Or is it?
Oppenheimer and Snyder pointed out that black holes are created when big masses, larger than the Sun, contract under gravitational collapse. The kernels that we want are much smaller than that. We need to be able to move them around the solar system, and the gravitational field of an object the mass of the Sun would tear the system apart. Unfortunately, there was no prescription in Oppenheimer’s work, or elsewhere, to allow us to make small black holes.
Stephen Hawking finally came to the rescue. Apart from being created by collapsing stars, he said, black holes could also be created in the extreme conditions of pressure that existed during the Big Bang that started our Universe. Small black holes, weighing no more than a hundredth of a milligram, could have been born then. Over billions of years, these could interact with each other to produce more massive black holes, of any size you care to mention. We seem to have the mechanism that will produce the kernels of the size we need.
Unfortunately, what Hawking gave he soon took away. In perhaps the biggest surprise of all in black hole theory, he showed that black holes are not black.
General relativity and quantum theory were both developed in this century, but they have never been combined in a satisfactory way. Physicists have known this and been uneasy about it for a long time. In attempting to move towards what John Wheeler terms the “fiery marriage of general relativity with quantum theory,” Hawking studied quantum mechanical effects in the vicinity of a black hole. He found that particles and radiation can (and must) be emitted from the hole. The smaller the hole, the faster the rate of radiation. He was able to relate the mass of the black hole to a temperature, and as one would expect a “hotter” black hole pours out radiation and particles much faster than a “cold” one. For a black hole the mass of the Sun, the associated temperature is lower than the background temperature of the Universe. Such a black hole receives more than it emits, so it will steadily increase in mass. However, for a small black hole, with the few billion tons of mass that we want in a kernel, the temperature is so high (ten billion degrees) that the black hole will radiate itself away in a gigantic and rapid burst of radiation and particles. Furthermore, a rapidly spinning kernel will preferentially radiate particles that decrease its spin, and a highly charged one will prefer to radiate charged particles that reduce its overall charge.
These results are so strange that in 1972 and 1973 Hawking spent a lot of time trying to find the mistake in his own analysis. Only when he had performed every check that he could think of was he finally forced to accept the conclusion: black holes aren’t black after all; and the smallest black holes are the least black.
That gives us a problem when we want to use power kernels in a story. First, the argument that they are readily available, as leftovers from the birth of the Universe, has been destroyed. Second, a Kerr-Newman black hole is a dangerous object to be near. It gives off high energy radiation and particles.
This is the point where the science of Kerr-Newman black holes stops and the science fiction begins. I assume in these stories that there is some as-yet-unknown natural process which creates sizeable black holes on a continuing basis. They can’t be created too close to Earth, or we would see them. However, there is plenty of room outside the known Solar System — perhaps in the region occupied by the long-period comets, from beyond the orbit of Pluto out to perhaps a light-year from the Sun.
Second, I assume that a kernel can be surrounded by a shield (not of matter, but of electromagnetic fields) which is able to reflect all the emitted particles and radiation back into the black hole. Humans can thus work close to the kernels without being fried in a storm of radiation and high-energy particles.
Even surrounded by such a shield, a rotating black hole would still be noticed by a nearby observer. Its gravitational field would still be felt, and it would also produce a curious effect known as “inertial dragging.”
We have pointed out that the inside of a black hole is completely shielded from the rest of the Universe, so that we can never know what is going on there. It is as though the inside of a black hole is a separate Universe, possibly with its own different physical laws. Inertial dragging adds to that idea. We are used to the notion that when we spin something around, we do it relative to a well-defined and fixed reference frame. Newton pointed out in his Principia Mathematica that a rotating bucket of water, from the shape of the water’s surface, provides evidence of an “absolute” rotation relative to the stars. This is true here on Earth, or over in the Andromeda Galaxy, or out in the Virgo Cluster. It is not true, however, near a rotating black hole. The closer that we get to one, the less that our usual absolute reference frame applies. The kernel defines its own absolute frame, one that rotates with it. Closer than a certain distance to the kernel (the “static limit” mentioned earlier) everything must revolve — dragged along and forced to adopt the rotating reference frame defined by the spinning black hole.
The McAndrew balanced drive.
This device makes a first appearance in the second chronicle, and is assumed in all the subsequent stories.
Let us begin with well-established science. Again it starts at the beginning of the century, in the work of Einstein. In 1908, he wrote as follows:
“We… assume the complete physical equivalence of a gravitational field and of a corresponding acceleration of the reference system…”
And in 1913:
“An observer enclosed in an elevator has no way to decide whether the elevator is at rest in a static gravitational field or whether the elevator is located in gravitation-free space in an accelerated motion that is maintained by forces acting on the elevator (equivalence hypothesis).”
This equivalence hypothesis, or equivalence principle, is central to general relativity. If you could be accelerated in one direction at a thousand gees, and simultaneously pulled in the other direction by an intense gravitational force producing a thousand gees, you would feel no force whatsoever. It would be just the same as if you were in free fall.
As McAndrew said, once you realize that fact, the rest is mere mechanics. You take a large circular disk of condensed matter (more on that in a moment), sufficient to produce a gravitational acceleration of, say, 50 gees on a test object (such as a human being) sitting on the middle of the plate. You also provide a drive that can accelerate the plate away from the human at 50 gees. The net force on the person at the middle of the plate is then zero. If you increase the acceleration of the plate gradually, from zero to 50 gees, then to remain comfortable the person must also be moved in gradually, starting well away from disk and finishing in contact with it. The life capsule must thus move in and out along the axis of the disk, depending on the ship’s acceleration: high acceleration, close to disk; low acceleration, far from disk.
There is one other variable of importance, and that is the tidal forces on the human passenger. These are caused by the changes in gravitational force with distance — it would be no good having a person’s head feeling a force of one gee, if his feet felt a force of thirty gees. Let us therefore insist that the rate of change of acceleration be no more than one gee per meter when the acceleration caused by the disk is 50 gees.
The gravitational acceleration produced along the axis of a thin circular disk of matter of total mass M and radius R is a textbook problem of classical potential theory. Taking the radius of the disk to be 50 meters, the gravitational acceleration acting on a test object at the center of the disk to be 50 gees, and the tidal force there to be one gee per meter, we can solve for the total mass M, together with the gravitational and tidal forces acting on a body at different distances Z along the axis of the disk.