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Astrophysics for People in a Hurry by Neil de Grasse Tyson


  • The highest grossing film of all time is by a famous director who set his story on a planet orbiting a distant star.
  • The universe is under no obligation to make sense to you. — NDT

1. The Greatest Story Ever Told

  • In the beginning, nearly fourteen billion years ago, all the space and all the matter and all the energy of the known universe was contained in a volume less than one-trillionth the size of the period that ends this sentence.
  • the four distinct forces we have come to know and love: with the weak force controlling radioactive decay, the strong force binding the atomic nucleus, the electromagnetic force binding molecules, and gravity binding bulk matter.
  • As the cosmos continued to expand and cool, growing larger than the size of our solar system, the temperature dropped rapidly below a trillion degrees Kelvin.
  • Without the billion-and-one to a billion imbalance between matter and antimatter, all mass in the universe would have self-annihilated, leaving a cosmos made of photons and nothing else — the ultimate let-there-be-light scenario.
  • By now, one second of time has passed. The universe has grown to a few light-years across, † † about the distance from the Sun to its closest neighboring stars.
  • Two minutes have now passed since the beginning. For another 380,000 years not much will happen to our particle soup.
  • But high-mass stars fortuitously explode, scattering their chemically enriched guts throughout the galaxy. After nine billion years of such enrichment, in an undistinguished part of the universe (the outskirts of the Virgo Supercluster) in an undistinguished galaxy (the Milky Way) in an undistinguished region (the Orion Arm), an undistinguished star (the Sun) was born.
  • Earth formed in a kind of Goldilocks zone around the Sun, where oceans remain largely in liquid form. Had Earth been much closer to the Sun, the oceans would have evaporated. Had Earth been much farther away, the oceans would have frozen. In either case, life as we know it would not have evolved.
  • A mere sixty-five million years ago (less than two percent of Earth’s past), a ten-trillion-ton asteroid hit what is now the Yucatan Peninsula and obliterated more than seventy percent of Earth’s flora and fauna — including all the famous outsized dinosaurs.
  • What we do know, and what we can assert without further hesitation, is that the universe had a beginning. The universe continues to evolve. And yes, every one of our body’s atoms is traceable to the big bang and to the thermonuclear furnaces within high-mass stars that exploded more than five billion years ago.
  • We are stardust brought to life, then empowered by the universe to figure itself out — and we have only just begun.

2. On Earth as in the Heavens

  • Newton had figured out that the force of gravity pulling ripe apples from their orchards also guides tossed objects along their curved trajectories and directs the Moon in its orbit around Earth.
  • It happens that we cannot see, touch, or taste the source of eighty-five percent of the gravity we measure in the universe. This mysterious dark matter, which remains undetected except for its gravitational pull on matter we see, may be composed of exotic particles that we have yet to discover or identify.
  • For ordinary household gravity, Newton’s law works just fine. It got us to the Moon and returned us safely to Earth in 1969. For black holes and the large-scale structure of the universe, we need general relativity.
  • No, your idea for a perpetual motion machine will never work; it violates well-tested laws of thermodynamics.
  • No, you can’t build a time machine that will enable you to go back and kill your mother before you were born — it violates causality laws.

  • Let There Be Light

  • This “ cosmic background ” is the incarnation of the leftover light from a dazzling, sizzling early universe, and can be assigned a temperature, based on what part of the spectrum the dominant photons represent.

  • The LED revolution in advanced lighting technology creates pure visible light without wasting wattage on invisible parts of the spectrum. That’s how you can get crazy-sounding sentences like: “ 7 Watts LED replaces 60 Watts Incandescent ” on the packaging.
  • Ordinary matter is what we are all made of. It has gravity and interacts with light. Dark matter is a mysterious substance that has gravity but does not interact with light in any known way. Dark energy is a mysterious pressure in the vacuum of space that acts in the opposite direction of gravity, forcing the universe to expand faster than it otherwise would.

4. Between the Galaxies

  • The nearest galaxy larger than our own is two million light-years away, beyond the stars that trace the constellation Andromeda.
  • Notice that the name for each system lacks reference to the existence of stars: Milky Way, Magellanic Clouds, Andromeda Nebula. All three were named before telescopes were invented, so they could not yet be resolved into their stellar constituencies
  • Aided by modern detectors, and modern theories, we have probed our cosmic countryside and revealed all manner of hard-to-detect things: dwarf galaxies, runaway stars, runaway stars that explode, million-degree X-ray-emitting gas, dark matter, faint blue galaxies, ubiquitous gas clouds, super-duper high-energy charged particles, and the mysterious quantum vacuum energy.
  • Quasars are super-luminous galaxy cores whose light has typically been traveling for billions of years across space before reaching our telescopes.

