The kernel has all the nice features admired by McAndrew. Because it is charged, you can move it about using electric and magnetic fields. Because you can add and withdraw rotational energy, you can use it as a power source and a power reservoir. A Schwarzschild black hole lacks these desirable qualities. As McAndrew says, it just sits there.

One might think that this is just the beginning. There could be black holes that have mass, charge, spin, axial asymmetry, dipole moments, quadrupole moments, and many other properties. It turns out that this is not the case. The only properties that a black hole can possess are mass, charge, spin and magnetic moment — and the last one is uniquely fixed by the other three.

This strange result, often stated as the theorem “A black hole has no hair” (i.e. no detailed structure) was established to most people’s satisfaction in a powerful series of papers in 1967-1972 by Werner Israel, Brandon Carter, and Stephen Hawking. A black hole is fixed uniquely by its mass, spin, and electric charge. Kernels are the end of the line, and they represent the most general kind of black hole that physics permits.

After 1965, more people were working on general relativity and gravitation, and other properties of the Kerr-Newman black holes rapidly followed. Some of them were very strange. For example, the Schwarzschild black hole has a characteristic surface associated with it, a sphere where the reddening of light becomes infinite, and from within which no information can ever be sent to the outside world. This surface has various names: the surface of infinite red shift, the trapping surface, the one-way membrane, and the event horizon. But the Kerr-Newman black holes turn out to have two characteristic surfaces associated with them, and the surface of infinite red shift is in this case distinct from the event horizon.

To visualize these surfaces, take a hamburger bun and hollow out the inside enough to let you put a round hamburger patty entirely within it. For a Kerr-Newman black hole, the outer surface of the bread (which is a sort of ellipsoid in shape) is the surface of infinite red shift, the “static limit” within which no particle can remain at rest, no matter how hard its rocket engines work. Inside the bun, the surface of the meat patty is a sphere, the “event horizon,” from which no light or particle can ever escape. We can never find out anything about what goes on within the meat patty’s surface, so its composition must be a complete mystery (you may have eaten hamburgers that left the same impression). For a rotating black hole, these bun and patty surfaces touch only at the north and south poles of the axis of rotation (the top and bottom centers of the bun). The really interesting region, however, is that between these two surfaces — the remaining bread, usually called the ergosphere. It has a property which allows the kernel to become a power kernel.

Roger Penrose (whom we will meet again in a later chronicle) pointed out in 1969 that it is possible for a particle to dive in towards a Kerr black hole, split in two when it is inside the ergosphere, and then have part of it ejected in such a way that it has more total energy than the whole particle that went in. If this is done, we have extracted energy from the black hole.

Where has that energy come from? Black holes may be mysterious, but we still do not expect that energy can be created from nothing.

Note that we said a Kerr black hole — not a Schwarzschild black hole. The energy we extract comes from the rotational energy of the spinning black hole, and if a hole is not spinning, no energy can possibly be extracted from it in this way. As McAndrew remarked, a Schwarzschild hole is dull, a dead object that cannot be used to provide power. A Kerr black hole, on the other hand, is one of the most efficient energy sources imaginable, better by far than most nuclear fission or fusion processes. (A Kerr-Newman black hole allows the same energy extraction process, but we have to be a little more careful, since only part of the ergosphere can be used.)

If a Kerr-Newman black hole starts out with only a little spin energy, the energy-extraction process can be worked in reverse, to provide more rotational energy — the process that McAndrew referred to as “spin-up” of the kernel. “Spin-down” is the opposite process, the one that extracts energy. A brief paper by Christodoulou in the Physical Review Letters of 1970 discussed the limits on this process, and pointed out that you could only spin-up a kernel to a certain limit, termed an “extreme” Kerr solution. Past that limit (which can never be achieved using the Penrose process) a solution can be written to the Einstein field equations. This was done by Tomimatsu and Sato, and presented in 1972 in another one-page paper in Physical Review Letters. It is a very odd solution indeed. It has no event horizon, which means that activities there are not shielded from the rest of the Universe as they are for the usual kernels. And it has what is referred to as a “naked singularity” associated with it, where cause and effect relationships no longer apply. This bizarre object was discussed by Gibbons and Russell-Clark, in 1973, in yet another paper in Physical Review Letters.

That seems to leave us in pretty good shape. Everything so far has been completely consistent with current physics. We have kernels that can be spun up and spun down by well-defined procedures — and if we allow that McAndrew could somehow take a kernel past the extreme form, we would indeed have something with a naked singularity. It seems improbable that such a physical situation could exist, but if it did, space-time there would be highly peculiar. The existence of certain space-time symmetry directions — called killing vectors — that we find for all usual Kerr-Newman black holes would not be guaranteed. Everything is fine.

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.