Astrophysics for Young People in a Hurry Read online

  Sounds unbelievable. But is it any more crazy than the first suggestions that Earth orbits the Sun? Back then, everyone thought Earth was the center of the universe. They thought the sky was basically a big dome. Now we know better. We know the Sun is one of a hundred billion stars in the Milky Way. And we know that the Milky Way is one of a hundred billion galaxies in the universe. Our home planet isn’t as special as we once thought. We were wrong about Earth, so maybe we’re wrong about dark matter, too.

  Some scientists suspect that dark matter is made of a ghostly group of particles we have not yet discovered. They’re using giant machines called particle accelerators to try to make bits of dark matter here on Earth. Other groups have designed laboratories deep underground. If dark matter particles happen to wander through space, and a few coast into Earth, these underground labs should be able to detect them. Again, this probably sounds unbelievable. But scientists once accomplished a similar feat with a ghostly little particle called the neutrino.

  In the 1930s, as scientists were trying to understand the atom, a few of the leading thinkers began to float the idea of a tiny particle that had little or no mass. They had no direct evidence of the particle at first. But certain atoms were releasing energy in an unknown way, and a few scientists suggested that these unknown particles were the culprits, carrying that energy away from the atom. Although they had no direct evidence of them, the scientists predicted the existence of neutrinos, particles that barely interact with matter. Then, a few decades later, another group of scientists discovered proof that these particles are real. Neutrinos have been tracked and counted in other experiments since then. Every second, a hundred billion neutrinos from the Sun pass through each thumbnail-sized patch of your body. And they do nothing.

  Giant underground detectors like this one, which is part of the world’s largest atom smasher, are helping scientists study the mystery of dark matter.

  What started out as a scientific hunch, a way of explaining something that didn’t make sense, turned out to be real. Maybe we will find a way to detect dark matter, as we did with the neutrino. Or maybe, more amazingly, we will discover that dark matter particles are something entirely different, and that they make use of some new, undiscovered force or forces.

  For now, we must remain content to carry dark matter along as a strange, invisible friend, using it to explain the universe’s strange behavior. This alone would provide more than enough work for curious astrophysicists. But dark matter is not the only grand unsolved cosmic mystery. We have another fascinating puzzle to solve.

  * Is that where all our lost socks have gone, too?

  6.

  Dark Energy

  When I was a kid, I was fascinated by a cartoon character named Mighty Mouse. Sure, he was a rodent, but he was always saving the day, and he had this fantastic, operatic voice. The little guy could sing. Plus, he was barrel chested and incredibly strong, and he could fly.

  As a curious young person, I couldn’t help wondering exactly how Mighty Mouse was capable of flight. He didn’t have wings. He didn’t have propellers or jet engines hiding in his belt. But he did have a cape. Superman, the other famous flying hero of the day, also wore a cape. Was that the secret? Did the power of flight really just lie in your choice of outfit?

  Soon I developed a theory: Capes give humans and mice the ability to fly.

  Although I wasn’t a scientist yet, I was starting to think like one. Science doesn’t thrive on theories alone. Theories need to be tested. So I needed to set up an experiment to test my idea. I found a cape for myself, tied it around my neck, and jumped as far as I could.

  I measured the distance of this cape-assisted jump.

  Then I removed the cape, jumped again, and measured that distance.

  There was no difference.

  I didn’t jump any farther with the cape. I definitely didn’t fly. But I did learn a valuable lesson: In science, a theory should match the evidence collected in experiments. Otherwise, it needs to be adjusted or tossed into the garbage bin of ideas. My guess that capes allow mice and humans to fly did not match my jumping experiment, so I had to abandon my theory and move on with my life, learning to fly like the rest of humanity, in large machines called airplanes.

  But sometimes even the wildest theories survive experimental tests. Albert Einstein hardly ever set foot in the laboratory. He was a pure theorist—a scientist who develops ideas about the way nature works. He perfected what’s called a “thought experiment,” in which you try to solve mysteries by using your imagination.

  When he was sixteen years old, for example, Einstein wondered what it would be like if he were to race alongside a beam of light. This is impossible, of course. We’ve already discussed the cosmic speed limit. But merely thinking about this strange idea kept Einstein busy for years, and eventually led to one of his biggest breakthroughs.

  Theorists like Einstein develop models of how the universe works. Using these models, they can make predictions. If the model is broken, then the observers—the scientists who use advanced instruments to study nature—will uncover a mismatch between the prediction and the evidence. The “model” of flight I developed as a kid insisted that capes allowed humans and mice to float through the air. Then I tested the model—without the need for advanced instruments—and discovered a mismatch between my theory and the evidence. I was disappointed, but scientists are generally pretty excited when they find one of these errors in another researcher’s model. We all like finding mistakes in someone else’s homework.

  Einstein developed one of the most powerful and far-reaching theoretical models ever, his general theory of relativity.* This model details how everything in the universe moves under the influence of gravity and how gravity shapes space itself. The general theory makes predictions that scientists are still testing today.