5. Dark Matter

  • we’ve now been waiting nearly a century for somebody to tell us why the bulk of all the gravitational force that we’ve measured in the universe — about eighty-five percent of it — arises from substances that do not otherwise interact with “ our ” matter or energy.
  • The Coma cluster, as we call it, is an isolated and richly populated ensemble of galaxies about 300 million light-years from Earth. Its thousand galaxies orbit the cluster’s center, moving in all directions like bees swarming a beehive.
  • Perhaps the “ missing mass ” needed to bind the Coma cluster’s galaxies does exist, but in some unknown, invisible form. Today, we’ve settled on the moniker “ dark matter, ” which makes no assertion that anything is missing, yet nonetheless implies that some new kind of matter must exist, waiting to be discovered.
  • Across the universe, the discrepancy averages to a factor of six: cosmic dark matter has about six times the total gravity of all the visible matter.
  • Further research has revealed that the dark matter cannot consist of ordinary matter that happens to be under-luminous, or nonluminous.
  • Detailed observations of the cosmic microwave background, which allow a separate test of this conclusion, verify the result: Dark matter and nuclear fusion don’t mix.
  • Dark matter exerts gravity according to the same rules that ordinary matter follows, but it does little else that might allow us to detect it. Of course, we are hamstrung in this analysis by not knowing what the dark matter is in the first place. If all mass has gravity, does all gravity have mass ? We don’t know.
  • Unlike sound waves, which consist of air vibrations, light waves were found to be self-propagating packets of energy requiring no assistance at all.
  • Science is not just about seeing, it’s about measuring, preferably with something that’s not your own eyes, which are inextricably conjoined with the baggage of your brain. That baggage is more often than not a satchel of preconceived ideas, post-conceived notions, and outright bias.
  • The copious flux of neutrinos from the Sun — two neutrinos for every helium nucleus fused from hydrogen in the Sun’s thermonuclear core — exit the Sun unfazed by the Sun itself, travel through the vacuum of space at nearly the speed of light, then pass through Earth as though it does not exist. The tally: night and day, a hundred billion neutrinos from the Sun pass through every thumbnail square of your body, every second, without a trace of interaction with your body’s atoms.

6. Dark Energy

  • Albert Einstein hardly ever set foot in the laboratory; he didn’t test phenomena or use elaborate equipment. He was a theorist who perfected the “ thought experiment, ” in which you engage nature through your imagination, by inventing a situation or model and then working out the consequences of some physical principle.
  • And that’s exactly what was observed. The gravitational waves of the first detection were generated by a collision of black holes in a galaxy 1.3 billion light-years away, and at a time when Earth was teeming with simple, single-celled organisms. While the ripple moved through space in all directions, Earth would, after another 800 million years, evolve complex life, including flowers and dinosaurs and flying creatures, as well as a branch of vertebrates called mammals. Among the mammals, a sub-branch would evolve frontal lobes and complex thought to accompany them. We call them primates. A single branch of these primates would develop a genetic mutation that allowed speech, and that branch — Homo sapiens — would invent agriculture and civilization and philosophy and art and science. All in the last ten thousand years. Ultimately, one of its twentieth-century scientists would invent relativity out of his head, and predict the existence of gravitational waves. A century later, technology capable of seeing these waves would finally catch up with the prediction, just days before that gravity wave, which had been traveling for 1.3 billion years, washed over Earth and was detected. Yes, Einstein was a badass.
  • Here was the first direct evidence that a repulsive force permeated the universe, opposing gravity, which is how and why the cosmological constant rose from the dead. Lambda suddenly acquired a physical reality that needed a name, and so “ dark energy ” took center stage in the cosmic drama,
  • The most accurate measurements to date reveal dark energy as the most prominent thing in town, currently responsible for 68 percent of all the mass-energy in the universe; dark matter comprises 27 percent, with regular matter comprising a mere 5 percent.
  • If omega is less than one, the actual mass-energy falls below the critical value, and the universe expands forever in every direction for all of time, taking on the shape of a saddle, in which initially parallel lines diverge. If omega equals one, the universe expands forever, but only barely so. In that case the shape is flat, preserving all the geometric rules we learned in high school about parallel lines. If omega exceeds one, parallel lines converge, and the universe curves back on itself, ultimately recollapsing into the fireball whence it came.
  • . As a consequence, anything not gravitationally bound to the neighborhood of the Milky Way galaxy will recede at ever-increasing speed, as part of the accelerating expansion of the fabric of space-time.
  • In a trillion or so years, anyone alive in our own galaxy may know nothing of other galaxies. Our observable universe will merely comprise a system of nearby, long-lived stars within the Milky Way. And beyond this starry night will lie an endless void — darkness in the face of the deep.
  • Behold my recurring nightmare: Are we, too, missing some basic pieces of the universe that once were ? What part of the cosmic history book has been marked “ access denied ” ? What remains absent from our theories and equations that ought to be there, leaving us groping for answers we may never find ?

