Like ordinary stars, planets, and other celestial bodies, black holes, which astronomers have been able to detect in recent years, have certain physical properties that distinguish them from the others. Yet because of a black hole's extraordinary nature, especially the fact that it is black and invisible, very few of its properties can be directly measured from the outside. "From the outside," John Gribbin explains,
you can [calculate] the mass of the hole from its gravitational attraction and the speed with which it rotates. If it has an electric charge, you could measure that as well. But those … properties are all you can ever measure. There is no way to tell what the matter that went into the hole was before it was swallowed up … whether it was a star, a great glob of water, or a pile of frozen TV dinners. There is no way to distinguish a black hole made of stellar material from one made of anything else, a property summed up by [scientists] in the expression "black holes have no hair [distinct, visible physical characteristics]," coined by [John] Wheeler and his colleague Kip Thorne in the early 1970s. 22
Still, even if black holes "have no hair," they do behave in certain definite and characteristic ways as they interact with the universe around them. By observing this behavior, scientists can theorize and draw conclusions about those properties of black holes that cannot be seen or measured directly. And the more people learn about these cosmic oddities, the more they will be able to reveal the hidden secrets of the universe. Moreover, learning as much as possible about the properties of black holes could conceivably prove beneficial to humanity. Someday it may be possible to harness and utilize some of the vast energies produced by these objects.
The first and probably most obvious measurable property of a black hole is its mass. Clearly, black holes must be extremely massive and also dense, since each consists of most of the matter of a giant star compressed into an unimaginably tiny space.
Yet how can scientists on Earth measure the mass of a black hole or other body lying trillions of miles away? That depends on whether any stars or other large bodies happen to lie near the black hole. If it is floating through space alone, far from any such objects, scientists will have no way to measure its mass.
In contrast, if the black hole and a star are orbiting each other (actually, each orbiting a common center of gravity), scientists can use the formula for universal gravitation to compute their masses. First, using sensitive instruments and mathematics, they measure the distance between the two objects. Then they compute their orbital velocity (the speed at which they move in orbit). Finally, they plug these figures into an equation that determines the mass. As Thomas Arny says, this method "can be used to find the mass of any body around which another object orbits. Thus, gravity becomes a tool for determining the mass of astronomical bodies." 23 In the case of black holes, scientists often express their masses in multiples of the Sun's mass. A black hole is said to contain 8, 12, 20, or some other number of solar masses.
The mass of a black hole, which is measurable, directly affects the nature of other properties of the hole that are not measurable. One of these is the size of its Schwarzschild radius. The easiest way to understand this fundamental property of a black hole is to visualize the hole moving through space. From time to time, it encounters gas, dust, asteroids, and other forms of matter, which are naturally attracted by its huge gravitational pull. When the matter gets close enough, it is torn apart and reduced to atoms; then it is sucked into the black hole, where the debris spirals into the bottomless gravity well, never to be seen again.
The crucial part of this scenario of annihilation is that, on its way into the black hole, the matter passes what might be called "the point of no return," which scientists call the event horizon. As long as the matter manages to stay outside the horizon, it has a chance of escaping. Once it crosses the horizon, however, it will disappear into the black hole's gravity well. The distance from the center of a black hole, called the singularity, to the event horizon is the Schwarzschild radius, named after German astronomer Karl Schwarzschild, who discovered it in 1915.
Mathematical equations show conclusively that this radius will vary according to a black hole's mass. The more massive the hole, the longer the Schwarzschild radius, and conversely, the less massive the hole, the shorter the radius. A black hole of one solar mass will have a Schwarzschild radius of 1.86 miles; and a hole of ten solar masses will have a radius of about 20 miles. In the latter case, therefore, the point of no return for any matter approaching the black hole lies 20 miles from the singularity, or center.
