"Abandon all hope, ye who enter here," would be an appropriate warning for any space traveler foolish enough to approach a black hole. Black holes are proposed by astrophysicists as regions of space where gravity is so strong that the black holes act like stellar vacuum cleaners, sucking in matter and energy from space and allowing nothing, not even light, to escape.
The American physicist John Wheeler coined the term "black hole" in 1969, but, in fact, the theory has been around for much longer. As far back as 1783, English astronomer John Michell suggested that if a star were massive enough, it would have such a strong gravitational field that any light leaving the star would immediately be dragged back to the star's surface.
Michell's theories were largely ignored until 1939, when physicists Robert Oppenheimer and Hartland S. Snyder demonstrated that, based upon Albert Einstein's general theory of relativity, it would be possible for a star to collapse to the point where it would become a black hole.
|This artist's concept of frame dragging in a black hole shows the curvature of space-time. No, it's not science fiction! Frame dragging is where the fabric of space (not just matter) is literally shifted by the gravitational pull of a black hole. A black hole is a region defined as the ultimate expression of gravity. (NASA)|
How a Star Ages
In order to understand how a star could collapse into a black hole, it is first important to understand the life cycle of a star. A star is, essentially, a giant fusion reactor. At the central core of the star, swirling atoms of hydrogen gas collide with one another and merge to form helium. In the process of fusing together, these hydrogen atoms release a tremendous amount of energy in the form of heat.
At the same time, the star as a whole is continuously struggling against the inward pull of gravity. The inward gravity is from the central core of the star, which is surrounded by a massive envelope of gas. This inward pull is so immense that the star is always on the verge of collapsing under its own weight.
What prevents the star from collapsing? Tremendous internal pressure that is generated by the extreme heat at the star's core, which pushes outward, counterbalancing the inward pull of gravity. In our own sun, for example, the temperature at the core is about 25,000,000 — Fhrenheit (14,000,000—C), generating pressure 100 billion times the air pressure at sea level on Earth.
After thousands of millions of years, however, a star comes to the end of its hydrogen fuel supply. It starts to cool and contract. What happens next will depend entirely on the mass of the star.
Small stars, such as our Sun, will collapse to form objects called white dwarfs. About the size of the planet Earth, white dwarfs resist further collapse with internal pressure caused by electrons spinning at near the velocity of light. White dwarfs are very dense objects: 1 cubic inch of white dwarf weighs several tons. But they are considered lightweights when compared to neutron stars.
Neutron stars are the evolutionary end products of larger stars — those 1.4 to 2 times as large as the Sun. Electrons cannot resist the greater gravitational collapse of such stars, and are pushed into atomic nuclei where they combine with protons to form uncharged, tightly packed neutrons. Neutron stars are only a few miles in diameter. They weigh about 1 million tons per cubic centimeter. They can resist further collapse only by invoking the strongest force in nature — appropriately called the "strong force" — the force that binds together an atomic nucleus.
The strong force halts the imploding matter so abruptly — in a tenth of a second — that the collapsed stellar core acts as an explosive charge. The resulting explosion in the star's outer regions is called a supernova. Such celestial fireworks, observed by Chinese astronomers in July 1054, produced the Crab nebula, a cloud of gas that still writhes and glows today, 4,000 light-years from Earth.
What happens to a dying star that is more than twice as large as the Sun? Even the strong force cannot halt its in-falling momentum. It collapses completely, beyond the neutron-star stage, to an even smaller, denser object. Back in 1939, Oppenheimer and Snyder calculated that the gravitational field at the surface of such an object would become so strong that even light (traveling at a speed of 186,282 miles — 299,792 kilometers — per second) would be unable to escape. According to Einstein's theory of relativity, nothing in the universe can travel faster than light. Therefore, if light cannot escape, neither can anything else. The collapsed star becomes what we call a black hole.
Perhaps the best way to visualize a black hole is to imagine, for a moment, that space is a flat rubber sheet. If you were to drop a steel ball on the sheet, the rubber would curve downward, forming a shallow hole. This, in a nutshell, is how Einstein interpreted gravity. According to Einstein, gravity exists because massive objects bend the fabric of space around them. If, for example, we rolled a small marble across our rubber sheet, it would roll around the top of the hole formed by the steel ball, much in the way that the Earth orbits around the Sun.
Now imagine that we could increase the weight of the steel ball that we dropped on our rubber sheet. As the weight increased, the ball would sag farther and farther downward, creating a deep "gravity" hole. Eventually the rubber would be stretched so tight that the top of the hole would pinch together, closing off from the outside world the region containing the steel ball. Similarly, a collapsed star could eventually become so dense that it would curve space completely around on itself, isolating it from the rest of the universe.