7. The Cosmos on the Table

  • Helium is widely recognized as an over-the-counter, low-density gas that, when inhaled, temporarily increases the vibrational frequency of your windpipe and larynx, making you sound like Mickey Mouse.
  • Macy’s Thanksgiving Day parade, making the department store second only to the U.S. military as the nation’s top user of the element.
  • As high-mass stars manufacture and accumulate iron in their cores, they are nearing death. Without a fertile source of energy, the star collapses under its own weight and instantly rebounds in a stupendous supernova explosion, outshining a billion suns for more than a week.
  • The metal mercury, liquid and runny at room temperature, and the planet Mercury, the fastest of all planets in the solar system, are both named for the speedy Roman messenger god of the same name.
  • One pound of plutonium will generate a half million kilowatt-hours of heat energy, enough to continuously power a household blender for a hundred years, or a human being for five times as long, if we ran on nuclear fuel instead of grocery-store food.

8. On Being Round

  • Using freshman-level calculus you can show that the one and only shape that has the smallest surface area for an enclosed volume is a perfect sphere. In fact, billions of dollars could be saved annually on packaging materials if all shipping boxes and all packages of food in the supermarket were spheres.
  • you should know that the spread in height from the deepest undersea trenches to the tallest mountains is about a dozen miles, yet Earth’s diameter is nearly eight thousand miles. So, contrary to what it looks like to teeny humans crawling on its surface, Earth, as a cosmic object, is remarkably smooth. If you had a super-duper, jumbo-gigantic finger, and you dragged it across Earth’s surface (oceans and all), Earth would feel as smooth as a cue ball.
  • Earth’s mountains are also puny when compared with some other mountains in the solar system. The largest on Mars, Olympus Mons, is 65,000 feet tall and nearly 300 miles wide at its base. It makes the highest of Earth's mountains look like molehills. The cosmic mountain-building recipe is simple: the weaker the gravity on the surface of an object, the higher its mountains can reach. Mount Everest is about as tall as a mountain on Earth can grow before the lower rock layers succumb to their own plasticity under the mountain’s weight.
  • Two famous celestial non-spheres are Phobos and Deimos, the Idaho potato – shaped moons of Mars. On thirteen-mile-long Phobos, the bigger of the two moons, an average person would weigh the same as five french fries.
  • The stars of the Milky Way galaxy trace a big, flat circle. With a diameter-to-thickness ratio of one hundred to one, our galaxy is flatter than the flattest flapjacks ever made. In fact, its proportions are better represented by a crépe or a tortilla.

9. Invisible Light

  • Herschel inadvertently discovered “ infra ” red light, a brand-new part of the spectrum found just “ below ” red, reported in the first of his four papers on the subject.
  • Filling out the entire electromagnetic spectrum, in order of low-energy and low-frequency to high-energy and high-frequency, we have: radio waves, microwaves, infrared, ROYGBIV, ultraviolet, X-rays, and gamma rays.
  • ALMA gives astrophysicists high-resolution access to categories of cosmic action unseen in other bands, such as the structure of collapsing gas clouds as they become nurseries from which stars are born.
  • Water may be fine for microwave cooking but it’s bad for astrophysicists, because the water vapor in Earth’s atmosphere chews up pristine microwave signals from across the galaxy and beyond.
  • Today, telescopes operate in every invisible part of the spectrum, some from the ground but most from space, where a telescope’s view is unimpeded by Earth’s absorptive atmosphere. We can now observe phenomena ranging from low-frequency radio waves a dozen meters long, crest to crest, to high-frequency gamma rays no longer than a quadrillionth of a meter.