German astronomer Karl Schwarzschild, who was born in 1873, made major contributions to knowledge about superdense objects and their effects on space and time. He became director of the Astrophysical Observatory in Potsdam in 1909. In 1915, while serving his country in World War I, he heard about Einstein's work on the theory of general relativity. Schwarzschild contacted Einstein and kept him informed about his own efforts to describe the geometry of spacetime around a superdense object occupying a single point, or singularity. Among Schwarzschild's mathematical discoveries was that the singularity would be separated from the event horizon by a certain distance, which scientists later named the Schwarzschild radius in his honor. Tragically, he contracted a skin disease while in the military and grew gravely ill. Einstein presented his colleague's groundbreaking ideas to the scientific community only months before Schwarzschild died in May 1916 at the age of forty-two.
Karl Schwarzschild worked out his equations for a hypothetical black hole that does not rotate on its axis. So scientists came to call a theoretical, nonspinning black hole a Schwarzschild black hole. This model worked well enough to calculate the distance from the singularity to the event horizon. But did it accurately describe the real state of a black hole? Most physicists felt that it did not. This is because they already knew that all of the bodies ever observed in outer space both rotate and possess a measurable property known as angular momentum.
Angular momentum is the tendency of a spinning object to keep on spinning. Even if the object gets larger or smaller, its original rotational energy will be preserved by altering the speed of rotation appropriately. For example, when a spinning ice skater extends his or her arms, in effect making the skater's body larger, the rate of spin slows down; by contrast, when the skater draws his or her arms in tight to the body, the rate of spin increases.
This same effect can be seen when a large star collapses into a neutron star. The original star rotates at a certain rate, perhaps once every twenty or thirty days. After the collapse, its angular momentum is transferred into the much smaller neutron star, which now spins around many times in a second. It stands to reason that a black hole will follow this same scenario. Isaac Asimov summarizes it this way:
When a star collapses, to make up for that, its speed of rotation must increase. The more extreme the collapse, the greater the gain in speed of rotation. A brand-new neutron star can spin as much as a thousand times a second. Black holes must spin more rapidly still. There's no way of avoiding that. We can say, then, that every black hole has mass and angular momentum. 24
Thus, what was needed after Schwarzschild introduced his calculations for nonspinning black holes was a mathematical solution that would describe the workings of black holes that rotate, as all black holes are believed to do. This goal was attained in 1963 by New Zealander astronomer Roy P. Kerr, who was then working at the University of Texas. Since that time, in Kerr's honor, it has become common for scientists to refer to spinning black holes as Kerr black holes. The first definite confirmation of these objects came in August 2001, when scientists at NASA's Goddard Space Flight Center (in Greenbelt, Maryland) detected the spin of a black hole lying about ten thousand light-years from Earth. (A light-year is the distance that light travels in a year, or about 6 trillion miles.)
The Kerr solution recognizes the singularity, the event horizon, the Schwarzschild radius, and other black hole properties found in the Schwarzschild solution. However, the rapid spin inherent in a Kerr black hole creates a considerably more complex and dynamic situation.
First, the singularity is not a single point, but a warped area of space shaped like a ring. Second, the event horizon, marking the boundary between the black hole and ordinary space beyond it, is moving in the same direction that the singularity is spinning. And as it moves, it drags part of the nearby region of space along with it. As Kip Thorne explains:
The hole's spin grabs hold of its surrounding space [shaped like the bell of a trumpet] and forces it to rotate in a tornado-like manner…. Far from a tornado's core, the air rotates slowly, and similarly, far from the hole's [event] horizon, space rotates slowly. Near the tornado's core the air rotates fast, and similarly, near the horizon space rotates fast. At the horizon, space is locked tightly onto the horizon. It rotates at precisely the same rate as the horizon spins. 25
At the same time, any matter that happens to lie in that area of rotating space is carried along in the moving current. The matter spins around the outside of the black hole and forms a flattened disk of material in a manner similar to that in which small pieces of rock and ice form a flattened disk of rings around the planet Saturn. Scientists call this spinning disk around a black hole an accretion disk.
