Astrophysics for Young People in a Hurry Read online

  The elements inside those giant stars would be completely useless were they to remain where they formed. But these stars were unstable. They exploded, sending their insides racing across the galaxy.

  Nine billion years after the beginning of the universe, in an average part of the universe in an average galaxy, an average star (the Sun) was born.

  How did it form? Gravity slowly pulled together an enormous gas cloud filled with particles and heavy elements packed with added protons and neutrons. As they orbited around one another, gravity forced them closer and closer until they collided and fused.

  What are elements?

  There are 118 known elements in the universe. Each one is made from just one type of atom. The main difference between each element is the number of protons it has packed into its core. Hydrogen, which has just one proton, is the most common element in the universe. If you add a proton to a hydrogen atom, you end up with a new element, helium.

  Once the Sun was born, this gas cloud still had plenty of cosmic ingredients remaining. The cloud provided enough matter to make several planets, hundreds of thousands of the space rocks known as asteroids, and billions of comets. Even then, there were leftovers, and this wandering junk slammed into the other cosmic objects.

  These crashes were so energetic they melted the surfaces of the rocky planets.

  As the amount of stuff whipping around the solar system decreased, there were fewer of these impacts, and the planet surfaces began to cool. The one we call Earth formed in a kind of Goldilocks zone around the Sun. Goldilocks, you remember, doesn’t like her porridge too hot or too cool. She wants it just right. Similarly, Earth formed at just the right distance from the Sun. Had Earth been much closer, the oceans would have evaporated. Had Earth been much farther away, the oceans would have frozen.

  Looking at Earth from 700 kilometers above the surface reveals why we call ours a blue planet.

  In either case, life as we know it would not have evolved.

  You would not be here, reading this book.

  The universe is now more than nine billion years old.

  Water trapped within the rocks that made up our young, hot planet was released into the skies. As Earth cooled, this water fell as rain, gradually creating the oceans. Within these oceans, by some method we have not yet discovered, simple molecules joined together and transformed into life.

  Humans are aerobic creatures. We require oxygen-rich air. The dominant players in these early oceans were simple anaerobic bacteria—microscopic life forms that don’t need oxygen to survive. Thankfully, those anaerobic bacteria released oxygen, pumping the air full of the stuff we humans would eventually need to thrive. This new, oxygen-rich atmosphere allowed more and more complex forms of life to arise.

  But life is fragile. Occasionally, large comets and asteroids crash into our planet and make an enormous mess.

  Sixty-five million years ago, a ten-trillion-ton asteroid hit what is now the Yucatan Peninsula, in Mexico. The space rock punched a hole in the surface that was one hundred and ten miles wide and twelve miles deep. The impact, and the dust and debris it sent up into the atmosphere, obliterated most of the life on Earth, including all the famous large dinosaurs.

  Extinction. The absolute end to the existence of a creature or life form.

  This catastrophe allowed our mammal ancestors to thrive, rather than continue to serve as snacks for T. rex. One big-brained branch of these mammals, that which we call primates, evolved a species (Homo sapiens) with enough smarts to invent methods and tools of science—and to figure out the origin and evolution of the universe.

  That’s us.

  What happened before the beginning?

  Astrophysicists have no idea. Or, rather, our most creative answers to this question have little or no grounding in experimental science. In other words, we can’t prove them. In response, some people insist that something must have started it all: a force greater than all others, a source from which everything issues. In the mind of these people, that something is, of course, God.

  But what if the universe was always there, in a state we have yet to identify—a multiverse, for instance, that continually creates new universes?

  Or what if the universe just popped into existence from nothing?

  Or what if everything we know and love was just a computer game created by a superintelligent species of aliens?

  These questions usually satisfy nobody. Yet they remind us that ignorance—not knowing—is the natural state of mind for a research scientist. Smart young people often hate to utter the words “I don’t know.” But scientists have to admit what we don’t know all the time. People who believe they know everything have neither looked for, nor stumbled upon, the boundary between what is known and what is unknown in the universe.

  That is where I hope to take you in the following chapters.

  What we do know for certain is that the universe had a beginning.

  We know the universe continues to change and evolve.

  And we know that every one of your body’s atoms can be traced back to the big bang and the ovens in the giant stars that launched their insides across the galaxies more than five billion years ago.

  We are stardust brought to life.

  The universe has given us the power to figure itself out—and we have only just begun.

  * The four forces are gravity, the strong force, the weak force, and electromagnetism. We’ll talk more about them later.

  2.

  How to Communicate with Aliens

  Imagine we land on another planet with a thriving alien civilization. The aliens might look nothing like us. They could have three legs. Or no legs at all. Their skin could be slimy and purple and they could be uglier than naked mole rats. Maybe they will be wonderful dancers. We just don’t know. The only thing we know for certain is that their world will be following the same laws of nature as our own.

  In science, we call this idea the universality of physical laws.