10. Between the Planets

  • Interplanetary space is so not-empty that Earth, during its 30 kilometer-per-second orbital journey, plows through hundreds of tons of meteors per day — most of them no larger than a grain of sand. Nearly all of them burn in Earth’s upper atmosphere,
  • At the rate we are discovering meteorites on Earth whose origin is Mars, we conclude that about a thousand tons of Martian rocks rain down on Earth each year.
  • Most of the solar system’s asteroids live and work in the main asteroid belt, a roughly flat zone between the orbits of Mars and Jupiter.
  • The Dutch-born American astronomer Gerard Kuiper advanced the idea that in the cold depths of space, beyond the orbit of Neptune, there reside frozen leftovers from the formation of the solar system. Without a massive planet upon which to fall, most of these comets will orbit the Sun for billions more years.
  • Earth’s Moon is about 1 / 400th the diameter of the Sun, but it is also 1 / 400th as far from us, making the Sun and the Moon the same size in the sky — a coincidence not shared by any other planet – moon combination in the solar system, allowing for uniquely photogenic total solar eclipses.
  • Jupiter’s moon Europa has enough H2O that its heating mechanism — the same one at work on Io — has melted the subsurface ice, leaving a warmed ocean below. If ever there was a next-best place to look for life, it’s here.
  • And on Earth, the aurora borealis and australis (the northern and southern lights) serve as intermittent reminders of how nice it is to have a protective atmosphere.
  • We have exploited the gravitational fields of planets for nearly every probe launched into space. The Cassini probe, for example, which visited Saturn, was gravitationally assisted twice by Venus, once by Earth (on a return flyby), and once by Jupiter. Like a multi-cushion billiard shot, trajectories from one planet to another are common. Our tiny probes would not otherwise have enough speed and energy from our rockets to reach their destination.

11. Exoplanet Earth

  • Radio waves and microwaves might work. Maybe our eavesdropping aliens have something like the 500-meter radio telescope in the Guizhou province of China. If they do, and if they tune to the right frequencies, they’ll certainly notice Earth — or rather, they’ll notice our modern civilization as one of the most luminous sources in the sky. Consider everything we’ve got that generates radio waves and microwaves: not only traditional radio itself, but also broadcast television, mobile phones, microwave ovens, garage-door openers, car-door unlockers, commercial radar, military radar, and communications satellites. We’re ablaze in long-frequency waves — spectacular evidence that something unusual is going on here, because in their natural state, small rocky planets emit hardly any radio waves at all.
  • Latest estimates, extrapolating from the current catalogs, suggests as many as forty billion Earth-like planets in the Milky Way alone. Those are the planets our descendants might want to visit someday, by choice, if not by necessity.

12. Reflections on the Cosmic Perspective

  • In all fairness to the fellow, powerful forces in society leave most of us susceptible. As was I, until the day I learned in biology class that more bacteria live and work in one centimeter of my colon than the number of people who have ever existed in the world.
  • I know what you’re thinking: we’re smarter than bacteria. No doubt about it, we’re smarter than every other living creature that ever ran, crawled, or slithered on Earth. But how smart is that ? We cook our food. We compose poetry and music. We do art and science. We’re good at math. Even if you’re bad at math, you’re probably much better at it than the smartest chimpanzee, whose genetic identity varies in only trifling ways from ours. Try as they might, primatologists will never get a chimpanzee to do long division, or trigonometry.
  • If small genetic differences between us and our fellow apes account for what appears to be a vast difference in intelligence, then maybe that difference in intelligence is not so vast after all. Imagine a life-form whose brainpower is to ours as ours is to a chimpanzee’s. To such a species, our highest mental achievements would be trivial. Their toddlers, instead of learning their ABCs on Sesame Street, would learn multivariable calculus on Boolean Boulevard.
  • Take water. It’s common, and vital. There are more molecules of water in a cup of the stuff than there are cups of water in all the world’s oceans.
  • No way around it: some of the water you just drank passed through the kidneys of Socrates, Genghis Khan, and Joan of Arc.
  • There are more stars in the universe than grains of sand on any beach, more stars than seconds have passed since Earth formed, more stars than words and sounds ever uttered by all the humans who ever lived.
  • Several separate lines of research, when considered together, have forced investigators to reassess who we think we are and where we think we came from. As we’ve already seen, when a large asteroid strikes a planet, the surrounding areas can recoil from the impact energy, catapulting rocks into space. From there, they can travel to — and land on — other planetary surfaces. Second, microorganisms can be hardy. Extremophiles on Earth can survive wide ranges of temperature, pressure, and radiation encountered during space travel. If the rocky ejecta from an impact hails from a planet with life, then microscopic fauna could have stowed away in the rocks ’ nooks and crannies. Third, recent evidence suggests that shortly after the formation of our solar system, Mars was weet, and perhaps fertile, even before Earth was. Collectively, these findings tell us it’s conceivable that life began on Mars and later seeded life on Earth, a process known as panspermia. So all Earthlings might — just might — be descendants of Martians.
  • Today, how easy it is to presume that one universe is all there is. Yet emerging theories of modern cosmology, as well as the continually reaffirmed improbability that anything is unique, require that we remain open to the latest assault on our plea for distinctiveness: the multiverse.
  • The cosmic perspective reminds us that in space, where there is no air, a flag will not wave — an indication that perhaps flag-waving and space exploration do not mix.