The material in the accretion disk plays an important role in another effect of a black hole's spin—the creation of two narrow but powerful jets of gas that appear to be shooting out of some black holes. (In reality, the jets do not come from inside the hole; instead, they originate in the disk, outside of the event horizon.)
Astronomers have advanced a number of convincing explanations to explain how these jets might form. In one, the tremendous pressures produced by the rapidly rotating gases in the accretion disk create two vortexes, whirlpools similar to the kind formed by water swirling down a drain. These vortexes shoot jets of hot gases outward at high speeds in opposite directions.
Thorne summarizes another possible scenario for the creation of these jets. This one involves a powerful magnetic field generated by the black hole's spin:
Magnetic field lines [invisible strands of magnetism] anchored in the [accretion] disk and
sticking out of it will be forced, by the disk's orbital motion, to spin around and around…. Electrical forces should anchor hot gas (plasma) onto the spinning field lines…. As the field lines spin, centrifugal forces [forces pushing outward] should fling the plasma outward along them to form two magnetized jets, one shooting outward and upward, the other outward and downward. 26
Scientists have also determined that when black holes, including spinning ones, interact with normal space, they can create strange time distortions. That is, the passage of time experienced by an observer located outside a hole looking in will be markedly different from that of an observer located inside a hole looking out.
Consider the example of two astronauts in a spaceship orbiting the black hole from a safe distance. One exits the ship and propels himself toward the hole, which draws him in. From the point of view of his friend aboard the ship, he will spiral inward increasingly slowly and eventually become frozen on the hole's spinning event horizon. Round and round he will go, getting closer and closer to the horizon for years, and indeed forever; but he will never seem to pass through it into the hole. However, the point of view of the astronaut who approaches the horizon will be quite different. Time will seem to pass normally for him; he will feel himself journey from the ship to the horizon and then cross over the horizon into the black hole, all in only a few minutes.
If humanity ever develops sufficiently advanced technology, this time differential, called time dilation, might be exploited to allow people to propel themselves forward in time. (It must be emphasized that such ventures would take place outside the black hole and be unrelated to the even weirder distortions of space and time that might exist inside.) Gribbin explains how such journeys might be accomplished. A group of astronauts would bid farewell to observers on a space station orbiting far from a black hole and fly a ship toward the hole's event horizon. "The longer the astronauts spend near the event horizon, and the closer they get to it," Gribbin says,
the stronger the effect will be. You don't even need enormously powerful rockets to take advantage of the effect, because the astronauts could use a judicious, short-lived blast on their rockets to set their spacecraft falling on an open orbit down into the region of highly distorted spacetime, leaving the observers behind…. The falling spacecraft would coast in … being accelerated by the gravity of the black hole up to the point of closest approach. Then, it would whip around the hole very sharply … and climb out again, now being slowed all the time by gravity. At the farthest distance from the hole, the astronauts could fire their rockets briefly again, to put the spacecraft back alongside the space station of the observers, who [would be] ready to compare clocks. 27
When the astronauts and observers do compare clocks, they will find a noticeable difference. Whereas the astronauts may have experienced the passage of only a few hours, the observers' clocks will record that several weeks or months have gone by. If the astronauts are careful to choose just the right orbit and speed around the event horizon, they might be able to leap ahead dozens, hundreds, or even thousands of years. They could not travel backward to their starting point, however, as the effects of time dilation work in only one direction—forward (at least in ordinary space). Also, in longer journeys through time, the original observers will grow old and die while the astronauts are away; so each time the travelers visit the space station, they will be greeted by a new group of observers.
Traveling forward in time by using the strange distortions of spacetime generated near the event horizon of a black hole may eventually become possible. Yet such undertakings would be risky and the results would be highly unpredictable and of questionable value. So a majority of people in an advanced future society may well feel that it makes more sense to exploit the unusual properties of black holes in more practical ways.