  If you wanted to talk to the aliens, you can bet they wouldn’t speak English or French or even Mandarin. Nor would you know whether shaking their hands would be considered a friendly greeting or a terrible insult. But if they are an advanced civilization, they will understand our shared physical laws. Short or tall, slimy or not, they’ll know about gravity. So your best hope is to find a way to communicate using the language of science.

  The scientific rules that define and shape our world are the same everywhere in the universe, from your backyard to the surface of Mars and beyond. Even the Star Wars movies, which take place in a galaxy far, far away, should stick to these laws, since the most distant galaxies remain part of our cosmos.

  Scientists did not always know that physical laws were universal. Until the year 1666, when a gentleman named Isaac Newton wrote down the law of gravitation, a kind of recipe for how gravity works, nobody had any reason to think that the scientific rules here at home were the same as they were everywhere else in the universe. Earth had earthly things going on and the heavens—the stars and planets—had heavenly things going on.

  In our own lives, rules can change from one place to the next. You might be allowed to stomp all around your house or apartment with your sneakers on. But if you go over to your friend’s place, the rules might demand that you slip off your shoes at the door to avoid tracking mud everywhere. Scientists used to think the cosmos operated the same way. Newton discovered that the universe works differently.

  The same laws apply everywhere.

  In 1665, people were fleeing the city of London to avoid a deadly infection known as the plague. Sir Isaac Newton joined them, retreating to his country estate in Lincolnshire. Away from the city, Newton had some time on his hands, so he did a little thinking. Staring at his orchard, he began wondering what kind of force pulled ripe apples from a tree. Why did they fall straight to the ground? By 1666, this question helped him work out the laws of gravity.

  The genius of Newton’s work was in realizing that
gravity did not merely yank apples down to the grass. He figured out that gravity also holds the Moon in orbit around Earth.

  Newton’s law of gravity steers planets, asteroids, and comets around the Sun.

  It prevents the hundreds of billions of stars in our Milky Way galaxy from spinning off into the cosmos.

  Gravity is not the only law with this kind of reach. Since Newton’s day, scientists have discovered many other physical laws that operate the same way everywhere. This universality of physical laws helps scientists make fantastic discoveries. We can study distant stars and planets and assume they follow the same rules.

  Sir Isaac Newton realized that gravity doesn’t just pull apples from trees. It also holds the Moon in orbit around Earth.

  After Newton, nineteenth-century astronomers used this idea to determine that the Sun is made of the same elements they’d been studying on Earth, including hydrogen, carbon, oxygen, nitrogen, calcium, and iron. They even found the traces of a new element in sunlight. Being of the Sun, the new substance was given a name derived from the Greek word helios (“the Sun”). Helium became the first and only member of the grand collection of elements known as the Periodic Table to be discovered someplace other than Earth. Many years later, birthday parties were forever changed as kids discovered they could suck in a draft of the gas from balloons and transform their voices to a cartoonish pitch.

  Okay, these laws work in the solar system, but do they operate throughout the galaxy?

  Across the universe?

  And were they around a million or even a few billion years ago?

  Step by step, the laws were tested.

  Astronomers found that nearby stars were also made of familiar building blocks like hydrogen and carbon. Later, while studying binary stars, or pairs of stars that circle each other like hesitant fighters in a boxing ring, astronomers once again discovered gravity’s influence. The same universal law that pulled Newton’s apples from his tree and prevents fifth-graders from dunking basketballs binds these pairs together and allows scientists to predict their movements.

  When gravity pulls two powerful stars closer together, the results can be explosive, as this artist’s version shows.

  So the laws work here, and far away. But how do we know they’ve always been true? Were these universal laws operating a million years ago?

  Yes. We know this because astrophysicists can see into the past.

  When you stare at Mars through a telescope, you’re not viewing the Red Planet as it is at that instant. The distance between Earth and Mars changes, but let’s say it’s one hundred and forty million miles away. That means the light has to travel one hundred and forty million miles to reach us, a trip that takes about twelve minutes for a light beam. Since it took twelve minutes for the light to reach your telescope, you’re actually seeing Mars as it was twelve minutes ago.

  The light from Mars travels through space before reaching our telescopes, so we actually see the planet as it was several minutes earlier.

  Astrophysicists have much bigger telescopes, so we can study objects much farther away, and the farther away we look in space, the farther back in time we see.

  I know what you’re thinking: Whoa.

  Yes, that is the appropriate response.

  We talk about the distance to faraway stars and galaxies in terms of light-years, or the time it takes a beam of light to travel from that object to our telescopes. So when we study a galaxy five billion light-years away, this means it took five billion years for the light to get here.

  In other words, we’re seeing that galaxy as it was five billion years ago.

  We literally look back in time, and we find that the most distant objects in the universe, which are billions of years old, follow the same rules we observe today. Across the cosmos, the universal laws have been hard at work since the beginning.

