So far, stellar and galactic black holes have been considered in light of how their major properties—extreme gravity, accretion disks, quasars, and so forth—affect matter, space, and time in the universe surrounding them. Very little has been said about what happens inside a black hole other than that matter is either crushed or falls down the hole's gravity well forever. This is partly because the inner workings of these cosmic oddities remain largely mysterious. Obviously, there is no known way to see into or directly measure the inside of a black hole.
But that has certainly not stopped people from trying to visualize what lies beyond the enigmatic event horizon. Ever since serious consideration of black holes began in the 1960s, various theories and mathematical equations have predicted that certain things are likely to exist or occur inside black holes. And when the theories have seemed inadequate, investigators have freely used their imaginations. They have wondered, for example, whether bodies outside the hole would be visible from the inside since they know that matter inside a black hole is not visible from the outside. Also, matter that enters a black hole disappears from the regular universe. Does this matter cease to exist, or does it somehow survive and reemerge somewhere else, either in this universe or another one? Moreover, if the matter can survive intact, might it be possible for people, too, to survive a trip into a black hole?
Not surprisingly, science fiction writers have frequently and colorfully exploited these and other bizarre possible qualities of the inner environments of black holes. Most often, they portray these superdense objects disturbing the fabric of space and finally tearing it, thereby creating a small opening. Such an opening and the invisible spatial tunnel it leads to are together commonly referred to as a wormhole. As of yet, wormholes are technically theoretical, although physicists believe they are likely to exist.
Some of the mathematical formulas associated with the theory of wormholes suggest that if one end of a hole is fixed and the other end is moving, each will end in a different time frame. In this excerpt from The Physics of Star Trek , physicist Lawrence M. Krauss tells how writers for Star Trek: Voyager correctly depicted this phenomenon.
Wormholes, as glorious as they would be for tunneling through vast distances in space, have an even more remarkable potential, glimpsed most recently in the Voyager episode "Eye of the Needle." In this episode, the Voyager crew discovered a small wormhole leading back to their own "alpha quadrant" of the galaxy. After communicating through it, they found to their horror that it led not to the alpha quadrant they knew and loved but to the alpha quadrant of a generation earlier. The two ends of the wormhole connected space at two different times! Well, this is another one of those instances in which the Voyager writers got it right. If wormholes exist, they can well be time machines! This startling realization has grown over the last decade, as various theorists … began to investigate the physics of wormholes a little more seriously.
Science fiction stories and films usually describe piloted spacecraft traveling through wormholes and emerging either in distant regions of the galaxy or in the past or future. In the popular television series Babylon 5 , for example, an interstellar space station floats near wormholes leading to various distant star systems. And in the final episode of the television series Star Trek: Voyager , Captain Janeway uses a wormhole to travel through both space and time in an effort to alter the past. Incredibly, in recent years physicists have shown that these kinds of journeys, though certainly not feasible using existing human technology, are theoretically possible.
Indeed, noted planetary scientist Carl Sagan learned of this possibility to his delight in the summer of 1985. At the time, he was working on a science fiction novel titled Contact and wanted his main character to traverse huge cosmic distances in very short time spans in a scientifically plausible way. Not being a specialist in general relativity, Sagan turned to one of the leading experts in that field, Kip Thorne, of the California Institute of Technology (or Caltech for short). "It occurred to me," Thorne later wrote, "that his novel could serve as a … tool for students studying general relativity." 46 With this in mind, Thorne accepted the challenge and enlisted the aid of two of his doctoral students, Michael Morris and Ulvi Yurtsever.