How far would a star have to collapse before it "disappeared" from the visible universe? Astronomers refer to that critical size as the "event horizon" (otherwise known as the "Schwarzschild radius," named after German physicist Karl Schwarzschild). The event horizon is the outer boundary of a black hole, the exact point at which light rays fail to escape. The horizon acts as a one-way membrane — light and matter can cross the horizon into a black hole, but once inside, the horizon can never be recrossed.
The size of the event horizon is proportional to the mass of the collapsing star. Typically, the event horizon of a star would be on the order of miles. (For example, a star 10 times as massive as our Sun would have a Schwarzschild radius of 18.6 miles — 30 kilometers.)
Yet, according to our current knowledge of theoretical physics, once a star starts collapsing, no known force can stop it. It will continue to shrink past its event horizon, smaller and smaller, until it becomes a "singularity" — a mathematical point with zero volume and infinite density. This singularity lies at the very center of a black hole.
Exploring a Black Hole
If an astronaut were to attempt to visit a black hole, it would be a one-way trip. Before the astronaut even arrived at the event horizon, he or she would encounter tremendous tidal forces exerted by the black hole. Imagine, for example, that the astronaut is falling feetfirst toward the hole. The gravitational force pulling on the legs would be considerably stronger than the gravitational force pulling on the head. The difference between those two forces would stretch the astronaut like a piece of taffy.
As if that weren't bad enough, every single atom in the astronaut's body would be pulled toward the singularity at the black hole's center. For the astronaut the sensation might be similar to being squeezed by a giant fist.
After being stretched and squeezed by the black hole's gravitational forces, our intrepid space traveler would resemble a strand of spaghetti, and would likely not be in the mood for any further exploration.
Let's imagine for a moment that we could instead send a robot probe to investigate the black hole, one that could somehow stay intact despite the tremendous tidal forces. For the sake of discussion, we will mount a clock and a light source on the outside of our probe.
If we were watching the robot back on Earth, we would notice a curious phenomenon. The light source mounted on the side of the probe would start to change color. If the light, for instance, started out green, it would turn yellow, and then red as it got closer and closer to the event horizon of the black hole.
This is because light is composed of particles known as photons. As the photons move away from the black hole, they expend some of their energy as they try to escape from the hole's tremendous gravitational pull. The closer they are to the event horizon, the more energy they need to pull away.
The energy of a photon is proportional to the frequency of its radiation. As a result, light that loses energy will have a reduced frequency, and therefore a longer wavelength. This effect is known as "gravitational redshift." When light has a long wavelength, it is red in color.
Eventually, as the robot probe moves closer and closer to the event horizon, the light source will seem to disappear from view. The wavelength of the light will have become so long that it can only be detected with infrared and radio telescopes.
Just above the event horizon of the black hole, the wavelength of the light will approach infinity. Theoretically, radiation from the light source would still reach us back on Earth, but by then the wavelengths would be so long that no known scientific instruments would be able to detect them.
Meanwhile, the clock mounted on the side of our robot probe would also be behaving rather oddly. According to Einstein's theory of relativity, time slows down in the presence of a strong gravitational field — at least as viewed by an outside observer. As the probe got nearer and nearer to the black hole, astronomers back on Earth would notice that the clock was ticking more and more slowly.
The clock would continue to slow down, until the probe arrived at the event horizon, at which point the clock would stop altogether. The probe would appear frozen in time, hovering at the brink of the black hole for the rest of eternity.
Relativity predicts, however, that from the perspective of the robot, time would not seem to be affected in any way. The probe would arrive at the event horizon and enter the black hole without the clock slowing down for even an instant. Yet our dutiful robot explorer would have only a fraction of a second to contemplate this peculiar law of nature, at which point it would be pulled toward the center of the black hole, where it would encounter the singularity and be crushed to infinite density.
Proving That Black Holes Exist
All of this might sound very strange, and, in fact, for many years the majority of astronomers and physicists were reluctant to believe it. (The prominent English astronomer Sir Arthur Eddington even declared that there must be "a law of Nature to prevent a star from behaving in this absurd way!") If astronomers were to believe in black holes, they wanted more than just mathematical equations on a blackboard; they wanted hard, physical evidence.
Such evidence became available in 1967, when two British astronomers, Jocelyn Bell and Antony Hewish, discovered objects in space that were emitting regular pulses of radio waves. At first the astronomers thought that they had made contact with an alien civilization in a distant galaxy. They even named the objects "LGMs," for Little Green Men. Eventually, however, astronomers came to the conclusion that the objects were rotating neutron stars, emitting radiation in the form of narrow beams. Like a celestial lighthouse, each time the neutron star spun toward Earth, astronomers could detect a pulse. Hence, these objects were named pulsars.
This was the first hard evidence that neutron stars actually exist. If a star could collapse into an object as small as a neutron star, it then seemed reasonable to assume that it could collapse to an even smaller size and become a black hole.
