High in the sky on a clear fall evening is the star group called Cygnus, the Swan. It is not hard to find. Look for a large pattern of stars in the shape of a cross, or a swan in flight.

 

A map of the constellation Cygnus is shown in Figure 1. On the map, the end of the swan's tail is marked by a bright star called Deneb. Slightly ahead of Deneb are three stars in a line. They represent the swan's body and the tips of its wings. Finally some distance away is a fifth, fairly bright star, Albireo. It marks the position of the great bird's head.

 

Now look halfway along the neck of the swan. There is a point on the map labeled Cygnus X-1. The next time you are outside on a clear, dark fall evening, locate Cygnus and gaze at the spot where Cygnus X-1 lies. Though you will not actually see anything, you will be looking at the exact point in space where scientists believe there may be a black hole (see Figure 2).

 

Crushed Out of Sight

Black holes are popular subjects in films, as well as in many science fiction stories. But there is a good chance that black holes really exist in space. They are places where the pull of gravity is so strong that nothing, not even light, can escape from them. Once inside a black hole, you could never come back out the same way. However, it is possible that you could escape by a different route and arrive at a totally different part of the Universe. What is more, a journey into a black hole might transport you through time, either into the far future or the remote past.

 

How can black holes be made? One way is by the explosion of very heavy stars (see Figure 3). A star that weighs 20 or 30 times as much as the Sun can shine brightly for only a few million years. Then it blows itself apart. During this huge explosion, known as a supernova, all the top layers of the old star are blasted away into space at high speed. However, the core, or central part, of the star may remain whole.

 

In a normal, middle-aged star, such as the Sun, the core is the place where light and heat are made. Here the temperature is incredibly high – many millions of degrees. The outward pressure of this light and heat prevents the inward force of gravity from squeezing the core any smaller. For most of a star's life, these two great forces struggle against one another in an evenly matched game of tug-of-war. But in a dead star, there is no longer any light pressure to oppose gravity. As a result, the core is squeezed tighter and tighter and gets smaller and smaller.

 

When average-sized stars, such as the Sun, reach the end of their lives, their cores shrink down to hot balls of squashed matter. These are called white dwarfs. Then another force, caused by particles of matter becoming too crowded together, stops gravity from crushing a white dwarf to an even smaller size.

 

In bigger, heavier stars, the force of gravity acting on the dead star's core is much stronger. Even after the supernova explosion has blown away much of the star's contents, the core that remains may be heavier than the Sun. If the core is more than three times as heavy as the Sun, nothing can prevent gravity from crushing the core smaller and smaller. From an original size of more than 20,000 miles across, the core is squashed in less than a tenth of a second to a ball only 25 miles across. At this stage, a tablespoonful of its matter would weigh the same as four billion full-grown elephants. But gravity squeezes it still smaller. In a fraction of a second, more than three sun's worth of star material becomes crammed into an incredibly tiny space. Now it may be no larger than the period at the end of this sentence.

 

Within a few miles of the totally crushed star, gravity is so strong that it will pull in anything that comes too close. And it will allow nothing to escape, not even a ray of light traveling at more than 186,000 miles per second. The region around the crushed star is completely black and invisible. That is why scientists call it a black hole.

 

The Mystery of Cygnus X-1

If black holes are black and invisible, then how can we ever know they are there? In fact, we cannot, unless there is something nearby that can be seen and upon which the black hole has a noticeable effect. This is the case with Cygnus X-1 (see Figure 4).

 

From observations made by instruments in space, scientists have discovered that huge amounts of X-rays – rays that carry a great deal of energy – are coming from the direction of Cygnus X-1. They have also found that a binary star lies in the same position as the source of the X-rays. A binary star consists of two stars that are circling around each other. One of these stars is much bigger and brighter than the Sun, but it can only be seen through a large telescope because it is so far away. Astronomers know it is there because of the "wobbles" it causes in the movement of its giant neighbor.

 

From the extent of the wobbles, astronomers think the dark star in Cygnus X-1 must weigh from five to eight times as much as the Sun. This fact alone suggests that it is likely to be a black hole. But the X-rays offer still stronger evidence. Careful studies of the X-rays have revealed that they are almost certainly coming from a whirlpool of extremely hot gas. This gas, scientists believe, has been stripped away from the bright giant star by the gravitational pull of a nearby black hole. Just before it disappears down the black hole, the gas is heated to more than 18 million °F. At that superhigh temperature, it gives off an intense X-ray glow.

 

Into the Black Hole

Even before scientists found signs of real black holes in the Universe, they had studied the mathematics of what black holes might be like inside. According to their theories, black holes may be like the entrances of tunnels that join different regions of space and time (see Figure 5). These strange tunnels are called wormholes. At the end of a wormhole is an exit known as a white hole. By going into a black hole, traveling along its wormhole, and then coming out the white hole at the other end, a spacecraft might be able to leap across huge distances of space and millions of years in time.