After exploring the mathematical possibilities, they informed Sagan that a spacetime geometry incorporating the concept of wormholes as cosmic gateways was theoretically possible. One gateway might allow matter to enter "hyperspace," a hypothetical region lying beyond normal space, and exit back into space at another similar portal. "To be sure," John Gribbin points out,
the physical requirements appear contrived and implausible. But that isn't the point. What matters is that there seems to be nothing in the laws of physics that forbids travel through wormholes. The science-fiction writers were right—hyperspace connections do, at least in theory, provide a means to travel to distant regions of the universe without spending thousands of years puttering along through ordinary flat space at less than the speed of light. 47
Sagan's inquiry and the Caltech team's calculations stimulated a sudden burst of interest in the scientific community. And since that time a good deal of research into wormholes and possible travel through them has been conducted. These efforts did not come out of a scientific vacuum, however. Decades before, a few scientists had considered the basic idea that wormholes might be a physical consequence of the warping of space by black holes. In 1916, shortly after Einstein's and Schwarzschild's equations for general relativity appeared, an Austrian scientist, Ludwig Flamm, examined them closely. Flamm pointed out that these equations allowed for some kind of invisible connection between two distinct regions of spacetime. German mathematician Hermann Weyl came to a similar conclusion in the 1920s.
In 1935, Einstein himself, working with a colleague, Nathan Rosen, explored the concept of this mysterious connection in more detail, including its relation to superdense objects. They conjectured that a sort of tunnel might exist inside a black hole. This tunnel, which would inhabit a region outside of normal space, might connect with another black hole somewhere else. For a while, researchers called such cosmic tunnels Einstein-Rosen bridges, after the men who first proposed them; only later did they acquire the name wormholes.
It should be emphasized that Einstein and Rosen did not mean to suggest that people could actually enter a black hole and use it as a gateway to somewhere else. They merely showed that mathematics did not forbid the existence of such tunnels. They did not pretend to know how big, how long, or how safe these tunnels might be, and in any case, the idea of traveling though them seemed irrelevant. For one thing, mathematical calculations indicated that any such wormhole would open up for no more than 1/10,000 of a second and then close. Indeed, it would not stay open long enough to allow even light to travel from one end of the tunnel to the other, so how could a much slower-moving spaceship get through? Also, all of these scientists agreed that any matter entering a black hole, including a person, would be demolished; even his or her atoms would be torn apart. So no space traveler who did fly into a black hole would survive long enough to make it to the wormhole, let alone travel through it.
However, in the 1960s new research began to alter this seemingly hopeless outlook. Flamm, Einstein, Rosen, and the others had based their calculations and opinions mainly on the workings of static, nonspinning Schwarzschild black holes. Yet as time went on, more and more scientists agreed that such bodies are theoretical constructs and do not exist in the real universe. When Roy Kerr introduced his mathematical solution for spinning black holes and it became clear that all such superdense bodies must be rotating, the picture of a black hole's interior changed. Now it could be seen that the singularity is shaped like a ring rather than an infinitely tiny point. And the mathematics for such rotating rings does suggest that matter can pass through them without being crushed. (Of course, this does not rule out the possibility that the matter will suffer other lethal effects, such as bombardment by deadly radiation.)
The idea that a ring singularity might be a portal to a wormhole in hyperspace opens up a host of intriguing possibilities for the geometry of the region inside a black hole. Among these is the notion that some of the basic properties of the normal universe will be reversed. In the case of an astronaut diving through the middle of a ring singularity, Gribbin explains,
the world is turned upside-down. The equations tell us that as you pass through the ring you enter a region of spacetime in which the product of your distance from the center of the ring and the force of gravity is negative . This might mean that gravity is behaving perfectly normally but you have entered a region of negative space in which it is possible to be, for example, "minus ten kilometers" away from the center of the hole. Even relativists [experts in general relativity] have trouble coming to terms with that possibility, so they usually interpret this negativity as meaning that gravity reverses as you pass through the ring, turning into a repulsive force that pushes you, instead of pulling. In the region of spacetime beyond the ring, the gravity of the black hole repels both matter and light away from itself. 48
Although most physicists agree, at least in theory, that Gribbin's astronaut could enter and experience the strange effects of hyperspace inside the black hole, they caution that it is by no means certain that he or she could do so safely. First, they warn, there are serious dangers lurking outside the event horizon. Even before entering the black hole, the astronaut would have to find some way of surviving the extreme tidal forces and searing radiation in the spinning accretion disk.