Indeed, black holes produce enormous amounts of raw energy. And it would certainly be advantageous for human beings to harness some of that energy, which could be converted to electricity to power homes, offices, public buildings, and even cars and ships. A number of scientists believe that such a goal will actually be attainable in the future. Admittedly, many difficult technical problems would have to be overcome before people could control and tap into these cosmic powerhouses. But after all, only a century ago space shuttles, artificial satellites, nuclear power, television, computers, and the Internet, all of which required the development of bold new technologies, did not exist.
Farming the energy of black holes would utilize the same basic principle used in existing types of energy production. Namely, when any kind of fuel is burned or destroyed, some of its mass is converted into energy, which people then exploit. When people burn oil or coal, only about 1 percent of the fuel's mass is converted into energy. Obviously, this is not very efficient and produces a lot of soot and other waste materials that pollute the environment. Even nuclear reactions, like those produced in nuclear power plants, convert only 2 or 3 percent of their mass into energy. By contrast, when matter is annihilated at the event horizon of a black hole, up to 30 percent of its mass becomes energy. In theory, people could stoke such a black hole furnace by firing asteroids and other space debris toward the hole, destroying the debris and thereby generating energy. If people could find a way to capture that energy, say by installing large collection grids around the hole, Earth could be provided with seemingly unlimited power.
On an even grander scale, given further technological advances, people might actually be able to create black holes from scratch. As John Taylor explains, this would yield mass-to-energy conversions considerably higher than 30 percent:
We can envisage a technologically very advanced intelligent civilization which goes in for black-hole farming. To do this, they would spread hydrogen or helium throughout a region of a galaxy, or concentrate some already there, so that large stars were formed rapidly. These would then be used as energy generators during their nuclear burning phase, and allowed to collapse to black holes, also collecting supernova energy emitted during the short implosion [collapse] time. The resulting black holes would then be brought together in pairs by suitable methods to obtain a large fraction of their available energy. The resulting single black hole would then be finally exhausted of all its remaining available rotational and electrical energy. The amount of energy available in this way would be enormous. 28
In 1974, English physicist Stephen Hawking surprised the scientific community by showing that black holes can give off radiation and thereby lose some of their mass. Some of the matter at the edge of the event horizon, he said, would consist of pairs of particles. One particle would be positively charged, the other negatively charged. In his book about Hawking, scholar Paul Strathern writes: "The black hole would attract the negative particle, while at the same time it would eject the positive particle. This would escape in the form of radiation." This radiation, now called Hawking radiation, would be in the form of heat. Its temperature would be "mere millionths of a degree above absolute zero," says Strathern, "but it would undeniably be there."
In his book Explorations , astronomer Thomas Arny points out that, because of this phenomenon, black holes must eventually evaporate. "However," he adds, "the time it takes for a solar-mass black hole to disappear by 'shining itself away' is very long—approximately 10 67 years! This is … vastly larger than the age of the universe—but the implications are important: even black holes evolve and 'die.'"
Of course, humans may never achieve the level of technology needed for such fantastic engineering projects. Also, they may find it too difficult and time-consuming to travel hundreds or thousands of light-years in search of stellar black holes to exploit. However, that does not necessarily rule out human exploitation of black hole–generated energy. A more modest and plausible approach would be to build instruments capable of detecting mini–black holes that stray through our solar system. Once found, such objects might be captured and mined in a manageable way. "A stream of frozen hydrogen pellets can … be aimed past the mini–black hole," says Asimov,
so that it skims the Schwarzschild radius without entering it. Tidal [intense gravitational] effects will heat the hydrogen to the point of fusion, so that helium will come through at the other end. The mini–black hole will then prove the simplest and most foolproof nuclear reactor possible, and the energy it produces can be stored and sent down to Earth. 29
These are only some of the ways in which the highly unusual properties of black holes may someday be exploited by humanity, or by other races of intelligent beings. Only time will tell.