It’s something my co-author Neil deGrasse Tyson has said many times on radio and TV. Indeed it is one of Neil’s signature sayings. When you are born, you become a citizen of the universe. It behooves you to look around, and get curious about your surroundings.
Neil felt a call from the universe on a first visit to the Hayden Planetarium when he was nine years old. As a city kid, he saw for the first time the glories of the nighttime sky displayed on the planetarium dome and decided at that moment to become an astronomer. Today he is the director of that institution.
In fact, we are all touched by the universe. The hydrogen in your body was forged in the birth of the universe itself, while the other elements in your body were made in distant, long-dead stars. When you call a friend on your mobile phone, you should thank astronomers. Mobile phone technology depends on Maxwell’s equations, whose verification depended on the fact that astronomers had already measured the speed of light. The GPS system that tells your phone where you are and helps you navigate, relies on Einstein’s theory of general relativity, which was verified by astronomers measuring the deflection of starlight passing near the Sun. Did you know there is an ultimate limit to how much information can ever be stored in a six-inch-diameter hard drive, and that it depends on black-hole physics? At a more mundane level, the seasons you experience every year depend directly on the tip of Earth’s axis relative to the plane of its orbit around the Sun.
One of the great triumphs of science in the last century was the discovery that the universe is expanding (and that it is 13.8 billion years old). The universe is stranger than you might think. Only 5% of the stuff of the universe is the material found in atoms. Fully 25% is “dark matter,” which is invisible, but whose existence can be inferred from its gravitational effects: holding clusters of galaxies together. It must be in the form of yet-to-be discovered elementary particles. The search is on to discover what they are.
The remaining 70% is “dark energy,” also invisible, but uniformly spread through the vacuum of space. We are used to thinking that a vacuum—empty space—should have a zero energy density. But we have discovered that the vacuum itself appears to have a uniform positive energy density. And if it does, it must have a uniform negative pressure as well, by the logic of Einstein’s theory of relativity. The room you are sitting in has a uniform pressure of 15 pounds per square inch. But you don’t notice it because it is uniform. Pressure differences are required to make the wind blow and knock you over. Since the negative pressure of dark energy is uniform, it exerts no hydrodynamic effects. But according to Einstein, it has gravitational effects, and because the pressure is negative it makes dark energy gravitationally repulsive. This is currently causing the universe to expand faster and faster.
This effect, observed in 1997, garnered the 2011 Nobel Prize in Physics. The Higgs Boson has recently been discovered at the Large Hadron Collider in Europe, bringing us one step closer to the hoped for theory of everything. The standard cosmological model, including normal atoms, dark matter, and dark energy, is now known with exquisite accuracy, thanks to results from the Hubble Space Telescope, the Sloan Digital Sky Survey, and the WMAP and Planck satellites.
The most exotic objects in the universe are black holes. Once you fall inside a black hole, you can never get back out. Astronomers have recently detected gravitational waves (ripples in the geometry of spacetime traveling at the speed of light) emitted when a 29-solar-mass black hole and a 36-solar-mass black hole collided. They coalesced to form a single, rotating 62-solar-mass black hole. Three solar masses worth of mass-energy was released in the form of gravitational radiation via Einstein’s famous formula E = mc2. You may know that this formula shows how a little bit of mass can be converted into a lot of energy, and that it is the secret behind the atomic bomb.
It has been exciting for us as astrophysicists to participate in this burgeoning of astronomical understanding. Neil is well known for his views on demoting Pluto from planetary status, ratified by the International Astronomical Union in a historic vote in 2006. It’s official now: our solar system has the Sun plus 8 planets, as well as asteroids, comets, and many icy bodies called Kuiper-belt objects, of which Pluto is one.
Meanwhile, thousands of new planets have been discovered circling other stars. Neil estimates that there may be as many as 1.8 billion Earthlike planets orbiting other stars within the Milky Way galaxy. How many of these may harbor extraterrestrial civilizations broadcasting in radio frequencies, available for us to detect today? Neil estimates it could be up to a hundred.
My co-author Michael Strauss discovered the most distant quasar known at the time. And I have worked on cosmic strings, the possibilities of time travel to the past in Einstein’s theory of general relativity, as well as how our universe may turn out to be part of an infinite multiverse. How did the universe originate? Are there extra, hidden dimensions? What will be the ultimate fate of the universe? These are questions astrophysicists are currently exploring which take us to the frontiers of physics.
Why is space colonization important, and why should we be doing it now while we have the chance? Will we be wise enough to become a multiplanet species with more chances for survival, or will we remain stranded on tiny Earth until some unfortunate event causes our extinction?
How big is the universe? You know how big a billion is. With a dollar you would be lucky to buy a hamburger. With a billion dollars you would be a billionaire. Shrink the Earth by a factor of a billion and it would be the size of a marble. At that scale—of 1/billion—the Moon would be a BB (1/8th of an inch across) sitting just 15 inches from the blue marble model of Earth. That’s as far as humans have traveled—to the Moon. The Sun would be a 55-inch-diameter beach ball sitting about 500 feet away. On this scale, the next nearest star, Proxima Centauri, would be placed 24,000 miles away. That gives you some idea of the challenges of interstellar travel. The radius of the visible universe, within the range of the Hubble Space Telescope, is indeed enormous, over 3 billion times further away than Proxima Centauri! Our Sun and Proxima Centauri are but two stars out of some 300 billion stars in the Milky Way galaxy. But our galaxy is not alone. There are 130 billion other galaxies within the range of the Hubble Space telescope, each containing over 100 billion stars.
The universe is awesome. This leaves many people thrilled, but feeling tiny and insignificant at the same time. But our aim is to empower you to understand the universe. That should make you feel strong. We have learned how gravity works, how stars evolve, and just how old the universe is. These are triumphs of human thought and observation—things that should make you proud to be a member of the human race.
Excerpted and adapted from Welcome to the Universe: An Astrophysical Tour by Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott. Reprinted by permission of the publisher, Princeton University Press.
J. Richard Gott is a Professor Emeritus of Astrophysical Sciences at Princeton University. He is noted for his contributions to cosmology and general relativity. He has received the Robert J. Trumpler Award, an Alfred P. Sloan Fellowship, the Astronomical League Award, and Princeton's President's Award for Distinguished Teaching. He was for many years Chair of the Judges for the Westinghouse and Intel Science Talent Search. His paper “On the Infall of Matter into Clusters of Galaxies and Some Effects on Their Evolution” co-authored with Jim Gunn has received over 1500 citations. He proposed that the clustering pattern of galaxies in the universe should be spongelike--a prediction now confirmed by numerous surveys. He discovered exact solutions to Einstein's field equations for the gravitational field around one cosmic string (in 1985) and two moving cosmic strings (in 1991). This second solution has been of particular interest because, if the strings move fast enough, at nearly the speed of light, time travel to the past can occur. His paper with Li-Xin Li, “Can the Universe Create Itself?” explores the idea of how the laws of physics may permit the universe to be its own mother. His book Time Travel in Einstein's Universe was selected by Booklist as one of four “Editors’ Choice” science books for 2001. He has published papers on map projections in Cartographica. His picture has appeared Time, Newsweek, and the New York Times. He wrote an article on time travel for Time magazine as part of its cover story on the future (April 10, 2000). His and Mario Juric’s Map of the Universe appeared in the New York Times (January 13, 2004), New Scientist, and Astronomy. Gott and Juric are in Guinness World Records 2006 for finding the largest structure in the universe: the Sloan Great Wall of Galaxies (1.37 billion light years long). Gott’s Copernican argument for space colonization was the subject of an article in the New York Times (July 17, 2007).
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