  When black holes collide, Einstein’s model predicts that they should release energy in the form of gravity waves that travel across the universe. Instead of moving through water, like the waves at a surfing beach, these cause ripples in space itself. And sure enough, scientists have caught waves from these ancient, far-off black hole collisions as they washed over Earth, proving Einstein correct.

  Every few years, lab scientists come up with better experiments to test Einstein’s theory. And every time, they show that he was right. Einstein wasn’t just the smartest kid in his class. He was one of the smartest people ever.

  But even he could make a mistake.

  In his day, people desperately wanted to prove Einstein wrong. His work challenged Newton’s ideas and some in the scientific community weren’t too excited about that. A group of them joined together to publish a 1931 book titled One Hundred Authors against Einstein. When he learned of the book, he responded that if he were wrong, then only one author would have been enough.

  General relativity was radically different from all previous thinking about gravity. According to general relativity, massive objects actually warp the space around them, causing distortions or dimples in the fabric of space and time.

  A small mass like an apple has very little effect. Something large, like a planet or a star, distorts space so much that straight lines bend. One of my former teachers, the twentieth-century American theoretical physicist John Archibald Wheeler, said, “Matter tells space how to curve; space tells matter how to move.”

  This new version of gravity, as defined by Einstein, doesn’t simply affect matter. Since gravity curves space itself, even light has to bend to gravity’s power, following warped paths around massive objects instead of straight lines. Einstein’s model described two kinds of gravity. One is the familiar kind: the attraction between Earth and a ball thrown into the air, or between the Sun and the planets. But general relativity also predicted another effect—a mysterious, antigravity pressure.

  Today, we know that our universe is expanding. Our galaxies are spreading farther and farther apart. Back then, the idea that our universe would be doing anything at all other than simply existing was beyond anyone’s imagination. Even Einstein figured the universe had to be stable, neither growing nor shrinking. But his model of the universe hinted that the universe should be either expanding or contracting. He guessed this had to be wrong. So he added a term he called the cosmological constant.

  The sole job of the cosmological constant was to work against gravity in Einstein’s model. If gravity was trying to pull the whole universe into one giant mass, the cosmological constant was pushing it apart.

  There was just one problem.

  Nobody had ever observed such a force in nature.

  In a way, Einstein cheated.

  Thirteen years after Einstein introduced his theory, the American astrophysicist Edwin P. Hubble discovered that the universe is not stable. Hubble had been studying distant galaxies. According to his work, these galaxies were not sitting in place. They were moving away from us! Not only that: Hubble had assembled convincing evidence that the more distant a galaxy, the faster it races away from the Milky Way.

  In other words, the universe is expanding.

  When he learned of the work, Einstein was embarrassed. He should have predicted it himself. He threw out the cosmological constant entirely, calling it his life’s “greatest blunder.” But that wasn’t the end of the story. Off and on over the decades, theorists would revive the cosmological constant. They’d ask what their ideas would look like in a universe that really did have this mysterious antigravity force.

  In 1998, science pulled Einstein’s greatest blunder out of its grave one last time.

  Early that year, two competing teams of astrophysicists made remarkable announcements. Both groups had been watching the exploding stars known as supernovas. Astronomers knew how they should behave, how bright they s
hould glow, and how far away they were supposed to be.

  But this group of supernovas was different.

  They were dimmer than expected.

  Two explanations were possible. Either those particular supernovas were different from all the other exploding stars that astrophysicists had studied in the past, or they were much farther away than scientists had predicted. And if they were farther away than we’d expected, then something was wrong with our models of the universe.

  Hubble’s work revealed that the universe was expanding, but these supernovas suggested that it was growing faster than we’d expected. And there was no easy way to make sense of this extra expansion without Einstein’s blunder, the cosmological constant. When astrophysicists dusted it off and put it back into Einstein’s general relativity, their observations of the universe matched his predictions.

  Those supernovas were right where they were supposed to be.

  This exploding star, Supernova 1987A, is pretty much a celebrity in astrophysics circles. Stars like this helped us realize that the universe is expanding.

  Are Scientists Competitive?

  Yes! Very much so. We’re just as competitive as athletes or chess champions. Generally, in science, you don’t want to get scooped. When Charles Darwin learned that another scientist, Alfred Russel Wallace, was developing some ideas that were similar to his own, he hurried to publish what would become known as his theory of evolution. He didn’t want Wallace to get the credit first. This kind of thing is true for any science, but in my opinion, the universe is big enough for us all. There’s plenty of research room.

  The two groups of astrophysicists studying those supernovas were both awarded the Nobel Prize for their work. In the world of science, that’s the equivalent of a tie.

  So Einstein was correct after all.

  Even when he thought he was wrong, he was right.

  The discovery of these speeding supernovas was the first direct evidence that a strange new force was at work throughout the universe, fighting gravity. The cosmological constant was real, and it needed a better name. Today we call it “dark energy.”

  The Universe as Hot Cocoa: A cup of hot cocoa with whipped cream and cinnamon on top. The cocoa takes up 68%, the whipped cream 27%, and the cinnamon 5%.

