George's Secret Key to the Universe

by Stephen Hawking

  “Not me,” said Whippet immediately.

  “Not me,” mumbled Tank.

  “It must be you then,” said Ringo, pointing at Zit.

  “Ringo, I haven’t … I didn’t … I wasn’t …” Zit was panicking now.

  “Very well,” said Dr. Reeper, glaring at the four of them. He put the money back in his pocket. “In that case, I think you’d better scram. Do you hear me? Scram!”

  Once the boys—who didn’t need telling twice—were gone, Dr. Reeper stood in the street, smiling to himself. It wasn’t a pleasant sight. Checking that no one else was coming or going, he went up to Eric’s front window and squinted through it. The curtains were drawn, so he only had a small opening to look through. He couldn’t see much, just two strangely shaped, shadowy figures, which seemed to be standing near some kind of doorway inside the house.

  “Interesting,” he muttered to himself. “Very, very interesting.”

  Suddenly, the temperature in the street dropped dramatically. For a second it felt as though air from the North Pole were blowing along the street. Strangely, the bitter wind seemed to be coming from under Eric’s front door, but as Dr. Reeper bent down to investigate, it stopped. When he went back to look through the window, the two figures had gone and there was no inside doorway to be seen.

  Dr. Reeper nodded to himself. “Ah, the chill of outer space—how I long to feel it,” he whispered, rubbing his hands together. “At last, Eric, I’ve found you! I knew you’d come back one day.”

  When he leaped over the threshold of the portal door, George found he was floating—not going up, not going down, just drifting in the huge, great darkness of outer space. He looked back toward the doorway, but the hole in space where it should have been had closed over as though it had never been. There was no way back now and the giant rock was getting closer all the time.

  “Hold my hand!” Annie shouted to George. As he gripped her hand in its space glove even harder, he started to feel as if they were falling down toward the comet. Moving faster and faster, as if they were on a giant Tilt-a-Whirl, George and Annie spiraled toward the huge rock, getting closer and closer all the time. Beneath them, they could see that one side of the comet, the part facing the Sun, was brightly lit. But the other side, which the Sun’s rays didn’t reach, was in darkness. Eventually they landed in a heap on a thick layer of icy, dust-covered rubble. Luckily they’d come down on the bright side of the comet, so they could see what lay around them.

  “Ha-ha-ha-ha!” Annie was laughing as she picked herself up. She hauled George to his feet and brushed bits of dirty ice and crumbly rock off him. “So?” she said. “Do you believe me now?”

  “Where are we?” said George, who was so surprised he forgot to be scared. George felt extremely light. He looked around and saw rock, ice, snow, and darkness. It was like standing on a giant dirty snowball someone had thrown into outer space. Stars blazed everywhere, their fiery glow quite different from the twinkling lights he saw from the Earth.

  “We’re having an adventure,” replied Annie. “On a comet. And it’s real—it isn’t a made-up story, is it?”

  “No, it isn’t,” admitted George. He patted her space suit awkwardly. “I’m sorry I didn’t believe you, Annie.”

  “That’s all right,” said Annie generously. “No one ever does. That’s why I had to show you. Look, George!” She waved an arm around. “You’re going to see the planets in the Solar System.” She started to pull a length of rope out of a pocket in her space suit. On the end of the rope was a spike, like a tent peg. Using her space boot, she jammed the spike into the ice on the comet’s surface.

  Watching her, George gave a tiny little jump for joy. Even though he was wearing the space suit that had seemed very heavy on Earth, he couldn’t believe how light he felt. So light that he thought he could leap as high as he wanted. He did another little jump across a little crack on the comet’s surface. This time he went up and forward, but he didn’t come down again. He seemed to be taking a giant leap, maybe hundreds of feet long! He’d never be able to find Annie again …

  “Help! Help!” George called through the helmet as his jump carried him farther and farther away, his arms whirling in the surrounding emptiness as he tried to make himself fall down onto the comet. But it was no good. Annie was far away in the distance now—he could only just see her when he looked back. The comet’s surface was passing quickly below him. He could see holes and little hills everywhere, but nothing that he could grab on to. But at last he seemed to fall. The ground was getting closer now, and as he landed he slid on the ice near the threshold between the bright and the dark side of the comet. In the distance, he saw Annie carefully running toward him.

