A

# COULD YOU EVER FLY TO THE STARS? 3. Through the Light Barrier

Figure 1. 'Warp drive'.

Figure 2. Albert Einstein.

Figure 3. Artist's impression of a black hole.

The fastest that most people ever travel on Earth is about 600 miles per hour, the cruising speed of a large airliner. Compare this with the 670 million miles per hour of a light ray, and it is clear that we have no experience of really high speeds at all.

Still, we can use our common sense. The only effect of traveling very fast would be to make the time of our journey shorter. Whether we travel at 5 miles per hour or 500 million miles per hour should not make any difference apart from getting to our destination quicker. Isn't that right?

No – it is wrong. In 1905, the brilliant German scientist Albert Einstein caused a revolution in the way we think about the universe. In his special theory of relativity, he showed that quantities such as mass, length, and time are not as fixed as they seem. In fact, very odd things start to happen when objects move relative to one another at speeds close to that of light.

## The Curious Case of the Mixed-Up Clocks

Suppose a spaceship makes a round trip to the nearest star, Proxima Centauri, at a steady nine-tenths of the speed of light. (We will assume that it comes straight back without stopping.) Then to everyone on Earth, the journey will seem to have taken about 9½ years. Yet, according to clocks carried aboard the spacecraft, only slightly more than 4 years will have gone by. Incredibly , the astronauts will have aged 5½ years less than their friends who stayed behind on Earth!

Strangely, as Einstein discovered, time slows down for objects traveling at extremely high speeds (see Figure 2). If someone on Earth with a powerful telescope could watch the clocks aboard a fast-moving starship, those clocks would seem to lose time. The faster the spacecraft went, the slower the clocks would seem to tick. Even the astronauts would appear to do everything in slow motion. Yet, from the astronauts own points of view, aboard the starship, all would appear normal.

Other odd changes take place, too, at high speeds. As view from Earth, a fast-moving spaceship would seem to shrink along its direction of motion. Also, its mass would increase. At nine-tenths the speed of light, a spacecraft would look less than half as long as it did at rest and twice as massive. Yet to the crew, the ship would appear unchanged. To them, dust particles and everything in space would appear more squashed together and more massive.

The changes that occur at such high speeds have an important bearing on future travel to the stars. They make it possible for astronauts to visit even very distant stars well within their own lifetimes. If the astronauts travel fast enough, they will age much more slowly than if they had stayed on Earth. For instance, a trip to a star 100 light-years away at 99 percent of the speed of light would take more than a century as measured by Earth clocks. But the astronauts who made such a journey would age by only 10 years. In starships that moved even faster, people could cross hundreds or thousands of light-years in just a few years as measured by the clocks aboard their spacecraft.

But when the crew returned from such a voyage, they would find that all their friends and relatives had long since died. The crew members would have aged by only a few years. Yet they would find that the Earth and everything on it would be centuries older than when they had left. It is hard to imagine that anyone would want to go back to a home planet that could no longer recognize.

## On the Trail of the Starship Enterprise

In the language of Star Trek, "warp factor one" means the speed of light. The starship Enterprise crashes through the light barrier with ease. But will that ever really be possible? According to Einstein's theory of relativity, the answer is no.

Einstein calculated that as an object gathers speed, it also gains mass. The more massive something is, the harder it is to push it to still higher speeds. Eventually, a time comes when the object has so much mass that it is impossible to make it go any faster. In fact, to make a spaceship reach exactly the speed of light would take more energy than there is in the whole universe.

Einstein's theory does not mean that a starship could not go very close to the speed of light. In principle, the interstellar ramjet could travel at 99 percent, or even 99.999 percent, of the speed of light. But it could never reach light-speed.

This limit on speed appears to mean that real starships cannot hop from star to star at warp factor one – never mind warp factor eight or nine. But there is one other possibility that may help us break through the light barrier.

## Into the Black Hole

There are some weird things in the depths of space. But perhaps the strangest of all is the BLACK HOLE (see Figure 3). A black hole is a place where the pull of gravity is so strong that nothing can escape from it – not even light.

Black holes are thought to form when stars much heavier than the Sun reach the end of their lives. When a massive star has used up all its available fuel for making new light and heat, the outer parts of it are blown away in a monstrous explosion called a supernova. The remains of the wrecked star – its burned-out core – may then shrink in the wink of an eye until it is smaller than the dot over this i.

Although the once-bright star has now become tiny, its gravitational pull, close by, is very strong. From within a radius of about 10 miles of the dead star, nothing can escape. This is the region of the black hole.

Scientists are not sure what black holes are like inside. They believe that if a spaceship fell into the kind of black hole that forms when a big star dies, that ship would be rapidly torn apart.

But there may be other kinds of black holes. Some of these may be much larger and more massive. The gravitational pull of a supermassive black hole would be extremely powerful. Yet, it might not prove as damaging to a spacecraft, because the difference in the pull between one end of the spacecraft and the other would be far less than in the case of a small black hole. A starship might be able to enter such an object without being destroyed. If so, that might create a remarkable opportunity. According to some suggestions, such a black hole could be used as a way to jump instantly to other, perhaps remote places in the universe.

At the moment it is just an idea. But black holes may be like the entrances to subway tunnels that connect may different places in space and time. The tunnels themselves are known as "wormholes." And just as a black hole could be the way into a wormhole, a "white hole" could be the way back out. In theory, a spaceship could travel along wormholes and pop up in a place tens or even millions of light-years away from where it entered. That journey could be made in a very short time. The spaceship might even appear in the far future or in the past, and then travel back to its own space and time by going back through the wormhole the other way. Traveling in this way, the speed of light would no longer be a barrier. But if such a journey through time and space is possible, it remains hundreds of years in the future.

NEXT PAGE •  PREVIOUS PAGE  •  CONTENTS