  The most accurate measurements to date reveal dark energy as the most prominent thing in town. The universe is built from a combination of matter and energy. When we add up all the mass-energy in the universe, dark energy is currently responsible for 68 percent. Dark matter makes up 27 percent. Regular matter represents just 5 percent of the universe.

  The ordinary stuff we see and feel and smell is just a sliver of the cosmos.

  So what is this mystery force? Nobody knows. The closest anybody has come is to guess that dark energy is created from the vacuum of space. In chapter 4, we discussed not just the dangers of intergalactic space, but all the action happening in these seemingly empty cosmic deserts. Particles and their opposites pop into existence and destroy each other. Each pair creates a little bit of outward pressure in the process. Maybe if you were to add up all those little nudges happening all across the universe, you’d end up with enough force to power dark energy.

  This is a reasonable idea. Unfortunately, when you estimate the total of this “vacuum pressure,” the result is stupidly large. Much larger than our estimates of the total value of dark energy. Not counting my Mighty Mouse experiment, it would be the biggest mismatch between theory and observation in the history of science. So “vacuum pressure” can’t be the source of dark energy’s power.

  Yes, we’re clueless.

  But not completely. Dark energy still arises from one of the best models of the universe we have ever developed: Einstein’s general relativity. It’s the cosmological constant. Whatever dark energy turns out to be, we already know how to measure it. We know how to predict its effects on the past, present, and future of the cosmos.

  Why It’s Thrilling to Be Clueless

  By this point you may have noticed that I’ve used this term “clueless” more than once. People often think of scientists as arrogant and always sure of themselves. But we love it when the universe stumps us. We love being clueless. It’s completely exciting. It’s what gets us to run to work every day. As a scientist, you learn to embrace ignorance, or not knowing. If you know all the answers, you’ve got nothing to work on, and you might as well just go home.

  And the hunt is on. Now that we know dark energy is real, teams of astrophysicists are racing to find its secrets. Maybe they will succeed. Or maybe we need an alternative to general relativity. There could be some theory of dark energy that awaits discovery by a clever person yet to be born. Or maybe that future genius is reading this very book right now.

  * You can call it GR. You’re in the club.

  7.

  My Favorite Elements

  In middle school, I asked my teacher what I thought was a simple question about the Periodic Table of Chemical Elements. You’ll find a poster of the Periodic Table on the wall of the average science classroom. At first glance, you could easily mistake it for a very confusing board game. But it’s not a game. The Periodic Table tells us about all 118 elements, or types of atoms, in the universe.

  Anyway, I asked my teacher where these elements came from.

  Earth’s crust, he replied.

  I’ll grant him that. It’s surely where the school supply lab got them. But that answer wasn’t enough for me. I wanted to know how the elements ended up in Earth’s crust. Yes, I was that kid,* and I figured the answer must be astronomical. The elements must have originated in space. But do you actually need to know the history of the universe to answer the question?

  Yes, you do.

  Normal matter is made of protons, neutrons, and electrons. The protons and neutrons bunch together into something called a nucleus. The electrons, meanwhile, orbit around the outside of the nucleus. Add them together and you get what we call an atom. An element is one or more of the same type of atom, with the same number and type of particles, and the simplest one of all is hydrogen. All it has is a single proton and a single electron. One or more of these hydrogen atoms added together is considered a hydrogen element.

  Hydrogen is one of only three of the naturally occurring elements—the ones we don’t make in a lab or in an experiment—that were manufactured in the big bang. The rest were forged in the high-temperature hearts and explosive remains of exploding stars. As a kind of guide to these elements, the Periodic Table is a seriously important piece of science. Yet every now and then, even a scientist can’t help thinking of it as a zoo of incredibly odd, one-of-a-kind animals conceived by Dr. Seuss. These elements, after all, are unbelievably strange.

  There’s sodium, a poisonous metal that you can cut with a butter knife. Elsewhere on the chart you’ll find chlorine, a smelly, deadly gas. The Periodic Table tells you that these two dangerous elements can be combined into one molecule. Sounds like a terrible idea. But when you add them together you make sodium chloride, better known as table salt.

  Or how about hydrogen and oxygen? The first is an explosive gas. The other helps materials burn. Add oxygen to a fire and it rages. Yet the Periodic Table tells us they can pair together. When you combine hydrogen and oxygen, you make liquid water, which puts out fires.

  The Periodic Table is filled with wonders. We could go through each element and review its many strange and fantastic qualities. But as you’ve probably realized by now, I prefer to focus on the stars. So allow me to offer a tour of the Periodic Table as viewed through the lens of an astrophysicist.

  The Most Popular Element in the Universe

  The lightest and simplest element, hydrogen was made entirely during the big bang. Out of the 94 elements found in nature, hydrogen dominates. Two out of every three atoms in the human body have hydrogen in them. Nine out of every ten atoms in the entire universe are hydrogen. Every second of every day, 4.5 billion tons of fast-moving hydrogen particles slam together within the fiery hot core of the Sun. These collisions provide the energy that helps the Sun shine.