  “If you can hear me, don’t jump again!” she was saying in a very urgent voice. “If you can hear me, don’t jump again! If you can—”

  “I won’t!” he called back as she reached him.

  “Don’t do that, George!” said Annie. “You could have landed on the dark side of the comet. I might never have found you! Now stand up—the boots have small spikes on their soles.” She sounded very grownup and not at all like the impish little girl he had met at Eric’s house. “A comet is different from the Earth. We weigh much less here than we do there, so when we jump, it can take us a long, long way. This is a different world. Oh, look!” she added, changing the subject. “We’re just in time!”

  “For what?” asked George.

  “For that!” Annie pointed to the other side of the comet.

  Behind the comet was a tail of ice and dust, which was getting steadily longer. As it grew, it caught the light from the faraway Sun and glistened in the wake of the comet, making it look as though thousands of diamonds were shining in outer space.

  “That’s beautiful,” whispered George.

  For a minute he and Annie just stood there in silence. As George watched the trail grow, he realized it was made up of bits of the bright side of the comet.

  MASS

  The mass of a body measures the force needed to move it or to change the way it moves. Mass is often measured by weighing the body, but mass and weight are not the same. The weight of an object is the force attracting it to another object, such as the Earth or the Moon, and it depends on the mass of both objects and the distance between them. You weigh slightly less on top of a mountain because you are farther from the center of the Earth.

  Because the mass of the Moon is much less than the mass of the Earth, an astronaut who weighs about 200 pounds (about 90 kg) on Earth would weigh only about 33 pounds (15 kg) on the Moon. So astronauts on the Moon could, with the correct training, beat any Earth-based long-jump record.

  Einstein was a German physicist who was born in 1879. He discovered that energy is equivalent to mass, according to the famous equation E=mc2, where “E” is energy, “m” is mass, and “c” is the speed of light. Because the speed of light is very large, Einstein and others realized that this equation suggested one could make an atom bomb, in which a small amount of mass is converted into a very large amount of energy in an explosion.

  Einstein also discovered that mass and energy curve space, creating gravity.

  “The rock’s melting!” said George in a panic, clutching Annie’s arm. “What will happen when there’s nothing left?”

  “Don’t worry.” Annie shook her head. “We’re just getting closer to the Sun. The Sun slowly warms up the bright side of the comet and the ice turns into gas. But it’s okay because there’s enough ice here for us to pass the Sun loads of times. Anyway, the rock under the ice won’t melt. So we won’t start falling through space, if that’s what you’re scared of.”

  “I’m not scared!” protested George, letting go of her arm very suddenly. “I was just asking.”

  “Then ask more interesting questions!” said Annie.

  “Like what?” asked George.

  “Like, what would happen if some of the rocks from the comet’s tail fell on the Earth?”

  George
kicked some dust around and then said reluctantly, “All right, what would happen?”

  “Now that is a good question!” said Annie, sounding pleased. “The rocks catch fire when they enter the Earth’s atmosphere, and from the ground, when we look up, they become what we call shooting stars, or meteors.”

  They stood and gazed until the comet’s tail got so long they couldn’t see the end of it. But as they were watching it, the comet seemed to start changing direction: All the stars in the background were moving. “What’s happening?” George asked.

  “Quick!” Annie replied. “We’ve only got a few seconds. Sit down, George.” She cleared two little spaces on the ice, speedily brushing the powder aside with her glove. Reaching into another pocket of her suit, she produced what looked like climbing hooks. “Sit down!” she ordered again. She screwed the hooks into the ground and then fastened them onto a longish piece of cord hanging from a buckle on George’s suit. “Just in case something hits you,” she added.