For the sake of argument, however, assume that the astronaut manages to invent special shielding to protect against these lethal effects. And he or she makes it across the event horizon and into the black hole in one piece. From that point on, it is far from certain that the astronaut will be able to make it back to his or her starting point in space and time. Astronomer Sagan addresses this problem in Contact:
As measured from Earth, it takes an infinite amount of time for us to pass through a black hole, and we could never, never return to Earth…. A Kerr-type tunnel can lead to grotesque causality violations [a breakdown of normal cause and effect]. With a modest change of trajectory [its path] inside the tunnel, one could emerge from the other end as early in the history of the universe as you might like—a picosecond [a small fraction of a second] after the Big Bang, for example. That would be a very disorderly universe. 49
In his acclaimed book Black Holes and Time Warps , noted physicist Kip Thorne describes the phenomenon called the matricide paradox this way: "If I have a time machine … I should be able to use it to go back in time and kill my mother before I was conceived, thereby preventing myself from being born and killing my mother." Obviously, the paradox lies in the fact that the murder appears to stop itself from happening.
Thorne credits Joe Polchinski, a physicist at the University of Texas in Austin, with supplying the following scientific description of how the paradox might work:
Take a wormhole that has been made into a time machine, and place its two mouths [each located at a black hole] at rest near each other out in interplanetary space. Then, if a billiard ball is launched toward the right mouth … with an appropriate initial velocity, the ball will enter the right mouth, travel backward in time, and fly out of the left mouth before it entered the right … and it will then hit its younger self, thereby preventing itself from ever entering the right mouth and hitting itself.
Still another challenge for the astronaut to overcome is the instability of the wormhole gateway and tunnel. A wormhole in a Kerr black hole might well remain open a good deal longer than the extremely short-lived version in a Schwarzschild black hole. However, a Kerr wormhole would still be highly fragile. Even the rather small gravitational effects created by the astronaut and his ship entering the tunnel might be enough to cause its collapse, which would simply crush the ship out of existence. According to Sagan:
There is an interior tunnel in the exact Kerr solution of the Einstein field equations, but it's unstable. The slightest perturbation would seal it off and convert the tunnel into a physical singularity through which nothing can pass. I have tried to imagine a superior civilization that would control the internal structure of a collapsing star to keep the interior tunnel stable. This is very difficult. The civilization would have to monitor and stabilize the tunnel forever. It would be especially difficult with something as large as [a spacecraft] falling through. 50
This tendency of wormholes to collapse easily seems at first glance to rule out using such tunnels as gateways to other places and times. And this is the problem that Thorne, Morris, and Yurtsever faced when they began tinkering with the mathematics of black holes at Sagan's request. They were able to overcome the problem because they used a fresh approach. Instead of treating wormholes as hypothetical objects and trying to predict how they would work if they did exist, they began with the assumption that a stable, traversable wormhole could exist. They described the likely geometry of such an entity; and finally, they applied the principles of general relativity to predict what kind of matter would be needed to keep it open and stable.