One problem remains. How do you find a black hole? They aren't as accommodating as neutron stars, in that they don't emit easily detectable beams of radiation. In fact, according to conventional theory, black holes don't emit anything at all.
Astronomers saw a way out of this dilemma. Black holes exert an enormous gravitational force on nearby objects. So although scientists can't see a black hole"in the flesh," so to speak, they can observe how it would affect its surrounding environment.
To date, binary-star systems offer the best hope for locating a black hole. Astronomers have detected many such systems, where two stars orbit around one another. In some cases the astronomers have observed only one visible star, which seemed to be in orbit around an unseen companion. It is possible that the companion might be a star too faint to be seen from Earth. It is also possible, however, that the second object could be a black hole.
If a black hole were part of a binary-star system, its enormous tidal forces would pull gaseous material off the surface of the neighboring star. Like water draining out of a bathtub, the gaseous material would slowly spiral into the black hole, forming a swirling disk of gas around the event horizon, a phenomenon that astronomers refer to as an accretion disk.
Within the accretion disk, compression and internal friction would heat the gas to temperatures as high as 1,800,000— F (1,000,000— C). When gas gets this hot, it radiates tremendous energy in the form of X rays detectable by astronomers.
In 1970, a United States artificial satellite, the Uhuru, was launched off the coast of East Africa. (Uhuru is the Swahili word for "freedom.") Its purpose was to detect sources of X rays while above the interference of Earth's atmosphere. Uhuru has found more than 100 stars emanating X-ray pulses. One of the most powerful X-ray sources was Cygnus X-1, located about 6,000 light-years from Earth.
Closer examination of Cygnus X-1 revealed it to be a binary-star system, with a supergiant star orbiting around an unseen companion. By measuring the velocity and the orbital period of the supergiant star, astronomers were able to roughly calculate the mass of the unseen object. The object was estimated to be at least six solar masses (six times the mass of the Sun), far too massive to be either a white dwarf or a neutron star. By 1974, astronomers concluded that Cygnus X-1 must contain a black hole.
In 1997, astronomers found in the core of the active galaxy NGC 6521 what appears to be a warped, dusty disk swirling around a supermassive black hole, giving them the first direct line of sight into the immediate environment of a black hole.
In January 2000, astronomers found what may be the closest black hole to Earth — a mere 1,600 light years away. Located near the center of the Milky Way in the direction of the constellation Sagittarius, the black hole emits gamma rays continuously rather than in flashes or bursts.
And deep within the core of the distant galaxy NGC 4395, astronomers discovered what may be a new type of mid-mass black hole, weighing perhaps as "little" as 10,000 to 100,000 Suns.
Perhaps the best black-hole candidate yet discovered is a binary-star system that goes by the uninspiring name A0620-00. Like Cygnus X-1, A0620-00 emits intense levels of X-ray radiation. The binary system has a visible orange dwarf star, which orbits around a dark, unseen mass. In the late 1980s, astronomers studied the motions of the orange dwarf and estimated that the star's dark companion was 3.2 times the mass of our Sun.
With a mass that large, the dark object was placed high on the black-hole suspect list. But in order to get a more accurate estimate, astronomers would have to measure the velocity of the dark object. At first that seemed impossible: How can you measure an object that you cannot even see?
The Hubble Telescope has found seemingly conclusive evidence for massive black holes at the cores of many galaxies throughout our universe. One such galaxy, known as M87, is bright enough to see with a small backyard telescope. Others are distant galaxies with highly energetic nuclei. In some of the galaxies, the Hubble Telescope has detected disks of material spiraling inward toward the black hole; in other galaxies, it has found beams of energetic radiation and gaseous knots being ejected at tremendous speeds.
English physicist Stephen Hawking suggested that radiation might not only exist in the vicinity of a black hole, but that it actually might be leaking from the hole itself. Energy leaking from a black hole? It sounds impossible. But Hawking says that black holes emit radiation in the form of subatomic particles that do not obey the traditional laws of physics. Such "virtual" particles, as Hawking calls them, can be created in pairs in empty space, only to instantly collide and annihilate each other. If such a pair were to come into being near a black hole, one particle would be sucked in, while the other would escape into space.
As a black hole loses energy, it would also lose a proportionate amount of mass. Hawking's theory suggests that there might come a time when a black hole will lose so much mass that it will no longer be able to curve space around itself. The black hole would cease to be a black hole, and the remaining mass would likely explode outward, with a force equivalent to millions of hydrogen bombs.
But don't look up in the sky expecting to see a fireworks display of exploding black holes. A large black hole lives a very long time. More specifically, it would take trillions upon trillions of years for it to lose enough energy to explode outward. The universe itself has been around for only 20 billion years.