  “Like what?” asked George.

  “Well, I don’t know. My dad normally does this part,” she replied. Next, she sat down behind George and did the same to herself. “Do you like roller coasters?” she asked him.

  “I don’t know,” said George, who had never been on one.

  “Well, you’re about to find out!” said Annie, laughing.

  The comet was definitely falling—or at least changing direction toward what seemed to be “down.” From the way the stars were moving all around him, George understood that the comet was falling very fast. But he couldn’t feel anything—he didn’t have butterflies in his stomach, and there was no rush of air blowing past him. It wasn’t at all how he had expected a ride on a roller coaster to feel. But he was starting to realize that things feel very different in outer space from the way they do on Earth.

  George closed his eyes for a moment, just to see if he could feel anything at all. But no, nothing. Suddenly, with his eyes closed, he realized that something in space must be pulling them and the comet toward it for the comet to change direction like that. George instinctively knew that this something was probably much, much bigger than the comet on which he and Annie were surfing through outer space.

  COMETS

  Comets are big, dirty, and not very round snowballs that travel around the Sun. They are made up of elements created in stars that exploded a long time before our Sun was born. It is believed that there are more than 100 billion of them, very far away from the Sun, waiting to come closer to us. But we can see them only when they come close enough to the Sun to have a shiny tail. We actually have seen only about 1,000 comets so far.

  The largest known comets have a central core of more than 20 miles (32 km) from one side to the other.

  When comets come close to the Sun, the ice in them turns into gas and releases the dust that was trapped inside. This dust is probably the oldest dust there is throughout the Solar System. It contains clues about our cosmic neighborhood at the very beginning of the life of all the planets, more than 6 billion years ago.

  Most of the time, comets circle around the Sun from very far away (much, much farther away than the Earth). Every now and then, one of them starts to travel toward the Sun. There are then two possibilities:

  1) Some, like Halley’s Comet, will get trapped by the Sun’s gravity. These comets will then keep orbiting the Sun until they melt completely or until they hit a planet. Halley’s Comet’s core is about 9.6 miles (16 km) long. It returns near enough to the Sun to melt down a bit and have a tail that can be seen by us about every 76 years. It was near us in 1986 and will be back in 2061. Some of the comets trapped by the Sun’s gravity return near the Sun much more rarely. The Hyakutake Comet, for instance, will travel for 110,000 years before coming back.

  2) Because they have too much speed or because they do not travel close enough to the Sun, some other comets, like Comet Swan, never come back. They pass by us once and then start an immense journey in outer space toward another star. These comets are cosmic wanderers. Their interstellar journey can take hundreds of thousands of years, sometimes less, sometimes even more.

  When George opened his eyes again, he saw a massive pale yellow planet with a belt of rings rising in the dark sky ahead of them. They sped along on the comet, heading for a point just above the rings. From far away, the rings looked like soft ribbons. Some were pale yellow, like the planet itself; others were darker.

  “This is Saturn,” said Annie. “And I saw it first.”

  “I know what it is!” replied George. “And what do you mean, ‘first’? I’m in front of you. I saw it first!”

  “No, you weren’t looking, you were too scared! You had your eyes shut!” Annie’s voice rang inside his helmet. “Ner-ner-na-ner-ner.”

  “No I didn’t!” protested George.

  “Shhh!” Annie interrupted him. “Did you know that Saturn is the second biggest of the planets that move around the Sun?”

  “Of course I knew,” lied George.

  “Oh really?” replied Annie. “Then if you knew that, you’ll know which is the biggest planet of all.”

  “Well … um … ,” said George, who had no idea. “It’s the Earth, isn’t it?”

  “Wrong!” trumpeted Annie. “The Earth is teeny-weeny, just like your silly little brain. The Earth is only number five.”

  “How do you know that?”

  “How do I know you’ve got a silly little brain?” said Annie sarcastically.

  “No, stupid,” said George furiously. “How do you know about the planets?”