  Of course, the universality of physical laws doesn’t mean that all things that happen in the cosmos take place here on Earth. Just because the laws are the same everywhere doesn’t mean everything is possible everywhere. For instance, I’d bet you’ve never greeted a black hole on the street.

  These cosmic monsters form when gravity collapses incredibly dense stars. Gravity sucks all the matter in the star into the center, leaving a hole in space where the star once shone. The gravitational force around these black holes is so powerful that not even light can escape them. If a cosmic pothole like that really did appear on the street, you wouldn’t be the only victim. The entire planet could be drawn into the vortex and disappear.

  But as powerful as they are, black holes still follow the laws of nature.

  Physical laws are not the only things that apply everywhere in the universe. These laws also depend on numbers called constants, which help scientists predict the effect a given law is going to have. The constant of gravitation, known as “big G,” helps scientists figure out how strong gravity will be in a given situation. For instance, we can use big G to help us estimate the surface gravity on Mars.

  Among all constants, though, the speed of light is the most famous. During the Apollo missions, it took astronauts about three days to fly to the Moon. If they’d traveled at light speed, the two-hundred-and-forty-thousand-mile trip would have taken a little more than a second. So why didn’t they? It’s impossible.

  No experiment has ever revealed an object of any form reaching the speed of light.

  No matter how fast we go, we will never outrun a beam of light.

  Humans accomplish the seemingly impossible all the time. We also underestimate our engineers and inventors. People once said we’d never fly. They insisted we’d never be able to travel to the Moon or split the atom. We have since accomplished all three. But in each case, no established laws of physics stood in the way.

  Going to the Moon was hard, but not impossible.

  The claim “We will never outrun a beam of light” is a totally different prediction. It flows from basic, time-tested physical principles. The universe might as well be posted with speed limit signs that read:

  The Speed of Light: It’s Not Just a Good Idea It’s the Law

  Aliens, no matter how advanced or intelligent, won’t be able to outrace a light beam either. But they will probably be familiar with these constants. All our scientific research, measurements, and observations of the cosmos suggest that the major constants, from the big G to the speed of light, and the physical laws that use them, never change with time or location.

  Maybe I seem a little too sure of myself. Scientists don’t know everything. Not even close. We don’t agree on everything, either. We argue as intensely as siblings. But when we do, our arguments focus on concepts and cosmic happenings we barely understand.

  When a universal physical law is involved, the debate is guaranteed to be brief.

  Yet not everyone grasps this idea.

  A few years ago I was having a hot cocoa at a dessert shop in Pasadena, California. Ordered it with whipped cream, of course. When my cocoa arrived at the table, I saw no trace of the stuff. After I told my waiter that my cocoa had no whipped cream, he insisted I couldn’t see it because it sank to the bottom.

  But whipped cream has low density. It floats on all the liquids we humans drink, including hot chocolate. Wherever you are in the universe, substances with low density will float in liquids with higher density. This is a universal law.

  So I offered the waiter two possible explanations: either somebody forgot to add the whipped cream to my hot cocoa or the universal laws of physics were different in his restaurant. Unconvinced, he brought over a dollop of whipped cream to demonstrate his claim. After bobbing once or twice the whipped cream rose to the top, safely afloat.

  What better proof do you need of the universality of physical law?

  3.

  Let There Be Light

  I met Superman once. This happened in the pages of a comic book, but it felt real. In this particular issue, “Star Light, Star Bright,” the Man of Steel has been busy fight
ing off a horde of alien invaders on Mars when he takes a break. He leaves the battle to his Justice League friends and flies back to Earth, all because he wants to see a star.

  That’s my kind of superhero.

  If you’re not familiar with Superman, he has bulletproof skin, eyes that can shoot lasers, the ability to fly, and a few other impressive capabilities. More importantly, though, he’s an alien. He was born on a planet called Krypton and traveled to Earth as an infant in a spacecraft. After his journey through space, he landed in a field in Kansas, met his new parents, Jonathan and Martha Kent, and got on with his life.

  Here’s a look at what happens after a star explodes, vomiting its insides across the galaxy in all directions.

  While he was on his way to Earth, though, Krypton was destroyed. The comics and movies offer different versions of how this happened, but in “Star Light, Star Bright,” Krypton’s sun goes supernova. The star explodes, roasting Superman’s home planet in the process.

  My contribution to the issue, besides lending myself as a character, complete with my moustache and my favorite astronomy-themed vest, was figuring out where Superman’s home might be located in our actual galaxy. The writers asked for my help and, after a little research, I picked a nice neighborhood in the constellation Corvus, about twenty-seven light-years from Earth. Again, that’s the distance it would take a beam of light to cover while racing across the universe for twenty-seven years.

  I picked Corvus as Superman’s home address because its light needs to travel for twenty-seven years before reaching us. This way, the light from its final moments didn’t arrive until he was an adult.