Yet it may be possible that very small black holes, formed in the early days of the universe, might be exploding just about now, releasing energy in the form of gamma rays, equivalent to about 100 million volts of electricity.
Astronomers are now searching the skies for just such bursts of gamma radiation. If found, then astronomers could verify what Stephen Hawking has been saying for the last 20 years: "Black holes ain't so black."
In 1895 H. G. Wells wrote a book about a device that could carry a man back and forth through time. The book was called The Time Machine, and for a century after it was published, the concept of time travel remained a favorite topic among writers of science fiction.
In 1988, however, science fiction moved closer to becoming science fact when American physicist Kip Thorne and his colleagues at the California Institute of Technology (Caltech) published a paper in the prestigious journal Physical Review Letters, titled "Wormholes, Time Machines, and the Weak Energy Condition."
Thorne didn't actually publish a blueprint for a do-it-yourself time machine. He speculated that an "arbitrarily advanced civilization" might be able to find a loophole in the laws of physics that would allow individuals to travel through time.
The loophole that Thorne had in mind is what physicists call a "wormhole." A wormhole is similar to a black hole, but with one noteworthy difference. At the bottom of a black hole, there is a singularity, a mathematical point of infinite mass through which nothing can pass.
A wormhole, by contrast, has no bottom. It has two "mouths" connected by a "throat." It is, essentially, a tunnel through space. A space traveler entering one mouth of a wormhole might emerge from the second mouth only a few seconds later, but halfway across the galaxy.
Einstein's equations predict that wormholes exist, although nobody has ever found one. American physicist John Wheeler has suggested that a good place to look for one would be at a submicroscopic level, where random fluctuations occur in the fabric of space-time. In such an environment, wormholes would spontaneously appear and collapse, giving space a frothy, foam-like appearance.
Kip Thorne suggests that an advanced civilization could pull a wormhole out of this foam, enlarge it, and then move its openings around the universe until the wormhole assumed a desired size, shape, and location. Unfortunately, once such a wormhole was created, it would be highly unstable. If a space traveler entered the wormhole, the throat might instantly pinch shut. Even moving at the speed of light, the space traveler might be unable to reach the other side of the wormhole before it collapsed around him or her.
In order to avoid such a catastrophe, the Caltech physicists recommend that our hypothetical advanced civilization thread the throat of the wormhole with what they call "exotic material." In order to prop open a wormhole a half a mile or so across, the material would have to possess a radial (outward) tension comparable to the pressure at the center of a neutron star. Kip Thorne believes that there is a 50–50 chance that the laws of physics permit such a substance to exist.
Once our "arbitrarily advanced scientists" finished building a safe, traversable wormhole, they would be ready to convert it into a time machine. At this point they would rely upon Albert Einstein's general theory of relativity. According to Einstein, time slows down for a moving object when it is measured by a stationary observer.
This is often illustrated with what is known as the "twin paradox." Imagine that you have twin brothers, named Bill and Ted, each 20 years old. Bill takes off in a spaceship, while Ted stays back on Earth. Bill's destination is a star 25 light-years away. (A light-year is the distance that a beam of light can travel in one year.) His spaceship can attain a speed of 99.9 percent of the speed of light. From Ted's point of view, Bill will be gone for 50 years (25 years to reach the star, plus 25 years to return). However, from Bill's point of view on board the spaceship, the entire trip will last only one year. This effect is known as "time dilation." When Bill returns to Earth, he will be only 21 years old, but his brother Ted will be a 70-year-old man.
Now, instead of two brothers, imagine that we are dealing with two mouths of a wormhole. Our advanced civilization could move one end of the wormhole, perhaps by using a heavy asteroid or a neutron star as a kind of gravitational tugboat. If the mouth of the wormhole were accelerated to a high enough speed and then returned to its original position, it would behave just like our space-traveling twin brother. A clock fixed to the moving mouth would tick more slowly than one at the stationary mouth.
For instance, the clock outside the accelerated mouth might read 12:00 noon, but the clock outside the stationary mouth would read 1:00 p.m. By passing from one mouth to the other, a space traveler could move back and forth through time.
How far could our traveler move through time? That would depend upon how long and how fast the wormhole mouth is accelerated. If the mouth were moved at 99.9 percent of the speed of light for 10 years, the time difference between the two mouths would be nine years and 10 months. Theoretically, if you accelerated a wormhole mouth fast enough and long enough, the time difference between the two mouths could be stretched across several centuries.
There is, however, a limitation to Kip Thorne's time machine. Common sense tells us you cannot travel back to a time before you created the wormhole and accelerated one of the mouths through space. After all, what we're doing is exploiting the relative rate at which time passes under the effects of speed. So, unfortunately, you could not pop back through time to visit the dinosaurs. Unless, of course, you were lucky enough to find a time hole that had already been constructed by an advanced civilization several million years ago.