years of stress at high temperatures tend to stretch the rock itself. During world stops, part of the Patrol's standard refurbishment procedure is to rotate the drive 90 degrees, distributing the stress along another axis.
Boats, in contrast, are metal-skin vessels. No matter how large, they mass less than a millionth as much as their big brothers. With their small dimensions and superior tensile strength, they are little affected by tidal stress, and can safely accelerate at dozens of gees. Ironically, such extreme acceleration is unnecessary, as boats can never approach interstellar velocities without frying their occupants. They are restricted to interplanetary work, except for a small role in interstellar exploration as unmanned probes . . .
Rigorous explanation of the Q drive requires an analysis based on Morganian gravity waves. In lay terms the projectors may be described as creating virtual masses having location, infinitesimal duration, and no dimensions. The Q ship is impelled by its efforts to fall into a hole that is constantly appearing in front of it and vanishing before the ship arrives. The Patrol decries the popular terms "bootstrap machine" and "celestial carrot" . . .
If a Q drive projector merely created a single transitory quasi mass, it would violate the laws of conservation, but a single quasi mass is no more possible than a single magnetic monopole. The two virtual masses have opposite signs, and may be thought of as quasi matter and quasi antimatter, with a net pseudomass of zero. As both matter and antimatter have the same gravitational results, both act to draw the ship toward them.
Caught within two steep gravity gradients, each of which defines a different apparent center of mass (ACM), a rock must still move as a unit, and therefore a single effective center of mass (ECM) may be defined. The ship is accelerated or decelerated by moving the Moganian projector forward or aft of the ECM—which may not correspond exactly with the rest center of mass (RCM) even when the ship is traveling at constant velocity. At such times, bodies—including human bodies—situated forward of the ECM are accelerated faster than the ship, and hence sense "down" as being toward the bow. Aft of the ECM, the apparent gravity field is reversed, in an interesting Einsteinian comparison of gravity and inertia. Apparent gravitational effects during acceleration are more complex . . . .
The quasi masses created are not as large as commonly believed. Typically, they are located about fifteen kilometers away from the ship. Assuming a ship diameter of five kilometers, an Earthlike mass would generate a gravitational field of almost 200,000 gee at the bow—somewhat excessive—and little more than half that at the rear. Add in the opposing effects of the rear quasi mass, and the result is a gravity gradient enough to disrupt any matter ever envisioned. A quasi mass equivalent to a small asteroid is adequate, and a Q ship may approach a planet without causing disaster, or even any appreciable tidal disturbance in the oceans. Remember though, that the quasi masses have no dimensions and thus constitute singularities. At close quarters the gravitational gradient becomes effectively infinite.
The aft quasi mass does more than maintain the laws of physics. Because space is never empty, a Q ship cannot just coast after reaching cruising velocity. The forward singularity must be maintained to defend the ship from impact with the galactic medium, and thus without the braking action of the rear quasi mass, Q ships would accelerate indefinitely to relativistic speeds. Once the desired velocity has been obtained, therefore, the projector is moved forward until the attractions balance, and net acceleration becomes zero. For deceleration, of course, the projector is merely moved farther forward yet.
Rocks are not designed to be nimble; space is so huge that they can normally just line up on their targets and go. Any significant change of course at high velocity would require displacing the forward singularity with respect to the line of flight, thus exposing the rock itself to the impact of the interstellar medium . . .
The Q drive itself should not be confused with pseudo-gravity, which is a short-range, low-intensity field used mainly to make shipboard life more comfortable for crew and passengers when the Q drive in not in use . . .
Ibid.