The result was seen as a major breakthrough in theoretical physics. The Caltech team's equations showed that some kind of matter would be needed to exert the pressures required to keep the wormhole stable and open long enough for travelers to pass through. But ordinary matter does not exert enough pressure to do the job. Instead, some kind of extra ordinary matter would be needed. Thorne called it exotic matter, or material, about which he later wrote:
I learned from the Einstein field equation, that, in order to gravitationally … push the wormhole's walls apart, the exotic material threading the wormhole must have a negative energy density [a state in which the material exerts no internal pressure, as material in the normal universe does]…. Because almost all forms of matter that we humans have ever encountered have positive average energy densities in everyone's reference frame, physicists have long suspected that exotic material cannot exist…. Then in 1974, came a great surprise. [Stephen] Hawking [determined] that vacuum fluctuations [random gravitational effects] near a hole's [event] horizon are exotic . … The horizon distorts the vacuum fluctuations away from the shapes they would have on Earth and by this distortion it makes their average energy density negative, that is, it makes them exotic. 51
Although exotic matter has not been proven categorically to exist, a number of scientists think that measurable quantities of it may have drifted through the early universe. Perhaps, they say, small amounts of it still exist here and there in the present universe. Possibly, an advanced race of beings could find a way to manufacture exotic matter out of ordinary matter.
If exotic matter does exist naturally or else can be manufactured, the problem of keeping a wormhole open and stable would be solved. And barring any other unforeseen impediments, it would be possible to travel through such a cosmic tunnel. But why would someone embark on such a journey? Assuming they manage to develop the advanced technology required to stabilize and manipulate wormholes, why would humans or other intelligent beings choose travel in hyperspace over travel in ordinary space?
First, the math suggests that wormholes might create shortcuts to distant locations. In other words, if it takes a thousand years for a fast-moving spaceship to reach a faraway planetary system, manipulating a wormhole in just the right manner might allow the ship to make the trip in a much shorter amount of time. Scientists often point to the analogy of a watermelon, the outer surface of which represents normal space. An ant walking on the watermelon represents a spacecraft on a long journey to a distant location on the opposite side of the watermelon. Even when moving as fast as it can, the ant requires three full minutes to complete the trip. Just before setting out, however, the creature sees a nearby hole, the mouth of a tunnel that appears to go straight into the heart of the watermelon. The ant thinks twice about entering the hole because it seems to lead into an unknown region very different from the familiar realm of the surface. But it takes a chance and crawls down the hole. Following the tunnel, the ant travels straight through the center of the watermelon and climbs out of another hole on the opposite side, right beside its destination. The route it chose is shorter and more direct than the one on the surface, so the trip took only a minute instead of three.
In a similar manner, people in spaceships might someday take advantage of wormholes to reach faraway locations faster. If so, they will also travel through time. Just as time slows down for objects and people traveling near the speed of light in normal space, time will behave strangely and changeably inside a black hole or in hyperspace. To an astronaut floating inside a black hole (if it is actually possible to do so), time would seem to pass quite normally. But from his or her point of view, the universe outside the event horizon would appear to move abnormally fast. The astronaut might remain in hyperspace for a week and then exit the wormhole to find that the world he or she left had aged ten thousand years.
Another possibility is that some wormholes might lead not to other parts of the known universe, but instead to unseen alternate universes. No direct evidence for such places has yet been found. But some scientists have suggested that the violent explosion
of the Big Bang could conceivably have created "bubbles," shells encasing distorted regions of space and time separate from and completely invisible to ordinary space and time. There is no guarantee that the known laws of physics would be the same inside these bubble universes. So any humans who entered such a place via a wormhole might die instantly, or at least find themselves in an unimaginably bizarre and hostile environment.
Invoking the image of the Big Bang suggests one final, mind-bending concept involving black holes. As near as scientists can tell, that tremendous primeval explosion expanded outward from a single, infinitely small point in spacetime, a point exactly like the theoretical singularity of a black hole. Perhaps the known universe emerged from a massive superdense object. And the atoms making up our bodies, Earth, the stars, and everything else we can see coalesced from the compact, swirling energies buried deep inside the guts of the greatest of all cosmic monsters. As Begelman and Rees put it:
Hidden from view inside their "horizons," [black holes] hold secrets that transcend the physics we understand. The central "singularity" involves the same physics that occurred at the initial instants of the Big Bang and will recur again if the universe recollapses. When we really understand black holes, we will understand the origin of the universe itself. 52