 

But two British scientists, Stephen Hawking and Roger Penrose, pointed out some problems with this wonderful way to travel. For one thing, there seems to be an energy barrier inside a black hole. No normal object, such as a spacecraft, could pass through this barrier without being torn to bits. The two scientists identified a second major problem. It appears that the wormhole tunnel would instantly squeeze shut if a piece of matter tried to move along it.

 

However, in 1988, new results were published by researchers Michael Morris, Kip Thorne, and Ulvi Yurtsever at the California Institute of Technology. These showed that a wormhole might be kept open with the help of two round plates that carried a charge of electricity. The plates would be located on either side of the "throat" leading into the wormhole. Results from even more recent research have shown that objects entering a spinning black hole might also be able to travel through time.

 

Yet, just because something is possible in theory does not mean it will quickly, or ever, become fact. We are still not certain that black holes exist. The evidence for them, though, is strong. If they do exist, then the nearest one is likely to be many trillions of miles away. Cygnus X-1 is about 10,000 light-years from Earth. One light-year is the distance that light travels in a year, or about 6 trillion miles. Cygnus X-1, then, lies about 60 thousand trillion miles away!

 

It is possible that there are closer black holes to Earth that we have not yet found. If they are not members of binary systems, then they would be extremely hard to detect. Still, it would be surprising if there were a black hole similar to Cygnus X-1 that was closer than a few hundred light-years to the Sun. At such a distance, it would be very difficult to reach such an object. And it would be even harder to use it as a time machine.

Black Holes, Large and Small

Much larger black holes, weighing millions or even billions of times as much as the Sun, are thought to lie in the center of galaxies. Galaxies are huge collections of stars arranged in spiral, round, oval, or irregular shapes. We live in a galaxy called the Milky Way. To explain the unusual amount of energy coming from the middle of large galaxies, scientists have proposed the idea of "supermassive" black holes. But these would lie even farther from Earth than black holes that formed from neighboring stars.

 

There may also be mini back holes. These may be smaller than a pea but with the mass, or amount of matter, of a mountain. It is also possible that scientists will someday be able to create their own tiny black holes in the laboratory. They might do this by directing extremely intense, pure beams of light into a tiny pellet of matter. If enough energy could be focused onto the pellet at one time, it would collapse to form a black hole so small that it could only be seen under a microscope.

 

But there is a problem with this plan. Small black holes would tend to rip apart any object that was sent into them. This would happen because the pull of gravity on anything approaching a mini black hole would be much greater at the front of the object than at the back.

 

In the case of a very large, massive black hole, the difference in gravitational pull across an approaching object would be much less. The supermassive black holes that may occupy the center of some galaxies also appear to be the only kind that human beings might be able to enter and survive.

 

Beyond the Light Barrier

According to Einstein's Special Theory of Relativity, no object can accelerate up to the speed of light. As an object goes faster, its mass increases. That makes it harder and harder to boost its speed further. To reach the speed of light, even a particle that started out with a tiny mass would require more energy than there is in the whole Universe.

 

Einstein's theory, though, does not say that faster-than-light particles cannot exist. It only says that if there are such particles, known as tachyons, then they can never travel at or less than light speed.

 

If tachyons did turn out to be real, they would behave in a very strange way. They would travel backward in time! This is another prediction of Einstein's theory. Any object that travels faster than light would seem to us to be moving into the past. As a result, a tachyon could be detected by an instrument before it was actually formed. On the practical side, tachyons might make possible an unusual kind of telephone. On this phone, calls could be sent into the past! So far, despite searches carried out by various groups of researchers, no real tachyon has yet been found.

 

The Prospect for Time Travel

Could you then build a time machine? With devices such as cameras and video recorders, it is already possible to review events from the past. Researchers are also making progress in learning how humans age and how the aging process might be slowed. Perhaps within 20 or 30 years, there will be methods to allow people to live longer. If so, they will see much more of the future.

 

Other forms of time travel will probably take longer to develop. It is unlikely that there will be any crew carrying spaceships that can fly close to the speed of light by the end of the twenty-first century. But such craft will be built. Then human beings who travel to the stars will go on journeys through time. They will leap hundreds, thousands, or even millions of years into the future during their own lifetimes.

 

Scientists today do not know if black holes will ever be used as a means to jump instantly into the remote past or future. The technical problems to be overcome, even if such journeys are possible, are among the most difficult imaginable. Yet the people who lived a century ago might have thought of human missions to the Moon in the same way.

 

Someday, in some form, the human race is likely to build a machine that can swiftly travel through time. Where that will lead us to, no one yet knows.

 

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