  “Because I’ve done this trip many, many times before,” said Annie, tossing her head as though throwing back her ponytail. “So let me tell you. And listen carefully,” she ordered. “There are eight planets orbiting the Sun. Four are huge and four are small. The huge ones are Jupiter, Saturn, Neptune, and Uranus. But the two biggest are so much bigger than the others that they are called the Giants. Saturn is the second of the giant planets, and the biggest one of all is Jupiter. The four small planets are Mars, Earth, Venus, and Mercury,” she continued, ticking them off on her fingers. “The Earth is the biggest of the small ones, but if you put these four together into a ball, you still wouldn’t get anything nearly as big as Saturn. Saturn is more than forty-five times bigger than these four small planets added together.”

  Annie was clearly delighted to be showing off about the planets. Even though he was very annoyed by how smug she was, George was secretly impressed. All he had ever done was dig potatoes and mess around with a pig in his backyard. It wasn’t much in comparison with riding around the Solar System on a comet.

  As Annie talked, the comet flew nearer and nearer to Saturn. They got so close that George could see that the rings were made not of ribbons but of ice, rocks, and stones. These were all different sizes, the smallest no bigger than a speck of dust, the largest about twelve feet long. Most of them were moving much too fast for George to catch one. But then he spotted a small chunk of rock calmly floating right next to him. A quick glance behind showed that Annie wasn’t looking. He reached out, snatched up the rock, and held it in his space glove! A real treasure from outer space! His heart was beating fast. The sound was so loud in his ears that he thought Annie must be able to hear it through the sound transmitter in his helmet. He suspected that taking things home from outer space was probably not allowed, so he hoped she hadn’t noticed.

  “George, are you all right?” asked Annie. “Why are you wriggling around like that?”

  George quickly thought of something to say to divert her attention from the rock he was trying to stuff into his pocket.

  THE SOLAR SYSTEM

  The Solar System is the cosmic family of our Sun. It comprises all the objects trapped by the Sun’s gravity: planets, dwarf planets, moons, comets, asteroids, and other small objects yet to be discovered. An object trapped by the Sun’s gravity is said to be in orbit around the Sun.

  Closest planet to the Sun: Mercury Mercury is 36 million mil
es (57.9 million km) away from the Sun on average

  Farthest planet from the Sun: Neptune Neptune is 2.8 billion miles (4.5 billion km) away from the Sun on average

  Distance of the Earth from the Sun: 93 million miles (149.6 million km) on average

  Number of planets: 8

  From closest to the Sun, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune

  Number of dwarf planets: 3

  From closest to farthest to the Sun, the dwarf planets are: Ceres, Pluto, and Eris

  Number of known planetary moons: 165 Mercury: 0; Venus: 0; Earth: 1; Mars: 2; Jupiter: 63; Saturn: 59; Uranus: 27; Neptune: 13

  Number of known comets: 1,000 (estimated real number:1,000,000,000,000,000)

  Farthest distance traveled by a man-made object: more than 9.3 billion miles (14.96 billion km). Nine billion three hundred million miles is the distance reached by Voyager 1 on August 15, 2006, at 10:13 a.m. (Greenwich Mean Time). This corresponds to exactly 100 times the distance from the Earth to the Sun. Voyager 1 is still traveling away.

  “Why did we change direction? Why did our comet move toward Saturn? Why didn’t we continue in a straight line?” he babbled.

  “Oh dear, you just don’t know anything at all, do you?” sighed Annie. “It’s lucky for you that I’m such a fount of useful scientific knowledge,” she added importantly. “We moved toward Saturn because we fell toward it. Just like an apple falls on Earth, just like we fell onto the comet when we arrived, just like the particles in space clouds fall onto each other and become balls that become stars. Everything falls toward everything throughout the Universe. And do you know the name of what causes this fall?”

  George didn’t have a clue.

  “It’s called gravity.”

  “So it’s because of gravity that we’re going to fall on Saturn now? And crash?”

  “No, silly! We’re moving way too fast to crash. We’re just flying by to say hello.”